// Go reference: server/jetstream_api.go — jsStreamSnapshotT and jsStreamRestoreT handlers.
// Snapshot creates a serialized byte representation of stream state; restore re-applies it.
// The async variants (HandleSnapshotAsync / HandleRestoreAsync) add stream name and chunk
// metadata to the response and provide richer error codes compared to the sync stubs.

using System.Text;
using NATS.Server.JetStream;
using NATS.Server.JetStream.Api;
using NATS.Server.JetStream.Api.Handlers;
using NATS.Server.JetStream.Models;

namespace NATS.Server.JetStream.Tests.JetStream.Api;

public class SnapshotApiTests
{
    // ---------------------------------------------------------------
    // Helpers
    // ---------------------------------------------------------------

    private static StreamManager CreateManagerWithStream(string streamName, string subjectPattern)
    {
        var sm = new StreamManager();
        sm.CreateOrUpdate(new StreamConfig
        {
            Name = streamName,
            Subjects = [subjectPattern],
        });
        return sm;
    }

    private static async Task AppendAsync(StreamManager sm, string subject, string payload)
    {
        var handle = sm.FindBySubject(subject);
        handle.ShouldNotBeNull();
        await handle!.Store.AppendAsync(subject, Encoding.UTF8.GetBytes(payload), default);
    }

    // ---------------------------------------------------------------
    // HandleSnapshot (sync, existing)
    // ---------------------------------------------------------------

    /// <summary>
    /// Go ref: jsStreamSnapshotT — snapshot of an existing stream returns a non-empty base64 payload.
    /// </summary>
    [Fact]
    public async Task HandleSnapshot_returns_base64_payload_for_existing_stream()
    {
        var sm = CreateManagerWithStream("ORDERS", "orders.>");
        await AppendAsync(sm, "orders.1", "hello");

        var response = StreamApiHandlers.HandleSnapshot("$JS.API.STREAM.SNAPSHOT.ORDERS", sm);

        response.Error.ShouldBeNull();
        response.Snapshot.ShouldNotBeNull();
        response.Snapshot!.Payload.ShouldNotBeNullOrEmpty();

        // Verify it is valid base64.
        var bytes = Convert.FromBase64String(response.Snapshot.Payload);
        bytes.ShouldNotBeEmpty();
    }

    /// <summary>
    /// Go ref: jsStreamSnapshotT — snapshot of a non-existent stream returns 404.
    /// </summary>
    [Fact]
    public void HandleSnapshot_returns_not_found_for_missing_stream()
    {
        var sm = new StreamManager();

        var response = StreamApiHandlers.HandleSnapshot("$JS.API.STREAM.SNAPSHOT.MISSING", sm);

        response.Error.ShouldNotBeNull();
        response.Error!.Code.ShouldBe(404);
    }

    // ---------------------------------------------------------------
    // HandleRestore (sync, existing)
    // ---------------------------------------------------------------

    /// <summary>
    /// Go ref: jsStreamRestoreT — restore with a valid base64 snapshot payload succeeds.
    /// </summary>
    [Fact]
    public async Task HandleRestore_succeeds_with_valid_payload()
    {
        var sm = CreateManagerWithStream("ORDERS", "orders.>");
        await AppendAsync(sm, "orders.1", "msg1");

        // Obtain a snapshot first.
        var snapshotResponse = StreamApiHandlers.HandleSnapshot("$JS.API.STREAM.SNAPSHOT.ORDERS", sm);
        snapshotResponse.Snapshot.ShouldNotBeNull();
        var base64 = snapshotResponse.Snapshot!.Payload;

