export module ra3.fs;

import std;

export import ra3.core;
import ra3.assets;

/**
 * Reading of the retail game's on-disk assets.
 *
 * Red Alert 3 ships its data in `BIG4` archives under `<install>\Data`, with
 * individual payloads compressed by EA's RefPack codec. The container and codec
 * are implemented by the vendored `libra3assets` (`ra3.assets`, a sibling of
 * `libenderlog`); this module is the thin adapter OpenRA3's loader talks to, so
 * the format knowledge lives in one place.
 *
 * No game data is ever written into the repository; callers point the loader at
 * their local install (`--game-dir` / `RA3_GAME_DIR`, default `C:\Red Alert 3`).
 */
export namespace ra3::fs {
    using ra3::core::uint32;
    using ra3::core::uint8;
    using ra3::core::usize;

    /** Thrown when a `BIG4` archive is malformed or an entry is missing. */
    class archive_error : public std::runtime_error {
    public:
        using std::runtime_error::runtime_error;
    };

    /** Thrown when a RefPack stream is malformed. */
    class refpack_error : public std::runtime_error {
    public:
        using std::runtime_error::runtime_error;
    };

    inline constexpr std::array<uint8, 4> big_magic{'B', 'I', 'G', '4'};
    inline constexpr uint8 refpack_mask = 0x3EU;
    inline constexpr uint8 refpack_magic2 = 0xFBU;

    /** One file inside a `BIG4` archive. */
    struct big_entry {
        std::string name;
        uint32 offset = 0;
        uint32 size = 0;
    };

    namespace detail {
        /** View a `uint8` range as bytes, the currency of `libra3assets`. */
        [[nodiscard]] inline auto as_bytes(std::span<const uint8> data) -> std::span<const std::byte> { return std::as_bytes(data); }

        /** Copy a `libra3assets` byte buffer into OpenRA3's `uint8` vector. */
        [[nodiscard]] inline auto to_u8(std::vector<std::byte> bytes) -> std::vector<uint8> {
            std::vector<uint8> out(bytes.size());
            if (!bytes.empty()) std::memcpy(out.data(), bytes.data(), bytes.size());
            return out;
        }
    }

    /** True when `data` starts with a RefPack header (`0b??010000`, `0xFB`). */
    [[nodiscard]] inline auto is_refpack(std::span<const uint8> data) -> bool { return ra3::assets::is_refpack(detail::as_bytes(data)); }

    /**
     * Decompress an EA RefPack stream.
     *
     * @param data Compressed stream, starting at the header byte.
     * @return The decompressed bytes.
     * @throws refpack_error if the stream is malformed or the length disagrees.
     */
    [[nodiscard]] inline auto refpack_decompress(std::span<const uint8> data) -> std::vector<uint8> {
        try {
            return detail::to_u8(ra3::assets::refpack_decompress(detail::as_bytes(data)));
        } catch (const ra3::assets::refpack_error &error) {
            throw refpack_error(error.what());
        }
    }

    /** Decompress `data` when it is RefPack, otherwise copy it unchanged. */
    [[nodiscard]] inline auto maybe_decompress(std::span<const uint8> data) -> std::vector<uint8> {
        if (!is_refpack(data)) return {data.begin(), data.end()};
        return refpack_decompress(data);
    }

    /**
     * Read the declared output size from a RefPack header without decompressing.
     *
     * @throws refpack_error if the stream is malformed.
     */
    [[nodiscard]] inline auto refpack_output_size(std::span<const uint8> data) -> uint32 {
        try {
            return ra3::assets::refpack_output_size(detail::as_bytes(data));
        } catch (const ra3::assets::refpack_error &error) {
            throw refpack_error(error.what());
        }
    }

    [[nodiscard]] constexpr auto read_be32(const uint8 *p) -> uint32 {
        return (static_cast<uint32>(p[0]) << 24U) | (static_cast<uint32>(p[1]) << 16U) | (static_cast<uint32>(p[2]) << 8U) | static_cast<uint32>(p[3]);
    }

