parser.inl

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// # This file is a part of toml++ and is subject to the the terms of the MIT license.
// # Copyright (c) Mark Gillard <mark.gillard@outlook.com.au>
// # See https://github.com/marzer/tomlplusplus/blob/master/LICENSE for the full license text.
//  SPDX-License-Identifier: MIT
#pragma once
 
#include "preprocessor.hpp"
// # {{
#if !TOML_IMPLEMENTATION
#error This is an implementation-only header.
#endif
// # }}
#if TOML_ENABLE_PARSER
 
#include "array.hpp"
#include "date_time.hpp"
#include "parse_error.hpp"
#include "parser.hpp"
#include "source_region.hpp"
#include "std_optional.hpp"
#include "table.hpp"
#include "unicode.hpp"
#include "value.hpp"
TOML_DISABLE_WARNINGS;
#include <fstream>
#include <istream>
#if TOML_INT_CHARCONV || TOML_FLOAT_CHARCONV
#include <charconv>
#endif
#if !TOML_INT_CHARCONV || !TOML_FLOAT_CHARCONV
#include <sstream>
#endif
#if !TOML_INT_CHARCONV
#include <iomanip>
#endif
TOML_ENABLE_WARNINGS;
#include "header_start.hpp"
 
// #---------------------------------------------------------------------------------------------------------------------
// # UTF8 STREAMS
// #---------------------------------------------------------------------------------------------------------------------
 
TOML_ANON_NAMESPACE_START {
  template <typename T>
  class utf8_byte_stream;
 
  TOML_INTERNAL_LINKAGE
  constexpr auto utf8_byte_order_mark = "\xEF\xBB\xBF"sv;
 
  template <typename Char>
  class utf8_byte_stream<std::basic_string_view<Char>> {
    static_assert(sizeof(Char) == 1);
 
   private:
    std::basic_string_view<Char> source_;
    size_t                       position_ = {};
 
   public:
    TOML_NODISCARD_CTOR
    explicit constexpr utf8_byte_stream(std::basic_string_view<Char> sv) noexcept  //
        : source_{sv} {
      // skip bom
      if (source_.length() >= 3u && memcmp(utf8_byte_order_mark.data(), source_.data(), 3u) == 0)
        position_ += 3u;
    }
 
    TOML_CONST_INLINE_GETTER
    constexpr bool error() const noexcept { return false; }
 
    TOML_PURE_INLINE_GETTER
    constexpr bool eof() const noexcept { return position_ >= source_.length(); }
 
    TOML_PURE_INLINE_GETTER
    explicit constexpr operator bool() const noexcept { return !eof(); }
 
    TOML_PURE_INLINE_GETTER
    constexpr bool peek_eof() const noexcept { return eof(); }
 
    TOML_NODISCARD
    TOML_ATTR(nonnull)
    size_t operator()(void* dest, size_t num) noexcept {
      TOML_ASSERT_ASSUME(!eof());
 
      num = impl::min(position_ + num, source_.length()) - position_;
      std::memcpy(dest, source_.data() + position_, num);
      position_ += num;
      return num;
    }
  };
 
  template <>
  class utf8_byte_stream<std::istream> {
   private:
    std::istream* source_;
 
   public:
    TOML_NODISCARD_CTOR
    explicit utf8_byte_stream(std::istream& stream) noexcept(!TOML_COMPILER_HAS_EXCEPTIONS)  //
        : source_{&stream} {
      if (!*this)  // eof, bad
        return;
 
      const auto initial_pos = source_->tellg();
      char       bom[3];
      source_->read(bom, 3);
      if (source_->bad() ||
          (source_->gcount() == 3 && memcmp(utf8_byte_order_mark.data(), bom, 3u) == 0))
        return;
 
      source_->clear();
      source_->seekg(initial_pos, std::istream::beg);
    }
 
    TOML_PURE_INLINE_GETTER
    bool error() const noexcept { return !!(source_->rdstate() & std::istream::badbit); }
 
    TOML_PURE_INLINE_GETTER
    bool eof() const noexcept { return !!(source_->rdstate() & std::istream::eofbit); }
 
    TOML_PURE_INLINE_GETTER
    explicit operator bool() const noexcept {
      return !(source_->rdstate() & (std::istream::badbit | std::istream::eofbit));
    }
 
    TOML_NODISCARD
    bool peek_eof() const noexcept(!TOML_COMPILER_HAS_EXCEPTIONS) {
      return eof() || source_->peek() == std::istream::traits_type::eof();
    }
 
    TOML_NODISCARD
    TOML_ATTR(nonnull)
    size_t operator()(void* dest, size_t num) noexcept(!TOML_COMPILER_HAS_EXCEPTIONS) {
      TOML_ASSERT(*this);
 
      source_->read(static_cast<char*>(dest), static_cast<std::streamsize>(num));
      return static_cast<size_t>(source_->gcount());
    }
  };
 
  struct utf8_codepoint {
    char32_t        value;
    char            bytes[4];
    size_t          count;
    source_position position;
 
    TOML_PURE_INLINE_GETTER
    constexpr operator const char32_t&() const noexcept { return value; }
 
    TOML_PURE_INLINE_GETTER
    constexpr const char32_t& operator*() const noexcept { return value; }
  };
  static_assert(std::is_trivial_v<utf8_codepoint>);
  static_assert(std::is_standard_layout_v<utf8_codepoint>);
 
  struct TOML_ABSTRACT_INTERFACE utf8_reader_interface {
    TOML_NODISCARD
    virtual const source_path_ptr& source_path() const noexcept = 0;
 
    TOML_NODISCARD
    virtual const utf8_codepoint* read_next() noexcept(!TOML_COMPILER_HAS_EXCEPTIONS) = 0;
 
    TOML_NODISCARD
    virtual bool peek_eof() const noexcept(!TOML_COMPILER_HAS_EXCEPTIONS) = 0;
 
#if !TOML_EXCEPTIONS
 
    TOML_NODISCARD
    virtual optional<parse_error>&& error() noexcept = 0;
 
#endif
 
    virtual ~utf8_reader_interface() noexcept = default;
  };
 
#if TOML_EXCEPTIONS
#define utf8_reader_error(...) throw parse_error(__VA_ARGS__)
#define utf8_reader_return_after_error(...) static_assert(true)
#define utf8_reader_error_check(...) static_assert(true)
#else
#define utf8_reader_error(...) err_.emplace(__VA_ARGS__)
#define utf8_reader_return_after_error(...) return __VA_ARGS__
#define utf8_reader_error_check(...)            \
  do {                                          \
    if TOML_UNLIKELY (err_) return __VA_ARGS__; \
  } while (false)
 
#endif
 
#if defined(__APPLE__) || defined(__MINGW32__) || defined(__MINGW64__)
#define TOML_OVERALIGNED
#else
#define TOML_OVERALIGNED alignas(32)
#endif
 
  template <typename T>
  class TOML_EMPTY_BASES utf8_reader final : public utf8_reader_interface {
   private:
    static constexpr size_t block_capacity = 32;
    utf8_byte_stream<T>     stream_;
    source_position         next_pos_ = {1, 1};
 
    impl::utf8_decoder decoder_;
    struct currently_decoding_t {
      char   bytes[4];
      size_t count;
    } currently_decoding_;
 
    struct codepoints_t {
      TOML_OVERALIGNED utf8_codepoint buffer[block_capacity];
      size_t                          current;
      size_t                          count;
    } codepoints_;
 
    source_path_ptr source_path_;
 
#if !TOML_EXCEPTIONS
    optional<parse_error> err_;
#endif
 
    bool read_next_block() noexcept(!TOML_COMPILER_HAS_EXCEPTIONS) {
      TOML_ASSERT(stream_);
 
      TOML_OVERALIGNED char raw_bytes[block_capacity];
      size_t                raw_bytes_read;
 
      // read the next raw (encoded) block in from the stream
      if constexpr (noexcept(stream_(raw_bytes, block_capacity)) || !TOML_EXCEPTIONS) {
        raw_bytes_read = stream_(raw_bytes, block_capacity);
      }
#if TOML_EXCEPTIONS
      else {
        try {
          raw_bytes_read = stream_(raw_bytes, block_capacity);
        } catch (const std::exception& exc) {
          throw parse_error{exc.what(), next_pos_, source_path_};
        } catch (...) {
          throw parse_error{"An unspecified error occurred", next_pos_, source_path_};
        }
      }
#endif  // TOML_EXCEPTIONS
 
      // handle a zero-byte read
      if TOML_UNLIKELY (!raw_bytes_read) {
        if (stream_.eof()) {
          // EOF only sets the error state if the decoder wants more input, otherwise
          // a zero-byte read might have just caused the underlying stream to realize it's exhaused
          // and set the EOF flag, and that's totally fine
          if (decoder_.needs_more_input())
            utf8_reader_error("Encountered EOF during incomplete utf-8 code point sequence",
                              next_pos_, source_path_);
        } else {
          utf8_reader_error("Reading from the underlying stream failed - zero bytes read",
                            next_pos_, source_path_);
        }
        return false;
      }
 
      TOML_ASSERT_ASSUME(raw_bytes_read);
      std::memset(&codepoints_, 0, sizeof(codepoints_));
 
      // helper for calculating decoded codepoint line+cols
      const auto calc_positions = [&]() noexcept {
        for (size_t i = 0; i < codepoints_.count; i++) {
          auto& cp    = codepoints_.buffer[i];
          cp.position = next_pos_;
 
          if (cp == U'\n') {
            next_pos_.line++;
            next_pos_.column = source_index{1};
          } else
            next_pos_.column++;
        }
      };
 
      // decide whether we need to use the UTF-8 decoder or if we can treat this block as plain
      // ASCII
      const auto ascii_fast_path =
          !decoder_.needs_more_input() && impl::is_ascii(raw_bytes, raw_bytes_read);
 
      // ASCII fast-path
      if (ascii_fast_path) {
        decoder_.reset();
        currently_decoding_.count = {};
 
        codepoints_.count = raw_bytes_read;
        for (size_t i = 0; i < codepoints_.count; i++) {
          auto& cp    = codepoints_.buffer[i];
          cp.value    = static_cast<char32_t>(raw_bytes[i]);
          cp.bytes[0] = raw_bytes[i];
          cp.count    = 1u;
        }
      }
 
      // UTF-8 slow-path
      else {
        // helper for getting precise error location
        const auto error_pos = [&]() noexcept -> const source_position& {  //
          return codepoints_.count ? codepoints_.buffer[codepoints_.count - 1u].position
                                   : next_pos_;
        };
 
        for (size_t i = 0; i < raw_bytes_read; i++) {
          decoder_(static_cast<uint8_t>(raw_bytes[i]));
          if TOML_UNLIKELY (decoder_.error()) {
            calc_positions();
            utf8_reader_error("Encountered invalid utf-8 sequence", error_pos(), source_path_);
            utf8_reader_return_after_error(false);
          }
 
          currently_decoding_.bytes[currently_decoding_.count++] = raw_bytes[i];
 
          if (decoder_.has_code_point()) {
            auto& cp = codepoints_.buffer[codepoints_.count++];
 
            cp.value = decoder_.codepoint;
            cp.count = currently_decoding_.count;
            std::memcpy(cp.bytes, currently_decoding_.bytes, currently_decoding_.count);
            currently_decoding_.count = {};
          } else if TOML_UNLIKELY (currently_decoding_.count == 4u) {
            calc_positions();
            utf8_reader_error("Encountered overlong utf-8 sequence", error_pos(), source_path_);
            utf8_reader_return_after_error(false);
          }
        }
        if TOML_UNLIKELY (decoder_.needs_more_input() && stream_.eof()) {
          calc_positions();
          utf8_reader_error("Encountered EOF during incomplete utf-8 code point sequence",
                            error_pos(), source_path_);
          utf8_reader_return_after_error(false);
        }
      }
 
      TOML_ASSERT_ASSUME(codepoints_.count);
      calc_positions();
 
      // handle general I/O errors
      // (down here so the next_pos_ benefits from calc_positions())
      if TOML_UNLIKELY (stream_.error()) {
        utf8_reader_error("An I/O error occurred while reading from the underlying stream",
                          next_pos_, source_path_);
        utf8_reader_return_after_error(false);
      }
 
      return true;
    }
 
   public:
    template <typename U, typename String = std::string_view>
    TOML_NODISCARD_CTOR explicit utf8_reader(U&& source, String&& source_path = {}) noexcept(
        std::is_nothrow_constructible_v<utf8_byte_stream<T>, U&&>)
        : stream_{static_cast<U&&>(source)} {
      currently_decoding_.count = {};
 
      codepoints_.current = {};
      codepoints_.count   = {};
 
      if (!source_path.empty())
        source_path_ = std::make_shared<const std::string>(static_cast<String&&>(source_path));
    }
 
    TOML_PURE_INLINE_GETTER
    const source_path_ptr& source_path() const noexcept final { return source_path_; }
 
    TOML_NODISCARD
    const utf8_codepoint* read_next() noexcept(!TOML_COMPILER_HAS_EXCEPTIONS) final {
      utf8_reader_error_check({});
 
      if (codepoints_.current == codepoints_.count) {
        if TOML_UNLIKELY (!stream_ || !read_next_block()) return nullptr;
 
        TOML_ASSERT_ASSUME(!codepoints_.current);
      }
      TOML_ASSERT_ASSUME(codepoints_.count);
      TOML_ASSERT_ASSUME(codepoints_.count <= block_capacity);
      TOML_ASSERT_ASSUME(codepoints_.current < codepoints_.count);
 
      return &codepoints_.buffer[codepoints_.current++];
    }
 
    TOML_NODISCARD
    bool peek_eof() const noexcept(!TOML_COMPILER_HAS_EXCEPTIONS) final {
      return stream_.peek_eof();
    }
 
#if !TOML_EXCEPTIONS
 
    TOML_NODISCARD
    optional<parse_error>&& error() noexcept final { return std::move(err_); }
 
#endif
  };
 
  template <typename Char>
  utf8_reader(std::basic_string_view<Char>, std::string_view)
      -> utf8_reader<std::basic_string_view<Char>>;
  template <typename Char>
  utf8_reader(std::basic_string_view<Char>, std::string&&)
      -> utf8_reader<std::basic_string_view<Char>>;
  template <typename Char>
  utf8_reader(std::basic_istream<Char>&, std::string_view) -> utf8_reader<std::basic_istream<Char>>;
  template <typename Char>
  utf8_reader(std::basic_istream<Char>&, std::string&&) -> utf8_reader<std::basic_istream<Char>>;
 
#if TOML_EXCEPTIONS
#define utf8_buffered_reader_error_check(...) static_assert(true)
#else
#define utf8_buffered_reader_error_check(...)              \
  do {                                                     \
    if TOML_UNLIKELY (reader_.error()) return __VA_ARGS__; \
  } while (false)
 
#endif
 
  class TOML_EMPTY_BASES utf8_buffered_reader {
   public:
    static constexpr size_t max_history_length = 128;
 
   private:
    static constexpr size_t history_buffer_size =
        max_history_length - 1;  //'head' is stored in the reader
    utf8_reader_interface& reader_;
    struct {
      utf8_codepoint buffer[history_buffer_size];
      size_t         count, first;
    } history_                             = {};
    const utf8_codepoint* head_            = {};
    size_t                negative_offset_ = {};
 
   public:
    TOML_NODISCARD_CTOR
    explicit utf8_buffered_reader(utf8_reader_interface& reader) noexcept  //
        : reader_{reader} {}
 
