Crafter.Review/implementations/Crafter.Review-Text.cpp

// SPDX-License-Identifier: AGPL-3.0-only
// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®

module;
#include <arpa/inet.h>
#include <sys/socket.h>

module Crafter.Review:Text_impl;
import :Text;
import :Unicode;
import :Json;
import std;

namespace Crafter::Review::Dyn {
    namespace {
        template <typename Table>
        bool InRuns(const Table& table, char32_t c) {
            const auto found = std::ranges::upper_bound(table, c, {}, [](const Unicode::Run& run) { return run.first; });
            if (found == table.begin()) return false;
            return c <= std::prev(found)->last;
        }

        template <typename Table>
        std::string_view Mapped(const Table& table, char32_t c) {
            const auto found = std::ranges::lower_bound(table, c, {}, [](const Unicode::Mapping& mapping) { return mapping.from; });
            if (found == table.end() || found->from != c) return {};
            return found->to;
        }

        void AppendUtf8(std::string& out, char32_t c) {
            if (c < 0x80) {
                out.push_back(static_cast<char>(c));
            } else if (c < 0x800) {
                out.push_back(static_cast<char>(0xC0 | (c >> 6)));
                out.push_back(static_cast<char>(0x80 | (c & 0x3F)));
            } else if (c < 0x10000) {
                out.push_back(static_cast<char>(0xE0 | (c >> 12)));
                out.push_back(static_cast<char>(0x80 | ((c >> 6) & 0x3F)));
                out.push_back(static_cast<char>(0x80 | (c & 0x3F)));
            } else {
                out.push_back(static_cast<char>(0xF0 | (c >> 18)));
                out.push_back(static_cast<char>(0x80 | ((c >> 12) & 0x3F)));
                out.push_back(static_cast<char>(0x80 | ((c >> 6) & 0x3F)));
                out.push_back(static_cast<char>(0x80 | (c & 0x3F)));
            }
        }

        // One code point from `bytes` at `at`: the code point and the bytes
        // it took, or U+FFFD and the length of the maximal subpart of an
        // ill-formed sequence, which one U+FFFD replaces.
        std::pair<char32_t, std::size_t> Next(std::string_view bytes, std::size_t at) {
            const auto lead = static_cast<std::uint8_t>(bytes[at]);
            if (lead < 0x80) return {lead, 1};
            std::size_t need = 0;
            char32_t value = 0;
            std::uint8_t low = 0x80;
            std::uint8_t high = 0xBF;
            if (lead >= 0xC2 && lead <= 0xDF) {
                need = 1;
                value = lead & 0x1F;
            } else if (lead >= 0xE0 && lead <= 0xEF) {
                need = 2;
                value = lead & 0x0F;
                if (lead == 0xE0) low = 0xA0;
                if (lead == 0xED) high = 0x9F;
            } else if (lead >= 0xF0 && lead <= 0xF4) {
                need = 3;
                value = lead & 0x07;
                if (lead == 0xF0) low = 0x90;
                if (lead == 0xF4) high = 0x8F;
            } else {
                return {0xFFFD, 1};
            }
            std::size_t used = 1;
            for (std::size_t i = 0; i < need; ++i) {
                if (at + used >= bytes.size()) return {0xFFFD, used};
                const auto next = static_cast<std::uint8_t>(bytes[at + used]);
                if (next < low || next > high) return {0xFFFD, used};
                value = (value << 6) | (next & 0x3F);
                ++used;
                low = 0x80;
                high = 0xBF;
            }
            return {value, used};
        }

        bool IsAsciiDigit(char32_t c) {
            return c >= '0' && c <= '9';
        }

        // ParseInt()'s and ParseFloat()'s first step: Unicode whitespace
        // stripped, then every decimal digit made ASCII and every space a plain
        // one.
        std::optional<std::string> AsciiNumber(std::string_view text) {
            std::u32string decoded = Decode(Strip(text));
            std::string out;
            for (const char32_t c : decoded) {
                if (c < 0x80) {
                    out.push_back(static_cast<char>(c));
                } else if (const std::int32_t digit = DecimalValue(c); digit >= 0) {
                    out.push_back(static_cast<char>('0' + digit));
                } else if (IsSpace(c)) {
                    out.push_back(' ');
                } else {
                    return std::nullopt;
                }
            }
            return out;
        }

        // A run of digits with single underscores between them, as ParseInt()
        // and ParseFloat() take them; the digits alone.
        std::optional<std::string> Digits(std::string_view text) {
            if (text.empty() || text.front() == '_' || text.back() == '_') return std::nullopt;
            std::string digits;
            char previous = '\0';
            for (const char c : text) {
                if (c == '_') {
                    if (previous == '_') return std::nullopt;
                } else if (c >= '0' && c <= '9') {
                    digits.push_back(c);
                } else {
                    return std::nullopt;
                }
                previous = c;
            }
            return digits;
        }

        std::string ToLowerAscii(std::string_view text) {
            std::string out(text);
            for (char& c : out) {
                if (c >= 'A' && c <= 'Z') c = static_cast<char>(c - 'A' + 'a');
            }
            return out;
        }

        // NormPath() on a string that is known to be a path.
        std::vector<std::string> PathComponents(std::string_view path) {
            std::vector<std::string> out;
            for (const std::string& piece : Split(path, "/")) {
                if (!piece.empty()) out.push_back(piece);
            }
            return out;
        }
    }

    std::u32string Decode(std::string_view utf8) {
        std::u32string out;
        out.reserve(utf8.size());
        for (std::size_t at = 0; at < utf8.size();) {
            const auto [c, used] = Next(utf8, at);
            out.push_back(c);
            at += used;
        }
        return out;
    }

    std::string Encode(std::u32string_view text) {
        std::string out;
        out.reserve(text.size());
        for (const char32_t c : text) AppendUtf8(out, c);
        return out;
    }

    std::string Encode(char32_t c) {
        std::string out;
        AppendUtf8(out, c);
        return out;
    }

    bool IsUtf8(std::string_view bytes) {
        for (std::size_t at = 0; at < bytes.size();) {
            const auto [c, used] = Next(bytes, at);
            if (c == 0xFFFD) {
                // A real U+FFFD in the input is three valid bytes.
                if (used != 3 || bytes.substr(at, 3) != "\xEF\xBF\xBD") return false;
            }
            at += used;
        }
        return true;
    }

    std::string DecodeReplace(std::string_view bytes) {
        if (IsUtf8(bytes)) return std::string(bytes);
        return Encode(Decode(bytes));
    }

    std::size_t Length(std::string_view text) {
        std::size_t count = 0;
        for (std::size_t at = 0; at < text.size(); ++count) at += Next(text, at).second;
        return count;
    }

    std::string Slice(std::string_view text, std::int64_t start, std::optional<std::int64_t> end) {
        const std::u32string decoded = Decode(text);
        const auto size = static_cast<std::int64_t>(decoded.size());
        auto clip = [size](std::int64_t index) {
            if (index < 0) index += size;
            return std::clamp<std::int64_t>(index, 0, size);
        };
        const std::int64_t from = clip(start);
        const std::int64_t to = end ? clip(*end) : size;
        if (to <= from) return {};
        return Encode(std::u32string_view(decoded).substr(static_cast<std::size_t>(from), static_cast<std::size_t>(to - from)));
    }

    std::pair<char32_t, std::size_t> CodePointAt(std::string_view text, std::size_t at) {
        return Next(text, at);
    }

    std::pair<std::size_t, std::size_t> SliceBounds(std::size_t size, std::int64_t start, std::optional<std::int64_t> end) {
        const auto count = static_cast<std::int64_t>(size);
        auto clip = [count](std::int64_t index) {
            if (index < 0) index += count;
            return std::clamp<std::int64_t>(index, 0, count);
        };
        const std::int64_t from = clip(start);
        const std::int64_t to = end ? clip(*end) : count;
        return {static_cast<std::size_t>(from), static_cast<std::size_t>(std::max(from, to))};
    }

