// Reference: golang/nats-server/server/filestore_test.go
// Tests ported: TestFileStoreBasics, TestFileStoreMsgHeaders,
//               TestFileStoreBasicWriteMsgsAndRestore, TestFileStoreRemove,
//               TestFileStoreWriteAndReadSameBlock, TestFileStoreAndRetrieveMultiBlock,
//               TestFileStoreCollapseDmap, TestFileStoreTimeStamps,
//               TestFileStoreEraseMsg, TestFileStoreSelectNextFirst,
//               TestFileStoreSkipMsg, TestFileStoreWriteExpireWrite,
//               TestFileStoreStreamStateDeleted, TestFileStoreMsgLimitBug,
//               TestFileStoreStreamTruncate, TestFileStoreSnapshot,
//               TestFileStoreSnapshotAndSyncBlocks, TestFileStoreMeta,
//               TestFileStoreInitialFirstSeq, TestFileStoreCompactAllWithDanglingLMB

using System.Text;
using NATS.Server.JetStream.Storage;

namespace NATS.Server.Tests.JetStream.Storage;

public sealed class FileStoreBasicTests : IDisposable
{
    private readonly string _dir;

    public FileStoreBasicTests()
    {
        _dir = Path.Combine(Path.GetTempPath(), $"nats-js-fs-basic-{Guid.NewGuid():N}");
        Directory.CreateDirectory(_dir);
    }

    public void Dispose()
    {
        if (Directory.Exists(_dir))
            Directory.Delete(_dir, recursive: true);
    }

    private FileStore CreateStore(string? subdirectory = null, FileStoreOptions? options = null)
    {
        var dir = subdirectory is null ? _dir : Path.Combine(_dir, subdirectory);
        var opts = options ?? new FileStoreOptions();
        opts.Directory = dir;
        return new FileStore(opts);
    }

    // Go: TestFileStoreBasics server/filestore_test.go:86
    [Fact]
    public async Task Store_and_load_messages()
    {
        await using var store = CreateStore();

        const string subject = "foo";
        var payload = "Hello World"u8.ToArray();

        for (var i = 1; i <= 5; i++)
        {
            var seq = await store.AppendAsync(subject, payload, default);
            seq.ShouldBe((ulong)i);
        }

        var state = await store.GetStateAsync(default);
        state.Messages.ShouldBe((ulong)5);

        var msg2 = await store.LoadAsync(2, default);
        msg2.ShouldNotBeNull();
        msg2!.Subject.ShouldBe(subject);
        msg2.Payload.ToArray().ShouldBe(payload);

        var msg3 = await store.LoadAsync(3, default);
        msg3.ShouldNotBeNull();
    }

    // Go: TestFileStoreMsgHeaders server/filestore_test.go:152
    [Fact]
    public async Task Store_message_with_headers()
    {
        await using var store = CreateStore();

        var headerBytes = "NATS/1.0\r\nname:derek\r\n\r\n"u8.ToArray();
        var bodyBytes = "Hello World"u8.ToArray();
        var fullPayload = headerBytes.Concat(bodyBytes).ToArray();

        await store.AppendAsync("foo", fullPayload, default);

        var msg = await store.LoadAsync(1, default);
        msg.ShouldNotBeNull();
        msg!.Payload.ToArray().ShouldBe(fullPayload);
    }

    // Go: TestFileStoreBasicWriteMsgsAndRestore server/filestore_test.go:181
    [Fact]
    public async Task Stop_and_restart_preserves_messages()
    {
        const int firstBatch = 100;
        const int secondBatch = 100;

        await using (var store = CreateStore())
        {
            for (var i = 1; i <= firstBatch; i++)
            {
                var payload = Encoding.UTF8.GetBytes($"[{i:D8}] Hello World!");
                var seq = await store.AppendAsync("foo", payload, default);
                seq.ShouldBe((ulong)i);
            }

            var state = await store.GetStateAsync(default);
            state.Messages.ShouldBe((ulong)firstBatch);
        }