    TOML_PURE_INLINE_GETTER
    const source_path_ptr& source_path() const noexcept { return reader_.source_path(); }
 
    TOML_NODISCARD
    const utf8_codepoint* read_next() noexcept(!TOML_COMPILER_HAS_EXCEPTIONS) {
      utf8_buffered_reader_error_check({});
 
      if (negative_offset_) {
        negative_offset_--;
 
        // an entry negative offset of 1 just means "replay the current head"
        if (!negative_offset_) return head_;
 
        // otherwise step back into the history buffer
        else
          return history_.buffer +
                 ((history_.first + history_.count - negative_offset_) % history_buffer_size);
      } else {
        // first character read from stream
        if TOML_UNLIKELY (!history_.count && !head_) head_ = reader_.read_next();
 
        // subsequent characters and not eof
        else if (head_) {
          if TOML_UNLIKELY (history_.count < history_buffer_size)
            history_.buffer[history_.count++] = *head_;
          else
            history_.buffer[(history_.first++ + history_buffer_size) % history_buffer_size] =
                *head_;
 
          head_ = reader_.read_next();
        }
 
        return head_;
      }
    }
 
    TOML_NODISCARD
    const utf8_codepoint* step_back(size_t count) noexcept {
      utf8_buffered_reader_error_check({});
 
      TOML_ASSERT_ASSUME(history_.count);
      TOML_ASSERT_ASSUME(negative_offset_ + count <= history_.count);
 
      negative_offset_ += count;
 
      return negative_offset_
                 ? history_.buffer +
                       ((history_.first + history_.count - negative_offset_) % history_buffer_size)
                 : head_;
    }
 
    TOML_NODISCARD
    bool peek_eof() const noexcept(!TOML_COMPILER_HAS_EXCEPTIONS) { return reader_.peek_eof(); }
 
#if !TOML_EXCEPTIONS
 
    TOML_NODISCARD
    optional<parse_error>&& error() noexcept { return reader_.error(); }
 
#endif
  };
}
TOML_ANON_NAMESPACE_END;
 
// #---------------------------------------------------------------------------------------------------------------------
// # PARSER INTERNAL IMPLEMENTATION
// #---------------------------------------------------------------------------------------------------------------------
 
#if TOML_EXCEPTIONS
#define TOML_RETURNS_BY_THROWING [[noreturn]]
#else
#define TOML_RETURNS_BY_THROWING
#endif
 
TOML_ANON_NAMESPACE_START {
  template <typename... T>
  TOML_CONST_GETTER TOML_INTERNAL_LINKAGE constexpr bool is_match(char32_t codepoint,
                                                                  T... vals) noexcept {
    static_assert((std::is_same_v<char32_t, T> && ...));
    return ((codepoint == vals) || ...);
  }
 
  template <uint64_t>
  struct parse_integer_traits;
  template <>
  struct parse_integer_traits<2> {
    static constexpr auto scope_qualifier  = "binary integer"sv;
    static constexpr auto is_digit         = impl::is_binary_digit;
    static constexpr auto is_signed        = false;
    static constexpr auto max_digits       = 63;
    static constexpr auto prefix_codepoint = U'b';
    static constexpr auto prefix           = "b"sv;
    static constexpr auto full_prefix      = "0b"sv;
  };
  template <>
  struct parse_integer_traits<8> {
    static constexpr auto scope_qualifier  = "octal integer"sv;
    static constexpr auto is_digit         = impl::is_octal_digit;
    static constexpr auto is_signed        = false;
    static constexpr auto max_digits       = 21;  // strlen("777777777777777777777")
    static constexpr auto prefix_codepoint = U'o';
    static constexpr auto prefix           = "o"sv;
    static constexpr auto full_prefix      = "0o"sv;
  };
  template <>
  struct parse_integer_traits<10> {
    static constexpr auto scope_qualifier = "decimal integer"sv;
    static constexpr auto is_digit        = impl::is_decimal_digit;
    static constexpr auto is_signed       = true;
    static constexpr auto max_digits      = 19;  // strlen("9223372036854775807")
    static constexpr auto full_prefix     = ""sv;
  };
  template <>
  struct parse_integer_traits<16> {
    static constexpr auto scope_qualifier  = "hexadecimal integer"sv;
    static constexpr auto is_digit         = impl::is_hexadecimal_digit;
    static constexpr auto is_signed        = false;
    static constexpr auto max_digits       = 16;  // strlen("7FFFFFFFFFFFFFFF")
    static constexpr auto prefix_codepoint = U'x';
    static constexpr auto prefix           = "x"sv;
    static constexpr auto full_prefix      = "0x"sv;
  };
 
  TOML_PURE_GETTER
  TOML_INTERNAL_LINKAGE
  std::string_view to_sv(node_type val) noexcept {
    return impl::node_type_friendly_names[impl::unwrap_enum(val)];
  }
 
  TOML_PURE_GETTER
  TOML_INTERNAL_LINKAGE
  std::string_view to_sv(const std::string& str) noexcept {
    return std::string_view{str};
  }
 
  TOML_CONST_GETTER
  TOML_INTERNAL_LINKAGE
  std::string_view to_sv(bool val) noexcept {
    using namespace std::string_view_literals;
 
    return val ? "true"sv : "false"sv;
  }
 
  TOML_PURE_GETTER
  TOML_INTERNAL_LINKAGE
  std::string_view to_sv(const utf8_codepoint& cp) noexcept {
    if (cp.value <= U'\x1F')
      return impl::control_char_escapes[cp.value];
    else if (cp.value == U'\x7F')
      return "\\u007F"sv;
    else
      return std::string_view{cp.bytes, cp.count};
  }
 
  TOML_PURE_GETTER
  TOML_INTERNAL_LINKAGE
  std::string_view to_sv(const utf8_codepoint* cp) noexcept {
    if (cp) return to_sv(*cp);
    return ""sv;
  }
 
  struct escaped_codepoint {
    const utf8_codepoint& cp;
  };
 
  template <typename T>
  TOML_ATTR(nonnull)
  TOML_INTERNAL_LINKAGE void concatenate(char*& write_pos, char* const buf_end,
                                         const T& arg) noexcept {
    if TOML_UNLIKELY (write_pos >= buf_end) return;
 
    using arg_type = impl::remove_cvref<T>;
 
    // string views
    if constexpr (std::is_same_v<arg_type, std::string_view>) {
      const auto max_chars = static_cast<size_t>(buf_end - write_pos);
      const auto len       = max_chars < arg.length() ? max_chars : arg.length();
      std::memcpy(write_pos, arg.data(), len);
      write_pos += len;
    }
 
    // doubles
    else if constexpr (std::is_same_v<arg_type, double>) {
#if TOML_FLOAT_CHARCONV
      const auto result = std::to_chars(write_pos, buf_end, arg);
      write_pos         = result.ptr;
#else
      std::ostringstream ss;
      ss.imbue(std::locale::classic());
      ss.precision(std::numeric_limits<arg_type>::max_digits10);
      ss << arg;
      concatenate(write_pos, buf_end, to_sv(std::move(ss).str()));
#endif
    }
 
    // 64-bit integers
    else if constexpr (impl::is_one_of<arg_type, int64_t, uint64_t>) {
#if TOML_INT_CHARCONV
      const auto result = std::to_chars(write_pos, buf_end, arg);
      write_pos         = result.ptr;
#else
      std::ostringstream ss;
      ss.imbue(std::locale::classic());
      using cast_type = std::conditional_t<std::is_signed_v<arg_type>, int64_t, uint64_t>;
      ss << static_cast<cast_type>(arg);
      concatenate(write_pos, buf_end, to_sv(std::move(ss).str()));
#endif
    }
 
    // escaped_codepoint
    else if constexpr (std::is_same_v<arg_type, escaped_codepoint>) {
      if (arg.cp.value <= U'\x7F')
        concatenate(write_pos, buf_end, to_sv(arg.cp));
      else {
        auto           val     = static_cast<uint_least32_t>(arg.cp.value);
        const auto     digits  = val > 0xFFFFu ? 8u : 4u;
        constexpr auto mask    = uint_least32_t{0xFu};
        char           buf[10] = {'\\', digits > 4 ? 'U' : 'u'};
        for (auto i = 2u + digits; i-- > 2u;) {
          const auto hexdig = val & mask;
          buf[i] = static_cast<char>(hexdig >= 0xAu ? ('A' + (hexdig - 0xAu)) : ('0' + hexdig));
          val >>= 4;
        }
        concatenate(write_pos, buf_end, std::string_view{buf, digits + 2u});
      }
    }
 
    // all other floats (fallback - coerce to double)
    else if constexpr (std::is_floating_point_v<arg_type>)
      concatenate(write_pos, buf_end, static_cast<double>(arg));
 
    // all other integers (fallback - coerce to (u)int64_t)
    else if constexpr (std::is_arithmetic_v<arg_type> && std::is_integral_v<arg_type>) {
      using cast_type = std::conditional_t<std::is_unsigned_v<arg_type>, uint64_t, int64_t>;
      concatenate(write_pos, buf_end, static_cast<cast_type>(arg));
    }
 
    else {
      static_assert(impl::always_false<T>,
                    "concatenate() inputs are limited to std::string_views, integers, floats, and "
                    "escaped_codepoint");
    }
  }
 
  struct error_builder {
    static constexpr std::size_t buf_size = 512;
    char                         buf[buf_size];
    char*                        write_pos = buf;
    char* const max_write_pos = buf + (buf_size - std::size_t{1});  // allow for null terminator
 
    TOML_NODISCARD_CTOR
    error_builder(std::string_view scope) noexcept {
      concatenate(write_pos, max_write_pos, "Error while parsing "sv);
      concatenate(write_pos, max_write_pos, scope);
      concatenate(write_pos, max_write_pos, ": "sv);
    }
 
    template <typename T>
    void append(const T& arg) noexcept {
      concatenate(write_pos, max_write_pos, arg);
    }
 
    TOML_RETURNS_BY_THROWING
    auto finish(const source_position& pos, const source_path_ptr& source_path) const {
      *write_pos = '\0';
 
#if TOML_EXCEPTIONS
      throw parse_error{buf, pos, source_path};
#else
      return parse_error{std::string(buf, static_cast<size_t>(write_pos - buf)), pos, source_path};
#endif
    }
 
    TOML_DELETE_DEFAULTS(error_builder);
  };
 
  struct parse_scope {
    std::string_view& storage_;
    std::string_view  parent_;
 
    TOML_NODISCARD_CTOR
    explicit parse_scope(std::string_view& current_scope, std::string_view new_scope) noexcept
        : storage_{current_scope}, parent_{current_scope} {
      storage_ = new_scope;
    }
 
    ~parse_scope() noexcept { storage_ = parent_; }
 
    TOML_DELETE_DEFAULTS(parse_scope);
  };
#define push_parse_scope_2(scope, line) parse_scope ps_##line(current_scope, scope)
#define push_parse_scope_1(scope, line) push_parse_scope_2(scope, line)
#define push_parse_scope(scope) push_parse_scope_1(scope, __LINE__)
 
  struct parse_key_buffer {
    std::string                            buffer;
    std::vector<std::pair<size_t, size_t>> segments;
    std::vector<source_position>           starts;
    std::vector<source_position>           ends;
 
    void clear() noexcept {
      buffer.clear();
      segments.clear();
      starts.clear();
      ends.clear();
    }
 
    void push_back(std::string_view segment, source_position b, source_position e) {
      segments.push_back({buffer.length(), segment.length()});
      buffer.append(segment);
      starts.push_back(b);
      ends.push_back(e);
    }
 
    TOML_PURE_INLINE_GETTER
    std::string_view operator[](size_t i) const noexcept {
      return std::string_view{buffer.c_str() + segments[i].first, segments[i].second};
    }
 
    TOML_PURE_INLINE_GETTER
    std::string_view back() const noexcept { return (*this)[segments.size() - 1u]; }
 
    TOML_PURE_INLINE_GETTER
    bool empty() const noexcept { return segments.empty(); }
 
    TOML_PURE_INLINE_GETTER
    size_t size() const noexcept { return segments.size(); }
  };
 
  struct depth_counter_scope {
    size_t& depth_;
 
    TOML_NODISCARD_CTOR
    explicit depth_counter_scope(size_t& depth) noexcept  //
        : depth_{depth} {
      depth_++;
    }
 
    ~depth_counter_scope() noexcept { depth_--; }
 
    TOML_DELETE_DEFAULTS(depth_counter_scope);
  };
 
  struct parsed_string {
    std::string_view value;
    bool             was_multi_line;
  };
 
  struct table_vector_scope {
    std::vector<table*>& tables;
 
    TOML_NODISCARD_CTOR
    explicit table_vector_scope(std::vector<table*>& tables_, table& tbl)  //
        : tables{tables_} {
      tables.push_back(&tbl);
    }
 
    ~table_vector_scope() noexcept { tables.pop_back(); }
 
    TOML_DELETE_DEFAULTS(table_vector_scope);
  };
}
TOML_ANON_NAMESPACE_END;
 
#if 1  // parser helper macros
 
// Q: "what the fuck is this? MACROS????"
// A: The parser needs to work in exceptionless mode (returning error objects directly)
//    and exception mode (reporting parse failures by throwing). Two totally different control
//    flows. These macros encapsulate the differences between the two modes so I can write code code
//    as though I was only targeting one mode and not want yeet myself into the sun.
//    They're all #undef'd at the bottom of the parser's implementation so they should be harmless
//    outside of toml++.
 
