// vdm/util/crc32.hpp — CRC-32 (IEEE 802.3 / zlib polynomial), header-only.
//
// Used to integrity-check the .veloxpart.meta resume sidecar (docs/04 §5). Standard
// reflected CRC-32 with 0xEDB88320, init/xorout 0xFFFFFFFF — byte-compatible with
// zlib's crc32() and `cksum -o3` — so the value is reproducible outside this codebase.
//
// This header compiles standalone.

#ifndef VDM_UTIL_CRC32_HPP
#define VDM_UTIL_CRC32_HPP

#include <array>
#include <cstddef>
#include <cstdint>

#include "vdm/util/bytes.hpp"

namespace vdm {

namespace detail {
inline constexpr std::array<std::uint32_t, 256> make_crc32_table() {
    std::array<std::uint32_t, 256> t{};
    for (std::uint32_t i = 0; i < 256; ++i) {
        std::uint32_t c = i;
        for (int k = 0; k < 8; ++k)
            c = (c & 1u) ? (0xEDB88320u ^ (c >> 1)) : (c >> 1);
        t[i] = c;
    }
    return t;
}
inline constexpr std::array<std::uint32_t, 256> kCrc32Table = make_crc32_table();
}  // namespace detail

// Incremental: pass the previous result back as `seed` to continue over split buffers.
[[nodiscard]] inline std::uint32_t crc32_update(std::uint32_t seed, ConstByteSpan data) noexcept {
    std::uint32_t c = seed ^ 0xFFFFFFFFu;
    for (std::byte b : data)
        c = detail::kCrc32Table[(c ^ std::to_integer<std::uint8_t>(b)) & 0xFFu] ^ (c >> 8);
    return c ^ 0xFFFFFFFFu;
}

[[nodiscard]] inline std::uint32_t crc32(ConstByteSpan data) noexcept {
    return crc32_update(0u, data);
}

}  // namespace vdm

#endif  // VDM_UTIL_CRC32_HPP