    namespace {
        template <typename Predicate>
        std::size_t RunOf(std::string_view text, std::size_t at, Predicate matches) {
            std::size_t end = at;
            while (end < text.size()) {
                const auto [c, used] = Next(text, end);
                if (!matches(c)) break;
                end += used;
            }
            return end - at;
        }
    }

    std::size_t DecimalRun(std::string_view text, std::size_t at) {
        return RunOf(text, at, [](char32_t c) { return IsDecimal(c); });
    }

    std::size_t SpaceRun(std::string_view text, std::size_t at) {
        return RunOf(text, at, [](char32_t c) { return IsSpace(c); });
    }

    std::size_t NonSpaceRun(std::string_view text, std::size_t at) {
        return RunOf(text, at, [](char32_t c) { return !IsSpace(c); });
    }

    bool IsSpace(char32_t c) {
        return std::ranges::binary_search(Unicode::Space, c);
    }

    bool IsLineBreak(char32_t c) {
        return std::ranges::binary_search(Unicode::LineBreak, c);
    }

    bool IsDecimal(char32_t c) {
        return DecimalValue(c) >= 0;
    }

    std::int32_t DecimalValue(char32_t c) {
        if (IsAsciiDigit(c)) return static_cast<std::int32_t>(c - '0');
        const auto found = std::ranges::upper_bound(Unicode::Decimal, c, {}, [](const Unicode::Run& run) { return run.first; });
        if (found == Unicode::Decimal.begin()) return -1;
        const Unicode::Run& run = *std::prev(found);
        if (c > run.last) return -1;
        return static_cast<std::int32_t>((c - run.first) % 10);
    }

    bool IsPrintable(char32_t c) {
        return InRuns(Unicode::Printable, c);
    }

    bool IsWord(char32_t c) {
        return InRuns(Unicode::Word, c);
    }

    char IgnoreCaseLetter(char32_t c) {
        if (c >= 'a' && c <= 'z') return static_cast<char>(c);
        if (c >= 'A' && c <= 'Z') return static_cast<char>(c - 'A' + 'a');
        const auto found = std::ranges::lower_bound(Unicode::IgnoreCase, c, {}, [](const Unicode::Fold& fold) { return fold.from; });
        if (found == Unicode::IgnoreCase.end() || found->from != c) return '\0';
        return found->letter;
    }

    std::string LStrip(std::string_view text) {
        std::size_t at = 0;
        while (at < text.size()) {
            const auto [c, used] = Next(text, at);
            if (!IsSpace(c)) break;
            at += used;
        }
        return std::string(text.substr(at));
    }

    std::string RStrip(std::string_view text) {
        const std::u32string decoded = Decode(text);
        std::size_t end = decoded.size();
        while (end > 0 && IsSpace(decoded[end - 1])) --end;
        if (end == decoded.size()) return std::string(text);
        return Encode(std::u32string_view(decoded).substr(0, end));
    }

    std::string Strip(std::string_view text) {
        return RStrip(LStrip(text));
    }

    std::string LStrip(std::string_view text, std::string_view chars) {
        const std::u32string set = Decode(chars);
        const std::u32string decoded = Decode(text);
        std::size_t start = 0;
        while (start < decoded.size() && set.contains(decoded[start])) ++start;
        return Encode(std::u32string_view(decoded).substr(start));
    }

    std::string RStrip(std::string_view text, std::string_view chars) {
        const std::u32string set = Decode(chars);
        const std::u32string decoded = Decode(text);
        std::size_t end = decoded.size();
        while (end > 0 && set.contains(decoded[end - 1])) --end;
        return Encode(std::u32string_view(decoded).substr(0, end));
    }

    std::string Strip(std::string_view text, std::string_view chars) {
        return RStrip(LStrip(text, chars), chars);
    }

    std::vector<std::string> Split(std::string_view text) {
        std::vector<std::string> out;
        std::string current;
        bool inWord = false;
        for (std::size_t at = 0; at < text.size();) {
            const auto [c, used] = Next(text, at);
            if (IsSpace(c)) {
                if (inWord) out.push_back(std::move(current));
                current.clear();
                inWord = false;
            } else {
                current.append(text.substr(at, used));
                inWord = true;
            }
            at += used;
        }
        if (inWord) out.push_back(std::move(current));
        return out;
    }

    std::vector<std::string> Split(std::string_view text, std::string_view separator, std::int64_t maxSplit) {
        if (separator.empty()) throw std::invalid_argument("empty separator");
        std::vector<std::string> out;
        std::size_t start = 0;
        while (maxSplit < 0 || static_cast<std::int64_t>(out.size()) < maxSplit) {
            const std::size_t found = text.find(separator, start);
            if (found == std::string_view::npos) break;
            out.emplace_back(text.substr(start, found - start));
            start = found + separator.size();
        }
        out.emplace_back(text.substr(start));
        return out;
    }

    std::vector<std::string> RSplit(std::string_view text, std::string_view separator, std::int64_t maxSplit) {
        if (separator.empty()) throw std::invalid_argument("empty separator");
        std::vector<std::string> out;
        std::size_t end = text.size();
        while (maxSplit < 0 || static_cast<std::int64_t>(out.size()) < maxSplit) {
            if (end < separator.size()) break;
            const std::size_t found = text.rfind(separator, end - separator.size());
            if (found == std::string_view::npos) break;
            out.emplace_back(text.substr(found + separator.size(), end - found - separator.size()));
            end = found;
        }
        out.emplace_back(text.substr(0, end));
        std::ranges::reverse(out);
        return out;
    }

    std::vector<std::string> SplitLines(std::string_view text, bool keepEnds) {
        std::vector<std::string> out;
        std::size_t start = 0;
        std::size_t at = 0;
        while (at < text.size()) {
            const auto [c, used] = Next(text, at);
            if (!IsLineBreak(c)) {
                at += used;
                continue;
            }
            std::size_t end = at + used;
            if (c == '\r' && end < text.size() && text[end] == '\n') ++end;
            out.emplace_back(text.substr(start, (keepEnds ? end : at) - start));
            start = end;
            at = end;
        }
        if (start < text.size()) out.emplace_back(text.substr(start));
        return out;
    }

    std::string Lower(std::string_view text) {
        std::string out;
        out.reserve(text.size());
        for (std::size_t at = 0; at < text.size();) {
            const auto [c, used] = Next(text, at);
            if (c < 0x80) {
                out.push_back(static_cast<char>(c >= 'A' && c <= 'Z' ? c - 'A' + 'a' : c));
            } else if (const std::string_view to = Mapped(Unicode::Lower, c); !to.empty()) {
                out.append(to);
            } else {
                AppendUtf8(out, c);
            }
            at += used;
        }
        return out;
    }

    std::string Upper(std::string_view text) {
        std::string out;
        out.reserve(text.size());
        for (std::size_t at = 0; at < text.size();) {
            const auto [c, used] = Next(text, at);
            if (c < 0x80) {
                out.push_back(static_cast<char>(c >= 'a' && c <= 'z' ? c - 'a' + 'A' : c));
            } else if (const std::string_view to = Mapped(Unicode::Upper, c); !to.empty()) {
                out.append(to);
            } else {
                AppendUtf8(out, c);
            }
            at += used;
        }
        return out;
    }

    std::size_t Count(std::string_view text, std::string_view sub) {
        if (sub.empty()) return Length(text) + 1;
        std::size_t count = 0;
        for (std::size_t at = text.find(sub); at != std::string_view::npos; at = text.find(sub, at + sub.size())) ++count;
        return count;
    }