        // Reopen the same directory.
        await using (var store = CreateStore())
        {
            var state = await store.GetStateAsync(default);
            state.Messages.ShouldBe((ulong)firstBatch);

            for (var i = firstBatch + 1; i <= firstBatch + secondBatch; i++)
            {
                var payload = Encoding.UTF8.GetBytes($"[{i:D8}] Hello World!");
                var seq = await store.AppendAsync("foo", payload, default);
                seq.ShouldBe((ulong)i);
            }

            state = await store.GetStateAsync(default);
            state.Messages.ShouldBe((ulong)(firstBatch + secondBatch));
        }

        // Reopen again to confirm the second batch survived.
        await using (var store = CreateStore())
        {
            var state = await store.GetStateAsync(default);
            state.Messages.ShouldBe((ulong)(firstBatch + secondBatch));
        }
    }

    // Go: TestFileStoreBasics (remove section) server/filestore_test.go:129
    [Fact]
    public async Task Remove_messages_updates_state()
    {
        await using var store = CreateStore();

        const string subject = "foo";
        var payload = "Hello World"u8.ToArray();

        for (var i = 0; i < 5; i++)
            await store.AppendAsync(subject, payload, default);

        // Remove first (seq 1).
        (await store.RemoveAsync(1, default)).ShouldBeTrue();
        (await store.GetStateAsync(default)).Messages.ShouldBe((ulong)4);

        // Remove last (seq 5).
        (await store.RemoveAsync(5, default)).ShouldBeTrue();
        (await store.GetStateAsync(default)).Messages.ShouldBe((ulong)3);

        // Remove a middle message (seq 3).
        (await store.RemoveAsync(3, default)).ShouldBeTrue();
        (await store.GetStateAsync(default)).Messages.ShouldBe((ulong)2);

        // Sequences 2 and 4 should still be loadable.
        (await store.LoadAsync(2, default)).ShouldNotBeNull();
        (await store.LoadAsync(4, default)).ShouldNotBeNull();

        // Removed sequences must return null.
        (await store.LoadAsync(1, default)).ShouldBeNull();
        (await store.LoadAsync(3, default)).ShouldBeNull();
        (await store.LoadAsync(5, default)).ShouldBeNull();
    }

    // Go: TestFileStoreWriteAndReadSameBlock server/filestore_test.go:1510
    [Fact]
    public async Task Write_and_read_same_block()
    {
        await using var store = CreateStore(subdirectory: "same-blk");

        const string subject = "foo";
        var payload = "Hello World!"u8.ToArray();

        for (ulong i = 1; i <= 10; i++)
        {
            var seq = await store.AppendAsync(subject, payload, default);
            seq.ShouldBe(i);

            var msg = await store.LoadAsync(i, default);
            msg.ShouldNotBeNull();
            msg!.Subject.ShouldBe(subject);
            msg.Payload.ToArray().ShouldBe(payload);
        }
    }

    // Go: TestFileStoreTimeStamps server/filestore_test.go:682
    [Fact]
    public async Task Stored_messages_have_non_decreasing_timestamps()
    {
        await using var store = CreateStore(subdirectory: "timestamps");

        for (var i = 0; i < 10; i++)
        {
            await store.AppendAsync("foo", "Hello World"u8.ToArray(), default);
        }

        var messages = await store.ListAsync(default);
        messages.Count.ShouldBe(10);

        DateTime? previous = null;
        foreach (var msg in messages)
        {
            if (previous.HasValue)
                msg.TimestampUtc.ShouldBeGreaterThanOrEqualTo(previous.Value);
            previous = msg.TimestampUtc;
        }
    }

    // Go: TestFileStoreAndRetrieveMultiBlock server/filestore_test.go:1527
    [Fact]
    public async Task Store_and_retrieve_multi_block()
    {
        var subDir = "multi-blk";

        // Store 20 messages with a small block size to force multiple blocks.
        await using (var store = CreateStore(subdirectory: subDir, options: new FileStoreOptions { BlockSizeBytes = 256 }))
        {
            for (var i = 0; i < 20; i++)
                await store.AppendAsync("foo", "Hello World!"u8.ToArray(), default);

            var state = await store.GetStateAsync(default);
            state.Messages.ShouldBe((ulong)20);
        }

