// Go ref: filestore.go:2204 (lastChecksum), filestore.go:8180 (validation in msgFromBufEx)
//
// Tests for per-block last-checksum tracking and read-path validation using XxHash64.
// The Go reference implementation tracks the last written checksum in msgBlock.lchk
// and validates each record's checksum during reads to detect storage corruption.

using NATS.Server.JetStream.Storage;

namespace NATS.Server.Tests.JetStream.Storage;

public sealed class FileStoreChecksumTests : IDisposable
{
    private readonly DirectoryInfo _dir = Directory.CreateTempSubdirectory("checksum-");

    public void Dispose() => _dir.Delete(recursive: true);

    // Go ref: filestore.go:2204 (msgBlock.lchk — last checksum field)
    [Fact]
    public void MsgBlock_tracks_last_checksum()
    {
        // Arrange / Act
        using var block = MsgBlock.Create(1, _dir.FullName, 1024 * 1024);
        block.Write("test", ReadOnlyMemory<byte>.Empty, "hello"u8.ToArray());

        // Assert
        block.LastChecksum.ShouldNotBeNull();
        block.LastChecksum!.Length.ShouldBe(8); // XxHash64 = 8 bytes
    }

    // Go ref: filestore.go:8180 (msgFromBufEx checksum validation)
    [Fact]
    public void MsgBlock_validates_checksum_on_read()
    {
        // Arrange
        using var block = MsgBlock.Create(1, _dir.FullName, 1024 * 1024);
        block.Write("test", ReadOnlyMemory<byte>.Empty, "hello"u8.ToArray());
        block.Flush();
        block.ClearCache(); // force disk read

        // Act — read should succeed with valid data
        var record = block.Read(1);

        // Assert
        record.ShouldNotBeNull();
        record!.Subject.ShouldBe("test");
        record.Payload.ToArray().ShouldBe("hello"u8.ToArray());
    }

    // Go ref: filestore.go:8180 (checksum mismatch → error path)
    [Fact]
    public void MsgBlock_detects_corrupted_record_on_disk_read()
    {
        // Arrange — write a record, flush, clear cache so next read goes to disk
        using var block = MsgBlock.Create(1, _dir.FullName, 1024 * 1024);
        block.Write("test", ReadOnlyMemory<byte>.Empty, "hello"u8.ToArray());
        block.Flush();
        block.ClearCache();

        // Corrupt a byte near the end of the block file (in the payload region)
        var files = Directory.GetFiles(_dir.FullName, "*.blk");
        files.Length.ShouldBe(1);
        var bytes = File.ReadAllBytes(files[0]);
        // Flip a bit in the payload area (10 bytes from end: past checksum + timestamp)
        bytes[^10] ^= 0xFF;
        File.WriteAllBytes(files[0], bytes);

        // Act / Assert — Decode should throw on checksum mismatch
        Should.Throw<InvalidDataException>(() => block.Read(1));
    }

    // Go ref: filestore.go:2204 (lchk updated on each write)
    [Fact]
    public void MsgBlock_checksum_chain_across_writes()
    {
        // Arrange
        using var block = MsgBlock.Create(1, _dir.FullName, 1024 * 1024);

        // Act — write three records, capture checksum after each
        block.Write("a", ReadOnlyMemory<byte>.Empty, "one"u8.ToArray());
        var checksum1 = block.LastChecksum?.ToArray();

        block.Write("b", ReadOnlyMemory<byte>.Empty, "two"u8.ToArray());
        var checksum2 = block.LastChecksum?.ToArray();

        block.Write("c", ReadOnlyMemory<byte>.Empty, "three"u8.ToArray());
        var checksum3 = block.LastChecksum?.ToArray();

        // Assert — each write produces a non-null checksum that changes
        checksum1.ShouldNotBeNull();
        checksum2.ShouldNotBeNull();
        checksum3.ShouldNotBeNull();
        checksum1.ShouldNotBe(checksum2!);
        checksum2.ShouldNotBe(checksum3!);
    }
}