Rack/include/dsp/fir.hpp

#pragma once
#include "dsp/functions.hpp"


namespace rack {

/** Perform a direct convolution
x[-len + 1] to x[0] must be defined
*/
inline float convolve(const float *x, const float *kernel, int len) {
	float y = 0.0;
	for (int i = 0; i < len; i++) {
		y += x[-i] * kernel[i];
	}
	return y;
}

inline void blackmanHarrisWindow(float *x, int n) {
	const float a0 = 0.35875;
	const float a1 = 0.48829;
	const float a2 = 0.14128;
	const float a3 = 0.01168;
	for (int i = 0; i < n; i++) {
		x[i] *= a0
			- a1 * cosf(2 * M_PI * i / (n - 1))
			+ a2 * cosf(4 * M_PI * i / (n - 1))
			- a3 * cosf(6 * M_PI * i / (n - 1));
	}
}

inline void boxcarFIR(float *x, int n, float cutoff) {
	for (int i = 0; i < n; i++) {
		float t = (float)i / (n - 1) * 2.0 - 1.0;
		x[i] = sinc(t * n * cutoff);
	}
}

} // namespace rack