VoxelForge/Standalone/Compat/Json.h

#pragma once

#include <charconv>
#include <fstream>
#include <variant>

namespace VFJson
{
    struct Value
    {
        using Array = std::vector<Value>;
        using Object = std::unordered_map<std::string, Value>;
        using Storage = std::variant<std::nullptr_t, bool, double, std::string, Array, Object>;
        Storage Data = nullptr;

        bool IsObject() const { return std::holds_alternative<Object>(Data); }
        bool IsArray() const { return std::holds_alternative<Array>(Data); }
        bool IsString() const { return std::holds_alternative<std::string>(Data); }
        bool IsNumber() const { return std::holds_alternative<double>(Data); }
        const Object& AsObject() const { static const Object Empty; return IsObject() ? std::get<Object>(Data) : Empty; }
        const Array& AsArray() const { static const Array Empty; return IsArray() ? std::get<Array>(Data) : Empty; }
        const Value& Get(std::string_view Key) const
        {
            static const Value Null;
            const Object& O = AsObject();
            auto It = O.find(std::string(Key));
            return It == O.end() ? Null : It->second;
        }
        double Number(double Fallback = 0.0) const { return IsNumber() ? std::get<double>(Data) : Fallback; }
        int64 Integer(int64 Fallback = 0) const { return IsNumber() ? static_cast<int64>(std::get<double>(Data)) : Fallback; }
        bool Boolean(bool Fallback = false) const { return std::holds_alternative<bool>(Data) ? std::get<bool>(Data) : Fallback; }
        const std::string& String() const { static const std::string Empty; return IsString() ? std::get<std::string>(Data) : Empty; }
    };