#define is_eof() !cp
#define assert_not_eof() TOML_ASSERT_ASSUME(cp != nullptr)
#define return_if_eof(...)                          \
  do {                                              \
    if TOML_UNLIKELY (is_eof()) return __VA_ARGS__; \
  } while (false)
 
#if TOML_EXCEPTIONS
#define is_error() false
#define return_after_error(...) TOML_UNREACHABLE
#define assert_not_error() static_assert(true)
#define return_if_error(...) static_assert(true)
#define return_if_error_or_eof(...) return_if_eof(__VA_ARGS__)
#else
#define is_error() !!err
#define return_after_error(...) return __VA_ARGS__
#define assert_not_error() TOML_ASSERT(!is_error())
#define return_if_error(...)                          \
  do {                                                \
    if TOML_UNLIKELY (is_error()) return __VA_ARGS__; \
  } while (false)
#define return_if_error_or_eof(...)                               \
  do {                                                            \
    if TOML_UNLIKELY (is_eof() || is_error()) return __VA_ARGS__; \
  } while (false)
#endif
 
#if defined(TOML_BREAK_AT_PARSE_ERRORS) && TOML_BREAK_AT_PARSE_ERRORS
#if defined(__has_builtin)
#if __has_builtin(__builtin_debugtrap)
#define parse_error_break() __builtin_debugtrap()
#elif __has_builtin(__debugbreak)
#define parse_error_break() __debugbreak()
#endif
#endif
#ifndef parse_error_break
#if TOML_MSVC || TOML_ICC
#define parse_error_break() __debugbreak()
#else
#define parse_error_break() TOML_ASSERT(false)
#endif
#endif
#else
#define parse_error_break() static_assert(true)
#endif
 
#define set_error_and_return(ret, ...)       \
  do {                                       \
    if (!is_error()) set_error(__VA_ARGS__); \
    return_after_error(ret);                 \
  } while (false)
 
#define set_error_and_return_default(...) set_error_and_return({}, __VA_ARGS__)
 
#define set_error_and_return_if_eof(...)                                                        \
  do {                                                                                          \
    if TOML_UNLIKELY (is_eof()) set_error_and_return(__VA_ARGS__, "encountered end-of-file"sv); \
  } while (false)
 
#define advance_and_return_if_error(...) \
  do {                                   \
    assert_not_eof();                    \
    advance();                           \
    return_if_error(__VA_ARGS__);        \
  } while (false)
 
#define advance_and_return_if_error_or_eof(...) \
  do {                                          \
    assert_not_eof();                           \
    advance();                                  \
    return_if_error(__VA_ARGS__);               \
    set_error_and_return_if_eof(__VA_ARGS__);   \
  } while (false)
 
#endif  // parser helper macros
 
TOML_IMPL_NAMESPACE_START {
  TOML_ABI_NAMESPACE_BOOL(TOML_EXCEPTIONS, impl_ex, impl_noex);
 
  class parser {
   private:
    static constexpr size_t max_nested_values = TOML_MAX_NESTED_VALUES;
 
    utf8_buffered_reader  reader;
    table                 root;
    source_position       prev_pos = {1, 1};
    const utf8_codepoint* cp       = {};
    std::vector<table*>   implicit_tables;
    std::vector<table*>   dotted_key_tables;
    std::vector<table*>   open_inline_tables;
    std::vector<array*>   table_arrays;
    parse_key_buffer      key_buffer;
    std::string           string_buffer;
    std::string           recording_buffer;  // for diagnostics
    bool                  recording = false, recording_whitespace = true;
    std::string_view      current_scope;
    size_t                nested_values = {};
#if !TOML_EXCEPTIONS
    mutable optional<parse_error> err;
#endif
 
    TOML_NODISCARD
    source_position current_position(source_index fallback_offset = 0) const noexcept {
      if (!is_eof()) return cp->position;
      return {prev_pos.line, static_cast<source_index>(prev_pos.column + fallback_offset)};
    }
 
    template <typename... T>
    TOML_RETURNS_BY_THROWING TOML_NEVER_INLINE void set_error_at(source_position pos,
                                                                 const T&... reason) const {
      static_assert(sizeof...(T) > 0);
      return_if_error();
 
      error_builder builder{current_scope};
      (builder.append(reason), ...);
 
      parse_error_break();
 
#if TOML_EXCEPTIONS
      builder.finish(pos, reader.source_path());
#else
      err.emplace(builder.finish(pos, reader.source_path()));
#endif
    }
 
    template <typename... T>
    TOML_RETURNS_BY_THROWING void set_error(const T&... reason) const {
      set_error_at(current_position(1), reason...);
    }
 
    void go_back(size_t count = 1) noexcept {
      return_if_error();
      TOML_ASSERT_ASSUME(count);
 
      cp       = reader.step_back(count);
      prev_pos = cp->position;
    }
 
    void advance() {
      return_if_error();
      assert_not_eof();
 
      prev_pos = cp->position;
      cp       = reader.read_next();
 
#if !TOML_EXCEPTIONS
      if (reader.error()) {
        err = std::move(reader.error());
        return;
      }
#endif
 
      if (recording && !is_eof()) {
        if (recording_whitespace || !is_whitespace(*cp))
          recording_buffer.append(cp->bytes, cp->count);
      }
    }
 
    void start_recording(bool include_current = true) noexcept {
      return_if_error();
 
      recording            = true;
      recording_whitespace = true;
      recording_buffer.clear();
      if (include_current && !is_eof()) recording_buffer.append(cp->bytes, cp->count);
    }
 
    void stop_recording(size_t pop_bytes = 0) noexcept {
      return_if_error();
 
      recording = false;
      if (pop_bytes) {
        if (pop_bytes >= recording_buffer.length())
          recording_buffer.clear();
        else if (pop_bytes == 1u)
          recording_buffer.pop_back();
        else
          recording_buffer.erase(recording_buffer.begin() +
                                     static_cast<ptrdiff_t>(recording_buffer.length() - pop_bytes),
                                 recording_buffer.end());
      }
    }
 
    bool consume_leading_whitespace() {
      return_if_error_or_eof({});
 
      bool consumed = false;
      while (!is_eof() && is_horizontal_whitespace(*cp)) {
        if TOML_UNLIKELY (!is_ascii_horizontal_whitespace(*cp))
          set_error_and_return_default("expected space or tab, saw '"sv, escaped_codepoint{*cp},
                                       "'"sv);
 
        consumed = true;
        advance_and_return_if_error({});
      }
      return consumed;
    }
 
    bool consume_line_break() {
      return_if_error_or_eof({});
 
      if TOML_UNLIKELY (is_match(*cp, U'\v', U'\f'))
        set_error_and_return_default(
            R"(vertical tabs '\v' and form-feeds '\f' are not legal line breaks in TOML)"sv);
 
      if (*cp == U'\r') {
        advance_and_return_if_error({});  // skip \r
 
        if TOML_UNLIKELY (is_eof())
          set_error_and_return_default("expected '\\n' after '\\r', saw EOF"sv);
 
        if TOML_UNLIKELY (*cp != U'\n')
          set_error_and_return_default("expected '\\n' after '\\r', saw '"sv,
                                       escaped_codepoint{*cp}, "'"sv);
      } else if (*cp != U'\n')
        return false;
 
      advance_and_return_if_error({});  // skip \n
      return true;
    }
 
    bool consume_rest_of_line() {
      return_if_error_or_eof({});
 
      do {
        if (is_ascii_vertical_whitespace(*cp))
          return consume_line_break();
        else
          advance();
        return_if_error({});
      } while (!is_eof());
 
      return true;
    }
 
    bool consume_comment() {
      return_if_error_or_eof({});
 
      if (*cp != U'#') return false;
 
      push_parse_scope("comment"sv);
 
      advance_and_return_if_error({});  // skip the '#'
 
      while (!is_eof()) {
        if (consume_line_break()) return true;
        return_if_error({});
 
#if TOML_LANG_AT_LEAST(1, 0, 0)
 
        // toml/issues/567 (disallow non-TAB control characters in comments)
        if TOML_UNLIKELY (is_nontab_control_character(*cp))
          set_error_and_return_default(
              "control characters other than TAB (U+0009) are explicitly prohibited in comments"sv);
 
        // toml/pull/720 (disallow surrogates in comments)
        else if TOML_UNLIKELY (is_unicode_surrogate(*cp))
          set_error_and_return_default(
              "unicode surrogates (U+D800 to U+DFFF) are explicitly prohibited in comments"sv);
#endif
 
        advance_and_return_if_error({});
      }
 
      return true;
    }
 
    TOML_NODISCARD
    bool consume_expected_sequence(std::u32string_view seq) {
      return_if_error({});
      TOML_ASSERT(!seq.empty());
 
      for (auto c : seq) {
        set_error_and_return_if_eof({});
        if (*cp != c) return false;
        advance_and_return_if_error({});
      }
      return true;
    }
 
    template <typename T>
    TOML_NODISCARD bool consume_digit_sequence(T* digits, size_t len) {
      return_if_error({});
      TOML_ASSERT_ASSUME(digits);
      TOML_ASSERT_ASSUME(len);
 
      for (size_t i = 0; i < len; i++) {
        set_error_and_return_if_eof({});
        if (!is_decimal_digit(*cp)) return false;
 
        digits[i] = static_cast<T>(*cp - U'0');
        advance_and_return_if_error({});
      }
      return true;
    }
 
    template <typename T>
    TOML_NODISCARD size_t consume_variable_length_digit_sequence(T* buffer, size_t max_len) {
      return_if_error({});
      TOML_ASSERT_ASSUME(buffer);
      TOML_ASSERT_ASSUME(max_len);
 
      size_t i = {};
      for (; i < max_len; i++) {
        if (is_eof() || !is_decimal_digit(*cp)) break;
 
        buffer[i] = static_cast<T>(*cp - U'0');
        advance_and_return_if_error({});
      }
      return i;
    }
 
    TOML_NODISCARD
    TOML_NEVER_INLINE
    std::string_view parse_basic_string(bool multi_line) {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(*cp == U'"');
      push_parse_scope("string"sv);
 
      // skip the '"'
      advance_and_return_if_error_or_eof({});
 
      // multi-line strings ignore a single line ending right at the beginning
      if (multi_line) {
        consume_line_break();
        return_if_error({});
        set_error_and_return_if_eof({});
      }
 
      auto& str = string_buffer;
      str.clear();
      bool escaped             = false;
      bool skipping_whitespace = false;
      do {
        if (escaped) {
          escaped = false;
 
          // handle 'line ending slashes' in multi-line mode
          if (multi_line && is_whitespace(*cp)) {
            consume_leading_whitespace();
 
            if TOML_UNLIKELY (!consume_line_break())
              set_error_and_return_default(
                  "line-ending backslashes must be the last non-whitespace character on the line"sv);
 
            skipping_whitespace = true;
            return_if_error({});
            continue;
          }
 
          bool skip_escaped_codepoint = true;
          assert_not_eof();
          switch (const auto escaped_codepoint = *cp) {
            // 'regular' escape codes
            case U'b':
              str += '\b';
              break;
            case U'f':
              str += '\f';
              break;
            case U'n':
              str += '\n';
              break;
            case U'r':
              str += '\r';
              break;
            case U't':
              str += '\t';
              break;
            case U'"':
              str += '"';
              break;
            case U'\\':
              str += '\\';
              break;
 
#if TOML_LANG_UNRELEASED  // toml/pull/790 (\e shorthand for \x1B)
            case U'e':
              str += '\x1B';
              break;
#else
            case U'e':
              set_error_and_return_default(
                  "escape sequence '\\e' is not supported in TOML 1.0.0 and earlier"sv);
#endif
 
#if TOML_LANG_UNRELEASED  // toml/pull/796 (\xHH unicode scalar sequences)
            case U'x':
              [[fallthrough]];
#else
            case U'x':
              set_error_and_return_default(
                  "escape sequence '\\x' is not supported in TOML 1.0.0 and earlier"sv);
#endif
 
            // unicode scalar sequences
            case U'u':
              [[fallthrough]];
            case U'U': {
              push_parse_scope("unicode scalar sequence"sv);
              advance_and_return_if_error_or_eof({});
              skip_escaped_codepoint = false;
 
              uint32_t place_value = escaped_codepoint == U'U'
                                         ? 0x10000000u
                                         : (escaped_codepoint == U'u' ? 0x1000u : 0x10u);
              uint32_t sequence_value{};
              while (place_value) {
                set_error_and_return_if_eof({});
 
                if TOML_UNLIKELY (!is_hexadecimal_digit(*cp))
                  set_error_and_return_default("expected hex digit, saw '"sv, to_sv(*cp), "'"sv);
 
                sequence_value += place_value * hex_to_dec(*cp);
                place_value /= 16u;
                advance_and_return_if_error({});
              }
 
              if TOML_UNLIKELY (is_unicode_surrogate(sequence_value))
                set_error_and_return_default(
                    "unicode surrogates (U+D800 - U+DFFF) are explicitly prohibited"sv);
              else if TOML_UNLIKELY (sequence_value > 0x10FFFFu)
                set_error_and_return_default("values greater than U+10FFFF are invalid"sv);
 
              if (sequence_value < 0x80) {
                str += static_cast<char>(sequence_value);
              } else if (sequence_value < 0x800u) {
                str += static_cast<char>((sequence_value >> 6) | 0xC0u);
                str += static_cast<char>((sequence_value & 0x3Fu) | 0x80u);
              } else if (sequence_value < 0x10000u) {
                str += static_cast<char>((sequence_value >> 12) | 0xE0u);
                str += static_cast<char>(((sequence_value >> 6) & 0x3Fu) | 0x80u);
                str += static_cast<char>((sequence_value & 0x3Fu) | 0x80u);
              } else if (sequence_value < 0x110000u) {
                str += static_cast<char>((sequence_value >> 18) | 0xF0u);
                str += static_cast<char>(((sequence_value >> 12) & 0x3Fu) | 0x80u);
                str += static_cast<char>(((sequence_value >> 6) & 0x3Fu) | 0x80u);
                str += static_cast<char>((sequence_value & 0x3Fu) | 0x80u);
              }
              break;
            }
 
              // ???
              TOML_UNLIKELY_CASE
            default:
              set_error_and_return_default("unknown escape sequence '\\"sv, to_sv(*cp), "'"sv);
          }
 
          if (skip_escaped_codepoint) advance_and_return_if_error_or_eof({});
        } else {
          // handle closing delimiters
          if (*cp == U'"') {
            if (multi_line) {
              size_t lookaheads             = {};
              size_t consecutive_delimiters = 1;
              do {
                advance_and_return_if_error({});
                lookaheads++;
                if (!is_eof() && *cp == U'"')
                  consecutive_delimiters++;
                else
                  break;
              } while (lookaheads < 4u);
 
              switch (consecutive_delimiters) {
                // """ " (one quote somewhere in a ML string)
                case 1:
                  str += '"';
                  skipping_whitespace = false;
                  continue;
 
                // """ "" (two quotes somewhere in a ML string)
                case 2:
                  str.append("\"\""sv);
                  skipping_whitespace = false;
                  continue;
 
                // """ """ (the end of the string)
                case 3:
                  return str;
 
                // """ """" (one at the end of the string)
                case 4:
                  str += '"';
                  return str;
 
                // """ """"" (two quotes at the end of the string)
                case 5:
                  str.append("\"\""sv);
                  advance_and_return_if_error({});  // skip the last '"'
                  return str;
 
                default:
                  TOML_UNREACHABLE;
              }
            } else {
              advance_and_return_if_error({});  // skip the closing delimiter
              return str;
            }
          }
 
          // handle escapes
          else if (*cp == U'\\') {
            advance_and_return_if_error_or_eof({});  // skip the '\'
            skipping_whitespace = false;
            escaped             = true;
            continue;
          }
 
          // handle line endings in multi-line mode
          if (multi_line && is_ascii_vertical_whitespace(*cp)) {
            consume_line_break();
            return_if_error({});
            if (!skipping_whitespace) str += '\n';
            continue;
          }
 
          // handle control characters
          if TOML_UNLIKELY (is_nontab_control_character(*cp))
            set_error_and_return_default(
                "unescaped control characters other than TAB (U+0009) are explicitly prohibited"sv);
 
#if TOML_LANG_AT_LEAST(1, 0, 0)
 
