Mesen-SX/Utilities/LowPassFilter.h

#pragma once
#include "stdafx.h"
#include <assert.h>
#include <numeric>

class LowPassFilter
{
private:
	uint8_t _prevSampleCounter = 0;
	int16_t _prevSamplesLeft[10] = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
	int16_t _prevSamplesRight[10] = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0};

	void UpdateSample(int16_t* buffer, size_t index, int strength, double volume, int16_t* _prevSamples)
	{
		if (strength > 0)
		{
			int32_t sum = std::accumulate(_prevSamples, _prevSamples + strength, 0);
			buffer[index] = (int16_t)((sum + buffer[index]) / (strength + 1) * volume);
			_prevSamples[_prevSampleCounter] = buffer[index];
		}
		else
		{
			buffer[index] = (int16_t)(buffer[index] * volume);
		}
	}

public:
	void ApplyFilter(int16_t* buffer, size_t sampleCount, int strength, double volume = 1.0f)
	{
		assert(strength <= 10);

		for (size_t i = 0; i < sampleCount * 2; i += 2)
		{
			UpdateSample(buffer, i, strength, volume, _prevSamplesLeft);
			UpdateSample(buffer, i + 1, strength, volume, _prevSamplesRight);
			if (strength > 0)
			{
				_prevSampleCounter = (_prevSampleCounter + 1) % strength;
			}
		}
	}
};