    class Parser
    {
    public:
        explicit Parser(std::string Source) : Text(std::move(Source)) {}
        bool Parse(Value& Out, std::string& Error)
        {
            SkipSpace();
            if (!ReadValue(Out, Error)) return false;
            SkipSpace();
            if (Position != Text.size()) { Error = "trailing characters after JSON value"; return false; }
            return true;
        }
    private:
        std::string Text;
        size_t Position = 0;
        void SkipSpace() { while (Position < Text.size() && static_cast<unsigned char>(Text[Position]) <= 0x20) ++Position; }
        bool Consume(char C) { SkipSpace(); if (Position < Text.size() && Text[Position] == C) { ++Position; return true; } return false; }
        static void AppendUtf8(std::string& Out, uint32 Code)
        {
            if (Code <= 0x7f) Out.push_back(static_cast<char>(Code));
            else if (Code <= 0x7ff) { Out.push_back(static_cast<char>(0xc0 | (Code >> 6))); Out.push_back(static_cast<char>(0x80 | (Code & 0x3f))); }
            else if (Code <= 0xffff) { Out.push_back(static_cast<char>(0xe0 | (Code >> 12))); Out.push_back(static_cast<char>(0x80 | ((Code >> 6) & 0x3f))); Out.push_back(static_cast<char>(0x80 | (Code & 0x3f))); }
            else { Out.push_back(static_cast<char>(0xf0 | (Code >> 18))); Out.push_back(static_cast<char>(0x80 | ((Code >> 12) & 0x3f))); Out.push_back(static_cast<char>(0x80 | ((Code >> 6) & 0x3f))); Out.push_back(static_cast<char>(0x80 | (Code & 0x3f))); }
        }
        bool Hex4(uint32& Out)
        {
            if (Position + 4 > Text.size()) return false;
            Out = 0;
            for (int32 I = 0; I < 4; ++I)
            {
                const char C = Text[Position++];
                const int32 N = (C >= '0' && C <= '9') ? C - '0' : (C >= 'a' && C <= 'f') ? C - 'a' + 10 : (C >= 'A' && C <= 'F') ? C - 'A' + 10 : -1;
                if (N < 0) return false;
                Out = (Out << 4) | static_cast<uint32>(N);
            }
            return true;
        }
        bool ReadString(std::string& Out, std::string& Error)
        {
            SkipSpace();
            if (Position >= Text.size() || Text[Position++] != '"') { Error = "expected JSON string"; return false; }
            while (Position < Text.size())
            {
                const unsigned char C = static_cast<unsigned char>(Text[Position++]);
                if (C == '"') return true;
                if (C < 0x20) { Error = "control character in JSON string"; return false; }
                if (C != '\\') { Out.push_back(static_cast<char>(C)); continue; }
                if (Position >= Text.size()) break;
                const char Escape = Text[Position++];
                switch (Escape)
                {
                case '"': Out.push_back('"'); break; case '\\': Out.push_back('\\'); break;
                case '/': Out.push_back('/'); break; case 'b': Out.push_back('\b'); break;
                case 'f': Out.push_back('\f'); break; case 'n': Out.push_back('\n'); break;
                case 'r': Out.push_back('\r'); break; case 't': Out.push_back('\t'); break;
                case 'u':
                {
                    uint32 Code = 0; if (!Hex4(Code)) { Error = "invalid JSON unicode escape"; return false; }
                    if (Code >= 0xd800 && Code <= 0xdbff)
                    {
                        if (Position + 2 > Text.size() || Text[Position++] != '\\' || Text[Position++] != 'u') { Error = "missing JSON low surrogate"; return false; }
                        uint32 Low = 0; if (!Hex4(Low) || Low < 0xdc00 || Low > 0xdfff) { Error = "invalid JSON low surrogate"; return false; }
                        Code = 0x10000 + ((Code - 0xd800) << 10) + (Low - 0xdc00);
                    }
                    AppendUtf8(Out, Code); break;
                }
                default: Error = "invalid JSON escape"; return false;
                }
            }
            Error = "unterminated JSON string"; return false;
        }
        bool ReadValue(Value& Out, std::string& Error)
        {
            SkipSpace();
            if (Position >= Text.size()) { Error = "unexpected end of JSON"; return false; }
            const char C = Text[Position];
            if (C == '"') { std::string S; if (!ReadString(S, Error)) return false; Out.Data = std::move(S); return true; }
            if (C == '{')
            {
                ++Position; Value::Object O; SkipSpace();
                if (Consume('}')) { Out.Data = std::move(O); return true; }
                for (;;)
                {
                    std::string Key; if (!ReadString(Key, Error) || !Consume(':')) { if (Error.empty()) Error = "expected colon in JSON object"; return false; }
                    Value V; if (!ReadValue(V, Error)) return false; O[std::move(Key)] = std::move(V);
                    if (Consume('}')) break; if (!Consume(',')) { Error = "expected comma in JSON object"; return false; }
                }
                Out.Data = std::move(O); return true;
            }
            if (C == '[')
            {
                ++Position; Value::Array A; SkipSpace();
                if (Consume(']')) { Out.Data = std::move(A); return true; }
                for (;;) { Value V; if (!ReadValue(V, Error)) return false; A.push_back(std::move(V)); if (Consume(']')) break; if (!Consume(',')) { Error = "expected comma in JSON array"; return false; } }
                Out.Data = std::move(A); return true;
            }
            auto Match = [&](std::string_view S) { if (Text.compare(Position, S.size(), S) == 0) { Position += S.size(); return true; } return false; };
            if (Match("true")) { Out.Data = true; return true; }
            if (Match("false")) { Out.Data = false; return true; }
            if (Match("null")) { Out.Data = nullptr; return true; }
            const size_t Start = Position;
            if (C == '-') ++Position;
            while (Position < Text.size() && Text[Position] >= '0' && Text[Position] <= '9') ++Position;
            if (Position < Text.size() && Text[Position] == '.') { ++Position; while (Position < Text.size() && Text[Position] >= '0' && Text[Position] <= '9') ++Position; }
            if (Position < Text.size() && (Text[Position] == 'e' || Text[Position] == 'E')) { ++Position; if (Position < Text.size() && (Text[Position] == '+' || Text[Position] == '-')) ++Position; while (Position < Text.size() && Text[Position] >= '0' && Text[Position] <= '9') ++Position; }
            if (Start == Position) { Error = "invalid JSON value"; return false; }
            char* End = nullptr; const double Number = std::strtod(Text.c_str() + Start, &End);
            if (End != Text.c_str() + Position || !std::isfinite(Number)) { Error = "invalid JSON number"; return false; }
            Out.Data = Number; return true;
        }
    };

    inline bool Load(const std::string& Path, Value& Out, std::string& Error)
    {
        std::ifstream Input(Path, std::ios::binary);
        if (!Input) { Error = "cannot open " + Path; return false; }
        std::string Text((std::istreambuf_iterator<char>(Input)), std::istreambuf_iterator<char>());
        return Parser(std::move(Text)).Parse(Out, Error);
    }
}