        // Restore back using the base64 bytes directly as the payload.
        var payloadBytes = Encoding.UTF8.GetBytes(base64);
        var response = StreamApiHandlers.HandleRestore(
            "$JS.API.STREAM.RESTORE.ORDERS",
            payloadBytes,
            sm);

        response.Error.ShouldBeNull();
        response.Success.ShouldBeTrue();
    }

    /// <summary>
    /// Go ref: jsStreamRestoreT — empty payload returns a 400 error.
    /// </summary>
    [Fact]
    public void HandleRestore_returns_error_for_empty_payload()
    {
        var sm = CreateManagerWithStream("ORDERS", "orders.>");

        var response = StreamApiHandlers.HandleRestore(
            "$JS.API.STREAM.RESTORE.ORDERS",
            ReadOnlySpan<byte>.Empty,
            sm);

        response.Error.ShouldNotBeNull();
        response.Error!.Code.ShouldBe(400);
    }

    /// <summary>
    /// Go ref: jsStreamRestoreT — bad subject token (no trailing stream name) returns 404.
    /// </summary>
    [Fact]
    public void HandleRestore_returns_not_found_for_bad_subject()
    {
        var sm = new StreamManager();
        // Subject without trailing token — ExtractTrailingToken returns null.
        var payload = Encoding.UTF8.GetBytes(Convert.ToBase64String([1, 2, 3]));

        var response = StreamApiHandlers.HandleRestore(
            "$JS.API.STREAM.RESTORE.",
            payload,
            sm);

        response.Error.ShouldNotBeNull();
        response.Error!.Code.ShouldBe(404);
    }

    // ---------------------------------------------------------------
    // HandleSnapshotAsync (new)
    // ---------------------------------------------------------------

    /// <summary>
    /// Go ref: jsStreamSnapshotT — async handler populates StreamName in the response.
    /// </summary>
    [Fact]
    public async Task HandleSnapshotAsync_includes_stream_name_in_response()
    {
        var sm = CreateManagerWithStream("EVENTS", "events.>");
        await AppendAsync(sm, "events.1", "data");

        var response = await StreamApiHandlers.HandleSnapshotAsync(
            "$JS.API.STREAM.SNAPSHOT.EVENTS",
            sm,
            CancellationToken.None);

        response.Error.ShouldBeNull();
        response.Snapshot.ShouldNotBeNull();
        response.Snapshot!.StreamName.ShouldBe("EVENTS");
    }

    /// <summary>
    /// Go ref: jsStreamSnapshotT — async handler sets NumChunks=1 and BlkSize equal to the
    /// length of the raw (pre-base64) snapshot bytes.
    /// </summary>
    [Fact]
    public async Task HandleSnapshotAsync_includes_chunk_metadata()
    {
        var sm = CreateManagerWithStream("EVENTS", "events.>");
        await AppendAsync(sm, "events.1", "payload-data");

        var response = await StreamApiHandlers.HandleSnapshotAsync(
            "$JS.API.STREAM.SNAPSHOT.EVENTS",
            sm,
            CancellationToken.None);

        response.Error.ShouldBeNull();
        var snap = response.Snapshot!;
        snap.NumChunks.ShouldBe(1);
        snap.BlkSize.ShouldBeGreaterThan(0);

        // BlkSize should match the raw snapshot byte count.
        var rawBytes = Convert.FromBase64String(snap.Payload);
        snap.BlkSize.ShouldBe(rawBytes.Length);
    }

    /// <summary>
    /// HandleSnapshotAsync returns 404 when the stream does not exist.
    /// </summary>
    [Fact]
    public async Task HandleSnapshotAsync_returns_not_found_for_missing_stream()
    {
        var sm = new StreamManager();

        var response = await StreamApiHandlers.HandleSnapshotAsync(
            "$JS.API.STREAM.SNAPSHOT.NOPE",
            sm,
            CancellationToken.None);

        response.Error.ShouldNotBeNull();
        response.Error!.Code.ShouldBe(404);
    }

    // ---------------------------------------------------------------
    // HandleRestoreAsync (new)
    // ---------------------------------------------------------------

