using System.Buffers;
using ErsatzTV.Core.Interfaces.Streaming;
using Microsoft.Extensions.Logging;
using Microsoft.IO;

namespace ErsatzTV.Infrastructure.Streaming;

/// <summary>
///     Fetches a remote graphics-engine image over HTTP with a bounded timeout and a bounded size.
/// </summary>
/// <remarks>
///     This runs during graphics-engine element initialization, which happens inside stream startup
///     while ffmpeg waits on the pipe — so an unbounded fetch stalls the tune. Every failure here is
///     surfaced as an exception and the calling element disables itself. (ersatztv#511)
/// </remarks>
public class HttpRemoteImageFetcher : IRemoteImageFetcher
{
    /// <summary>Named <see cref="HttpClient" /> configured with the redirect cap in Startup.</summary>
    public const string HttpClientName = "RemoteImage";

    /// <summary>
    ///     Covers the whole exchange — connect, headers AND body — because the body read happens
    ///     outside <see cref="HttpClient.Timeout" /> under <see cref="HttpCompletionOption.ResponseHeadersRead" />,
    ///     so a slow-drip host would otherwise hang forever. (docs/decisions.md, #289)
    /// </summary>
    internal static readonly TimeSpan DefaultFetchTimeout = TimeSpan.FromSeconds(10);

    /// <summary>
    ///     Wire-size cap. A channel logo or overlay is orders of magnitude smaller than this.
    ///     This bounds transfer and buffering only — it is NOT decode-bomb protection, since a bomb
    ///     is by definition small on the wire. That check lives in
    ///     <c>ImageElementBase.DecodeRemoteImage</c>, which reads the declared dimensions and frame
    ///     count from the header before the decoder allocates.
    /// </summary>
    internal const long MaxImageBytes = 10 * 1024 * 1024;

    private readonly TimeSpan _fetchTimeout;
    private readonly IHttpClientFactory _httpClientFactory;
    private readonly ILogger<HttpRemoteImageFetcher> _logger;
    private readonly RecyclableMemoryStreamManager _memoryStreamManager;

    public HttpRemoteImageFetcher(
        IHttpClientFactory httpClientFactory,
        RecyclableMemoryStreamManager memoryStreamManager,
        ILogger<HttpRemoteImageFetcher> logger)
        : this(httpClientFactory, memoryStreamManager, logger, DefaultFetchTimeout)
    {
    }

    // the timeout is only parameterized so a test can prove the deadline fires without waiting the
    // real ten seconds; production always goes through the constructor above.
    internal HttpRemoteImageFetcher(
        IHttpClientFactory httpClientFactory,
        RecyclableMemoryStreamManager memoryStreamManager,
        ILogger<HttpRemoteImageFetcher> logger,
        TimeSpan fetchTimeout)
    {
        _httpClientFactory = httpClientFactory;
        _memoryStreamManager = memoryStreamManager;
        _logger = logger;
        _fetchTimeout = fetchTimeout;
    }

    public async Task<Stream> Fetch(Uri uri, CancellationToken cancellationToken)
    {
        using var timeoutCts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
        timeoutCts.CancelAfter(_fetchTimeout);

        try
        {
            using HttpClient client = _httpClientFactory.CreateClient(HttpClientName);

            // set here, not only in the DI registration, so the deadline cannot silently widen to
            // HttpClient's 100s default if that registration is ever dropped or reordered. the
            // factory hands back a fresh wrapper each call, so mutating it is safe.
            client.Timeout = Timeout.InfiniteTimeSpan;
            using HttpResponseMessage response = await client.GetAsync(
                uri,
                HttpCompletionOption.ResponseHeadersRead,
                timeoutCts.Token);

            response.EnsureSuccessStatusCode();

            string mediaType = response.Content.Headers.ContentType?.MediaType;
            if (!IsAcceptableMediaType(mediaType))
            {
                throw new InvalidOperationException(
                    $"Remote image {uri} returned content type '{mediaType}', which is not an image");
            }

            // the advertised length is a cheap early reject; it is not trusted, because it can be
            // absent or a lie. the copy below is what actually enforces the cap.
            long? contentLength = response.Content.Headers.ContentLength;
            if (contentLength > MaxImageBytes)
            {
                throw new InvalidOperationException(
                    $"Remote image {uri} advertises {contentLength} bytes, over the {MaxImageBytes} byte limit");
            }

            await using Stream source = await response.Content.ReadAsStreamAsync(timeoutCts.Token);
            return await CopyCapped(source, uri, timeoutCts.Token);
        }
        catch (OperationCanceledException canceled) when (!cancellationToken.IsCancellationRequested)
        {
            // OUR deadline fired, not the caller's. re-thrown as a TimeoutException so the element's
            // warning names the real cause; caller cancellation (shutdown / client disconnect) still
            // propagates as OperationCanceledException.
            _logger.LogWarning(
                "Timed out after {Seconds}s fetching remote image {Uri}",
                _fetchTimeout.TotalSeconds,
                uri);

            throw new TimeoutException($"Timed out fetching remote image {uri}", canceled);
        }
    }

    // a missing content type is allowed (some hosts omit it) and octet-stream is allowed (a common
    // default for statically served files). anything else that is positively NOT an image -- an html
    // error page, say -- is rejected before it reaches the decoder.
    private static bool IsAcceptableMediaType(string mediaType) =>
        string.IsNullOrWhiteSpace(mediaType)
        || mediaType.StartsWith("image/", StringComparison.OrdinalIgnoreCase)
        || mediaType.Equals("application/octet-stream", StringComparison.OrdinalIgnoreCase);

    private async Task<Stream> CopyCapped(Stream source, Uri uri, CancellationToken cancellationToken)
    {
        MemoryStream buffer = _memoryStreamManager.GetStream(nameof(HttpRemoteImageFetcher));
        byte[] chunk = ArrayPool<byte>.Shared.Rent(81920);

        try
        {
            int read;
            while ((read = await source.ReadAsync(chunk.AsMemory(), cancellationToken)) > 0)
            {
                if (buffer.Length + read > MaxImageBytes)
                {
                    throw new InvalidOperationException(
                        $"Remote image {uri} exceeds the {MaxImageBytes} byte limit");
                }

                await buffer.WriteAsync(chunk.AsMemory(0, read), cancellationToken);
            }
        }
        catch
        {
            await buffer.DisposeAsync();
            throw;
        }
        finally
        {
            ArrayPool<byte>.Shared.Return(chunk);
        }

        buffer.Position = 0;
        return buffer;
    }
}