          // handle surrogates in strings
          if TOML_UNLIKELY (is_unicode_surrogate(*cp))
            set_error_and_return_default(
                "unescaped unicode surrogates (U+D800 to U+DFFF) are explicitly prohibited"sv);
#endif
 
          if (multi_line) {
            if (!skipping_whitespace || !is_horizontal_whitespace(*cp)) {
              skipping_whitespace = false;
              str.append(cp->bytes, cp->count);
            }
          } else
            str.append(cp->bytes, cp->count);
 
          advance_and_return_if_error({});
        }
      } while (!is_eof());
 
      set_error_and_return_default("encountered end-of-file"sv);
    }
 
    TOML_NODISCARD
    TOML_NEVER_INLINE
    std::string_view parse_literal_string(bool multi_line) {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(*cp == U'\'');
      push_parse_scope("literal string"sv);
 
      // skip the delimiter
      advance_and_return_if_error_or_eof({});
 
      // multi-line strings ignore a single line ending right at the beginning
      if (multi_line) {
        consume_line_break();
        return_if_error({});
        set_error_and_return_if_eof({});
      }
 
      auto& str = string_buffer;
      str.clear();
      do {
        return_if_error({});
 
        // handle closing delimiters
        if (*cp == U'\'') {
          if (multi_line) {
            size_t lookaheads             = {};
            size_t consecutive_delimiters = 1;
            do {
              advance_and_return_if_error({});
              lookaheads++;
              if (!is_eof() && *cp == U'\'')
                consecutive_delimiters++;
              else
                break;
            } while (lookaheads < 4u);
 
            switch (consecutive_delimiters) {
              // ''' ' (one quote somewhere in a ML string)
              case 1:
                str += '\'';
                continue;
 
              // ''' '' (two quotes somewhere in a ML string)
              case 2:
                str.append("''"sv);
                continue;
 
              // ''' ''' (the end of the string)
              case 3:
                return str;
 
              // ''' '''' (one at the end of the string)
              case 4:
                str += '\'';
                return str;
 
              // ''' ''''' (two quotes at the end of the string)
              case 5:
                str.append("''"sv);
                advance_and_return_if_error({});  // skip the last '
                return str;
 
              default:
                TOML_UNREACHABLE;
            }
          } else {
            advance_and_return_if_error({});  // skip the closing delimiter
            return str;
          }
        }
 
        // handle line endings in multi-line mode
        if (multi_line && is_ascii_vertical_whitespace(*cp)) {
          consume_line_break();
          return_if_error({});
          str += '\n';
          continue;
        }
 
        // handle control characters
        if TOML_UNLIKELY (is_nontab_control_character(*cp))
          set_error_and_return_default(
              "control characters other than TAB (U+0009) are explicitly prohibited"sv);
 
#if TOML_LANG_AT_LEAST(1, 0, 0)
 
        // handle surrogates in strings
        if TOML_UNLIKELY (is_unicode_surrogate(*cp))
          set_error_and_return_default(
              "unicode surrogates (U+D800 - U+DFFF) are explicitly prohibited"sv);
#endif
 
        str.append(cp->bytes, cp->count);
        advance_and_return_if_error({});
      } while (!is_eof());
 
      set_error_and_return_default("encountered end-of-file"sv);
    }
 
    TOML_NODISCARD
    TOML_NEVER_INLINE
    parsed_string parse_string() {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(is_string_delimiter(*cp));
      push_parse_scope("string"sv);
 
      // get the first three characters to determine the string type
      const auto first = cp->value;
      advance_and_return_if_error_or_eof({});
      const auto second = cp->value;
      advance_and_return_if_error({});
      const auto third = cp ? cp->value : U'\0';
 
      // if we were eof at the third character then first and second need to be
      // the same string character (otherwise it's an unterminated string)
      if (is_eof()) {
        if (second == first) return {};
 
        set_error_and_return_default("encountered end-of-file"sv);
      }
 
      // if the first three characters are all the same string delimiter then
      // it's a multi-line string.
      else if (first == second && first == third) {
        return {first == U'\'' ? parse_literal_string(true) : parse_basic_string(true), true};
      }
 
      // otherwise it's just a regular string.
      else {
        // step back two characters so that the current
        // character is the string delimiter
        go_back(2u);
 
        return {first == U'\'' ? parse_literal_string(false) : parse_basic_string(false), false};
      }
    }
 
    TOML_NODISCARD
    TOML_NEVER_INLINE
    std::string_view parse_bare_key_segment() {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(is_bare_key_character(*cp));
 
      string_buffer.clear();
 
      while (!is_eof()) {
        if (!is_bare_key_character(*cp)) break;
 
        string_buffer.append(cp->bytes, cp->count);
        advance_and_return_if_error({});
      }
 
      return string_buffer;
    }
 
    TOML_NODISCARD
    TOML_NEVER_INLINE
    bool parse_boolean() {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(is_match(*cp, U't', U'f', U'T', U'F'));
      push_parse_scope("boolean"sv);
 
      start_recording(true);
      auto result = is_match(*cp, U't', U'T');
      if (!consume_expected_sequence(result ? U"true"sv : U"false"sv))
        set_error_and_return_default("expected '"sv, to_sv(result), "', saw '"sv,
                                     to_sv(recording_buffer), "'"sv);
      stop_recording();
 
      if (cp && !is_value_terminator(*cp))
        set_error_and_return_default("expected value-terminator, saw '"sv, to_sv(*cp), "'"sv);
 
      return result;
    }
 
    TOML_NODISCARD
    TOML_NEVER_INLINE
    double parse_inf_or_nan() {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(is_match(*cp, U'i', U'n', U'I', U'N', U'+', U'-'));
      push_parse_scope("floating-point"sv);
 
      start_recording(true);
      const bool negative = *cp == U'-';
      if (negative || *cp == U'+') advance_and_return_if_error_or_eof({});
 
      const bool inf = is_match(*cp, U'i', U'I');
      if (!consume_expected_sequence(inf ? U"inf"sv : U"nan"sv))
        set_error_and_return_default("expected '"sv, inf ? "inf"sv : "nan"sv, "', saw '"sv,
                                     to_sv(recording_buffer), "'"sv);
      stop_recording();
 
      if (cp && !is_value_terminator(*cp))
        set_error_and_return_default("expected value-terminator, saw '"sv, to_sv(*cp), "'"sv);
 
      return inf ? (negative ? -std::numeric_limits<double>::infinity()
                             : std::numeric_limits<double>::infinity())
                 : std::numeric_limits<double>::quiet_NaN();
    }
 
    TOML_NODISCARD
    TOML_NEVER_INLINE
    double parse_float() {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(is_match(*cp, U'+', U'-', U'.') || is_decimal_digit(*cp));
      push_parse_scope("floating-point"sv);
 
      // sign
      const int sign = *cp == U'-' ? -1 : 1;
      if (is_match(*cp, U'+', U'-')) advance_and_return_if_error_or_eof({});
 
      // consume value chars
      char                  chars[utf8_buffered_reader::max_history_length];
      size_t                length       = {};
      const utf8_codepoint* prev         = {};
      bool                  seen_decimal = false, seen_exponent = false;
      char                  first_integer_part = '\0';
      while (!is_eof() && !is_value_terminator(*cp)) {
        if (*cp == U'_') {
          if (!prev || !is_decimal_digit(*prev))
            set_error_and_return_default("underscores may only follow digits"sv);
 
          prev = cp;
          advance_and_return_if_error_or_eof({});
          continue;
        } else if TOML_UNLIKELY (prev && *prev == U'_' && !is_decimal_digit(*cp))
          set_error_and_return_default("underscores must be followed by digits"sv);
        else if TOML_UNLIKELY (length == sizeof(chars))
          set_error_and_return_default(
              "exceeds length limit of "sv, sizeof(chars), " digits"sv,
              (seen_exponent ? ""sv : " (consider using exponent notation)"sv));
        else if (*cp == U'.') {
          // .1
          // -.1
          // +.1 (no integer part)
          if (!first_integer_part)
            set_error_and_return_default("expected decimal digit, saw '.'"sv);
 
          // 1.0e+.10 (exponent cannot have '.')
          else if (seen_exponent)
            set_error_and_return_default("expected exponent decimal digit or sign, saw '.'"sv);
 
          // 1.0.e+.10
          // 1..0
          // (multiple '.')
          else if (seen_decimal)
            set_error_and_return_default("expected decimal digit or exponent, saw '.'"sv);
 
          seen_decimal = true;
        } else if (is_match(*cp, U'e', U'E')) {
          if (prev && !is_decimal_digit(*prev))
            set_error_and_return_default("expected decimal digit, saw '"sv, to_sv(*cp), "'"sv);
 
          // 1.0ee+10 (multiple 'e')
          else if (seen_exponent)
            set_error_and_return_default("expected decimal digit, saw '"sv, to_sv(*cp), "'"sv);
 
          seen_decimal  = true;  // implied
          seen_exponent = true;
        } else if (is_match(*cp, U'+', U'-')) {
          // 1.-0 (sign in mantissa)
          if (!seen_exponent)
            set_error_and_return_default("expected decimal digit or '.', saw '"sv, to_sv(*cp),
                                         "'"sv);
 
          // 1.0e1-0 (misplaced exponent sign)
          else if (!is_match(*prev, U'e', U'E'))
            set_error_and_return_default("expected exponent digit, saw '"sv, to_sv(*cp), "'"sv);
        } else if (is_decimal_digit(*cp)) {
          if (!seen_decimal) {
            if (!first_integer_part)
              first_integer_part = static_cast<char>(cp->bytes[0]);
            else if (first_integer_part == '0')
              set_error_and_return_default("leading zeroes are prohibited"sv);
          }
        } else
          set_error_and_return_default("expected decimal digit, saw '"sv, to_sv(*cp), "'"sv);
 
        chars[length++] = static_cast<char>(cp->bytes[0]);
        prev            = cp;
        advance_and_return_if_error({});
      }
 
      // sanity-check ending state
      if (prev) {
        if (*prev == U'_') {
          set_error_and_return_if_eof({});
          set_error_and_return_default("underscores must be followed by digits"sv);
        } else if (is_match(*prev, U'e', U'E', U'+', U'-', U'.')) {
          set_error_and_return_if_eof({});
          set_error_and_return_default("expected decimal digit, saw '"sv, to_sv(*cp), "'"sv);
        }
      }
 
      // convert to double
      double result;
#if TOML_FLOAT_CHARCONV
      {
        auto fc_result = std::from_chars(chars, chars + length, result);
        switch (fc_result.ec) {
          TOML_LIKELY_CASE
          case std::errc{}:  // ok
            return result * sign;
 
          case std::errc::invalid_argument:
            set_error_and_return_default("'"sv, std::string_view{chars, length},
                                         "' could not be interpreted as a value"sv);
            break;
 
          case std::errc::result_out_of_range:
            set_error_and_return_default("'"sv, std::string_view{chars, length},
                                         "' is not representable in 64 bits"sv);
            break;
 
          default:  //??
            set_error_and_return_default(
                "an unspecified error occurred while trying to interpret '"sv,
                std::string_view{chars, length}, "' as a value"sv);
        }
      }
#else
      {
        std::stringstream ss;
        ss.imbue(std::locale::classic());
        ss.write(chars, static_cast<std::streamsize>(length));
        if ((ss >> result))
          return result * sign;
        else
          set_error_and_return_default("'"sv, std::string_view{chars, length},
                                       "' could not be interpreted as a value"sv);
      }
#endif
    }
 
    TOML_NODISCARD
    TOML_NEVER_INLINE
    double parse_hex_float() {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(is_match(*cp, U'0', U'+', U'-'));
      push_parse_scope("hexadecimal floating-point"sv);
 
#if TOML_LANG_UNRELEASED  // toml/issues/562 (hexfloats)
 
      // sign
      const int sign = *cp == U'-' ? -1 : 1;
      if (is_match(*cp, U'+', U'-')) advance_and_return_if_error_or_eof({});
 
      // '0'
      if (*cp != U'0') set_error_and_return_default(" expected '0', saw '"sv, to_sv(*cp), "'"sv);
      advance_and_return_if_error_or_eof({});
 
      // 'x' or 'X'
      if (!is_match(*cp, U'x', U'X'))
        set_error_and_return_default("expected 'x' or 'X', saw '"sv, to_sv(*cp), "'"sv);
      advance_and_return_if_error_or_eof({});
 
      // <HEX DIGITS> ([.]<HEX DIGITS>)? [pP] [+-]? <DEC DIGITS>
 
      // consume value fragments
      struct fragment {
        char   chars[24];
        size_t length;
        double value;
      };
      fragment fragments[] = {
          {},  // mantissa, whole part
          {},  // mantissa, fractional part
          {}   // exponent
      };
      fragment*             current_fragment = fragments;
      const utf8_codepoint* prev             = {};
      int                   exponent_sign    = 1;
      while (!is_eof() && !is_value_terminator(*cp)) {
        if (*cp == U'_') {
          if (!prev || !is_hexadecimal_digit(*prev))
            set_error_and_return_default("underscores may only follow digits"sv);
 
          prev = cp;
          advance_and_return_if_error_or_eof({});
          continue;
        } else if (prev && *prev == U'_' && !is_hexadecimal_digit(*cp))
          set_error_and_return_default("underscores must be followed by digits"sv);
        else if (*cp == U'.') {
          // 0x10.0p-.0 (exponent cannot have '.')
          if (current_fragment == fragments + 2)
            set_error_and_return_default("expected exponent digit or sign, saw '.'"sv);
 
          // 0x10.0.p-0 (multiple '.')
          else if (current_fragment == fragments + 1)
            set_error_and_return_default("expected hexadecimal digit or exponent, saw '.'"sv);
 
          else
            current_fragment++;
        } else if (is_match(*cp, U'p', U'P')) {
          // 0x10.0pp-0 (multiple 'p')
          if (current_fragment == fragments + 2)
            set_error_and_return_default("expected exponent digit or sign, saw '"sv, to_sv(*cp),
                                         "'"sv);
 
          // 0x.p-0 (mantissa is just '.')
          else if (fragments[0].length == 0u && fragments[1].length == 0u)
            set_error_and_return_default("expected hexadecimal digit, saw '"sv, to_sv(*cp), "'"sv);
 
          else
            current_fragment = fragments + 2;
        } else if (is_match(*cp, U'+', U'-')) {
          // 0x-10.0p-0 (sign in mantissa)
          if (current_fragment != fragments + 2)
            set_error_and_return_default("expected hexadecimal digit or '.', saw '"sv, to_sv(*cp),
                                         "'"sv);
 
          // 0x10.0p0- (misplaced exponent sign)
          else if (!is_match(*prev, U'p', U'P'))
            set_error_and_return_default("expected exponent digit, saw '"sv, to_sv(*cp), "'"sv);
 
          else
            exponent_sign = *cp == U'-' ? -1 : 1;
        } else if (current_fragment < fragments + 2 && !is_hexadecimal_digit(*cp))
          set_error_and_return_default("expected hexadecimal digit or '.', saw '"sv, to_sv(*cp),
                                       "'"sv);
        else if (current_fragment == fragments + 2 && !is_decimal_digit(*cp))
          set_error_and_return_default("expected exponent digit or sign, saw '"sv, to_sv(*cp),
                                       "'"sv);
        else if (current_fragment->length == sizeof(fragment::chars))
          set_error_and_return_default("fragment exceeeds maximum length of "sv,
                                       sizeof(fragment::chars), " characters"sv);
        else
          current_fragment->chars[current_fragment->length++] = static_cast<char>(cp->bytes[0]);
 
        prev = cp;
        advance_and_return_if_error({});
      }
 
      // sanity-check ending state
      if (current_fragment != fragments + 2 || current_fragment->length == 0u) {
        set_error_and_return_if_eof({});
        set_error_and_return_default("missing exponent"sv);
      } else if (prev && *prev == U'_') {
        set_error_and_return_if_eof({});
        set_error_and_return_default("underscores must be followed by digits"sv);
      }
 
