natsdotnet/src/NATS.Server/JetStream/Api/Handlers/DirectApiHandlers.cs

using System.Text;
using System.Text.Json;

namespace NATS.Server.JetStream.Api.Handlers;

public static class DirectApiHandlers
{
    private const string Prefix = JetStreamApiSubjects.DirectGet;

    public static JetStreamApiResponse HandleGet(string subject, ReadOnlySpan<byte> payload, StreamManager streamManager)
    {
        var streamName = ExtractTrailingToken(subject, Prefix);
        if (streamName == null)
            return JetStreamApiResponse.NotFound(subject);

        var sequence = ParseSequence(payload);
        if (sequence == 0)
            return JetStreamApiResponse.ErrorResponse(400, "sequence required");

        var message = streamManager.GetMessage(streamName, sequence);
        if (message == null)
            return JetStreamApiResponse.NotFound(subject);

        return new JetStreamApiResponse
        {
            DirectMessage = new JetStreamDirectMessage
            {
                Sequence = message.Sequence,
                Subject = message.Subject,
                Payload = Encoding.UTF8.GetString(message.Payload.Span),
            },
        };
    }

    private static string? ExtractTrailingToken(string subject, string prefix)
    {
        if (!subject.StartsWith(prefix, StringComparison.Ordinal))
            return null;

        var token = subject[prefix.Length..].Trim();
        return token.Length == 0 ? null : token;
    }

    private static ulong ParseSequence(ReadOnlySpan<byte> payload)
    {
        if (payload.IsEmpty)
            return 0;

        try
        {
            using var doc = JsonDocument.Parse(payload.ToArray());
            if (doc.RootElement.TryGetProperty("seq", out var seqEl) && seqEl.TryGetUInt64(out var sequence))
                return sequence;
        }
        catch (JsonException)
        {
        }

        return 0;
    }
}