    /// <summary>
    /// Go ref: jsStreamRestoreT — async restore validates the base64 payload and succeeds.
    /// </summary>
    [Fact]
    public async Task HandleRestoreAsync_validates_base64_payload()
    {
        var sm = CreateManagerWithStream("ORDERS", "orders.>");
        await AppendAsync(sm, "orders.1", "hello");

        // Take a snapshot, then restore it using the async path.
        var snapResp = await StreamApiHandlers.HandleSnapshotAsync(
            "$JS.API.STREAM.SNAPSHOT.ORDERS",
            sm,
            CancellationToken.None);
        snapResp.Snapshot.ShouldNotBeNull();
        var base64Payload = Encoding.UTF8.GetBytes(snapResp.Snapshot!.Payload);

        var response = await StreamApiHandlers.HandleRestoreAsync(
            "$JS.API.STREAM.RESTORE.ORDERS",
            base64Payload,
            sm,
            CancellationToken.None);

        response.Error.ShouldBeNull();
        response.Success.ShouldBeTrue();
    }

    /// <summary>
    /// HandleRestoreAsync returns 400 when given an empty payload array.
    /// </summary>
    [Fact]
    public async Task HandleRestoreAsync_returns_error_for_empty_payload()
    {
        var sm = CreateManagerWithStream("ORDERS", "orders.>");

        var response = await StreamApiHandlers.HandleRestoreAsync(
            "$JS.API.STREAM.RESTORE.ORDERS",
            [],
            sm,
            CancellationToken.None);

        response.Error.ShouldNotBeNull();
        response.Error!.Code.ShouldBe(400);
    }

    // ---------------------------------------------------------------
    // Round-trip
    // ---------------------------------------------------------------

    /// <summary>
    /// Go ref: jsStreamSnapshotT / jsStreamRestoreT — full snapshot-then-restore round-trip:
    /// messages written before snapshot are recoverable after restore.
    /// </summary>
    [Fact]
    public async Task Snapshot_round_trip_create_and_restore()
    {
        var sm = CreateManagerWithStream("LOGS", "logs.>");
        await AppendAsync(sm, "logs.a", "alpha");
        await AppendAsync(sm, "logs.b", "beta");
        await AppendAsync(sm, "logs.c", "gamma");

        var stateBefore = await sm.GetStateAsync("LOGS", default);
        stateBefore.Messages.ShouldBe(3UL);

        // Snapshot via async handler.
        var snapResp = await StreamApiHandlers.HandleSnapshotAsync(
            "$JS.API.STREAM.SNAPSHOT.LOGS",
            sm,
            CancellationToken.None);
        snapResp.Error.ShouldBeNull();
        var base64Payload = Encoding.UTF8.GetBytes(snapResp.Snapshot!.Payload);

        // Restore via async handler.
        var restoreResp = await StreamApiHandlers.HandleRestoreAsync(
            "$JS.API.STREAM.RESTORE.LOGS",
            base64Payload,
            sm,
            CancellationToken.None);
        restoreResp.Error.ShouldBeNull();
        restoreResp.Success.ShouldBeTrue();

        // State should still be consistent (restore does not clear — it re-applies).
        var stateAfter = await sm.GetStateAsync("LOGS", default);
        stateAfter.Messages.ShouldBeGreaterThanOrEqualTo(3UL);
    }

    // ---------------------------------------------------------------
    // Subject extraction
    // ---------------------------------------------------------------

    /// <summary>
    /// Go ref: jsStreamSnapshotT — the stream name is correctly extracted from the API subject.
    /// </summary>
    [Fact]
    public async Task HandleSnapshot_extracts_stream_name_from_subject()
    {
        var sm = CreateManagerWithStream("MY_STREAM", "mystream.>");

        var response = await StreamApiHandlers.HandleSnapshotAsync(
            "$JS.API.STREAM.SNAPSHOT.MY_STREAM",
            sm,
            CancellationToken.None);

        response.Error.ShouldBeNull();
        response.Snapshot.ShouldNotBeNull();
        response.Snapshot!.StreamName.ShouldBe("MY_STREAM");
    }
}