      // calculate values for the three fragments
      for (int fragment_idx = 0; fragment_idx < 3; fragment_idx++) {
        auto&          f    = fragments[fragment_idx];
        const uint32_t base = fragment_idx == 2 ? 10u : 16u;
 
        // left-trim zeroes
        const char* c   = f.chars;
        size_t      sig = {};
        while (f.length && *c == '0') {
          f.length--;
          c++;
          sig++;
        }
        if (!f.length) continue;
 
        // calculate value
        auto place = 1u;
        for (size_t i = 0; i < f.length - 1u; i++) place *= base;
        uint32_t val{};
        while (place) {
          if (base == 16)
            val += place * hex_to_dec(*c);
          else
            val += place * static_cast<uint32_t>(*c - '0');
          if (fragment_idx == 1) sig++;
          c++;
          place /= base;
        }
        f.value = static_cast<double>(val);
 
        // shift the fractional part
        if (fragment_idx == 1) {
          while (sig--) f.value /= base;
        }
      }
 
      return (fragments[0].value + fragments[1].value) *
             pow(2.0, fragments[2].value * exponent_sign) * sign;
 
#else  // !TOML_LANG_UNRELEASED
 
      set_error_and_return_default(
          "hexadecimal floating-point values are not supported "
          "in TOML 1.0.0 and earlier"sv);
 
#endif  // !TOML_LANG_UNRELEASED
    }
 
    template <uint64_t base>
    TOML_NODISCARD TOML_NEVER_INLINE int64_t parse_integer() {
      return_if_error({});
      assert_not_eof();
      using traits = parse_integer_traits<base>;
      push_parse_scope(traits::scope_qualifier);
 
      [[maybe_unused]] int64_t sign = 1;
      if constexpr (traits::is_signed) {
        sign = *cp == U'-' ? -1 : 1;
        if (is_match(*cp, U'+', U'-')) advance_and_return_if_error_or_eof({});
      }
 
      if constexpr (base == 10) {
        if (!traits::is_digit(*cp))
          set_error_and_return_default("expected expected digit or sign, saw '"sv, to_sv(*cp),
                                       "'"sv);
      } else {
        // '0'
        if (*cp != U'0') set_error_and_return_default("expected '0', saw '"sv, to_sv(*cp), "'"sv);
        advance_and_return_if_error_or_eof({});
 
        // 'b', 'o', 'x'
        if (*cp != traits::prefix_codepoint)
          set_error_and_return_default("expected '"sv, traits::prefix, "', saw '"sv, to_sv(*cp),
                                       "'"sv);
        advance_and_return_if_error_or_eof({});
 
        if (!traits::is_digit(*cp))
          set_error_and_return_default("expected digit, saw '"sv, to_sv(*cp), "'"sv);
      }
 
      // consume digits
      char                  digits[utf8_buffered_reader::max_history_length];
      size_t                length = {};
      const utf8_codepoint* prev   = {};
      while (!is_eof() && !is_value_terminator(*cp)) {
        if (*cp == U'_') {
          if (!prev || !traits::is_digit(*prev))
            set_error_and_return_default("underscores may only follow digits"sv);
 
          prev = cp;
          advance_and_return_if_error_or_eof({});
          continue;
        } else if TOML_UNLIKELY (prev && *prev == U'_' && !traits::is_digit(*cp))
          set_error_and_return_default("underscores must be followed by digits"sv);
        else if TOML_UNLIKELY (!traits::is_digit(*cp))
          set_error_and_return_default("expected digit, saw '"sv, to_sv(*cp), "'"sv);
        else if TOML_UNLIKELY (length == sizeof(digits))
          set_error_and_return_default("exceeds length limit of "sv, sizeof(digits), " digits"sv);
        else
          digits[length++] = static_cast<char>(cp->bytes[0]);
 
        prev = cp;
        advance_and_return_if_error({});
      }
 
      // sanity check ending state
      if (prev && *prev == U'_') {
        set_error_and_return_if_eof({});
        set_error_and_return_default("underscores must be followed by digits"sv);
      }
 
      // single digits can be converted trivially
      if (length == 1u) {
        int64_t result;
 
        if constexpr (base == 16)
          result = static_cast<int64_t>(hex_to_dec(digits[0]));
        else
          result = static_cast<int64_t>(digits[0] - '0');
 
        if constexpr (traits::is_signed) result *= sign;
 
        return result;
      }
 
      // bin, oct and hex allow leading zeroes so trim them first
      const char* end = digits + length;
      const char* msd = digits;
      if constexpr (base != 10) {
        while (msd < end && *msd == '0') msd++;
        if (msd == end) return 0ll;
      }
 
      // decimal integers do not allow leading zeroes
      else {
        if TOML_UNLIKELY (digits[0] == '0')
          set_error_and_return_default("leading zeroes are prohibited"sv);
      }
 
      // range check
      if TOML_UNLIKELY (static_cast<size_t>(end - msd) > traits::max_digits)
        set_error_and_return_default("'"sv, traits::full_prefix, std::string_view{digits, length},
                                     "' is not representable in 64 bits"sv);
 
      // do the thing
      {
        uint64_t result = {};
        {
          uint64_t power = 1;
          while (--end >= msd) {
            if constexpr (base == 16)
              result += power * hex_to_dec(*end);
            else
              result += power * static_cast<uint64_t>(*end - '0');
 
            power *= base;
          }
        }
 
        // range check
        static constexpr auto i64_max =
            static_cast<uint64_t>((std::numeric_limits<int64_t>::max)());
        if TOML_UNLIKELY (result > i64_max + (sign < 0 ? 1u : 0u))
          set_error_and_return_default("'"sv, traits::full_prefix, std::string_view{digits, length},
                                       "' is not representable in 64 bits"sv);
 
        if constexpr (traits::is_signed) {
          // avoid signed multiply UB when parsing INT64_MIN
          if TOML_UNLIKELY (sign < 0 && result == i64_max + 1u)
            return (std::numeric_limits<int64_t>::min)();
 
          return static_cast<int64_t>(result) * sign;
        } else
          return static_cast<int64_t>(result);
      }
    }
 
    TOML_NODISCARD
    TOML_NEVER_INLINE
    date parse_date(bool part_of_datetime = false) {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(is_decimal_digit(*cp));
      push_parse_scope("date"sv);
 
      // "YYYY"
      uint32_t digits[4];
      if (!consume_digit_sequence(digits, 4u))
        set_error_and_return_default("expected 4-digit year, saw '"sv, to_sv(cp), "'"sv);
      const auto year         = digits[3] + digits[2] * 10u + digits[1] * 100u + digits[0] * 1000u;
      const auto is_leap_year = (year % 4u == 0u) && ((year % 100u != 0u) || (year % 400u == 0u));
      set_error_and_return_if_eof({});
 
      // '-'
      if (*cp != U'-') set_error_and_return_default("expected '-', saw '"sv, to_sv(*cp), "'"sv);
      advance_and_return_if_error_or_eof({});
 
      // "MM"
      if (!consume_digit_sequence(digits, 2u))
        set_error_and_return_default("expected 2-digit month, saw '"sv, to_sv(cp), "'"sv);
      const auto month = digits[1] + digits[0] * 10u;
      if (month == 0u || month > 12u)
        set_error_and_return_default("expected month between 1 and 12 (inclusive), saw "sv, month);
      const auto max_days_in_month =
          month == 2u ? (is_leap_year ? 29u : 28u)
                      : (month == 4u || month == 6u || month == 9u || month == 11u ? 30u : 31u);
      set_error_and_return_if_eof({});
 
      // '-'
      if (*cp != U'-') set_error_and_return_default("expected '-', saw '"sv, to_sv(*cp), "'"sv);
      advance_and_return_if_error_or_eof({});
 
      // "DD"
      if (!consume_digit_sequence(digits, 2u))
        set_error_and_return_default("expected 2-digit day, saw '"sv, to_sv(cp), "'"sv);
      const auto day = digits[1] + digits[0] * 10u;
      if (day == 0u || day > max_days_in_month)
        set_error_and_return_default("expected day between 1 and "sv, max_days_in_month,
                                     " (inclusive), saw "sv, day);
 
      if (!part_of_datetime && !is_eof() && !is_value_terminator(*cp))
        set_error_and_return_default("expected value-terminator, saw '"sv, to_sv(*cp), "'"sv);
 
      return {year, month, day};
    }
 
    TOML_NODISCARD
    TOML_NEVER_INLINE
    time parse_time(bool part_of_datetime = false) {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(is_decimal_digit(*cp));
      push_parse_scope("time"sv);
 
      static constexpr size_t max_digits = 64;  // far more than necessary but needed to allow
                                                // fractional millisecond truncation per the spec
      uint32_t digits[max_digits];
 
      // "HH"
      if (!consume_digit_sequence(digits, 2u))
        set_error_and_return_default("expected 2-digit hour, saw '"sv, to_sv(cp), "'"sv);
      const auto hour = digits[1] + digits[0] * 10u;
      if (hour > 23u)
        set_error_and_return_default("expected hour between 0 to 59 (inclusive), saw "sv, hour);
      set_error_and_return_if_eof({});
 
      // ':'
      if (*cp != U':') set_error_and_return_default("expected ':', saw '"sv, to_sv(*cp), "'"sv);
      advance_and_return_if_error_or_eof({});
 
      // "MM"
      if (!consume_digit_sequence(digits, 2u))
        set_error_and_return_default("expected 2-digit minute, saw '"sv, to_sv(cp), "'"sv);
      const auto minute = digits[1] + digits[0] * 10u;
      if (minute > 59u)
        set_error_and_return_default("expected minute between 0 and 59 (inclusive), saw "sv,
                                     minute);
      auto time = toml::time{hour, minute};
 
      // ':'
      if constexpr (TOML_LANG_UNRELEASED)  // toml/issues/671 (allow omission of seconds)
      {
        if (is_eof() || is_value_terminator(*cp) ||
            (part_of_datetime && is_match(*cp, U'+', U'-', U'Z', U'z')))
          return time;
      } else
        set_error_and_return_if_eof({});
      if (*cp != U':') set_error_and_return_default("expected ':', saw '"sv, to_sv(*cp), "'"sv);
      advance_and_return_if_error_or_eof({});
 
      // "SS"
      if (!consume_digit_sequence(digits, 2u))
        set_error_and_return_default("expected 2-digit second, saw '"sv, to_sv(cp), "'"sv);
      const auto second = digits[1] + digits[0] * 10u;
      if (second > 59u)
        set_error_and_return_default("expected second between 0 and 59 (inclusive), saw "sv,
                                     second);
      time.second = static_cast<decltype(time.second)>(second);
 
      // '.' (early-exiting is allowed; fractional is optional)
      if (is_eof() || is_value_terminator(*cp) ||
          (part_of_datetime && is_match(*cp, U'+', U'-', U'Z', U'z')))
        return time;
      if (*cp != U'.') set_error_and_return_default("expected '.', saw '"sv, to_sv(*cp), "'"sv);
      advance_and_return_if_error_or_eof({});
 
      // "FFFFFFFFF"
      size_t digit_count = consume_variable_length_digit_sequence(digits, max_digits);
      if (!digit_count) {
        set_error_and_return_if_eof({});
        set_error_and_return_default("expected fractional digits, saw '"sv, to_sv(*cp), "'"sv);
      } else if (!is_eof()) {
        if (digit_count == max_digits && is_decimal_digit(*cp))
          set_error_and_return_default("fractional component exceeds maximum precision of "sv,
                                       max_digits);
        else if (!part_of_datetime && !is_value_terminator(*cp))
          set_error_and_return_default("expected value-terminator, saw '"sv, to_sv(*cp), "'"sv);
      }
      uint32_t value = 0u;
      uint32_t place = 1u;
      for (auto i = impl::min<size_t>(digit_count, 9u); i-- > 0u;) {
        value += digits[i] * place;
        place *= 10u;
      }
      for (auto i = digit_count; i < 9u; i++)  // implicit zeros
        value *= 10u;
      time.nanosecond = value;
      return time;
    }
 
    TOML_NODISCARD
    TOML_NEVER_INLINE
    date_time parse_date_time() {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(is_decimal_digit(*cp));
      push_parse_scope("date-time"sv);
 
      // "YYYY-MM-DD"
      auto date = parse_date(true);
      set_error_and_return_if_eof({});
 
      // ' ', 'T' or 't'
      if (!is_match(*cp, U' ', U'T', U't'))
        set_error_and_return_default("expected space, 'T' or 't', saw '"sv, to_sv(*cp), "'"sv);
      advance_and_return_if_error_or_eof({});
 
      // "HH:MM:SS.FFFFFFFFF"
      auto time = parse_time(true);
      return_if_error({});
 
      // no offset
      if (is_eof() || is_value_terminator(*cp)) return {date, time};
 
      // zero offset ('Z' or 'z')
      time_offset offset{};
      if (is_match(*cp, U'Z', U'z')) advance_and_return_if_error({});
 
      // explicit offset ("+/-HH:MM")
      else if (is_match(*cp, U'+', U'-')) {
        push_parse_scope("date-time offset"sv);
 
        // sign
        int sign = *cp == U'-' ? -1 : 1;
        advance_and_return_if_error_or_eof({});
 
        // "HH"
        int digits[2];
        if (!consume_digit_sequence(digits, 2u))
          set_error_and_return_default("expected 2-digit hour, saw '"sv, to_sv(cp), "'"sv);
        const auto hour = digits[1] + digits[0] * 10;
        if (hour > 23)
          set_error_and_return_default("expected hour between 0 and 23 (inclusive), saw "sv, hour);
        set_error_and_return_if_eof({});
 
        // ':'
        if (*cp != U':') set_error_and_return_default("expected ':', saw '"sv, to_sv(*cp), "'"sv);
        advance_and_return_if_error_or_eof({});
 
        // "MM"
        if (!consume_digit_sequence(digits, 2u))
          set_error_and_return_default("expected 2-digit minute, saw '"sv, to_sv(cp), "'"sv);
        const auto minute = digits[1] + digits[0] * 10;
        if (minute > 59)
          set_error_and_return_default("expected minute between 0 and 59 (inclusive), saw "sv,
                                       minute);
        offset.minutes = static_cast<decltype(offset.minutes)>((hour * 60 + minute) * sign);
      }
 
      if (!is_eof() && !is_value_terminator(*cp))
        set_error_and_return_default("expected value-terminator, saw '"sv, to_sv(*cp), "'"sv);
 
      return {date, time, offset};
    }
 
    TOML_NODISCARD
    node_ptr parse_array();
 
    TOML_NODISCARD
    node_ptr parse_inline_table();
 
