using System.Security.Cryptography;

namespace MusicBridge.Agent.Media;

internal sealed record Artwork(string Id, string ContentType, byte[] Bytes);

// Only the current cover is retained. IDs are content hashes, never file paths or URLs.
internal sealed class ArtworkCache
{
    public const int MaxBytes = 2 * 1024 * 1024;
    private Artwork? current;
    public Artwork? Current => Volatile.Read(ref current);
    public Artwork? Get(string id) => Current is { } artwork && artwork.Id == id ? artwork : null;
    public void Clear() => Volatile.Write(ref current, null);

    public async Task<Artwork> LoadAsync(Stream stream, CancellationToken cancellationToken)
    {
        Clear(); // A failed/new cover must not leave the previous track's image attached.
        using var output = new MemoryStream();
        var buffer = new byte[16 * 1024];
        while (true)
        {
            var count = await stream.ReadAsync(buffer, cancellationToken);
            if (count == 0) break;
            if (output.Length + count > MaxBytes) throw new InvalidDataException("Обложка больше 2 МиБ.");
            output.Write(buffer, 0, count);
        }
        var bytes = output.ToArray();
        var type = DetectType(bytes) ?? throw new InvalidDataException("Неподдерживаемый формат обложки.");
        var artwork = new Artwork(Convert.ToHexString(SHA256.HashData(bytes)), type, bytes);
        Volatile.Write(ref current, artwork);
        return artwork;
    }

    private static string? DetectType(ReadOnlySpan<byte> data)
    {
        if (data.StartsWith(new byte[] { 137, 80, 78, 71, 13, 10, 26, 10 })) return "image/png";
        if (data.StartsWith(new byte[] { 255, 216, 255 })) return "image/jpeg";
        if (data.StartsWith("GIF87a"u8) || data.StartsWith("GIF89a"u8)) return "image/gif";
        if (data.Length >= 12 && data[..4].SequenceEqual("RIFF"u8) && data.Slice(8, 4).SequenceEqual("WEBP"u8)) return "image/webp";
        return null;
    }
}