sf_out.cc

// g++ -o /tmp/sf_out sf_out.cc -Wall -lsndfile -lm

#include <assert.h>
#include <math.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdint.h>
#include <sndfile.h>

int main (int argc, char** argv) 
{
	SF_INFO  _info;
	memset (&_info, 0, sizeof (_info));

	_info.channels = 1;
	_info.samplerate = 48000;
	_info.format = SF_FORMAT_WAV | SF_FORMAT_FLOAT;

	/* tempo ramp */
	double   bpm_start      = 120;
	double   bpm_end        = 60;
	uint32_t duration_beats = 16;

	
	/* with 0 <= t <= 1 and bpm_start > bpm_end
	 * bpm[t] = bpm_start * exp (-t * log (bpm_start / bpm_end))
	 *
	 * bpm[t0,t1] = \frac {\int_{t0}^{t1} bpm[t] \,dt}} {t1 - t0}
	 */

	SNDFILE* sf_out = sf_open ("/tmp/out.wav", SFM_WRITE, &_info);

	const float one = 1.0;
	const float zero = 0.0;

	assert (bpm_start > bpm_end);
	const double lg_bpm = log (bpm_start / bpm_end);

	for (uint32_t b = 0; b < duration_beats; ++b) {
		sf_write_float (sf_out, &one, 1);

		double b0 = b / (double) duration_beats;
		double b1 = (b + 1) / (double) duration_beats;
		double bpm_b0 = bpm_start * exp (-b0 * lg_bpm);
		double bpm_b1 = bpm_start * exp (-b1 * lg_bpm);
		/* calculate area under exp curve between bpm[b0], bpm[b1], the divide by distance */
		double bpm_dist = (1.0 / lg_bpm) * (bpm_b0 - bpm_b1) / (b1 - b0);

		int spb = _info.samplerate * 60.0 / bpm_dist;
		for (int i = 1; i < spb; ++i) {
			sf_write_float (sf_out, &zero, 1);
		}
	}

	/* add fixed beat at the end */
	uint32_t spb_end = _info.samplerate * 60 / bpm_end;
	for (uint32_t b = 0; b < duration_beats; ++b) {
		sf_write_float (sf_out, &one, 1);
		for (uint32_t i = 1; i < spb_end; ++i) {
			sf_write_float (sf_out, &zero, 1);
		}
	}

	sf_close (sf_out);
}