    TOML_NODISCARD
    node_ptr parse_value_known_prefixes() {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(!is_control_character(*cp));
      TOML_ASSERT_ASSUME(*cp != U'_');
 
      switch (cp->value) {
        // arrays
        case U'[':
          return parse_array();
 
        // inline tables
        case U'{':
          return parse_inline_table();
 
        // floats beginning with '.'
        case U'.':
          return node_ptr{new value{parse_float()}};
 
        // strings
        case U'"':
          [[fallthrough]];
        case U'\'':
          return node_ptr{new value{parse_string().value}};
 
        default: {
          const auto cp_upper = static_cast<uint_least32_t>(cp->value) & ~0x20u;
 
          // bools
          if (cp_upper == 70u || cp_upper == 84u)  // F or T
            return node_ptr{new value{parse_boolean()}};
 
          // inf/nan
          else if (cp_upper == 73u || cp_upper == 78u)  // I or N
            return node_ptr{new value{parse_inf_or_nan()}};
 
          else
            return nullptr;
        }
      }
      TOML_UNREACHABLE;
    }
 
    TOML_NODISCARD
    node_ptr parse_value() {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(!is_value_terminator(*cp));
      push_parse_scope("value"sv);
 
      const depth_counter_scope depth_counter{nested_values};
      if TOML_UNLIKELY (nested_values > max_nested_values)
        set_error_and_return_default("exceeded maximum nested value depth of "sv, max_nested_values,
                                     " (TOML_MAX_NESTED_VALUES)"sv);
 
      // check if it begins with some control character
      // (note that this will also fail for whitespace but we're assuming we've
      // called consume_leading_whitespace() before calling parse_value())
      if TOML_UNLIKELY (is_control_character(*cp))
        set_error_and_return_default("unexpected control character"sv);
 
      // underscores at the beginning
      else if (*cp == U'_')
        set_error_and_return_default("values may not begin with underscores"sv);
 
      const auto begin_pos = cp->position;
      node_ptr   val;
 
      do {
        TOML_ASSERT_ASSUME(!is_control_character(*cp));
        TOML_ASSERT_ASSUME(*cp != U'_');
 
        // detect the value type and parse accordingly,
        // starting with value types that can be detected
        // unambiguously from just one character.
 
        val = parse_value_known_prefixes();
        return_if_error({});
        if (val) break;
 
        // value types from here down require more than one character to unambiguously identify
        // so scan ahead and collect a set of value 'traits'.
        enum TOML_CLOSED_FLAGS_ENUM value_traits : int {
          has_nothing  = 0,
          has_digits   = 1,
          has_b        = 1 << 1,  // as second char only (0b)
          has_e        = 1 << 2,  // only float exponents
          has_o        = 1 << 3,  // as second char only (0o)
          has_p        = 1 << 4,  // only hexfloat exponents
          has_t        = 1 << 5,
          has_x        = 1 << 6,  // as second or third char only (0x, -0x, +0x)
          has_z        = 1 << 7,
          has_colon    = 1 << 8,
          has_plus     = 1 << 9,
          has_minus    = 1 << 10,
          has_dot      = 1 << 11,
          begins_sign  = 1 << 12,
          begins_digit = 1 << 13,
          begins_zero  = 1 << 14,
 
          signs_msk  = has_plus | has_minus,
          bdigit_msk = has_digits | begins_digit,
          bzero_msk  = bdigit_msk | begins_zero,
        };
        value_traits traits    = has_nothing;
        const auto   has_any   = [&](auto t) noexcept { return (traits & t) != has_nothing; };
        const auto   has_none  = [&](auto t) noexcept { return (traits & t) == has_nothing; };
        const auto   add_trait = [&](auto t) noexcept {
          traits = static_cast<value_traits>(traits | t);
        };
 
        // examine the first character to get the 'begins with' traits
        // (good fail-fast opportunity; all the remaining types begin with numeric digits or signs)
        if (is_decimal_digit(*cp)) {
          add_trait(begins_digit);
          if (*cp == U'0') add_trait(begins_zero);
        } else if (is_match(*cp, U'+', U'-'))
          add_trait(begins_sign);
        else
          break;
 
        // scan the rest of the value to determine the remaining traits
        char32_t   chars[utf8_buffered_reader::max_history_length];
        size_t     char_count = {}, advance_count = {};
        bool       eof_while_scanning = false;
        const auto scan               = [&]() noexcept(!TOML_COMPILER_HAS_EXCEPTIONS) {
          if (is_eof()) return;
          TOML_ASSERT_ASSUME(!is_value_terminator(*cp));
 
          do {
            if (const auto c = **cp; c != U'_') {
              chars[char_count++] = c;
 
              if (is_decimal_digit(c))
                add_trait(has_digits);
              else if (is_ascii_letter(c)) {
                TOML_ASSERT_ASSUME((c >= U'a' && c <= U'z') || (c >= U'A' && c <= U'Z'));
                switch (static_cast<char32_t>(c | 32u)) {
                  case U'b':
                    if (char_count == 2u && has_any(begins_zero)) add_trait(has_b);
                    break;
 
                  case U'e':
                    if (char_count > 1u &&
                        has_none(has_b | has_o | has_p | has_t | has_x | has_z | has_colon) &&
                        (has_none(has_plus | has_minus) || has_any(begins_sign)))
                      add_trait(has_e);
                    break;
 
                  case U'o':
                    if (char_count == 2u && has_any(begins_zero)) add_trait(has_o);
                    break;
 
                  case U'p':
                    if (has_any(has_x)) add_trait(has_p);
                    break;
 
                  case U'x':
                    if ((char_count == 2u && has_any(begins_zero)) ||
                        (char_count == 3u && has_any(begins_sign) && chars[1] == U'0'))
                      add_trait(has_x);
                    break;
 
                  case U't':
                    add_trait(has_t);
                    break;
                  case U'z':
                    add_trait(has_z);
                    break;
                }
              } else if (c <= U':') {
                TOML_ASSERT_ASSUME(c < U'0' || c > U'9');
                switch (c) {
                  case U'+':
                    add_trait(has_plus);
                    break;
                  case U'-':
                    add_trait(has_minus);
                    break;
                  case U'.':
                    add_trait(has_dot);
                    break;
                  case U':':
                    add_trait(has_colon);
                    break;
                }
              }
            }
 
            advance_and_return_if_error();
            advance_count++;
            eof_while_scanning = is_eof();
          } while (advance_count < (utf8_buffered_reader::max_history_length - 1u) && !is_eof() &&
                   !is_value_terminator(*cp));
        };
        scan();
        return_if_error({});
 
        // force further scanning if this could have been a date-time with a space instead of a T
        if (char_count == 10u                                     //
            && (traits | begins_zero) == (bzero_msk | has_minus)  //
            && chars[4] == U'-'                                   //
            && chars[7] == U'-'                                   //
            && !is_eof()                                          //
            && *cp == U' ') {
          const auto pre_advance_count = advance_count;
          const auto pre_scan_traits   = traits;
          chars[char_count++]          = *cp;
          add_trait(has_t);
 
          const auto backpedal = [&]() noexcept {
            go_back(advance_count - pre_advance_count);
            advance_count = pre_advance_count;
            traits        = pre_scan_traits;
            char_count    = 10u;
          };
 
          advance_and_return_if_error({});
          advance_count++;
 
          if (is_eof() || !is_decimal_digit(*cp))
            backpedal();
          else {
            chars[char_count++] = *cp;
 
            advance_and_return_if_error({});
            advance_count++;
 
            scan();
            return_if_error({});
 
            if (char_count == 12u) backpedal();
          }
        }
 
        // set the reader back to where we started
        go_back(advance_count);
 
        // if after scanning ahead we still only have one value character,
        // the only valid value type is an integer.
        if (char_count == 1u) {
          if (has_any(begins_digit)) {
            val.reset(new value{static_cast<int64_t>(chars[0] - U'0')});
            advance();  // skip the digit
            break;
          }
 
          // anything else would be ambiguous.
          else
            set_error_and_return_default(eof_while_scanning ? "encountered end-of-file"sv
                                                            : "could not determine value type"sv);
        }
 
        // now things that can be identified from two or more characters
        return_if_error({});
        TOML_ASSERT_ASSUME(char_count >= 2u);
 
        // do some 'fuzzy matching' where there's no ambiguity, since that allows the specific
        // typed parse functions to take over and show better diagnostics if there's an issue
        // (as opposed to the fallback "could not determine type" message)
        if (has_any(has_p))
          val.reset(new value{parse_hex_float()});
        else if (has_any(has_x | has_o | has_b)) {
          int64_t     i;
          value_flags flags;
          if (has_any(has_x)) {
            i     = parse_integer<16>();
            flags = value_flags::format_as_hexadecimal;
          } else if (has_any(has_o)) {
            i     = parse_integer<8>();
            flags = value_flags::format_as_octal;
          } else  // has_b
          {
            i     = parse_integer<2>();
            flags = value_flags::format_as_binary;
          }
          return_if_error({});
 
          val.reset(new value{i});
          val->ref_cast<int64_t>().flags(flags);
        } else if (has_any(has_e) || (has_any(begins_digit) && chars[1] == U'.'))
          val.reset(new value{parse_float()});
        else if (has_any(begins_sign)) {
          // single-digit signed integers
          if (char_count == 2u && has_any(has_digits)) {
            val.reset(
                new value{static_cast<int64_t>(chars[1] - U'0') * (chars[0] == U'-' ? -1LL : 1LL)});
            advance();  // skip the sign
            advance();  // skip the digit
            break;
          }
 
          // simple signed floats (e.g. +1.0)
          if (is_decimal_digit(chars[1]) && chars[2] == U'.') val.reset(new value{parse_float()});
 
          // signed infinity or nan
          else if (is_match(chars[1], U'i', U'n', U'I', U'N'))
            val.reset(new value{parse_inf_or_nan()});
        }
 
        return_if_error({});
        if (val) break;
 
        // match trait masks against what they can match exclusively.
        // all correct value parses will come out of this list, so doing this as a switch is likely
        // to be a better friend to the optimizer on the success path (failure path can be slow but
        // that doesn't matter much).
        switch (unwrap_enum(traits)) {
          // binary integers
          // 0b10
          case bzero_msk | has_b:
            val.reset(new value{parse_integer<2>()});
            val->ref_cast<int64_t>().flags(value_flags::format_as_binary);
            break;
 
          // octal integers
          // 0o10
          case bzero_msk | has_o:
            val.reset(new value{parse_integer<8>()});
            val->ref_cast<int64_t>().flags(value_flags::format_as_octal);
            break;
 
          // decimal integers
          // 00
          // 10
          // +10
          // -10
          case bzero_msk:
            [[fallthrough]];
          case bdigit_msk:
            [[fallthrough]];
          case begins_sign | has_digits | has_minus:
            [[fallthrough]];
          case begins_sign | has_digits | has_plus: {
            // if the value was so long we exhausted the history buffer it's reasonable to assume
            // there was more and the value's actual type is impossible to identify without making
            // the buffer bigger (since it could have actually been a float), so emit an error.
            //
            // (this will likely only come up during fuzzing and similar scenarios)
            static constexpr size_t max_numeric_value_length =
                utf8_buffered_reader::max_history_length - 2u;
            if TOML_UNLIKELY (!eof_while_scanning && advance_count > max_numeric_value_length)
              set_error_and_return_default(
                  "numeric value too long to identify type - cannot exceed "sv,
                  max_numeric_value_length, " characters"sv);
 
            val.reset(new value{parse_integer<10>()});
            break;
          }
 
          // hexadecimal integers
          // 0x10
          case bzero_msk | has_x:
            val.reset(new value{parse_integer<16>()});
            val->ref_cast<int64_t>().flags(value_flags::format_as_hexadecimal);
            break;
 
          // decimal floats
          // 0e1
          // 0e-1
          // 0e+1
          // 0.0
          // 0.0e1
          // 0.0e-1
          // 0.0e+1
          case bzero_msk | has_e:
            [[fallthrough]];
          case bzero_msk | has_e | has_minus:
            [[fallthrough]];
          case bzero_msk | has_e | has_plus:
            [[fallthrough]];
          case bzero_msk | has_dot:
            [[fallthrough]];
          case bzero_msk | has_dot | has_e:
            [[fallthrough]];
          case bzero_msk | has_dot | has_e | has_minus:
            [[fallthrough]];
          case bzero_msk | has_dot | has_e | has_plus:
            [[fallthrough]];
          // 1e1
          // 1e-1
          // 1e+1
          // 1.0
          // 1.0e1
          // 1.0e-1
          // 1.0e+1
          case bdigit_msk | has_e:
            [[fallthrough]];
          case bdigit_msk | has_e | has_minus:
            [[fallthrough]];
          case bdigit_msk | has_e | has_plus:
            [[fallthrough]];
          case bdigit_msk | has_dot:
            [[fallthrough]];
          case bdigit_msk | has_dot | has_e:
            [[fallthrough]];
          case bdigit_msk | has_dot | has_e | has_minus:
            [[fallthrough]];
          case bdigit_msk | has_dot | has_e | has_plus:
            [[fallthrough]];
          // +1e1
          // +1.0
          // +1.0e1
          // +1.0e+1
          // +1.0e-1
          // -1.0e+1
          case begins_sign | has_digits | has_e | has_plus:
            [[fallthrough]];
          case begins_sign | has_digits | has_dot | has_plus:
            [[fallthrough]];
          case begins_sign | has_digits | has_dot | has_e | has_plus:
            [[fallthrough]];
          case begins_sign | has_digits | has_dot | has_e | signs_msk:
            [[fallthrough]];
          // -1e1
          // -1e+1
          // +1e-1
          // -1.0
          // -1.0e1
          // -1.0e-1
          case begins_sign | has_digits | has_e | has_minus:
            [[fallthrough]];
          case begins_sign | has_digits | has_e | signs_msk:
            [[fallthrough]];
          case begins_sign | has_digits | has_dot | has_minus:
            [[fallthrough]];
          case begins_sign | has_digits | has_dot | has_e | has_minus:
            val.reset(new value{parse_float()});
            break;
 
          // hexadecimal floats
          // 0x10p0
          // 0x10p-0
          // 0x10p+0
          case bzero_msk | has_x | has_p:
            [[fallthrough]];
          case bzero_msk | has_x | has_p | has_minus:
            [[fallthrough]];
          case bzero_msk | has_x | has_p | has_plus:
            [[fallthrough]];
          // -0x10p0
          // -0x10p-0
          // +0x10p0
          // +0x10p+0
          // -0x10p+0
          // +0x10p-0
          case begins_sign | has_digits | has_x | has_p | has_minus:
            [[fallthrough]];
          case begins_sign | has_digits | has_x | has_p | has_plus:
            [[fallthrough]];
          case begins_sign | has_digits | has_x | has_p | signs_msk:
            [[fallthrough]];
          // 0x10.1p0
          // 0x10.1p-0
          // 0x10.1p+0
          case bzero_msk | has_x | has_dot | has_p:
            [[fallthrough]];
          case bzero_msk | has_x | has_dot | has_p | has_minus:
            [[fallthrough]];
          case bzero_msk | has_x | has_dot | has_p | has_plus:
            [[fallthrough]];
          // -0x10.1p0
          // -0x10.1p-0
          // +0x10.1p0
          // +0x10.1p+0
          // -0x10.1p+0
          // +0x10.1p-0
          case begins_sign | has_digits | has_x | has_dot | has_p | has_minus:
            [[fallthrough]];
          case begins_sign | has_digits | has_x | has_dot | has_p | has_plus:
            [[fallthrough]];
          case begins_sign | has_digits | has_x | has_dot | has_p | signs_msk:
            val.reset(new value{parse_hex_float()});
            break;
 
