using System.Buffers.Binary;

namespace DeepDrftContent.Processors.Opus;

/// <summary>
/// The single derived sidecar artifact per track (§3.4a B, recommended design): the Opus setup header
/// (<c>OpusHead</c> + <c>OpusTags</c>) followed by the granule→byte seek index. The client fetches this
/// once on track load and parses it into its <c>OpusSeekData</c>, so it always has both the setup bytes
/// (to prepend to any mid-stream slice) and the accurate seek transfer function before it ever issues a
/// Range fetch — including a window that opens away from byte 0 (UC9).
/// </summary>
/// <param name="SetupHeaderBytes">The verbatim OpusHead + OpusTags pages.</param>
/// <param name="SeekIndex">The bucketed granule→byte seek index.</param>
public sealed record OpusSidecar(byte[] SetupHeaderBytes, OggOpusSeekIndex SeekIndex)
{
    /// <summary>
    /// Serializes to <c>[uint32 setupHeaderLength][setup-header bytes][seek-index blob]</c>. The
    /// length prefix lets the client split the two regions with one read; the seek-index blob carries
    /// its own self-describing header (<see cref="OggOpusSeekIndex.ToBytes"/>), so it needs no trailing
    /// length.
    /// </summary>
    public byte[] ToBytes()
    {
        var indexBytes = SeekIndex.ToBytes();
        var bytes = new byte[4 + SetupHeaderBytes.Length + indexBytes.Length];
        var span = bytes.AsSpan();

        BinaryPrimitives.WriteUInt32LittleEndian(span[..4], (uint)SetupHeaderBytes.Length);
        SetupHeaderBytes.CopyTo(span.Slice(4));
        indexBytes.CopyTo(span.Slice(4 + SetupHeaderBytes.Length));

        return bytes;
    }

    /// <summary>
    /// Parses a blob produced by <see cref="ToBytes"/>. Returns null on any structural inconsistency
    /// (short blob, length prefix that overruns, or an unparseable index) — the format is exact, so a
    /// malformed blob is corruption.
    /// </summary>
    public static OpusSidecar? FromBytes(ReadOnlySpan<byte> bytes)
    {
        if (bytes.Length < 4)
            return null;

        var setupLength = BinaryPrimitives.ReadUInt32LittleEndian(bytes[..4]);
        var indexStart = 4 + (long)setupLength;
        if (bytes.Length < indexStart)
            return null;

        var setupHeader = bytes.Slice(4, (int)setupLength).ToArray();
        var index = OggOpusSeekIndex.FromBytes(bytes.Slice((int)indexStart));
        if (index is null)
            return null;

        return new OpusSidecar(setupHeader, index);
    }
}