using System;
using System.Buffers.Binary;

namespace AcDream.Core.Audio;

/// <summary>
/// Decodes the AC <c>Wave</c> dat (`0x0A000000..0x0A00FFFF`) into playable
/// PCM data.
///
/// <para>
/// On-disk format (r05 §2.1 + ACE.DatLoader.FileTypes.Wave + ACViewer's
/// Wave.cs:32-72):
/// <code>
///   int32  headerSize
///   int32  dataSize
///   byte[headerSize]  Header      // raw WAVEFORMATEX — NO RIFF wrapper
///   byte[dataSize]    Data        // raw sample bytes
/// </code>
/// </para>
///
/// <para>
/// Header byte 0 (low byte of <c>WAVEFORMATEX.wFormatTag</c>):
/// <list type="bullet">
///   <item><description><c>0x01</c> linear PCM — feeds straight into OpenAL.</description></item>
///   <item><description><c>0x55</c> MPEG Layer 3 — retail decoded via winmm ACM.</description></item>
///   <item><description><c>0x02</c> Microsoft ADPCM — retail decoded via winmm ACM.</description></item>
/// </list>
/// </para>
///
/// <para>
/// Our decoder currently handles the PCM case (the vast majority of AC's
/// ~3500 waves). MP3 / ADPCM decoding is best handled by a managed
/// decoder added later (NAudio / MediaToolkit / custom). Unsupported
/// formats return <c>null</c> and the caller logs + skips.
/// </para>
/// </summary>
public static class WaveDecoder
{
    /// <summary>
    /// Format tag from the first 2 bytes of the WAVEFORMATEX header.
    /// Retail and ACE map identically; we use these exact values for logs.
    /// </summary>
    public enum WaveFormatTag : ushort
    {
        Unknown     = 0x0000,
        Pcm         = 0x0001,  // Microsoft PCM
        Adpcm       = 0x0002,  // Microsoft ADPCM (rare in AC)
        Mp3         = 0x0055,  // MPEGLAYER3 (common for music-ish cues)
    }

    /// <summary>
    /// Parse a <c>Wave</c> dat record into a <see cref="WaveData"/> PCM
    /// container. Returns <c>null</c> when the source format is compressed
    /// (MP3 / ADPCM) and we don't yet have a decoder wired up, OR when the
    /// header is malformed.
    /// </summary>
    /// <param name="header">Raw WAVEFORMATEX bytes (at least 14 bytes).</param>
    /// <param name="data">Raw payload bytes following the header.</param>
    public static WaveData? Decode(byte[] header, byte[] data)
    {
        ArgumentNullException.ThrowIfNull(header);
        ArgumentNullException.ThrowIfNull(data);

        if (header.Length < 14) return null;

        // WAVEFORMATEX fields (little-endian):
        //   wFormatTag      u16 @ 0
        //   nChannels       u16 @ 2
        //   nSamplesPerSec  u32 @ 4
        //   nAvgBytesPerSec u32 @ 8
        //   nBlockAlign     u16 @12
        //   wBitsPerSample  u16 @14 (present if headerSize >= 16)
        //   cbSize          u16 @16 (optional extra-data length)
        var h = header.AsSpan();
        ushort fmtTag   = BinaryPrimitives.ReadUInt16LittleEndian(h);
        ushort channels = BinaryPrimitives.ReadUInt16LittleEndian(h.Slice(2));
        uint   sampleRate = BinaryPrimitives.ReadUInt32LittleEndian(h.Slice(4));
        ushort bitsPer    = header.Length >= 16
            ? BinaryPrimitives.ReadUInt16LittleEndian(h.Slice(14))
            : (ushort)16;

        if (channels == 0 || sampleRate == 0)
            return null;

        // Only PCM is natively supported here. Compressed paths require a
        // secondary decoder (MP3/ADPCM); caller logs + skips when null.
        if ((WaveFormatTag)fmtTag != WaveFormatTag.Pcm)
            return null;

        // For PCM the Data array IS the sample buffer (no framing needed).
        double duration = bitsPer > 0 && channels > 0
            ? data.Length * 8.0 / (double)(sampleRate * channels * bitsPer)
            : 0.0;

        return new WaveData
        {
            ChannelCount  = channels,
            SampleRate    = (int)sampleRate,
            BitsPerSample = bitsPer == 0 ? 16 : bitsPer,
            PcmBytes      = data,
            Duration      = TimeSpan.FromSeconds(duration),
        };
    }

    /// <summary>
    /// Peek only the format tag byte. Useful for logging which formats
    /// failed to decode without allocating a <see cref="WaveData"/>.
    /// </summary>
    public static WaveFormatTag PeekFormat(byte[] header)
    {
        if (header is null || header.Length < 2) return WaveFormatTag.Unknown;
        ushort fmtTag = BinaryPrimitives.ReadUInt16LittleEndian(header);
        return (WaveFormatTag)fmtTag;
    }
}