          // times
          // HH:MM
          // HH:MM:SS
          // HH:MM:SS.FFFFFF
          case bzero_msk | has_colon:
            [[fallthrough]];
          case bzero_msk | has_colon | has_dot:
            [[fallthrough]];
          case bdigit_msk | has_colon:
            [[fallthrough]];
          case bdigit_msk | has_colon | has_dot:
            val.reset(new value{parse_time()});
            break;
 
          // local dates
          // YYYY-MM-DD
          case bzero_msk | has_minus:
            [[fallthrough]];
          case bdigit_msk | has_minus:
            val.reset(new value{parse_date()});
            break;
 
          // date-times
          // YYYY-MM-DDTHH:MM
          // YYYY-MM-DDTHH:MM-HH:MM
          // YYYY-MM-DDTHH:MM+HH:MM
          // YYYY-MM-DD HH:MM
          // YYYY-MM-DD HH:MM-HH:MM
          // YYYY-MM-DD HH:MM+HH:MM
          // YYYY-MM-DDTHH:MM:SS
          // YYYY-MM-DDTHH:MM:SS-HH:MM
          // YYYY-MM-DDTHH:MM:SS+HH:MM
          // YYYY-MM-DD HH:MM:SS
          // YYYY-MM-DD HH:MM:SS-HH:MM
          // YYYY-MM-DD HH:MM:SS+HH:MM
          case bzero_msk | has_minus | has_colon | has_t:
            [[fallthrough]];
          case bzero_msk | signs_msk | has_colon | has_t:
            [[fallthrough]];
          case bdigit_msk | has_minus | has_colon | has_t:
            [[fallthrough]];
          case bdigit_msk | signs_msk | has_colon | has_t:
            [[fallthrough]];
          // YYYY-MM-DDTHH:MM:SS.FFFFFF
          // YYYY-MM-DDTHH:MM:SS.FFFFFF-HH:MM
          // YYYY-MM-DDTHH:MM:SS.FFFFFF+HH:MM
          // YYYY-MM-DD HH:MM:SS.FFFFFF
          // YYYY-MM-DD HH:MM:SS.FFFFFF-HH:MM
          // YYYY-MM-DD HH:MM:SS.FFFFFF+HH:MM
          case bzero_msk | has_minus | has_colon | has_dot | has_t:
            [[fallthrough]];
          case bzero_msk | signs_msk | has_colon | has_dot | has_t:
            [[fallthrough]];
          case bdigit_msk | has_minus | has_colon | has_dot | has_t:
            [[fallthrough]];
          case bdigit_msk | signs_msk | has_colon | has_dot | has_t:
            [[fallthrough]];
          // YYYY-MM-DDTHH:MMZ
          // YYYY-MM-DD HH:MMZ
          // YYYY-MM-DDTHH:MM:SSZ
          // YYYY-MM-DD HH:MM:SSZ
          // YYYY-MM-DDTHH:MM:SS.FFFFFFZ
          // YYYY-MM-DD HH:MM:SS.FFFFFFZ
          case bzero_msk | has_minus | has_colon | has_z | has_t:
            [[fallthrough]];
          case bzero_msk | has_minus | has_colon | has_dot | has_z | has_t:
            [[fallthrough]];
          case bdigit_msk | has_minus | has_colon | has_z | has_t:
            [[fallthrough]];
          case bdigit_msk | has_minus | has_colon | has_dot | has_z | has_t:
            val.reset(new value{parse_date_time()});
            break;
        }
      } while (false);
 
      if (!val) {
        set_error_at(begin_pos, "could not determine value type"sv);
        return_after_error({});
      }
 
      val->source_ = {begin_pos, current_position(1), reader.source_path()};
      return val;
    }
 
    TOML_NEVER_INLINE
    bool parse_key() {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(is_bare_key_character(*cp) || is_string_delimiter(*cp));
      push_parse_scope("key"sv);
 
      key_buffer.clear();
      recording_whitespace = false;
 
      while (!is_error()) {
        std::string_view key_segment;
        const auto       key_begin = current_position();
 
        // bare_key_segment
        if (is_bare_key_character(*cp)) key_segment = parse_bare_key_segment();
 
        // "quoted key segment"
        else if (is_string_delimiter(*cp)) {
          const auto begin_pos = cp->position;
 
          recording_whitespace = true;
          parsed_string str    = parse_string();
          recording_whitespace = false;
          return_if_error({});
 
          if (str.was_multi_line) {
            set_error_at(begin_pos, "multi-line strings are prohibited in "sv,
                         key_buffer.empty() ? ""sv : "dotted "sv, "keys"sv);
            return_after_error({});
          } else
            key_segment = str.value;
        }
 
        // ???
        else
          set_error_and_return_default(
              "expected bare key starting character or string delimiter, saw '"sv, to_sv(*cp),
              "'"sv);
 
        const auto key_end = current_position();
 
        // whitespace following the key segment
        consume_leading_whitespace();
 
        // store segment
        key_buffer.push_back(key_segment, key_begin, key_end);
 
        // eof or no more key to come
        if (is_eof() || *cp != U'.') break;
 
        // was a dotted key - go around again
        advance_and_return_if_error_or_eof({});
        consume_leading_whitespace();
        set_error_and_return_if_eof({});
      }
      return_if_error({});
 
      return true;
    }
 
    TOML_NODISCARD
    key make_key(size_t segment_index) const {
      TOML_ASSERT(key_buffer.size() > segment_index);
 
      return key{key_buffer[segment_index],
                 source_region{key_buffer.starts[segment_index], key_buffer.ends[segment_index],
                               root.source().path}};
    }
 
    TOML_NODISCARD
    TOML_NEVER_INLINE
    table* parse_table_header() {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(*cp == U'[');
      push_parse_scope("table header"sv);
 
      const source_position header_begin_pos = cp->position;
      source_position       header_end_pos;
      bool                  is_arr = false;
 
      // parse header
      {
        // skip first '['
        advance_and_return_if_error_or_eof({});
 
        // skip past any whitespace that followed the '['
        const bool had_leading_whitespace = consume_leading_whitespace();
        set_error_and_return_if_eof({});
 
        // skip second '[' (if present)
        if (*cp == U'[') {
          if (had_leading_whitespace)
            set_error_and_return_default(
                "[[array-of-table]] brackets must be contiguous (i.e. [ [ this ] ] is prohibited)"sv);
 
          is_arr = true;
          advance_and_return_if_error_or_eof({});
 
          // skip past any whitespace that followed the '['
          consume_leading_whitespace();
          set_error_and_return_if_eof({});
        }
 
        // check for a premature closing ']'
        if (*cp == U']')
          set_error_and_return_default("tables with blank bare keys are explicitly prohibited"sv);
 
        // get the actual key
        start_recording();
        parse_key();
        stop_recording(1u);
        return_if_error({});
 
        // skip past any whitespace that followed the key
        consume_leading_whitespace();
        return_if_error({});
        set_error_and_return_if_eof({});
 
        // consume the closing ']'
        if (*cp != U']') set_error_and_return_default("expected ']', saw '"sv, to_sv(*cp), "'"sv);
        if (is_arr) {
          advance_and_return_if_error_or_eof({});
          if (*cp != U']') set_error_and_return_default("expected ']', saw '"sv, to_sv(*cp), "'"sv);
        }
        advance_and_return_if_error({});
        header_end_pos = current_position(1);
 
        // handle the rest of the line after the header
        consume_leading_whitespace();
        if (!is_eof() && !consume_comment() && !consume_line_break())
          set_error_and_return_default("expected a comment or whitespace, saw '"sv, to_sv(cp),
                                       "'"sv);
      }
      TOML_ASSERT(!key_buffer.empty());
 
      // check if each parent is a table/table array, or can be created implicitly as a table.
      table* parent = &root;
      for (size_t i = 0, e = key_buffer.size() - 1u; i < e; i++) {
        const std::string_view segment = key_buffer[i];
        auto                   pit     = parent->lower_bound(segment);
 
        // parent already existed
        if (pit != parent->end() && pit->first == segment) {
          node& p = pit->second;
 
          if (auto tbl = p.as_table()) {
            // adding to closed inline tables is illegal
            if (tbl->is_inline() &&
                !impl::find(open_inline_tables.begin(), open_inline_tables.end(), tbl))
              set_error_and_return_default("cannot insert '"sv, to_sv(recording_buffer),
                                           "' into existing inline table"sv);
 
            parent = tbl;
          } else if (auto arr = p.as_array();
                     arr && impl::find(table_arrays.begin(), table_arrays.end(), arr)) {
            // table arrays are a special case;
            // the spec dictates we select the most recently declared element in the array.
            TOML_ASSERT(!arr->empty());
            TOML_ASSERT(arr->back().is_table());
            parent = &arr->back().ref_cast<table>();
          } else {
            if (!is_arr && p.type() == node_type::table)
              set_error_and_return_default("cannot redefine existing table '"sv,
                                           to_sv(recording_buffer), "'"sv);
            else
              set_error_and_return_default("cannot redefine existing "sv, to_sv(p.type()), " '"sv,
                                           to_sv(recording_buffer), "' as "sv,
                                           is_arr ? "array-of-tables"sv : "table"sv);
          }
        }
 
        // need to create a new implicit table
        else {
          pit       = parent->emplace_hint<table>(pit, make_key(i));
          table& p  = pit->second.ref_cast<table>();
          p.source_ = {header_begin_pos, header_end_pos, reader.source_path()};
 
          implicit_tables.push_back(&p);
          parent = &p;
        }
      }
 
      const auto last_segment = key_buffer.back();
      auto       it           = parent->lower_bound(last_segment);
 
      // if there was already a matching node some sanity checking is necessary;
      // this is ok if we're making an array and the existing element is already an array (new
      // element) or if we're making a table and the existing element is an implicitly-created table
      // (promote it), otherwise this is a redefinition error.
      if (it != parent->end() && it->first == last_segment) {
        node& matching_node = it->second;
        if (auto arr = matching_node.as_array();
            is_arr && arr && impl::find(table_arrays.begin(), table_arrays.end(), arr)) {
          table& tbl  = arr->emplace_back<table>();
          tbl.source_ = {header_begin_pos, header_end_pos, reader.source_path()};
          return &tbl;
        }
 
        else if (auto tbl = matching_node.as_table(); !is_arr && tbl && !implicit_tables.empty()) {
          if (auto found = impl::find(implicit_tables.begin(), implicit_tables.end(), tbl); found) {
            bool ok = true;
            if (!tbl->empty()) {
              for (auto& [_, child] : *tbl) {
                if (!child.is_table() && !child.is_array_of_tables()) {
                  ok = false;
                  break;
                }
              }
            }
 
            if (ok) {
              implicit_tables.erase(implicit_tables.cbegin() + (found - implicit_tables.data()));
              tbl->source_.begin = header_begin_pos;
              tbl->source_.end   = header_end_pos;
              return tbl;
            }
          }
        }
 
        // if we get here it's a redefinition error.
        if (!is_arr && matching_node.type() == node_type::table) {
          set_error_at(header_begin_pos, "cannot redefine existing table '"sv,
                       to_sv(recording_buffer), "'"sv);
          return_after_error({});
        } else {
          set_error_at(header_begin_pos, "cannot redefine existing "sv, to_sv(matching_node.type()),
                       " '"sv, to_sv(recording_buffer), "' as "sv,
                       is_arr ? "array-of-tables"sv : "table"sv);
          return_after_error({});
        }
      }
 
      // there was no matching node, sweet - we can freely instantiate a new table/table array.
      else {
        auto last_key = make_key(key_buffer.size() - 1u);
 
        // if it's an array we need to make the array and it's first table element,
        // set the starting regions, and return the table element
        if (is_arr) {
          it             = parent->emplace_hint<array>(it, std::move(last_key));
          array& tbl_arr = it->second.ref_cast<array>();
          table_arrays.push_back(&tbl_arr);
          tbl_arr.source_ = {header_begin_pos, header_end_pos, reader.source_path()};
 
          table& tbl  = tbl_arr.emplace_back<table>();
          tbl.source_ = {header_begin_pos, header_end_pos, reader.source_path()};
          return &tbl;
        }
 
        // otherwise we're just making a table
        else {
          it          = parent->emplace_hint<table>(it, std::move(last_key));
          table& tbl  = it->second.ref_cast<table>();
          tbl.source_ = {header_begin_pos, header_end_pos, reader.source_path()};
          return &tbl;
        }
      }
    }
 
    TOML_NEVER_INLINE
    bool parse_key_value_pair_and_insert(table* tbl) {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(is_string_delimiter(*cp) || is_bare_key_character(*cp));
      push_parse_scope("key-value pair"sv);
 
      // read the key into the key buffer
      start_recording();
      parse_key();
      stop_recording(1u);
      return_if_error({});
      TOML_ASSERT(key_buffer.size() >= 1u);
 
      // skip past any whitespace that followed the key
      consume_leading_whitespace();
      set_error_and_return_if_eof({});
 
      // '='
      if (*cp != U'=') set_error_and_return_default("expected '=', saw '"sv, to_sv(*cp), "'"sv);
      advance_and_return_if_error_or_eof({});
 
      // skip past any whitespace that followed the '='
      consume_leading_whitespace();
      return_if_error({});
      set_error_and_return_if_eof({});
 
      // check that the next character could actually be a value
      if (is_value_terminator(*cp))
        set_error_and_return_default("expected value, saw '"sv, to_sv(*cp), "'"sv);
 
      // if it's a dotted kvp we need to spawn the parent sub-tables if necessary,
      // and set the target table to the second-to-last one in the chain
      if (key_buffer.size() > 1u) {
        for (size_t i = 0; i < key_buffer.size() - 1u; i++) {
          const std::string_view segment = key_buffer[i];
          auto                   pit     = tbl->lower_bound(segment);
 
          // parent already existed
          if (pit != tbl->end() && pit->first == segment) {
            table* p = pit->second.as_table();
 
            // redefinition
            if TOML_UNLIKELY (!p ||
                              !(impl::find(dotted_key_tables.begin(), dotted_key_tables.end(), p) ||
                                impl::find(implicit_tables.begin(), implicit_tables.end(), p))) {
              set_error_at(key_buffer.starts[i], "cannot redefine existing "sv,
                           to_sv(pit->second.type()), " as dotted key-value pair"sv);
              return_after_error({});
            }
 
            tbl = p;
          }
 
          // need to create a new implicit table
          else {
            pit       = tbl->emplace_hint<table>(pit, make_key(i));
            table& p  = pit->second.ref_cast<table>();
            p.source_ = pit->first.source();
 
            dotted_key_tables.push_back(&p);
            tbl = &p;
          }
        }
      }
 