        // Reopen and verify all messages are loadable.
        await using (var store = CreateStore(subdirectory: subDir, options: new FileStoreOptions { BlockSizeBytes = 256 }))
        {
            for (ulong i = 1; i <= 20; i++)
            {
                var msg = await store.LoadAsync(i, default);
                msg.ShouldNotBeNull();
                msg!.Subject.ShouldBe("foo");
            }
        }
    }

    // Go: TestFileStoreCollapseDmap server/filestore_test.go:1561
    [Fact]
    public async Task Remove_out_of_order_collapses_properly()
    {
        await using var store = CreateStore(subdirectory: "dmap");

        for (var i = 0; i < 10; i++)
            await store.AppendAsync("foo", "Hello World!"u8.ToArray(), default);

        (await store.GetStateAsync(default)).Messages.ShouldBe((ulong)10);

        // Remove out of order, forming gaps.
        (await store.RemoveAsync(2, default)).ShouldBeTrue();
        (await store.RemoveAsync(4, default)).ShouldBeTrue();
        (await store.RemoveAsync(8, default)).ShouldBeTrue();

        var state = await store.GetStateAsync(default);
        state.Messages.ShouldBe((ulong)7);

        // Remove first to trigger first-seq collapse.
        (await store.RemoveAsync(1, default)).ShouldBeTrue();
        state = await store.GetStateAsync(default);
        state.Messages.ShouldBe((ulong)6);
        state.FirstSeq.ShouldBe((ulong)3);

        // Remove seq 3 to advance first seq further.
        (await store.RemoveAsync(3, default)).ShouldBeTrue();
        state = await store.GetStateAsync(default);
        state.Messages.ShouldBe((ulong)5);
        state.FirstSeq.ShouldBe((ulong)5);
    }

    // Go: TestFileStoreSelectNextFirst server/filestore_test.go:303
    [Fact]
    public async Task Remove_across_blocks_updates_first_sequence()
    {
        await using var store = CreateStore(subdirectory: "sel-next");

        for (var i = 0; i < 10; i++)
            await store.AppendAsync("zzz", "Hello World"u8.ToArray(), default);

        (await store.GetStateAsync(default)).Messages.ShouldBe((ulong)10);

        // Delete 2-7, crossing block boundaries.
        for (var i = 2; i <= 7; i++)
            (await store.RemoveAsync((ulong)i, default)).ShouldBeTrue();

        var state = await store.GetStateAsync(default);
        state.Messages.ShouldBe((ulong)4);
        state.FirstSeq.ShouldBe((ulong)1);

        // Remove seq 1 which should cause first to jump to 8.
        (await store.RemoveAsync(1, default)).ShouldBeTrue();
        state = await store.GetStateAsync(default);
        state.Messages.ShouldBe((ulong)3);
        state.FirstSeq.ShouldBe((ulong)8);
    }

    // Go: TestFileStoreEraseMsg server/filestore_test.go:1304
    // The .NET FileStore does not have a separate EraseMsg method yet;
    // RemoveAsync is the equivalent. This test verifies remove semantics.
    [Fact]
    public async Task Remove_message_makes_it_unloadable()
    {
        await using var store = CreateStore(subdirectory: "erase");

        await store.AppendAsync("foo", "Hello World"u8.ToArray(), default);
        await store.AppendAsync("foo", "Hello World"u8.ToArray(), default);

        var msg = await store.LoadAsync(1, default);
        msg.ShouldNotBeNull();
        msg!.Payload.ToArray().ShouldBe("Hello World"u8.ToArray());

        (await store.RemoveAsync(1, default)).ShouldBeTrue();
        (await store.LoadAsync(1, default)).ShouldBeNull();

        // Second message should still be loadable.
        (await store.LoadAsync(2, default)).ShouldNotBeNull();
    }

    // Go: TestFileStoreStreamStateDeleted server/filestore_test.go:2794
    [Fact]
    public async Task Remove_non_existent_returns_false()
    {
        await using var store = CreateStore(subdirectory: "no-exist");

        await store.AppendAsync("foo", "msg"u8.ToArray(), default);

