export module ra3.core;

import std;

/**
 * Fundamental engine types shared by every OpenRA3 module.
 *
 * The vocabulary mirrors SAGE 2.0 (the engine Red Alert 3 runs on) as it
 * appears in the CnC Generals source, with layouts cross-checked against the
 * retail `ra3_1.12.game` binary through Ghidra.
 */
export namespace ra3::core {
    inline constexpr int version_major = 0;
    inline constexpr int version_minor = 3;
    inline constexpr int version_patch = 0;

    using real = float;
    using int32 = std::int32_t;
    using uint32 = std::uint32_t;
    using uint16 = std::uint16_t;
    using uint8 = std::uint8_t;
    using usize = std::size_t;

    /** SAGE advances the simulation on a fixed 30 Hz logic tick. */
    inline constexpr int logic_frames_per_second = 30;
    inline constexpr real logic_frame_seconds = 1.0F / static_cast<real>(logic_frames_per_second);

    /**
     * A world-space position or direction, matching SAGE's `Coord3D`.
     */
    struct coord3d {
        real x = 0.0F;
        real y = 0.0F;
        real z = 0.0F;

        auto operator+(const coord3d &rhs) const -> coord3d { return {x + rhs.x, y + rhs.y, z + rhs.z}; }
        auto operator-(const coord3d &rhs) const -> coord3d { return {x - rhs.x, y - rhs.y, z - rhs.z}; }
        auto operator*(real scalar) const -> coord3d { return {x * scalar, y * scalar, z * scalar}; }
        auto operator==(const coord3d &) const -> bool = default;

        /** Squared length, cheaper than `length()` when only comparing. */
        [[nodiscard]] auto length_squared() const -> real { return x * x + y * y + z * z; }

        [[nodiscard]] auto length() const -> real;

        [[nodiscard]] auto distance_to(const coord3d &other) const -> real { return (*this - other).length(); }

        /** Unit vector, or the zero vector when this one is degenerate. */
        [[nodiscard]] auto normalized() const -> coord3d;
    };

    /**
     * Integer grid coordinates, matching SAGE's `ICoord2D`.
     */
    struct coord2d {
        int32 x = 0;
        int32 y = 0;

        auto operator==(const coord2d &) const -> bool = default;
    };

    /**
     * An 8-bit-per-channel RGB colour.
     */
    struct rgb_color {
        uint8 r = 0;
        uint8 g = 0;
        uint8 b = 0;

        auto operator==(const rgb_color &) const -> bool = default;
    };

    /**
     * Derive the 32-bit identifier SAGE associates with an ASCII name.
     *
     * The retail `NameKeyGenerator` hash is still being recovered; this
     * deterministic, case-insensitive FNV-1a stand-in keeps the interface
     * stable until exact parity is proven against the binary.
     */
    constexpr auto make_name_key(std::string_view name) -> uint32 {
        uint32 key = 2166136261U;
        for (const auto ch: name) {
            auto c = static_cast<unsigned char>(ch);
            if (c >= 'a' && c <= 'z') c = static_cast<unsigned char>(c - 'a' + 'A');
            key ^= c;
            key *= 16777619U;
        }
        return key;
    }

    /**
     * An owned ASCII identifier carrying a cached `NameKey`.
     */
    class ascii_string {
    public:
        ascii_string() = default;
        ascii_string(std::string_view text) : text_(text) {}
        explicit ascii_string(std::string text) : text_(std::move(text)) {}

        [[nodiscard]] auto view() const -> std::string_view { return text_; }
        [[nodiscard]] auto empty() const -> bool { return text_.empty(); }
        [[nodiscard]] auto c_str() const -> const char * { return text_.c_str(); }
        [[nodiscard]] auto key() const -> uint32 { return make_name_key(text_); }

        auto operator==(const ascii_string &) const -> bool = default;
        auto operator<=>(const ascii_string &) const = default;

        friend auto operator<<(std::ostream &out, const ascii_string &value) -> std::ostream &;

    private:
        std::string text_;
    };

    inline auto operator<<(std::ostream &out, const ascii_string &value) -> std::ostream & {
        return out << value.text_;
    }