      // ensure this isn't a redefinition
      const std::string_view last_segment = key_buffer.back();
      auto                   it           = tbl->lower_bound(last_segment);
      if (it != tbl->end() && it->first == last_segment) {
        set_error("cannot redefine existing "sv, to_sv(it->second.type()), " '"sv,
                  to_sv(recording_buffer), "'"sv);
        return_after_error({});
      }
 
      // create the key first since the key buffer will likely get overwritten during value parsing
      // (inline tables)
      auto last_key = make_key(key_buffer.size() - 1u);
 
      // now we can actually parse the value
      node_ptr val = parse_value();
      return_if_error({});
 
      tbl->emplace_hint<node_ptr>(it, std::move(last_key), std::move(val));
      return true;
    }
 
    void parse_document() {
      assert_not_error();
      assert_not_eof();
      push_parse_scope("root table"sv);
 
      table* current_table = &root;
 
      do {
        return_if_error();
 
        // leading whitespace, line endings, comments
        if (consume_leading_whitespace() || consume_line_break() || consume_comment()) continue;
        return_if_error();
 
        // [tables]
        // [[table array]]
        if (*cp == U'[') current_table = parse_table_header();
 
        // bare_keys
        // dotted.keys
        // "quoted keys"
        else if (is_bare_key_character(*cp) || is_string_delimiter(*cp)) {
          push_parse_scope("key-value pair"sv);
 
          parse_key_value_pair_and_insert(current_table);
 
          // handle the rest of the line after the kvp
          // (this is not done in parse_key_value_pair() because that is also used for inline
          // tables)
          consume_leading_whitespace();
          return_if_error();
          if (!is_eof() && !consume_comment() && !consume_line_break())
            set_error("expected a comment or whitespace, saw '"sv, to_sv(cp), "'"sv);
        }
 
        else  // ??
          set_error("expected keys, tables, whitespace or comments, saw '"sv, to_sv(cp), "'"sv);
      } while (!is_eof());
 
      auto eof_pos     = current_position(1);
      root.source_.end = eof_pos;
      if (current_table && current_table != &root &&
          current_table->source_.end <= current_table->source_.begin)
        current_table->source_.end = eof_pos;
    }
 
    static void update_region_ends(node& nde) noexcept {
      const auto type = nde.type();
      if (type > node_type::array) return;
 
      if (type == node_type::table) {
        auto& tbl = nde.ref_cast<table>();
        if (tbl.is_inline())  // inline tables (and all their inline descendants) are already
                              // correctly terminated
          return;
 
        auto end = nde.source_.end;
        for (auto&& [k, v] : tbl) {
          TOML_UNUSED(k);
          update_region_ends(v);
          if (end < v.source_.end) end = v.source_.end;
        }
      } else  // arrays
      {
        auto& arr = nde.ref_cast<array>();
        auto  end = nde.source_.end;
        for (auto&& v : arr) {
          update_region_ends(v);
          if (end < v.source_.end) end = v.source_.end;
        }
        nde.source_.end = end;
      }
    }
 
   public:
    parser(utf8_reader_interface&& reader_)  //
        : reader{reader_} {
      root.source_ = {prev_pos, prev_pos, reader.source_path()};
 
      if (!reader.peek_eof()) {
        cp = reader.read_next();
 
#if !TOML_EXCEPTIONS
        if (reader.error()) {
          err = std::move(reader.error());
          return;
        }
#endif
 
        if (cp) parse_document();
      }
 
      update_region_ends(root);
    }
 
    TOML_NODISCARD
    operator parse_result() && noexcept {
#if TOML_EXCEPTIONS
 
      return {std::move(root)};
 
#else
 
      if (err)
        return parse_result{*std::move(err)};
      else
        return parse_result{std::move(root)};
 
#endif
    }
  };
 
  TOML_EXTERNAL_LINKAGE
  node_ptr parser::parse_array() {
    return_if_error({});
    assert_not_eof();
    TOML_ASSERT_ASSUME(*cp == U'[');
    push_parse_scope("array"sv);
 
    // skip opening '['
    advance_and_return_if_error_or_eof({});
 
    node_ptr                    arr_ptr{new array{}};
    array&                      arr = arr_ptr->ref_cast<array>();
    enum class TOML_CLOSED_ENUM parse_type : int { none, comma, val };
    parse_type                  prev = parse_type::none;
 
    while (!is_error()) {
      while (consume_leading_whitespace() || consume_line_break() || consume_comment()) continue;
      set_error_and_return_if_eof({});
 
      // commas - only legal after a value
      if (*cp == U',') {
        if (prev == parse_type::val) {
          prev = parse_type::comma;
          advance_and_return_if_error_or_eof({});
          continue;
        }
        set_error_and_return_default("expected value or closing ']', saw comma"sv);
      }
 
      // closing ']'
      else if (*cp == U']') {
        advance_and_return_if_error({});
        break;
      }
 
      // must be a value
      else {
        if (prev == parse_type::val) {
          set_error_and_return_default("expected comma or closing ']', saw '"sv, to_sv(*cp), "'"sv);
          continue;
        }
        prev = parse_type::val;
 
        auto val = parse_value();
        return_if_error({});
 
        if (!arr.capacity()) arr.reserve(4u);
        arr.emplace_back<node_ptr>(std::move(val));
      }
    }
 
    return_if_error({});
    return arr_ptr;
  }
 
  TOML_EXTERNAL_LINKAGE
  node_ptr parser::parse_inline_table() {
    return_if_error({});
    assert_not_eof();
    TOML_ASSERT_ASSUME(*cp == U'{');
    push_parse_scope("inline table"sv);
 
    // skip opening '{'
    advance_and_return_if_error_or_eof({});
 
    node_ptr tbl_ptr{new table{}};
    table&   tbl = tbl_ptr->ref_cast<table>();
    tbl.is_inline(true);
    table_vector_scope table_scope{open_inline_tables, tbl};
 
    enum class TOML_CLOSED_ENUM parse_type : int { none, comma, kvp };
    parse_type                  prev = parse_type::none;
    while (!is_error()) {
      if constexpr (TOML_LANG_UNRELEASED)  // toml/issues/516 (newlines/trailing commas in inline
                                           // tables)
      {
        while (consume_leading_whitespace() || consume_line_break() || consume_comment()) continue;
      } else {
        while (consume_leading_whitespace()) continue;
      }
      return_if_error({});
      set_error_and_return_if_eof({});
 
      // commas - only legal after a key-value pair
      if (*cp == U',') {
        if (prev == parse_type::kvp) {
          prev = parse_type::comma;
          advance_and_return_if_error_or_eof({});
        } else
          set_error_and_return_default("expected key-value pair or closing '}', saw comma"sv);
      }
 
      // closing '}'
      else if (*cp == U'}') {
        if constexpr (!TOML_LANG_UNRELEASED)  // toml/issues/516 (newlines/trailing commas in inline
                                              // tables)
        {
          if (prev == parse_type::comma) {
            set_error_and_return_default(
                "expected key-value pair, saw closing '}' (dangling comma)"sv);
            continue;
          }
        }
        advance_and_return_if_error({});
        break;
      }
 
      // key-value pair
      else if (is_string_delimiter(*cp) || is_bare_key_character(*cp)) {
        if (prev == parse_type::kvp)
          set_error_and_return_default("expected comma or closing '}', saw '"sv, to_sv(*cp), "'"sv);
        else {
          prev = parse_type::kvp;
          parse_key_value_pair_and_insert(&tbl);
        }
      }
 
      else
        set_error_and_return_default("expected key or closing '}', saw '"sv, to_sv(*cp), "'"sv);
    }
 
    return_if_error({});
    return tbl_ptr;
  }
 
  TOML_ABI_NAMESPACE_END;  // TOML_EXCEPTIONS
}
TOML_IMPL_NAMESPACE_END;
 
#undef TOML_RETURNS_BY_THROWING
#undef advance_and_return_if_error
#undef advance_and_return_if_error_or_eof
#undef assert_not_eof
#undef assert_not_error
#undef is_eof
#undef is_error
#undef parse_error_break
#undef push_parse_scope
#undef push_parse_scope_1
#undef push_parse_scope_2
#undef return_after_error
#undef return_if_eof
#undef return_if_error
#undef return_if_error_or_eof
#undef set_error_and_return
#undef set_error_and_return_default
#undef set_error_and_return_if_eof
#undef utf8_buffered_reader_error_check
#undef utf8_reader_error
#undef utf8_reader_error_check
#undef utf8_reader_return_after_error
 
// #---------------------------------------------------------------------------------------------------------------------
// # PARSER PUBLIC IMPLEMENTATION
// #---------------------------------------------------------------------------------------------------------------------
 
TOML_ANON_NAMESPACE_START {
  TOML_NODISCARD
  TOML_INTERNAL_LINKAGE
  parse_result do_parse(utf8_reader_interface && reader) {
    return impl::parser{std::move(reader)};
  }
 
  TOML_NODISCARD
  TOML_INTERNAL_LINKAGE
  parse_result do_parse_file(std::string_view file_path) {
#if TOML_EXCEPTIONS
#define TOML_PARSE_FILE_ERROR(msg, path) \
  throw parse_error{msg, source_position{}, std::make_shared<const std::string>(std::move(path))}
#else
#define TOML_PARSE_FILE_ERROR(msg, path)                                           \
  return parse_result {                                                            \
    parse_error {                                                                  \
      msg, source_position{}, std::make_shared<const std::string>(std::move(path)) \
    }                                                                              \
  }
#endif
 
    std::string file_path_str(file_path);
 
    // open file with a custom-sized stack buffer
    std::ifstream         file;
    TOML_OVERALIGNED char file_buffer[sizeof(void*) * 1024u];
    file.rdbuf()->pubsetbuf(file_buffer, sizeof(file_buffer));
#if TOML_WINDOWS
    file.open(impl::widen(file_path_str).c_str(),
              std::ifstream::in | std::ifstream::binary | std::ifstream::ate);
#else
    file.open(file_path_str, std::ifstream::in | std::ifstream::binary | std::ifstream::ate);
#endif
    if (!file.is_open())
      TOML_PARSE_FILE_ERROR("File could not be opened for reading", file_path_str);
 
    // get size
    const auto file_size = file.tellg();
    if (file_size == -1) TOML_PARSE_FILE_ERROR("Could not determine file size", file_path_str);
    file.seekg(0, std::ifstream::beg);
 
    // read the whole file into memory first if the file isn't too large
    constexpr auto large_file_threshold = 1024 * 1024 * 2;  // 2 MB
    if (file_size <= large_file_threshold) {
      std::vector<char> file_data;
      file_data.resize(static_cast<size_t>(file_size));
      file.read(file_data.data(), static_cast<std::streamsize>(file_size));
      return parse(std::string_view{file_data.data(), file_data.size()}, std::move(file_path_str));
    }
 
    // otherwise parse it using the streams
    else
      return parse(file, std::move(file_path_str));
 
#undef TOML_PARSE_FILE_ERROR
  }
}
TOML_ANON_NAMESPACE_END;
 
TOML_NAMESPACE_START {
  TOML_ABI_NAMESPACE_BOOL(TOML_EXCEPTIONS, ex, noex);
 
  TOML_EXTERNAL_LINKAGE
  parse_result TOML_CALLCONV parse(std::string_view doc, std::string_view source_path) {
    return TOML_ANON_NAMESPACE::do_parse(TOML_ANON_NAMESPACE::utf8_reader{doc, source_path});
  }
 
  TOML_EXTERNAL_LINKAGE
  parse_result TOML_CALLCONV parse(std::string_view doc, std::string && source_path) {
    return TOML_ANON_NAMESPACE::do_parse(
        TOML_ANON_NAMESPACE::utf8_reader{doc, std::move(source_path)});
  }
 
  TOML_EXTERNAL_LINKAGE
  parse_result TOML_CALLCONV parse(std::istream & doc, std::string_view source_path) {
    return TOML_ANON_NAMESPACE::do_parse(TOML_ANON_NAMESPACE::utf8_reader{doc, source_path});
  }
 
  TOML_EXTERNAL_LINKAGE
  parse_result TOML_CALLCONV parse(std::istream & doc, std::string && source_path) {
    return TOML_ANON_NAMESPACE::do_parse(
        TOML_ANON_NAMESPACE::utf8_reader{doc, std::move(source_path)});
  }
 
  TOML_EXTERNAL_LINKAGE
  parse_result TOML_CALLCONV parse_file(std::string_view file_path) {
    return TOML_ANON_NAMESPACE::do_parse_file(file_path);
  }
 
#if TOML_HAS_CHAR8
 
  TOML_EXTERNAL_LINKAGE
  parse_result TOML_CALLCONV parse(std::u8string_view doc, std::string_view source_path) {
    return TOML_ANON_NAMESPACE::do_parse(TOML_ANON_NAMESPACE::utf8_reader{doc, source_path});
  }
 
  TOML_EXTERNAL_LINKAGE
  parse_result TOML_CALLCONV parse(std::u8string_view doc, std::string && source_path) {
    return TOML_ANON_NAMESPACE::do_parse(
        TOML_ANON_NAMESPACE::utf8_reader{doc, std::move(source_path)});
  }
 
  TOML_EXTERNAL_LINKAGE
  parse_result TOML_CALLCONV parse_file(std::u8string_view file_path) {
    std::string file_path_str;
    file_path_str.resize(file_path.length());
    memcpy(file_path_str.data(), file_path.data(), file_path.length());
    return TOML_ANON_NAMESPACE::do_parse_file(file_path_str);
  }
 
#endif  // TOML_HAS_CHAR8
 
#if TOML_ENABLE_WINDOWS_COMPAT
 
  TOML_EXTERNAL_LINKAGE
  parse_result TOML_CALLCONV parse(std::string_view doc, std::wstring_view source_path) {
    return TOML_ANON_NAMESPACE::do_parse(
        TOML_ANON_NAMESPACE::utf8_reader{doc, impl::narrow(source_path)});
  }
 
  TOML_EXTERNAL_LINKAGE
  parse_result TOML_CALLCONV parse(std::istream & doc, std::wstring_view source_path) {
    return TOML_ANON_NAMESPACE::do_parse(
        TOML_ANON_NAMESPACE::utf8_reader{doc, impl::narrow(source_path)});
  }
 
  TOML_EXTERNAL_LINKAGE
  parse_result TOML_CALLCONV parse_file(std::wstring_view file_path) {
    return TOML_ANON_NAMESPACE::do_parse_file(impl::narrow(file_path));
  }
 
#endif  // TOML_ENABLE_WINDOWS_COMPAT
 
#if TOML_HAS_CHAR8 && TOML_ENABLE_WINDOWS_COMPAT
 
  TOML_EXTERNAL_LINKAGE
  parse_result TOML_CALLCONV parse(std::u8string_view doc, std::wstring_view source_path) {
    return TOML_ANON_NAMESPACE::do_parse(
        TOML_ANON_NAMESPACE::utf8_reader{doc, impl::narrow(source_path)});
  }
 
#endif  // TOML_HAS_CHAR8 && TOML_ENABLE_WINDOWS_COMPAT
 
  TOML_ABI_NAMESPACE_END;  // TOML_EXCEPTIONS
}
TOML_NAMESPACE_END;
 
#undef TOML_OVERALIGNED
#include "header_end.hpp"
#endif  // TOML_ENABLE_PARSER