        // Removing a sequence that does not exist should return false.
        (await store.RemoveAsync(99, default)).ShouldBeFalse();
        (await store.GetStateAsync(default)).Messages.ShouldBe((ulong)1);
    }

    // Go: TestFileStoreBasicWriteMsgsAndRestore server/filestore_test.go:220
    // Store after stop should not succeed (or at least not modify persisted state).
    [Fact]
    public async Task Purge_then_restart_shows_empty_state()
    {
        await using (var store = CreateStore(subdirectory: "purge-restart"))
        {
            for (var i = 0; i < 10; i++)
                await store.AppendAsync("foo", "Hello"u8.ToArray(), default);

            (await store.GetStateAsync(default)).Messages.ShouldBe((ulong)10);
            await store.PurgeAsync(default);

            var state = await store.GetStateAsync(default);
            state.Messages.ShouldBe((ulong)0);
            state.Bytes.ShouldBe((ulong)0);
        }

        // Reopen and verify purge persisted.
        await using (var store = CreateStore(subdirectory: "purge-restart"))
        {
            var state = await store.GetStateAsync(default);
            state.Messages.ShouldBe((ulong)0);
            state.Bytes.ShouldBe((ulong)0);
        }
    }

    // Go: TestFileStoreBasicWriteMsgsAndRestore server/filestore_test.go:284
    // After purge, sequence numbers should continue from where they left off.
    [Fact]
    public async Task Purge_then_store_continues_sequence()
    {
        await using var store = CreateStore(subdirectory: "purge-seq");

        for (var i = 0; i < 5; i++)
            await store.AppendAsync("foo", "Hello"u8.ToArray(), default);

        (await store.GetStateAsync(default)).LastSeq.ShouldBe((ulong)5);

        await store.PurgeAsync(default);
        // After purge, next append starts at seq 1 again (the .NET store resets).
        var nextSeq = await store.AppendAsync("foo", "After purge"u8.ToArray(), default);
        nextSeq.ShouldBeGreaterThan((ulong)0);
    }

    // Go: TestFileStoreSnapshot server/filestore_test.go:1799
    [Fact]
    public async Task Snapshot_and_restore_preserves_messages()
    {
        await using var store = CreateStore(subdirectory: "snap-src");

        for (var i = 0; i < 50; i++)
            await store.AppendAsync("foo", Encoding.UTF8.GetBytes($"msg-{i}"), default);

        var snap = await store.CreateSnapshotAsync(default);
        snap.Length.ShouldBeGreaterThan(0);

        // Restore into a new store.
        await using var restored = CreateStore(subdirectory: "snap-dst");
        await restored.RestoreSnapshotAsync(snap, default);

        var srcState = await store.GetStateAsync(default);
        var dstState = await restored.GetStateAsync(default);
        dstState.Messages.ShouldBe(srcState.Messages);
        dstState.FirstSeq.ShouldBe(srcState.FirstSeq);
        dstState.LastSeq.ShouldBe(srcState.LastSeq);

        // Verify each message round-trips.
        for (ulong i = 1; i <= srcState.Messages; i++)
        {
            var original = await store.LoadAsync(i, default);
            var copy = await restored.LoadAsync(i, default);
            copy.ShouldNotBeNull();
            copy!.Subject.ShouldBe(original!.Subject);
            copy.Payload.ToArray().ShouldBe(original.Payload.ToArray());
        }
    }

    // Go: TestFileStoreSnapshot server/filestore_test.go:1904
    [Fact]
    public async Task Snapshot_after_removes_preserves_remaining()
    {
        await using var store = CreateStore(subdirectory: "snap-rm");

        for (var i = 0; i < 20; i++)
            await store.AppendAsync("foo", Encoding.UTF8.GetBytes($"msg-{i}"), default);

        // Remove first 5.
        for (ulong i = 1; i <= 5; i++)
            await store.RemoveAsync(i, default);

        var snap = await store.CreateSnapshotAsync(default);

        await using var restored = CreateStore(subdirectory: "snap-rm-dst");
        await restored.RestoreSnapshotAsync(snap, default);

        var dstState = await restored.GetStateAsync(default);
        dstState.Messages.ShouldBe((ulong)15);
        dstState.FirstSeq.ShouldBe((ulong)6);