    /**
     * A deterministic pseudo-random source.
     *
     * SAGE separates the logic and client random streams so replays stay in
     * sync; this is the logic-stream stand-in. The retail generator constants
     * are pending recovery, so this uses a standard 32-bit LCG.
     */
    class random {
    public:
        explicit random(uint32 seed = 1U) : state_(seed == 0U ? 1U : seed) {}

        auto seed(uint32 value) -> void { state_ = (value == 0U ? 1U : value); }
        [[nodiscard]] auto state() const -> uint32 { return state_; }

        auto next_uint() -> uint32 {
            state_ = state_ * 1664525U + 1013904223U;
            return state_;
        }

        /** Uniform integer in the inclusive range `[lo, hi]`. */
        auto next_int(int32 lo, int32 hi) -> int32 {
            if (hi <= lo) return lo;
            const auto span = static_cast<uint32>(hi - lo + 1);
            return lo + static_cast<int32>(this->next_uint() % span);
        }

        /** Uniform real in `[0, 1)`. */
        auto next_real() -> real { return static_cast<real>(this->next_uint() >> 8) / static_cast<real>(1U << 24); }

    private:
        uint32 state_;
    };

    /**
     * Message types carried by the engine's `MessageStream`.
     *
     * `new_game` is confirmed against the retail binary: the skirmish BEGIN
     * handler appends type `0x2` to start a match.
     */
    enum class message_id : uint32 {
        invalid = 0x0,
        new_game = 0x2,
        clear_game_data = 0x3,
        begin_block = 0x5,
        end_block = 0x6,
    };

    [[nodiscard]] constexpr auto to_string(message_id id) -> std::string_view {
        switch (id) {
        case message_id::invalid:
            return "Invalid";
        case message_id::new_game:
            return "NewGame";
        case message_id::clear_game_data:
            return "ClearGameData";
        case message_id::begin_block:
            return "BeginBlock";
        case message_id::end_block:
            return "EndBlock";
        }
        return "Unknown";
    }

    /**
     * A single entry in the message stream.
     *
     * The retail `MessageStream::appendMessage` allocates a 0x74-byte node
     * (recovered at `0x0060c4a0`); this is its owning equivalent.
     */
    struct game_message {
        message_id id = message_id::invalid;
        uint32 player_index = 0;
        std::vector<uint8> payload;
    };

    /**
     * A FIFO of `game_message`s drained once per logic frame.
     *
     * In the retail engine the stream is an intrusive doubly linked list
     * (head at `+0x24`, tail at `+0x28` of the stream object); this is the
     * behaviourally equivalent owning version.
     */
    class message_stream {
    public:
        auto append(message_id id, uint32 player_index = 0U) -> game_message & {
            auto &slot = messages_.emplace_back();
            slot.id = id;
            slot.player_index = player_index;
            return slot;
        }

        [[nodiscard]] auto empty() const -> bool { return messages_.empty(); }
        [[nodiscard]] auto size() const -> usize { return messages_.size(); }
        [[nodiscard]] auto pending() const -> std::span<const game_message> { return messages_; }

        auto clear() -> void { messages_.clear(); }

        /** Remove and return the oldest message, if any. */
        auto pop() -> std::optional<game_message> {
            if (messages_.empty()) return std::nullopt;
            auto front = std::move(messages_.front());
            messages_.erase(messages_.begin());
            return front;
        }

    private:
        std::vector<game_message> messages_;
    };

    inline auto coord3d::length() const -> real { return std::sqrt(this->length_squared()); }

    inline auto coord3d::normalized() const -> coord3d {
        const auto len = this->length();
        if (len <= std::numeric_limits<real>::epsilon()) return {0.0F, 0.0F, 0.0F};
        return *this * (1.0F / len);
    }
}