dft_example.c

/* $(CC) dft_example.c -lm -g -Wall */
#include <stdio.h>
#include <complex.h>
#include <math.h>

#define BUCKETS 16

/* 
 * XXX: looses any phase information.
 * len samples go in and len freqs come out.
 */
void dft(int *samples, int *freqs, int len) {
	for(int i = 0; i < len; i++) {
		for(int j = 0; j < len; j++) {
			freqs[i] += samples[j] * cexp((- I * 2 * M_PI * i * j)/len);
		}
	}
}

/* 
 * XXX: looses any phase information.
 * len freqs go in and len samples come out.
 * */
void inv_dft(int *freqs, int *samples, int len) {
	for(int i = 0; i < len; i++) {
		for(int j = 0; j < len; j++) {
			samples[i] += freqs[j] * cexp(( I * 2 * M_PI * i * j)/len);
		}
		samples[i] = samples[i]/len;
	}
}

int main(void) {

	int samples[BUCKETS] = {0};
	int freqs[BUCKETS] = {0};

	float peak = 2;
	for(int i = 0; i < BUCKETS; i++) {
		samples[i] = 128*sin(i/((float)BUCKETS)*2.0 * M_PI * peak);
	}

	//freqs[2] = 128;
	//inv_dft(freqs, samples, BUCKETS);
	dft(samples, freqs, BUCKETS);
	puts("samples");
	for(int i = 0; i < BUCKETS; i++) {
		printf("%d\n", samples[i]);
	}
	puts("freqs");
	for(int i = 0; i < BUCKETS; i++) {
		printf("%d\n", freqs[i]);
	}
	return 0;
}