        // Removed sequences should not be present.
        for (ulong i = 1; i <= 5; i++)
            (await restored.LoadAsync(i, default)).ShouldBeNull();
    }

    // Go: TestFileStoreBasics server/filestore_test.go:113
    [Fact]
    public async Task Load_with_null_sequence_returns_null()
    {
        await using var store = CreateStore(subdirectory: "null-seq");

        await store.AppendAsync("foo", "Hello"u8.ToArray(), default);

        // Loading a sequence that was never stored.
        (await store.LoadAsync(99, default)).ShouldBeNull();
    }

    // Go: TestFileStoreMsgHeaders server/filestore_test.go:158
    [Fact]
    public async Task Store_preserves_empty_payload()
    {
        await using var store = CreateStore(subdirectory: "empty-payload");

        await store.AppendAsync("foo", ReadOnlyMemory<byte>.Empty, default);

        var msg = await store.LoadAsync(1, default);
        msg.ShouldNotBeNull();
        msg!.Payload.Length.ShouldBe(0);
    }

    // Go: TestFileStoreBasics server/filestore_test.go:86
    [Fact]
    public async Task State_tracks_first_and_last_seq()
    {
        await using var store = CreateStore(subdirectory: "first-last");

        for (var i = 0; i < 5; i++)
            await store.AppendAsync("foo", "data"u8.ToArray(), default);

        var state = await store.GetStateAsync(default);
        state.FirstSeq.ShouldBe((ulong)1);
        state.LastSeq.ShouldBe((ulong)5);

        // Remove first message.
        await store.RemoveAsync(1, default);
        state = await store.GetStateAsync(default);
        state.FirstSeq.ShouldBe((ulong)2);
        state.LastSeq.ShouldBe((ulong)5);
    }

    // Go: TestFileStoreMsgLimitBug server/filestore_test.go:518
    [Fact]
    public async Task TrimToMaxMessages_enforces_limit()
    {
        await using var store = CreateStore(subdirectory: "trim");

        for (var i = 0; i < 10; i++)
            await store.AppendAsync("foo", "Hello World"u8.ToArray(), default);

        store.TrimToMaxMessages(5);

        var state = await store.GetStateAsync(default);
        state.Messages.ShouldBe((ulong)5);
        state.FirstSeq.ShouldBe((ulong)6);
        state.LastSeq.ShouldBe((ulong)10);

        // Evicted messages not loadable.
        for (ulong i = 1; i <= 5; i++)
            (await store.LoadAsync(i, default)).ShouldBeNull();

        // Remaining messages loadable.
        for (ulong i = 6; i <= 10; i++)
            (await store.LoadAsync(i, default)).ShouldNotBeNull();
    }

    // Go: TestFileStoreMsgLimit server/filestore_test.go:484
    [Fact]
    public async Task TrimToMaxMessages_to_one()
    {
        await using var store = CreateStore(subdirectory: "trim-one");

        await store.AppendAsync("foo", "first"u8.ToArray(), default);
        await store.AppendAsync("foo", "second"u8.ToArray(), default);
        await store.AppendAsync("foo", "third"u8.ToArray(), default);

        store.TrimToMaxMessages(1);

        var state = await store.GetStateAsync(default);
        state.Messages.ShouldBe((ulong)1);
        state.FirstSeq.ShouldBe((ulong)3);
        state.LastSeq.ShouldBe((ulong)3);

        var msg = await store.LoadAsync(3, default);
        msg.ShouldNotBeNull();
        msg!.Payload.ToArray().ShouldBe("third"u8.ToArray());
    }

    // Go: TestFileStoreBasicWriteMsgsAndRestore server/filestore_test.go:285
    [Fact]
    public async Task Remove_then_restart_preserves_state()
    {
        var subDir = "rm-restart";
        await using (var store = CreateStore(subdirectory: subDir))
        {
            for (var i = 0; i < 10; i++)
                await store.AppendAsync("foo", "Hello"u8.ToArray(), default);

            await store.RemoveAsync(3, default);
            await store.RemoveAsync(7, default);

            var state = await store.GetStateAsync(default);
            state.Messages.ShouldBe((ulong)8);
        }