    std::string Replace(std::string_view text, std::string_view from, std::string_view to) {
        if (from.empty()) return std::string(text);
        std::string out;
        std::size_t start = 0;
        for (std::size_t at = text.find(from); at != std::string_view::npos; at = text.find(from, start)) {
            out.append(text.substr(start, at - start));
            out.append(to);
            start = at + from.size();
        }
        out.append(text.substr(start));
        return out;
    }

    std::string Join(std::span<const std::string> parts, std::string_view separator) {
        std::string out;
        for (std::size_t i = 0; i < parts.size(); ++i) {
            if (i > 0) out.append(separator);
            out.append(parts[i]);
        }
        return out;
    }

    std::string LJust(std::string_view text, std::size_t width) {
        const std::size_t length = Length(text);
        std::string out(text);
        if (length < width) out.append(width - length, ' ');
        return out;
    }

    std::string ReadText(const std::filesystem::path& path) {
        std::ifstream in(path, std::ios::binary);
        if (!in || std::filesystem::is_directory(path)) {
            throw Error("FileNotFoundError", std::format("[Errno 2] No such file or directory: {}", Repr(std::string_view(path.string()))));
        }
        const std::string bytes((std::istreambuf_iterator<char>(in)), std::istreambuf_iterator<char>());
        if (const std::optional<std::string> error = Utf8DecodeError(bytes)) throw Error("UnicodeDecodeError", *error);
        return UniversalNewlines(bytes);
    }

    std::optional<std::string> Utf8DecodeError(std::string_view bytes) {
        // The decoder's reasons: a bad lead byte is "invalid start byte"; the
        // k-th continuation byte out of range is "invalid continuation byte"
        // over k bytes; the input ending inside a sequence is "unexpected end
        // of data" over the rest of it.
        for (std::size_t at = 0; at < bytes.size();) {
            const auto lead = static_cast<std::uint8_t>(bytes[at]);
            std::size_t need = 0;
            std::uint8_t low = 0x80;
            std::uint8_t high = 0xBF;
            if (lead < 0x80) {
                ++at;
                continue;
            }
            if (lead >= 0xC2 && lead <= 0xDF) {
                need = 1;
            } else if (lead >= 0xE0 && lead <= 0xEF) {
                need = 2;
                if (lead == 0xE0) low = 0xA0;
                if (lead == 0xED) high = 0x9F;
            } else if (lead >= 0xF0 && lead <= 0xF4) {
                need = 3;
                if (lead == 0xF0) low = 0x90;
                if (lead == 0xF4) high = 0x8F;
            }
            std::string_view reason;
            std::size_t length = 1;
            if (need == 0) {
                reason = "invalid start byte";
            } else {
                for (std::size_t k = 1; k <= need; ++k) {
                    if (at + k >= bytes.size()) {
                        reason = "unexpected end of data";
                        length = bytes.size() - at;
                        break;
                    }
                    const auto next = static_cast<std::uint8_t>(bytes[at + k]);
                    if (next < low || next > high) {
                        reason = "invalid continuation byte";
                        length = k;
                        break;
                    }
                    low = 0x80;
                    high = 0xBF;
                }
            }
            if (reason.empty()) {
                at += need + 1;
                continue;
            }
            if (length == 1) return std::format("'utf-8' codec can't decode byte 0x{:02x} in position {}: {}", lead, at, reason);
            return std::format("'utf-8' codec can't decode bytes in position {}-{}: {}", at, at + length - 1, reason);
        }
        return std::nullopt;
    }

    namespace {
        struct JsonFailure {
            std::string_view message;
            std::size_t position;
        };

        bool IsJsonSpace(char32_t c) {
            return c == ' ' || c == '\t' || c == '\n' || c == '\r';
        }

        bool IsJsonDigit(char32_t c) {
            return c >= '0' && c <= '9';
        }

        std::int32_t HexDigit(char32_t c) {
            if (c >= '0' && c <= '9') return static_cast<std::int32_t>(c - '0');
            if (c >= 'a' && c <= 'f') return static_cast<std::int32_t>(c - 'a' + 10);
            if (c >= 'A' && c <= 'F') return static_cast<std::int32_t>(c - 'A' + 10);
            return -1;
        }

        // A JSON scanner that fails where and why the recorded messages say,
        // step for step: a string, an object, an array, a number, then any
        // value. "Expecting value" is a position where no value starts.
        class JsonScanner {
        public:
            explicit JsonScanner(std::u32string_view text) : s(text) {}

            std::optional<JsonFailure> Value(std::size_t idx, std::size_t& next) {
                if (idx >= s.size()) return JsonFailure{"Expecting value", idx};
                switch (s[idx]) {
                    case '"':
                        return String(idx + 1, next);
                    case '{':
                        return Object(idx + 1, next);
                    case '[':
                        return Array(idx + 1, next);
                    case 'n':
                        if (Word(idx, U"null", next)) return std::nullopt;
                        break;
                    case 't':
                        if (Word(idx, U"true", next)) return std::nullopt;
                        break;
                    case 'f':
                        if (Word(idx, U"false", next)) return std::nullopt;
                        break;
                    case 'N':
                        if (Word(idx, U"NaN", next)) return std::nullopt;
                        break;
                    case 'I':
                        if (Word(idx, U"Infinity", next)) return std::nullopt;
                        break;
                    case '-':
                        if (Word(idx, U"-Infinity", next)) return std::nullopt;
                        break;
                    default:
                        break;
                }
                return Number(idx, next);
            }

        private:
            std::u32string_view s;

            bool Word(std::size_t idx, std::u32string_view word, std::size_t& next) const {
                if (s.substr(idx, word.size()) != word) return false;
                next = idx + word.size();
                return true;
            }

            std::size_t SkipSpace(std::size_t idx) const {
                while (idx < s.size() && IsJsonSpace(s[idx])) ++idx;
                return idx;
            }

            std::optional<JsonFailure> String(std::size_t end, std::size_t& next) const {
                const std::size_t begin = end - 1;
                const std::size_t len = s.size();
                while (true) {
                    std::size_t at = end;
                    char32_t c = 0;
                    for (; at < len; ++at) {
                        c = s[at];
                        if (c == '"' || c == '\\') break;
                        if (c <= 0x1F) return JsonFailure{"Invalid control character at", at};
                    }
                    if (at >= len) return JsonFailure{"Unterminated string starting at", begin};
                    ++at;
                    if (c == '"') {
                        next = at;
                        return std::nullopt;
                    }
                    if (at == len) return JsonFailure{"Unterminated string starting at", begin};
                    c = s[at];
                    if (c != 'u') {
                        end = at + 1;
                        if (!std::u32string_view(U"\"\\/bfnrt").contains(c)) return JsonFailure{"Invalid \\escape", end - 2};
                        continue;
                    }
                    ++at;
                    end = at + 4;
                    if (end >= len) return JsonFailure{"Invalid \\uXXXX escape", at - 1};
                    std::uint32_t unit = 0;
                    for (; at < end; ++at) {
                        const std::int32_t digit = HexDigit(s[at]);
                        if (digit < 0) return JsonFailure{"Invalid \\uXXXX escape", end - 5};
                        unit = (unit << 4) | static_cast<std::uint32_t>(digit);
                    }
                    if (unit >= 0xD800 && unit <= 0xDBFF && end + 6 < len && s[at] == '\\' && s[at + 1] == 'u') {
                        at += 2;
                        end += 6;
                        std::uint32_t second = 0;
                        for (; at < end; ++at) {
                            const std::int32_t digit = HexDigit(s[at]);
                            if (digit < 0) return JsonFailure{"Invalid \\uXXXX escape", end - 5};
                            second = (second << 4) | static_cast<std::uint32_t>(digit);
                        }
                        if (second < 0xDC00 || second > 0xDFFF) end -= 6;
                    }
                }
            }

