st-to-bpm.c

/* Semitone to BPM converter, for ANSI C, by Zoe Blade, 2020-01-16 */

#include <stdio.h>
#include <stdlib.h>

#define SEMITONE 1.059463 /* 2^(1/12), as there are 12 equally
                             logarithmically spaced semitones
                             in an octave */

float semitoneToBeatsPerMinute(int beatsPerMinute, int semitones) {
	float shiftedBeatsPerMinute;

	shiftedBeatsPerMinute = beatsPerMinute;

	if (semitones > 0) {
		/* Going up */

		while (semitones > 0) {
			shiftedBeatsPerMinute *= SEMITONE;
			semitones--;
		}

		return shiftedBeatsPerMinute;
	} else if (semitones == 0) {
		/* Staying still */
		return shiftedBeatsPerMinute;
	} else {
		/* Going down */
		semitones *= -1; /* Make semitones a positive equivalent number */

		while (semitones > 0) {
			shiftedBeatsPerMinute /= SEMITONE;
			semitones--;
		}

		return shiftedBeatsPerMinute;
	}

	return 0.0;
}

void semitonesToBeatsPerMinute(int beatsPerMinute) {
	int semitones;
	float shiftedBeatsPerMinute;

	for (semitones = -5; semitones < 6; semitones++) {
		shiftedBeatsPerMinute = semitoneToBeatsPerMinute(beatsPerMinute, semitones);
		printf("%3d\t       %+1d\t%7.3f\n", beatsPerMinute, semitones, shiftedBeatsPerMinute);
	}

	return;
}

int main(int argc, char *argv[]) {
	int beatsPerMinute;

	printf("BPM\tSemitones\t    BPM\n");
	printf("===\t=========\t=======\n\n");

	if (argc == 1) {
		for (beatsPerMinute = 80; beatsPerMinute < 145; beatsPerMinute += 5) {
			semitonesToBeatsPerMinute(beatsPerMinute);
		}
	} else {
		semitonesToBeatsPerMinute(atoi(argv[1]));
	}

	return 0;
}