using System;

namespace ADOFAIRenderer.Util.Audio;

/// <summary>
/// Splits the audio sample stream into per-video-frame sample counts so that audio and video stay aligned without drift.
///
/// The frame rate is converted to an exact fraction Numerator / Denominator (<see cref="RationalUtil.Double2MaxPrecisionInt"/>).
/// Frame k receives floor((k + 1) * sampleRate * Denominator / Numerator) - floor(k * sampleRate * Denominator / Numerator) samples:
/// the remainder of each division is carried into the next frame, so the sum over any number of frames equals the exact
/// sample count of that duration rounded down (for example 44100 Hz at 60 FPS gives 735 every frame; at 144 FPS three frames of 306
/// are followed by one of 307, 306.25 on average).
/// Used by the render thread only (RenderManager.WriteAudioForFrame, once per frame sent to the encoder).
/// </summary>
internal class AudioSyncCalculator
{
    /// <summary>Audio sample rate in Hz (samples per channel per second).</summary>
    private readonly long _sampleRate;

    /// <summary>Video frame rate as a fraction (frames per second = Numerator / Denominator).</summary>
    private readonly (long Numerator, long Denominator) _fpsRational;

    /// <summary>Total samples handed out since construction or <see cref="Reset"/> (kept for debugging, not read).</summary>
    // ReSharper disable once NotAccessedField.Local
    private long _currentSample;

    /// <summary>Remainder of the last division, in units of 1 / Numerator samples (0 ~ Numerator - 1).</summary>
    private long _remainder;
        
    /// <param name="sampleRate">Audio sample rate in Hz.</param>
    /// <param name="fps">Video frame rate in frames per second (the encoded frame rate).</param>
    public AudioSyncCalculator(long sampleRate, double fps)
    {
        _sampleRate = sampleRate;
        _fpsRational = RationalUtil.Double2MaxPrecisionInt(fps);
    }

    /// <summary>Restarts the sequence at frame 0 (the remainder is cleared).</summary>
    public void Reset()
    {
        _currentSample = 0;
        _remainder = 0;
    }

    /// <summary>Returns the sample count (frames per channel) of the next video frame and advances to the following frame.</summary>
    public int GetNextFrameSamples()
    {
        // samplesNeeded = floor((sampleRate * Denominator + remainder) / Numerator); the new remainder carries the fraction.
        var numerator = _sampleRate * _fpsRational.Denominator + _remainder;
        var samplesNeeded = numerator / _fpsRational.Numerator;

        _remainder = numerator % _fpsRational.Numerator;
        _currentSample += samplesNeeded;
        return (int)samplesNeeded;
    }
}