using System.Threading.Channels;

namespace NucleicBroker;

/// <summary>
/// The single writer of the broker's stdout: responses and notifications from any thread are
/// enqueued (never blocking the caller — wslc raises stdio events on WinRT threads that must
/// not stall on the pipe, docs/WINDOWS_PORT.md §3.3) and drained by one loop that also
/// coalesces bursts: consecutive stdio chunks for the same (proc, stream) queued at drain
/// time merge into one `proc.stdout`/`proc.stderr` notification, cutting per-line overhead
/// for agents' high-rate NDJSON without adding any timer or latency (only already-queued
/// items merge). Ordering is preserved end-to-end — one channel, one drain loop — so
/// `proc.exit` can never overtake the output that preceded it.
/// </summary>
public sealed class OutboundWriter
{
    private abstract record Item;
    private sealed record JsonLine(string Line) : Item;
    private sealed record ProcChunk(long ProcId, bool Stderr, byte[] Bytes) : Item;

    private readonly Channel<Item> channel = Channel.CreateUnbounded<Item>(
        new UnboundedChannelOptions { SingleReader = true });
    private readonly TextWriter output;
    private readonly Task loop;

    public OutboundWriter(TextWriter output)
    {
        this.output = output;
        loop = Task.Run(DrainAsync);
    }

    public void EnqueueJson(string line) => channel.Writer.TryWrite(new JsonLine(line));

    public void EnqueueProcOutput(long procId, bool stderr, ReadOnlySpan<byte> chunk) =>
        channel.Writer.TryWrite(new ProcChunk(procId, stderr, chunk.ToArray()));

    /// <summary>Close the queue, drain everything, and flush. Idempotent.</summary>
    public async Task CompleteAsync()
    {
        channel.Writer.TryComplete();
        await loop.ConfigureAwait(false);
    }

    private async Task DrainAsync()
    {
        var reader = channel.Reader;
        var batch = new List<Item>(64);
        while (await reader.WaitToReadAsync().ConfigureAwait(false))
        {
            batch.Clear();
            while (batch.Count < 256 && reader.TryRead(out var item)) batch.Add(item);

            for (var i = 0; i < batch.Count; i++)
            {
                if (batch[i] is not ProcChunk head)
                {
                    await output.WriteLineAsync(((JsonLine)batch[i]).Line).ConfigureAwait(false);
                    continue;
                }
                // Merge the run of chunks for the same (proc, stream) into one notification.
                using var merged = new MemoryStream();
                merged.Write(head.Bytes);
                while (i + 1 < batch.Count
                    && batch[i + 1] is ProcChunk next
                    && next.ProcId == head.ProcId && next.Stderr == head.Stderr)
                {
                    merged.Write(next.Bytes);
                    i++;
                }
                var line = Rpc.Notification(
                    head.Stderr ? "proc.stderr" : "proc.stdout",
                    new { procId = head.ProcId, b64 = Convert.ToBase64String(merged.ToArray()) });
                await output.WriteLineAsync(line).ConfigureAwait(false);
            }
            await output.FlushAsync().ConfigureAwait(false);
        }
        await output.FlushAsync().ConfigureAwait(false);
    }
}