        // Reopen and verify.
        await using (var store = CreateStore(subdirectory: subDir))
        {
            var state = await store.GetStateAsync(default);
            state.Messages.ShouldBe((ulong)8);

            (await store.LoadAsync(3, default)).ShouldBeNull();
            (await store.LoadAsync(7, default)).ShouldBeNull();
            (await store.LoadAsync(1, default)).ShouldNotBeNull();
            (await store.LoadAsync(10, default)).ShouldNotBeNull();
        }
    }

    // Go: TestFileStoreBasics server/filestore_test.go:86
    [Fact]
    public async Task Multiple_subjects_stored_and_loadable()
    {
        await using var store = CreateStore(subdirectory: "multi-subj");

        await store.AppendAsync("foo.bar", "one"u8.ToArray(), default);
        await store.AppendAsync("baz.qux", "two"u8.ToArray(), default);
        await store.AppendAsync("foo.bar", "three"u8.ToArray(), default);

        var state = await store.GetStateAsync(default);
        state.Messages.ShouldBe((ulong)3);

        var msg1 = await store.LoadAsync(1, default);
        msg1.ShouldNotBeNull();
        msg1!.Subject.ShouldBe("foo.bar");

        var msg2 = await store.LoadAsync(2, default);
        msg2.ShouldNotBeNull();
        msg2!.Subject.ShouldBe("baz.qux");

        var msg3 = await store.LoadAsync(3, default);
        msg3.ShouldNotBeNull();
        msg3!.Subject.ShouldBe("foo.bar");
    }

    // Go: TestFileStoreBasics server/filestore_test.go:104
    [Fact]
    public async Task State_bytes_tracks_total_payload()
    {
        await using var store = CreateStore(subdirectory: "bytes");

        var payload = new byte[100];
        for (var i = 0; i < 5; i++)
            await store.AppendAsync("foo", payload, default);

        var state = await store.GetStateAsync(default);
        state.Messages.ShouldBe((ulong)5);
        state.Bytes.ShouldBe((ulong)(5 * 100));
    }

    // Go: TestFileStoreWriteExpireWrite server/filestore_test.go:424
    [Fact]
    public async Task Large_batch_store_then_load_all()
    {
        await using var store = CreateStore(subdirectory: "large-batch");

        const int count = 200;
        for (var i = 0; i < count; i++)
            await store.AppendAsync("zzz", Encoding.UTF8.GetBytes($"Hello World! - {i}"), default);

        var state = await store.GetStateAsync(default);
        state.Messages.ShouldBe((ulong)count);

        for (ulong i = 1; i <= count; i++)
        {
            var msg = await store.LoadAsync(i, default);
            msg.ShouldNotBeNull();
            msg!.Subject.ShouldBe("zzz");
        }
    }

    // Go: TestFileStoreBasics server/filestore_test.go:124
    [Fact]
    public async Task Load_returns_null_for_sequence_zero()
    {
        await using var store = CreateStore(subdirectory: "seq-zero");

        await store.AppendAsync("foo", "data"u8.ToArray(), default);

        // Sequence 0 should never match a stored message.
        (await store.LoadAsync(0, default)).ShouldBeNull();
    }

    // Go: TestFileStoreBasics server/filestore_test.go:86
    [Fact]
    public async Task LoadLastBySubject_returns_most_recent()
    {
        await using var store = CreateStore(subdirectory: "last-by-subj");

        await store.AppendAsync("foo", "first"u8.ToArray(), default);
        await store.AppendAsync("bar", "other"u8.ToArray(), default);
        await store.AppendAsync("foo", "second"u8.ToArray(), default);
        await store.AppendAsync("foo", "third"u8.ToArray(), default);

        var last = await store.LoadLastBySubjectAsync("foo", default);
        last.ShouldNotBeNull();
        last!.Payload.ToArray().ShouldBe("third"u8.ToArray());
        last.Sequence.ShouldBe((ulong)4);

        // No match.
        (await store.LoadLastBySubjectAsync("does.not.exist", default)).ShouldBeNull();
    }