            std::optional<JsonFailure> Object(std::size_t idx, std::size_t& next) {
                idx = SkipSpace(idx);
                if (idx >= s.size() || s[idx] != '}') {
                    while (true) {
                        if (idx >= s.size() || s[idx] != '"') return JsonFailure{"Expecting property name enclosed in double quotes", idx};
                        std::size_t after = 0;
                        if (std::optional<JsonFailure> failure = String(idx + 1, after)) return failure;
                        idx = SkipSpace(after);
                        if (idx >= s.size() || s[idx] != ':') return JsonFailure{"Expecting ':' delimiter", idx};
                        idx = SkipSpace(idx + 1);
                        if (std::optional<JsonFailure> failure = Value(idx, after)) return failure;
                        idx = SkipSpace(after);
                        if (idx < s.size() && s[idx] == '}') break;
                        if (idx >= s.size() || s[idx] != ',') return JsonFailure{"Expecting ',' delimiter", idx};
                        const std::size_t comma = idx;
                        idx = SkipSpace(idx + 1);
                        if (idx < s.size() && s[idx] == '}') return JsonFailure{"Illegal trailing comma before end of object", comma};
                    }
                }
                next = idx + 1;
                return std::nullopt;
            }

            std::optional<JsonFailure> Array(std::size_t idx, std::size_t& next) {
                idx = SkipSpace(idx);
                if (idx >= s.size() || s[idx] != ']') {
                    while (true) {
                        std::size_t after = 0;
                        if (std::optional<JsonFailure> failure = Value(idx, after)) return failure;
                        idx = SkipSpace(after);
                        if (idx < s.size() && s[idx] == ']') break;
                        if (idx >= s.size() || s[idx] != ',') return JsonFailure{"Expecting ',' delimiter", idx};
                        const std::size_t comma = idx;
                        idx = SkipSpace(idx + 1);
                        if (idx < s.size() && s[idx] == ']') return JsonFailure{"Illegal trailing comma before end of array", comma};
                    }
                }
                next = idx + 1;
                return std::nullopt;
            }

            std::optional<JsonFailure> Number(std::size_t start, std::size_t& next) const {
                const std::size_t len = s.size();
                std::size_t idx = start;
                if (s[idx] == '-') {
                    ++idx;
                    if (idx >= len) return JsonFailure{"Expecting value", start};
                }
                if (s[idx] >= '1' && s[idx] <= '9') {
                    ++idx;
                    while (idx < len && IsJsonDigit(s[idx])) ++idx;
                } else if (s[idx] == '0') {
                    ++idx;
                } else {
                    return JsonFailure{"Expecting value", start};
                }
                if (idx + 1 < len && s[idx] == '.' && IsJsonDigit(s[idx + 1])) {
                    idx += 2;
                    while (idx < len && IsJsonDigit(s[idx])) ++idx;
                }
                if (idx + 1 < len && (s[idx] == 'e' || s[idx] == 'E')) {
                    const std::size_t exponent = idx;
                    ++idx;
                    if (idx + 1 < len && (s[idx] == '-' || s[idx] == '+')) ++idx;
                    while (idx < len && IsJsonDigit(s[idx])) ++idx;
                    if (!IsJsonDigit(s[idx - 1])) idx = exponent;
                }
                next = idx;
                return std::nullopt;
            }
        };
    }

    std::optional<std::string> JsonDecodeError(std::string_view text) {
        const std::u32string s = Decode(text);
        JsonScanner scanner(s);
        std::size_t start = 0;
        while (start < s.size() && IsJsonSpace(s[start])) ++start;
        std::size_t end = 0;
        std::optional<JsonFailure> failure;
        if (s.starts_with(U'\uFEFF')) {
            failure = JsonFailure{"Unexpected UTF-8 BOM (decode using utf-8-sig)", 0};
        } else {
            failure = scanner.Value(start, end);
        }
        if (!failure) {
            while (end < s.size() && IsJsonSpace(s[end])) ++end;
            if (end != s.size()) failure = JsonFailure{"Extra data", end};
        }
        if (!failure) return std::nullopt;
        const std::size_t pos = failure->position;
        const std::u32string_view before = std::u32string_view(s).substr(0, pos);
        const auto line = std::ranges::count(before, U'\n') + 1;
        const std::size_t lastNewline = before.rfind(U'\n');
        const std::size_t column = lastNewline == std::u32string_view::npos ? pos + 1 : pos - lastNewline;
        return std::format("{}: line {} column {} (char {})", failure->message, line, column, pos);
    }

    Json LoadJsonFile(const std::filesystem::path& path) {
        const std::string text = ReadText(path);
        if (const std::optional<std::string> error = JsonDecodeError(text)) throw Error("JSONDecodeError", *error);
        return ParseLoose(text);
    }

    std::string UniversalNewlines(std::string_view text) {
        if (!text.contains('\r')) return std::string(text);
        std::string out;
        out.reserve(text.size());
        for (std::size_t i = 0; i < text.size(); ++i) {
            if (text[i] == '\r') {
                out.push_back('\n');
                if (i + 1 < text.size() && text[i + 1] == '\n') ++i;
            } else {
                out.push_back(text[i]);
            }
        }
        return out;
    }

    std::string FloatRepr(double value) {
        if (std::isnan(value)) return "nan";
        if (std::isinf(value)) return value > 0 ? "inf" : "-inf";
        // The shortest round-trip digits, in to_chars' scientific form, then
        // laid out: fixed from 1e-4 up to 1e16, with an exponent otherwise.
        std::array<char, 64> buffer {};
        const auto [end, error] = std::to_chars(buffer.data(), buffer.data() + buffer.size(), value, std::chars_format::scientific);
        const std::string_view scientific(buffer.data(), end);
        const bool negative = scientific.starts_with('-');
        const std::string_view body = negative ? scientific.substr(1) : scientific;
        const std::size_t e = body.find('e');
        std::string digits;
        for (const char c : body.substr(0, e)) {
            if (c != '.') digits.push_back(c);
        }
        std::int32_t exponent = 0;
        std::from_chars(body.data() + e + 1 + (body[e + 1] == '+' ? 1 : 0), body.data() + body.size(), exponent);
        while (digits.size() > 1 && digits.back() == '0') digits.pop_back();
        const std::int32_t decimalPoint = exponent + 1;
        std::string out = negative ? "-" : "";
        if (decimalPoint <= -4 || decimalPoint > 16) {
            out += digits.substr(0, 1);
            if (digits.size() > 1) out += std::format(".{}", digits.substr(1));
            const std::int32_t shown = decimalPoint - 1;
            out += std::format("e{}{:02}", shown < 0 ? '-' : '+', shown < 0 ? -shown : shown);
            return out;
        }
        if (decimalPoint <= 0) {
            out += std::format("0.{}{}", std::string(static_cast<std::size_t>(-decimalPoint), '0'), digits);
        } else if (static_cast<std::size_t>(decimalPoint) >= digits.size()) {
            out += std::format("{}{}.0", digits, std::string(static_cast<std::size_t>(decimalPoint) - digits.size(), '0'));
        } else {
            out += std::format("{}.{}", digits.substr(0, static_cast<std::size_t>(decimalPoint)), digits.substr(static_cast<std::size_t>(decimalPoint)));
        }
        return out;
    }

    double Round(double value, std::int32_t digits) {
        if (!std::isfinite(value)) return value;
        // to_chars with a precision rounds the exact binary value correctly,
        // half to even on an exact tie.
        std::array<char, 400> buffer {};
        const auto [end, error] = std::to_chars(buffer.data(), buffer.data() + buffer.size(), value, std::chars_format::fixed, digits);
        if (error != std::errc()) return value;
        double rounded = value;
        std::from_chars(buffer.data(), end, rounded);
        return rounded;
    }

    std::int64_t RoundToInt(double value) {
        return static_cast<std::int64_t>(std::nearbyint(value));
    }

