Metronome: Difference between revisions

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 The visual indicator can simply be a blinking red or green area of the screen (depending on whether a high or low beat is being produced), and the metronome can be implemented using a terminal display, or optionally, a graphical display, depending on the language capabilities. If the language has no facility to output sound, then it is permissible for this to implemented using just the visual indicator.
+=={{header|C}}==
+Using <code>usleep</code> with self correcting delays.  Audio is the bell character, which will definitely drive one insane (but I'm ok: my computer doesn't have the bell device).  Invoke with <code>./a.out [beats_per_minute]</code>, default to 60.
+<lang c>#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;
+}</lang>