/*
 * fps-meter - Example program about how to measure frames per seconds
 *
 * Copyright (C) 2013  Antonio Ospite <ospite@studenti.unina.it>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <stdio.h>
#include <time.h>

#ifdef DEBUG
#define dbg(...)                     \
	do {                         \
		printf(__VA_ARGS__); \
		printf("\n");        \
		fflush(stdout);      \
	} while(0)
#else
#define dbg(...) do {} while(0)
#endif

#define NSEC_PER_SEC 1000000000

#define timespecsub(a, b, result)                                \
	do {                                                     \
		(result)->tv_sec = (a)->tv_sec - (b)->tv_sec;    \
		(result)->tv_nsec = (a)->tv_nsec - (b)->tv_nsec; \
		if ((result)->tv_nsec < 0) {                     \
			--(result)->tv_sec;                      \
			(result)->tv_nsec += 1000000000;         \
		}                                                \
	} while(0)

int main(void)
{
	struct timespec time_start;
	struct timespec time_end;
	struct timespec elapsed;

	struct timespec tmp;

	unsigned int frames = 0;
	double nsecs = 0;
	float fps;

	while (1) {
		clock_gettime(CLOCK_MONOTONIC, &time_start);
		dbg("Start time: s: %ld, ns: %ld", time_start.tv_sec, time_start.tv_nsec);

		/* simulate some workload of about 60fps */
		dbg("Doing some work, which takes about %d ns", NSEC_PER_SEC / 60);
		tmp.tv_sec = 0;
		tmp.tv_nsec = NSEC_PER_SEC / 60;
		clock_nanosleep(CLOCK_MONOTONIC, 0, &tmp, NULL);

		clock_gettime(CLOCK_MONOTONIC, &time_end);
		dbg("End time: s: %ld, ns: %ld", time_end.tv_sec, time_end.tv_nsec);
		timespecsub(&time_end, &time_start, &elapsed);
		dbg("Elapsed s: %ld ns: %ld", elapsed.tv_sec, elapsed.tv_nsec);

		frames++;
		nsecs += (elapsed.tv_sec * NSEC_PER_SEC + elapsed.tv_nsec);
		if (nsecs >= NSEC_PER_SEC) {
			/* 
			 * if each frame takes less than a second, then just
			 * printing 'frames' is enough, but if we want to
			 * cover the case when a frame may take more than
			 * a second, some calculations have to be done.
			 */
			fps = frames / (nsecs / NSEC_PER_SEC);
			printf("(frames: %d, nsecs: %f) FPS: %.2f\n", frames, nsecs, fps);
			nsecs = 0;
			frames = 0;
		}
	}

	return 0;
}