    std::optional<std::int64_t> ParseInt(std::string_view text) {
        const std::optional<std::string> ascii = AsciiNumber(text);
        if (!ascii || ascii->empty()) return std::nullopt;
        std::string_view body = *ascii;
        bool negative = false;
        if (body.front() == '+' || body.front() == '-') {
            negative = body.front() == '-';
            body.remove_prefix(1);
        }
        const std::optional<std::string> digits = Digits(body);
        if (!digits) return std::nullopt;
        std::int64_t value = 0;
        for (const char c : *digits) {
            if (value > (std::numeric_limits<std::int64_t>::max() - (c - '0')) / 10) return std::nullopt;
            value = value * 10 + (c - '0');
        }
        return negative ? -value : value;
    }

    std::optional<double> ParseFloat(std::string_view text) {
        const std::optional<std::string> ascii = AsciiNumber(text);
        if (!ascii || ascii->empty()) return std::nullopt;
        std::string_view body = *ascii;
        bool negative = false;
        if (body.front() == '+' || body.front() == '-') {
            negative = body.front() == '-';
            body.remove_prefix(1);
        }
        const std::string word = ToLowerAscii(body);
        if (word == "inf" || word == "infinity") return negative ? -std::numeric_limits<double>::infinity() : std::numeric_limits<double>::infinity();
        if (word == "nan") return std::numeric_limits<double>::quiet_NaN();
        // digits [. digits] [e [sign] digits], one of the two digit runs
        // present, underscores only between digits.
        const std::size_t exponentAt = body.find_first_of("eE");
        const std::string_view mantissa = body.substr(0, exponentAt);
        const std::size_t dot = mantissa.find('.');
        const std::string_view whole = mantissa.substr(0, dot);
        const std::string_view fraction = dot == std::string_view::npos ? std::string_view() : mantissa.substr(dot + 1);
        if (whole.empty() && fraction.empty()) return std::nullopt;
        std::string normal = negative ? "-" : "";
        if (!whole.empty()) {
            const std::optional<std::string> digits = Digits(whole);
            if (!digits) return std::nullopt;
            normal += *digits;
        }
        if (dot != std::string_view::npos) {
            normal += '.';
            if (!fraction.empty()) {
                const std::optional<std::string> digits = Digits(fraction);
                if (!digits) return std::nullopt;
                normal += *digits;
            }
        }
        if (exponentAt != std::string_view::npos) {
            std::string_view exponent = body.substr(exponentAt + 1);
            normal += 'e';
            if (!exponent.empty() && (exponent.front() == '+' || exponent.front() == '-')) {
                normal += exponent.front();
                exponent.remove_prefix(1);
            }
            const std::optional<std::string> digits = Digits(exponent);
            if (!digits) return std::nullopt;
            normal += *digits;
        }
        // strtod, not from_chars: an exponent out of range is inf or 0.0, not
        // an error. The process runs in the "C" locale.
        return std::strtod(normal.c_str(), nullptr);
    }

    std::string IntError(std::string_view text) {
        return std::format("invalid literal for int() with base 10: {}", Repr(text));
    }

    std::string FloatError(std::string_view text) {
        return std::format("could not convert string to float: {}", Repr(text));
    }

    std::string Repr(std::string_view text) {
        const char quote = text.contains('\'') && !text.contains('"') ? '"' : '\'';
        std::string out(1, quote);
        for (std::size_t at = 0; at < text.size();) {
            const auto [c, used] = Next(text, at);
            if (c == static_cast<char32_t>(quote) || c == '\\') {
                out.push_back('\\');
                out.push_back(static_cast<char>(c));
            } else if (c == '\t') {
                out += "\\t";
            } else if (c == '\n') {
                out += "\\n";
            } else if (c == '\r') {
                out += "\\r";
            } else if (c < 0x20 || c == 0x7F) {
                out += std::format("\\x{:02x}", static_cast<std::uint32_t>(c));
            } else if (c < 0x7F) {
                out.push_back(static_cast<char>(c));
            } else if (IsPrintable(c)) {
                AppendUtf8(out, c);
            } else if (c <= 0xFF) {
                out += std::format("\\x{:02x}", static_cast<std::uint32_t>(c));
            } else if (c <= 0xFFFF) {
                out += std::format("\\u{:04x}", static_cast<std::uint32_t>(c));
            } else {
                out += std::format("\\U{:08x}", static_cast<std::uint32_t>(c));
            }
            at += used;
        }
        out.push_back(quote);
        return out;
    }

    std::string ReprValue(const Json& value) {
        if (value.is_null()) return "None";
        if (value.is_boolean()) return value.get<bool>() ? "True" : "False";
        if (value.is_number_float()) return FloatRepr(value.get<double>());
        if (value.is_number_unsigned()) return std::to_string(value.get<std::uint64_t>());
        if (value.is_number_integer()) return std::to_string(value.get<std::int64_t>());
        if (value.is_string()) return Repr(std::string_view(value.get_ref<const std::string&>()));
        if (value.is_array()) {
            std::string out = "[";
            bool first = true;
            for (const Json& item : value) {
                if (!first) out += ", ";
                first = false;
                out += ReprValue(item);
            }
            return std::format("{}]", out);
        }
        std::string out = "{";
        bool first = true;
        for (auto it = value.begin(); it != value.end(); ++it) {
            if (!first) out += ", ";
            first = false;
            out += std::format("{}: {}", Repr(std::string_view(it.key())), ReprValue(it.value()));
        }
        return std::format("{}}}", out);
    }

    namespace {
        // One element of a translated shell pattern.
        struct GlobToken {
            enum class Kind { Star, Any, Literal, Class, Never };
            Kind kind;
            char32_t literal = 0;
            bool negate = false;
            std::vector<std::pair<char32_t, char32_t>> ranges;
        };

        // A bracket expression, from the text between [ and ] — escapes and
        // all — read as a regular expression reads a character class.
        GlobToken ClassFrom(const std::u32string& content) {
            GlobToken token { .kind = GlobToken::Kind::Class };
            std::u32string_view rest(content);
            if (rest.starts_with(U'!')) {
                token.negate = true;
                rest.remove_prefix(1);
            }
            auto item = [&rest]() {
                char32_t c = rest.front();
                rest.remove_prefix(1);
                if (c == U'\\' && !rest.empty()) {
                    c = rest.front();
                    rest.remove_prefix(1);
                }
                return c;
            };
            while (!rest.empty()) {
                const char32_t first = item();
                if (rest.size() >= 2 && rest.front() == U'-') {
                    rest.remove_prefix(1);
                    const char32_t last = item();
                    token.ranges.emplace_back(first, last);
                } else {
                    token.ranges.emplace_back(first, first);
                }
            }
            return token;
        }

        std::u32string EscapeClassChunk(std::u32string_view chunk, bool hyphens) {
            std::u32string out;
            for (const char32_t c : chunk) {
                if (c == U'\\' || (hyphens && c == U'-')) out.push_back(U'\\');
                out.push_back(c);
            }
            return out;
        }

