Metronome - musical tool.

If you are a musician, one of the most useful tools is a metronome. It graphically displays the beats for you to follow while playing music.One particular program is all text based so it will run on a variety of systems even without a sound card. To invoke the program, you use the program name followed by the speed you want to use.

Then the program will start. The program uses a simple cursor to mark the time of the beats.

The program seems to do some compensation to keep the metronome accurate.
To build the program, you would:

$ gcc metronome.c -o metronome

Here is the code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdint.h>
#include <signal.h>
#include <time.h>
#include <sys/time.h>

struct timeval start, last;

inline int64_t tv_to_u(struct timeval s)
{
    return s.tv_sec * 1000000 + s.tv_usec;
}

inline struct timeval u_to_tv(int64_t x)
{
    struct timeval s;
    s.tv_sec = x / 1000000;
    s.tv_usec = x % 1000000;
    return s;
}

void draw(int dir, int64_t period, int64_t cur, int64_t next)
{
    int len = 40 * (next - cur) / period;
    int s, i;

    if (len > 20) len = 40 - len;
    s = 20 + (dir ? len : -len);

    printf("\033[H");
    for (i = 0; i <= 40; i++) putchar(i == 20 ? '|': i == s ? '#' : '-');
}

void beat(int delay)
{
    struct timeval tv = start;
    int dir = 0;
    int64_t d = 0, corr = 0, slp, cur, next = tv_to_u(start) + delay;
    int64_t draw_interval = 20000;
    printf("\033[H\033[J");
    while (1) {
        gettimeofday(&tv, 0);
        slp = next - tv_to_u(tv) - corr;
        usleep(slp);
        gettimeofday(&tv, 0);

        putchar(7); /* bell */
        fflush(stdout);

        printf("\033[5;1Hdrift: %d compensate: %d (usec)   ",
            (int)d, (int)corr);
        dir = !dir;

        cur = tv_to_u(tv);
        d = cur - next;
        corr = (corr + d) / 2;
        next += delay;

        while (cur + d + draw_interval < next) {
            usleep(draw_interval);
            gettimeofday(&tv, 0);
            cur = tv_to_u(tv);
            draw(dir, delay, cur, next);
            fflush(stdout);
        }
    }
}

int main(int c, char**v)
{
    int bpm;

    if (c < 2 || (bpm = atoi(v[1])) <= 0) bpm = 60;
    if (bpm > 600) {
        fprintf(stderr, "frequency %d too high\n", bpm);
        exit(1);
    }

    gettimeofday(&start, 0);
    last = start;
    beat(60 * 1000000 / bpm);

    return 0;
}

Good luck and have fun.