    [[nodiscard]] constexpr auto read_le32(const uint8 *p) -> uint32 {
        return static_cast<uint32>(p[0]) | (static_cast<uint32>(p[1]) << 8U) | (static_cast<uint32>(p[2]) << 16U) | (static_cast<uint32>(p[3]) << 24U);
    }

    /**
     * A parsed `BIG4` archive.
     *
     * The index and payloads are held by a `ra3::assets::big_archive`; this
     * adapter exposes the OpenRA3-facing surface (`big_entry`, `uint8` buffers)
     * over it. Payloads are RefPack-decompressed on request.
     */
    class big_archive {
    public:
        /**
         * Parse a `BIG4` archive.
         *
         * @param path Archive path.
         * @throws archive_error if the file is missing, not `BIG4`, or truncated.
         */
        [[nodiscard]] static auto open(const std::filesystem::path &path) -> big_archive {
            try {
                return big_archive{ra3::assets::big_archive::open(path), path};
            } catch (const ra3::assets::asset_error &error) {
                throw archive_error(error.what());
            }
        }

        [[nodiscard]] auto path() const -> const std::filesystem::path & { return path_; }
        [[nodiscard]] auto entries() const -> const std::vector<big_entry> & { return entries_; }
        [[nodiscard]] auto size() const -> usize { return entries_.size(); }
        [[nodiscard]] auto contains(std::string_view name) const -> bool { return archive_.contains(name); }

        /** Entry names whose path contains `needle`, in index order. */
        [[nodiscard]] auto find(std::string_view needle) const -> std::vector<const big_entry *> {
            std::vector<const big_entry *> matches;
            for (const auto &entry: entries_) {
                if (entry.name.find(needle) != std::string::npos) matches.push_back(&entry);
            }
            return matches;
        }

        /**
         * Read an entry's payload.
         *
         * @param name Entry name (backslash-separated, case-sensitive).
         * @param decompress RefPack-decompress the payload when true.
         * @throws archive_error if the entry is missing or unreadable.
         */
        [[nodiscard]] auto read(std::string_view name, bool decompress = true) const -> std::vector<uint8> {
            try {
                return detail::to_u8(archive_.read(name, decompress));
            } catch (const ra3::assets::refpack_error &error) {
                throw refpack_error(error.what());
            } catch (const ra3::assets::asset_error &error) {
                throw archive_error(error.what());
            }
        }

        /** Read the first `count` stored bytes of an entry (no decompression). */
        [[nodiscard]] auto read_prefix(std::string_view name, usize count) const -> std::vector<uint8> {
            try {
                return detail::to_u8(archive_.read_prefix(name, count));
            } catch (const ra3::assets::asset_error &error) {
                throw archive_error(error.what());
            }
        }

    private:
        big_archive(ra3::assets::big_archive archive, std::filesystem::path path) : archive_(std::move(archive)), path_(std::move(path)) {
            entries_.reserve(archive_.size());
            for (const auto &entry: archive_.entries()) entries_.push_back({entry.name, entry.offset, entry.size});
        }

        ra3::assets::big_archive archive_;
        std::filesystem::path path_;
        std::vector<big_entry> entries_;
    };

    /**
     * Locate a Red Alert 3 installation.
     *
     * Resolution order: the explicit argument, then `$RA3_GAME_DIR`, then the
     * default `C:\Red Alert 3`. A directory qualifies only if it has a `Data`
     * subdirectory.
     *
     * @return The install root, or `std::nullopt` when none is found.
     */
    [[nodiscard]] inline auto find_game_dir(const std::optional<std::filesystem::path> &explicit_dir = std::nullopt) -> std::optional<std::filesystem::path> {
        return ra3::assets::find_game_dir(explicit_dir);
    }
}