        // A shell pattern, translated token by token.
        std::vector<GlobToken> Translate(const std::u32string& pattern) {
            std::vector<GlobToken> tokens;
            const std::size_t n = pattern.size();
            std::size_t i = 0;
            while (i < n) {
                const char32_t c = pattern[i++];
                if (c == U'*') {
                    if (tokens.empty() || tokens.back().kind != GlobToken::Kind::Star) tokens.push_back({ .kind = GlobToken::Kind::Star });
                    while (i < n && pattern[i] == U'*') ++i;
                } else if (c == U'?') {
                    tokens.push_back({ .kind = GlobToken::Kind::Any });
                } else if (c == U'[') {
                    std::size_t j = i;
                    if (j < n && pattern[j] == U'!') ++j;
                    if (j < n && pattern[j] == U']') ++j;
                    while (j < n && pattern[j] != U']') ++j;
                    if (j >= n) {
                        tokens.push_back({ .kind = GlobToken::Kind::Literal, .literal = U'[' });
                        continue;
                    }
                    std::u32string stuff = pattern.substr(i, j - i);
                    if (!stuff.contains(U'-')) {
                        stuff = EscapeClassChunk(stuff, false);
                    } else {
                        std::vector<std::u32string> chunks;
                        std::size_t start = i;
                        std::size_t k = pattern[i] == U'!' ? i + 2 : i + 1;
                        while (true) {
                            const std::size_t found = k < j ? pattern.find(U'-', k) : std::u32string::npos;
                            if (found == std::u32string::npos || found >= j) break;
                            chunks.push_back(pattern.substr(start, found - start));
                            start = found + 1;
                            k = found + 3;
                        }
                        const std::u32string chunk = pattern.substr(start, j - start);
                        if (!chunk.empty()) {
                            chunks.push_back(chunk);
                        } else {
                            chunks.back() += U'-';
                        }
                        // A reversed range is dropped: it matches nothing.
                        for (std::size_t m = chunks.size() - 1; m > 0; --m) {
                            if (chunks[m - 1].back() > chunks[m].front()) {
                                chunks[m - 1] = chunks[m - 1].substr(0, chunks[m - 1].size() - 1) + chunks[m].substr(1);
                                chunks.erase(chunks.begin() + static_cast<std::ptrdiff_t>(m));
                            }
                        }
                        stuff.clear();
                        for (std::size_t m = 0; m < chunks.size(); ++m) {
                            if (m > 0) stuff += U'-';
                            stuff += EscapeClassChunk(chunks[m], true);
                        }
                    }
                    i = j + 1;
                    if (stuff.empty()) {
                        tokens.push_back({ .kind = GlobToken::Kind::Never });
                    } else if (stuff == U"!") {
                        tokens.push_back({ .kind = GlobToken::Kind::Any });
                    } else {
                        tokens.push_back(ClassFrom(stuff));
                    }
                } else {
                    tokens.push_back({ .kind = GlobToken::Kind::Literal, .literal = c });
                }
            }
            return tokens;
        }

        bool Matches(const GlobToken& token, char32_t c) {
            switch (token.kind) {
                case GlobToken::Kind::Any:
                    return true;
                case GlobToken::Kind::Literal:
                    return token.literal == c;
                case GlobToken::Kind::Class: {
                    const bool in = std::ranges::any_of(token.ranges, [c](const auto& range) { return c >= range.first && c <= range.second; });
                    return in != token.negate;
                }
                default:
                    return false;
            }
        }
    }

    bool FnMatch(std::string_view name, std::string_view pattern) {
        const std::vector<GlobToken> tokens = Translate(Decode(pattern));
        const std::u32string text = Decode(name);
        // The classic two-pointer match: every token but * consumes exactly
        // one character, so backtracking only ever returns to the last star.
        std::size_t t = 0;
        std::size_t s = 0;
        std::optional<std::size_t> starToken;
        std::size_t starText = 0;
        while (s < text.size()) {
            if (t < tokens.size() && tokens[t].kind == GlobToken::Kind::Star) {
                starToken = t++;
                starText = s;
            } else if (t < tokens.size() && Matches(tokens[t], text[s])) {
                ++t;
                ++s;
            } else if (starToken) {
                t = *starToken + 1;
                s = ++starText;
            } else {
                return false;
            }
        }
        while (t < tokens.size() && tokens[t].kind == GlobToken::Kind::Star) ++t;
        return t == tokens.size();
    }

    std::optional<std::string> SplitUrl::Hostname() const {
        std::string hostinfo = netloc.substr(netloc.rfind('@') == std::string::npos ? 0 : netloc.rfind('@') + 1);
        std::string hostname;
        if (const std::size_t open = hostinfo.find('['); open != std::string::npos) {
            const std::string bracketed = hostinfo.substr(open + 1);
            hostname = bracketed.substr(0, bracketed.find(']'));
        } else {
            hostname = hostinfo.substr(0, hostinfo.find(':'));
        }
        if (hostname.empty()) return std::nullopt;
        // A scoped IPv6 address's zone is left as written.
        const std::size_t percent = hostname.find('%');
        if (percent == std::string::npos) return Lower(hostname);
        return Lower(hostname.substr(0, percent)) + hostname.substr(percent);
    }

    std::optional<std::string> SplitUrl::Username() const {
        const std::size_t at = netloc.rfind('@');
        if (at == std::string::npos) return std::nullopt;
        const std::string userinfo = netloc.substr(0, at);
        return userinfo.substr(0, userinfo.find(':'));
    }

    std::optional<std::string> SplitUrl::Password() const {
        const std::size_t at = netloc.rfind('@');
        if (at == std::string::npos) return std::nullopt;
        const std::string userinfo = netloc.substr(0, at);
        const std::size_t colon = userinfo.find(':');
        if (colon == std::string::npos) return std::nullopt;
        return userinfo.substr(colon + 1);
    }

    std::optional<std::int32_t> SplitUrl::Port() const {
        std::string hostinfo = netloc.substr(netloc.rfind('@') == std::string::npos ? 0 : netloc.rfind('@') + 1);
        std::string port;
        if (const std::size_t open = hostinfo.find('['); open != std::string::npos) {
            const std::string bracketed = hostinfo.substr(open + 1);
            const std::size_t close = bracketed.find(']');
            if (close != std::string::npos) {
                const std::string after = bracketed.substr(close + 1);
                if (const std::size_t colon = after.find(':'); colon != std::string::npos) port = after.substr(colon + 1);
            }
        } else if (const std::size_t colon = hostinfo.find(':'); colon != std::string::npos) {
            port = hostinfo.substr(colon + 1);
        }
        if (port.empty()) return std::nullopt;
        if (!std::ranges::all_of(port, [](char c) { return c >= '0' && c <= '9'; })) {
            throw std::invalid_argument(std::format("Port could not be cast to integer value as {}", Repr(port)));
        }
        std::int64_t value = 0;
        for (const char c : port) {
            value = value * 10 + (c - '0');
            if (value > 65535) break;
        }
        if (value > 65535) throw std::invalid_argument("Port out of range 0-65535");
        return static_cast<std::int32_t>(value);
    }

    namespace {
        // ipaddress.ip_address's verdict on what sits between brackets, as
        // far as urlsplit needs one: an IPv4 address is refused, anything that
        // is not an IPv6 address is invalid.
        void CheckBracketedHost(const std::string& host) {
            if (host.starts_with('v')) {
                // \Av[a-fA-F0-9]+\..+\z
                std::size_t i = 1;
                while (i < host.size() && std::isxdigit(static_cast<std::uint8_t>(host[i]))) ++i;
                if (i == 1 || i >= host.size() || host[i] != '.' || i + 1 >= host.size()) {
                    throw std::invalid_argument("IPvFuture address is invalid");
                }
                return;
            }
            std::array<unsigned char, 16> address {};
            std::string withoutZone = host.substr(0, host.find('%'));
            if (inet_pton(AF_INET, withoutZone.c_str(), address.data()) == 1) {
                throw std::invalid_argument("An IPv4 address cannot be in brackets");
            }
            if (inet_pton(AF_INET6, withoutZone.c_str(), address.data()) != 1) {
                throw std::invalid_argument(std::format("{} does not appear to be an IPv4 or IPv6 address", Repr(host)));
            }
        }
    }

    SplitUrl UrlSplit(std::string_view input) {
        std::string url;
        for (const char c : input) {
            if (c != '\t' && c != '\r' && c != '\n') url.push_back(c);
        }
        std::size_t lead = 0;
        while (lead < url.size() && static_cast<std::uint8_t>(url[lead]) <= 0x20) ++lead;
        url.erase(0, lead);