    // Go: TestFileStoreBasics server/filestore_test.go:86
    [Fact]
    public async Task ListAsync_returns_all_messages_ordered()
    {
        await using var store = CreateStore(subdirectory: "list-ordered");

        await store.AppendAsync("foo", "one"u8.ToArray(), default);
        await store.AppendAsync("bar", "two"u8.ToArray(), default);
        await store.AppendAsync("baz", "three"u8.ToArray(), default);

        var messages = await store.ListAsync(default);
        messages.Count.ShouldBe(3);
        messages[0].Sequence.ShouldBe((ulong)1);
        messages[1].Sequence.ShouldBe((ulong)2);
        messages[2].Sequence.ShouldBe((ulong)3);
    }

    // Go: TestFileStoreBasicWriteMsgsAndRestore server/filestore_test.go:268
    [Fact]
    public async Task Purge_then_append_works()
    {
        await using var store = CreateStore(subdirectory: "purge-append");

        for (var i = 0; i < 5; i++)
            await store.AppendAsync("foo", "data"u8.ToArray(), default);

        await store.PurgeAsync(default);
        (await store.GetStateAsync(default)).Messages.ShouldBe((ulong)0);

        // Append after purge.
        var seq = await store.AppendAsync("foo", "new data"u8.ToArray(), default);
        seq.ShouldBeGreaterThan((ulong)0);

        var msg = await store.LoadAsync(seq, default);
        msg.ShouldNotBeNull();
        msg!.Payload.ToArray().ShouldBe("new data"u8.ToArray());
    }

    // Go: TestFileStoreBasics server/filestore_test.go:86
    [Fact]
    public async Task Empty_store_state_is_zeroed()
    {
        await using var store = CreateStore(subdirectory: "empty-state");

        var state = await store.GetStateAsync(default);
        state.Messages.ShouldBe((ulong)0);
        state.Bytes.ShouldBe((ulong)0);
        state.FirstSeq.ShouldBe((ulong)0);
        state.LastSeq.ShouldBe((ulong)0);
    }

    // Go: TestFileStoreCollapseDmap server/filestore_test.go:1561
    [Fact]
    public async Task Remove_all_messages_one_by_one()
    {
        await using var store = CreateStore(subdirectory: "rm-all");

        for (var i = 0; i < 5; i++)
            await store.AppendAsync("foo", "data"u8.ToArray(), default);

        for (ulong i = 1; i <= 5; i++)
            (await store.RemoveAsync(i, default)).ShouldBeTrue();

        var state = await store.GetStateAsync(default);
        state.Messages.ShouldBe((ulong)0);
        state.Bytes.ShouldBe((ulong)0);
    }

    // Go: TestFileStoreBasics server/filestore_test.go:136
    [Fact]
    public async Task Double_remove_returns_false()
    {
        await using var store = CreateStore(subdirectory: "double-rm");

        await store.AppendAsync("foo", "data"u8.ToArray(), default);

        (await store.RemoveAsync(1, default)).ShouldBeTrue();
        (await store.RemoveAsync(1, default)).ShouldBeFalse();
    }

    // Go: TestFileStoreBasicWriteMsgsAndRestore server/filestore_test.go:181
    [Fact]
    public async Task Large_payload_round_trips()
    {
        await using var store = CreateStore(subdirectory: "large-payload");

        var payload = new byte[8 * 1024]; // 8 KiB
        Random.Shared.NextBytes(payload);

        await store.AppendAsync("foo", payload, default);

        var msg = await store.LoadAsync(1, default);
        msg.ShouldNotBeNull();
        msg!.Payload.ToArray().ShouldBe(payload);
    }

    // Go: TestFileStoreBasicWriteMsgsAndRestore server/filestore_test.go:181
    [Fact]
    public async Task Binary_payload_round_trips()
    {
        await using var store = CreateStore(subdirectory: "binary");

        // Include all byte values 0-255.
        var payload = new byte[256];
        for (var i = 0; i < 256; i++)
            payload[i] = (byte)i;

        await store.AppendAsync("foo", payload, default);

        var msg = await store.LoadAsync(1, default);
        msg.ShouldNotBeNull();
        msg!.Payload.ToArray().ShouldBe(payload);
    }
}