// Fuzz target for the .veloxpart.meta reader — the file AGENT-CORE §5 calls
// attacker-adjacent (it lives in a world-writable-ish download directory). The reader
// must be total: no crash, no over-read, no unbounded allocation, on ANY byte string.
//
//   clang++ -std=c++23 -fsanitize=fuzzer,address,undefined ... (see CMakeLists.txt)
//   ./fuzz_veloxpart -max_len=8192 corpus/veloxpart/

#include <cstddef>
#include <cstdint>
#include <vector>

#include "vdm/meta/veloxpart.hpp"
#include "vdm/util/bytes.hpp"
#include "vdm/util/crc32.hpp"

using vdm::ConstByteSpan;
using vdm::meta::parse_veloxpart;
using vdm::meta::serialize_veloxpart;
using vdm::meta::VeloxPart;

namespace {

void check_roundtrip_stable(const VeloxPart &vp) {
    // A value the reader accepted must serialize and re-parse to an equal value —
    // otherwise the reader is accepting something the writer can't reproduce.
    auto image = serialize_veloxpart(vp);
    auto again = parse_veloxpart(ConstByteSpan(image.data(), image.size()));
    if (!again.has_value() || !(again.value() == vp))
        __builtin_trap();
}

}  // namespace

extern "C" int LLVMFuzzerTestOneInput(const std::uint8_t *data, std::size_t size) {
    ConstByteSpan raw(reinterpret_cast<const std::byte *>(data), size);

    // 1. Raw bytes straight in — most inputs die at the magic or CRC check.
    if (auto r = parse_veloxpart(raw); r.has_value())
        check_roundtrip_stable(r.value());

    // 2. Same bytes with a valid CRC-32 appended, so the field parser is actually
    //    reached and the ByteReader bounds checks (and load_le's precondition behind
    //    them) get exercised on structurally-plausible-but-hostile input.
    std::vector<std::byte> with_crc(raw.begin(), raw.end());
    std::uint32_t c = vdm::crc32(raw);
    for (int i = 0; i < 4; ++i)
        with_crc.push_back(static_cast<std::byte>((c >> (8 * i)) & 0xFF));
    if (auto r = parse_veloxpart(ConstByteSpan(with_crc.data(), with_crc.size())); r.has_value())
        check_roundtrip_stable(r.value());

    return 0;
}