        SplitUrl parts;
        const std::size_t colon = url.find(':');
        auto isAlpha = [](char c) { return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z'); };
        auto isSchemeChar = [&](char c) { return isAlpha(c) || (c >= '0' && c <= '9') || c == '+' || c == '-' || c == '.'; };
        if (colon != std::string::npos && colon > 0 && isAlpha(url[0])
            && std::ranges::all_of(std::string_view(url).substr(0, colon), isSchemeChar)) {
            parts.scheme = ToLowerAscii(std::string_view(url).substr(0, colon));
            url.erase(0, colon + 1);
        }
        if (url.starts_with("//")) {
            std::size_t end = url.find_first_of("/?#", 2);
            if (end == std::string::npos) end = url.size();
            parts.netloc = url.substr(2, end - 2);
            url.erase(0, end);
            const bool open = parts.netloc.contains('[');
            const bool close = parts.netloc.contains(']');
            if (open != close) throw std::invalid_argument("Invalid IPv6 URL");
            if (open && close) {
                // _check_bracketed_netloc: the brackets may sit in the user
                // part, and then the host is checked as an address all the same.
                const std::string hostAndPort = parts.netloc.substr(parts.netloc.rfind('@') == std::string::npos ? 0 : parts.netloc.rfind('@') + 1);
                const std::size_t bracket = hostAndPort.find('[');
                if (bracket == std::string::npos) {
                    CheckBracketedHost(hostAndPort.substr(0, hostAndPort.find(':')));
                } else {
                    if (bracket != 0) throw std::invalid_argument("Invalid IPv6 URL");
                    const std::string bracketed = hostAndPort.substr(1);
                    const std::size_t closing = bracketed.find(']');
                    const std::string after = closing == std::string::npos ? std::string() : bracketed.substr(closing + 1);
                    if (!after.empty() && !after.starts_with(':')) throw std::invalid_argument("Invalid IPv6 URL");
                    CheckBracketedHost(bracketed.substr(0, closing));
                }
            }
        }
        if (const std::size_t hash = url.find('#'); hash != std::string::npos) {
            parts.fragment = url.substr(hash + 1);
            url.erase(hash);
        }
        if (const std::size_t question = url.find('?'); question != std::string::npos) {
            parts.query = url.substr(question + 1);
            url.erase(question);
        }
        parts.path = std::move(url);
        return parts;
    }

    std::string UrlUnsplit(std::string_view scheme, std::string_view netloc, std::string_view path, std::string_view query, std::string_view fragment) {
        // The schemes that get `//` before an empty netloc.
        constexpr std::array<std::string_view, 27> UsesNetloc = {
            "", "ftp", "http", "gopher", "nntp", "telnet", "imap", "wais", "file", "mms", "https", "shttp", "snews", "prospero",
            "rtsp", "rtsps", "rtspu", "rsync", "svn", "svn+ssh", "sftp", "nfs", "git", "git+ssh", "ws", "wss", "itms-services",
        };
        bool hasNetloc = !netloc.empty();
        if (!hasNetloc && !scheme.empty() && std::ranges::contains(UsesNetloc, scheme) && (path.empty() || path.starts_with('/'))) {
            hasNetloc = true;
        }
        std::string url(path);
        if (hasNetloc) {
            if (!url.empty() && !url.starts_with('/')) url.insert(0, "/");
            url = std::format("//{}{}", netloc, url);
        } else if (url.starts_with("//")) {
            url.insert(0, "//");
        }
        if (!scheme.empty()) url = std::format("{}:{}", scheme, url);
        if (!query.empty()) url = std::format("{}?{}", url, query);
        if (!fragment.empty()) url = std::format("{}#{}", url, fragment);
        return url;
    }

    std::string Unquote(std::string_view text) {
        if (!text.contains('%')) return std::string(text);
        // Percent sequences become bytes; runs of bytes are decoded as UTF-8
        // with bad bytes replaced; everything else is kept as it is.
        std::string out;
        std::string bytes;
        auto hexValue = [](char c) -> std::int32_t {
            if (c >= '0' && c <= '9') return c - '0';
            if (c >= 'a' && c <= 'f') return c - 'a' + 10;
            if (c >= 'A' && c <= 'F') return c - 'A' + 10;
            return -1;
        };
        for (std::size_t i = 0; i < text.size();) {
            if (text[i] == '%' && i + 2 < text.size() && hexValue(text[i + 1]) >= 0 && hexValue(text[i + 2]) >= 0) {
                bytes.push_back(static_cast<char>(hexValue(text[i + 1]) * 16 + hexValue(text[i + 2])));
                i += 3;
                continue;
            }
            if (!bytes.empty()) {
                out += DecodeReplace(bytes);
                bytes.clear();
            }
            out.push_back(text[i]);
            ++i;
        }
        if (!bytes.empty()) out += DecodeReplace(bytes);
        return out;
    }

    NaturalKey Natural(std::string_view name) {
        NaturalKey key;
        std::string text;
        std::int64_t number = 0;
        bool inDigits = false;
        for (std::size_t at = 0; at < name.size();) {
            const auto [c, used] = Next(name, at);
            const std::int32_t digit = DecimalValue(c);
            if (digit >= 0) {
                if (!inDigits) {
                    key.parts.emplace_back(text);
                    text.clear();
                    number = 0;
                    inDigits = true;
                }
                number = number * 10 + digit;
            } else {
                if (inDigits) {
                    key.parts.emplace_back(number);
                    inDigits = false;
                }
                text.append(name.substr(at, used));
            }
            at += used;
        }
        if (inDigits) {
            key.parts.emplace_back(number);
            text.clear();
        }
        key.parts.emplace_back(text);
        return key;
    }

    std::string NormPath(std::string_view path) {
        if (path.empty()) return ".";
        std::size_t leading = 0;
        while (leading < path.size() && path[leading] == '/') ++leading;
        // POSIX gives exactly two leading slashes a meaning of their own.
        const std::string prefix = leading == 0 ? "" : (leading == 2 ? "//" : "/");
        std::vector<std::string> kept;
        for (const std::string& component : PathComponents(path)) {
            if (component == ".") continue;
            if (component == "..") {
                if (!kept.empty() && kept.back() != "..") {
                    kept.pop_back();
                } else if (prefix.empty()) {
                    kept.push_back(component);
                }
                continue;
            }
            kept.push_back(component);
        }
        std::string out = prefix + Join(kept, "/");
        return out.empty() ? std::string(".") : out;
    }

    std::string AbsPath(std::string_view path) {
        if (path.starts_with('/')) return NormPath(path);
        return NormPath(JoinPath(std::filesystem::current_path().string(), path));
    }

    std::string RelPath(std::string_view path, std::string_view start) {
        if (path.empty()) throw std::invalid_argument("no path specified");
        const std::vector<std::string> from = PathComponents(AbsPath(start.empty() ? "." : start));
        const std::vector<std::string> to = PathComponents(AbsPath(path));
        std::size_t common = 0;
        while (common < from.size() && common < to.size() && from[common] == to[common]) ++common;
        std::vector<std::string> relative(from.size() - common, "..");
        relative.insert(relative.end(), to.begin() + static_cast<std::ptrdiff_t>(common), to.end());
        if (relative.empty()) return ".";
        return Join(relative, "/");
    }

    std::string BaseName(std::string_view path) {
        return std::string(path.substr(path.rfind('/') == std::string_view::npos ? 0 : path.rfind('/') + 1));
    }

    std::string DirName(std::string_view path) {
        const std::size_t slash = path.rfind('/');
        if (slash == std::string_view::npos) return "";
        std::string head(path.substr(0, slash + 1));
        if (!std::ranges::all_of(head, [](char c) { return c == '/'; })) {
            while (!head.empty() && head.back() == '/') head.pop_back();
        }
        return head;
    }

    std::string JoinPath(std::string_view a, std::string_view b) {
        if (b.starts_with('/')) return std::string(b);
        if (a.empty() || a.ends_with('/')) return std::string(a) + std::string(b);
        return std::format("{}/{}", std::string(a), std::string(b));
    }

    std::string ErrnoText(std::int32_t code) {
        return std::format("[Errno {}] {}", code, std::strerror(code));
    }

