using System.Buffers.Binary;
using System.Net.Sockets;
using System.Text;
using Big.Application;

namespace Big.Infrastructure;

/// <summary>
/// Scans a document with clamd over its INSTREAM protocol (S-29, ADR-0036): <c>zINSTREAM\0</c>, the
/// document as one big-endian length-prefixed chunk, a zero-length terminator, then one reply —
/// <c>stream: OK</c> or <c>stream: &lt;signature&gt; FOUND</c>. Anything else (an ERROR reply, a refused
/// connection, a timeout) is <see cref="ScanVerdict.Unavailable"/>, so the caller fails closed.
/// </summary>
public sealed class ClamdDocumentScanner(ClamAvOptions options) : IDocumentScanner
{
    public async Task<ScanVerdict> ScanAsync(byte[] content, CancellationToken ct = default)
    {
        ArgumentNullException.ThrowIfNull(content);
        using var timeout = CancellationTokenSource.CreateLinkedTokenSource(ct);
        timeout.CancelAfter(options.Timeout);

        string reply;
        try
        {
            using var client = new TcpClient();
            await client.ConnectAsync(options.Host, options.Port, timeout.Token);
            var stream = client.GetStream();

            var length = new byte[4];
            BinaryPrimitives.WriteInt32BigEndian(length, content.Length);
            await stream.WriteAsync("zINSTREAM\0"u8.ToArray(), timeout.Token);
            await stream.WriteAsync(length, timeout.Token);
            await stream.WriteAsync(content, timeout.Token);
            await stream.WriteAsync(new byte[4], timeout.Token);

            using var reader = new StreamReader(stream, Encoding.ASCII);
            reply = (await reader.ReadToEndAsync(timeout.Token)).TrimEnd('\0', '\n');
        }
        catch (Exception e) when (e is SocketException or IOException
                                  || (e is OperationCanceledException && !ct.IsCancellationRequested))
        {
            return ScanVerdict.Unavailable;
        }

        if (reply == "stream: OK") return ScanVerdict.Clean;
        return reply.EndsWith(" FOUND", StringComparison.Ordinal) ? ScanVerdict.Infected : ScanVerdict.Unavailable;
    }
}