    Error::Error(std::string_view kind, const std::string& message) : std::runtime_error(message), kind(kind) {}

    const std::string& Error::Kind() const {
        return kind;
    }

    bool Truthy(const Json& value) {
        switch (value.type()) {
            case Json::value_t::null:
                return false;
            case Json::value_t::boolean:
                return value.get<bool>();
            case Json::value_t::number_integer:
                return value.get<std::int64_t>() != 0;
            case Json::value_t::number_unsigned:
                return value.get<std::uint64_t>() != 0;
            case Json::value_t::number_float:
                // NaN is true: it is not 0.0.
                return value.get<double>() != 0.0;
            case Json::value_t::string:
                return !value.get_ref<const std::string&>().empty();
            case Json::value_t::array:
            case Json::value_t::object:
                return !value.empty();
            default:
                return true;
        }
    }

    const Json& Or(const Json& value, const Json& fallback) {
        return Truthy(value) ? value : fallback;
    }

    const Json& EmptyDict() {
        static const Json empty = Json::object();
        return empty;
    }

    const Json& EmptyList() {
        static const Json empty = Json::array();
        return empty;
    }

    bool Has(const Json& dict, std::string_view key) {
        return dict.is_object() && dict.find(key) != dict.end();
    }

    const Json& Get(const Json& dict, std::string_view key) {
        static const Json none;
        if (!dict.is_object()) throw Error("AttributeError", std::format("'{}' object has no attribute 'get'", TypeName(dict)));
        const auto found = dict.find(key);
        return found == dict.end() ? none : *found;
    }

    const Json& At(const Json& dict, std::string_view key) {
        if (dict.is_object()) {
            const auto found = dict.find(key);
            if (found == dict.end()) throw Error("KeyError", Repr(key));
            return *found;
        }
        if (dict.is_array()) throw Error("TypeError", "list indices must be integers or slices, not str");
        if (dict.is_string()) throw Error("TypeError", "string indices must be integers, not 'str'");
        throw Error("TypeError", std::format("'{}' object is not subscriptable", TypeName(dict)));
    }

    std::string_view TypeName(const Json& value) {
        switch (value.type()) {
            case Json::value_t::null:
                return "NoneType";
            case Json::value_t::boolean:
                return "bool";
            case Json::value_t::number_integer:
            case Json::value_t::number_unsigned:
                return "int";
            case Json::value_t::number_float:
                return "float";
            case Json::value_t::string:
                return "str";
            case Json::value_t::array:
                return "list";
            case Json::value_t::object:
                return "dict";
            default:
                return "bytes";
        }
    }

    bool IsInt(const Json& value) {
        return value.is_boolean() || value.is_number_integer();
    }

    bool IsNumber(const Json& value) {
        return value.is_boolean() || value.is_number();
    }

    std::string Str(const Json& value) {
        if (value.is_string()) return value.get<std::string>();
        return ReprValue(value);
    }

    std::string StrOr(const Json& value, std::string_view fallback) {
        return Truthy(value) ? Str(value) : std::string(fallback);
    }

    namespace {
        // An int of a JSON value: a bool is 0 or 1. Unsigned values beyond
        // the signed range keep their own type in Equal().
        std::int64_t IntOf(const Json& value) {
            if (value.is_boolean()) return value.get<bool>() ? 1 : 0;
            if (value.is_number_unsigned()) return static_cast<std::int64_t>(value.get<std::uint64_t>());
            return value.get<std::int64_t>();
        }

        // An integer equals a float exactly, never by rounding the integer to
        // a double.
        bool IntEqualsFloat(const Json& integer, double f) {
            if (!std::isfinite(f) || f != std::floor(f)) return false;
            if (integer.is_number_unsigned() && integer.get<std::uint64_t>() > static_cast<std::uint64_t>(std::numeric_limits<std::int64_t>::max())) {
                if (f < 0.0 || f >= 18446744073709551616.0) return false;
                return static_cast<std::uint64_t>(f) == integer.get<std::uint64_t>();
            }
            if (f < -9223372036854775808.0 || f >= 9223372036854775808.0) return false;
            return static_cast<std::int64_t>(f) == IntOf(integer);
        }
    }

    bool Equal(const Json& a, const Json& b) {
        if (IsNumber(a) && IsNumber(b)) {
            if (a.is_number_float() && b.is_number_float()) return a.get<double>() == b.get<double>();
            if (a.is_number_float()) return IntEqualsFloat(b, a.get<double>());
            if (b.is_number_float()) return IntEqualsFloat(a, b.get<double>());
            const bool bigA = a.is_number_unsigned() && a.get<std::uint64_t>() > static_cast<std::uint64_t>(std::numeric_limits<std::int64_t>::max());
            const bool bigB = b.is_number_unsigned() && b.get<std::uint64_t>() > static_cast<std::uint64_t>(std::numeric_limits<std::int64_t>::max());
            if (bigA || bigB) return bigA && bigB && a.get<std::uint64_t>() == b.get<std::uint64_t>();
            return IntOf(a) == IntOf(b);
        }
        if (a.type() != b.type()) return false;
        if (a.is_array()) {
            if (a.size() != b.size()) return false;
            for (std::size_t i = 0; i < a.size(); ++i) {
                if (!Equal(a[i], b[i])) return false;
            }
            return true;
        }
        if (a.is_object()) {
            if (a.size() != b.size()) return false;
            for (auto it = a.begin(); it != a.end(); ++it) {
                const auto other = b.find(it.key());
                if (other == b.end() || !Equal(it.value(), *other)) return false;
            }
            return true;
        }
        return a == b;
    }

    bool In(const Json& item, const Json& container) {
        if (container.is_array()) {
            return std::ranges::any_of(container, [&](const Json& element) { return Equal(item, element); });
        }
        if (container.is_object()) {
            if (item.is_array() || item.is_object()) throw Error("TypeError", std::format("unhashable type: '{}'", TypeName(item)));
            return item.is_string() && container.find(item.get_ref<const std::string&>()) != container.end();
        }
        if (container.is_string()) {
            if (!item.is_string()) throw Error("TypeError", std::format("'in <string>' requires string as left operand, not {}", TypeName(item)));
            return container.get_ref<const std::string&>().contains(item.get_ref<const std::string&>());
        }
        throw Error("TypeError", std::format("argument of type '{}' is not a container or iterable", TypeName(container)));
    }

    Json Add(const Json& a, const Json& b) {
        if (!IsNumber(a) || !IsNumber(b)) {
            throw Error("TypeError", std::format("unsupported operand type(s) for +: '{}' and '{}'", TypeName(a), TypeName(b)));
        }
        if (a.is_number_float() || b.is_number_float()) {
            auto asDouble = [](const Json& value) { return value.is_number_float() ? value.get<double>() : static_cast<double>(IntOf(value)); };
            return Json(asDouble(a) + asDouble(b));
        }
        return Json(IntOf(a) + IntOf(b));
    }

    double Sum(std::span<const double> values) {
        if (values.empty()) return 0.0;
        // sum() starts from the int 0; 0 + x is x (and 0.0 for -0.0).
        double total = 0.0 + values.front();
        double compensation = 0.0;
        for (const double x : values.subspan(1)) {
            const double t = total + x;
            if (std::fabs(total) >= std::fabs(x)) {
                compensation += (total - t) + x;
            } else {
                compensation += (x - t) + total;
            }
            total = t;
        }
        if (compensation != 0.0 && std::isfinite(compensation)) total += compensation;
        return total;
    }
}