#include <stdint.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/time.h>
#include <signal.h>

#define log(...) fprintf(stderr, __VA_ARGS__)
#define WATCHER "watcher"

static struct proc {
	uint64_t deadline;
	char filename[32 - sizeof(uint64_t)];
} procs[NPROC];

static pid_t
spawn_watcher(char *filename)
{
	char *argv[3] = { WATCHER, filename, NULL };
	pid_t pid;

	pid = fork();
	if (pid == 0) {
		if (execv(WATCHER, argv)) {
			log("Error spawning '%s': %s\n",
					WATCHER, strerror(errno));
			exit(EXIT_FAILURE);
		}
	}

	if (pid < 0)
		log("Error forking: %s\n", strerror(errno));
	
	return pid;
}

static int
touch(char *filename, int flags)
{
	int fd = open(filename, flags, 0644);
	if (fd < 0 || close(fd)) {
		log("Error touching '%s': %s\n",
				filename, strerror(errno));
		return -1;
	}
	return 0;
}

static uint64_t 
now(void)
{
	struct timeval tv;
	(void)gettimeofday(&tv, NULL);
	return ((uint64_t)tv.tv_sec * 1000000) + (uint64_t)tv.tv_usec;
}

static uint64_t
random_delay(void)
{
	return (uint64_t)DELAY_MIN + (random() / (RAND_MAX/(DELAY_MAX - DELAY_MIN)));
}

int
main()
{
	unsigned int i;
	struct proc *next;

	if (signal(SIGCHLD, SIG_IGN)) {
		log("Error setting SIGCHLD handler: %s\n",
				strerror(errno));
		return EXIT_FAILURE;
	}

	for (i = 0; i < NPROC; i++) {
		(void)sprintf(procs[i].filename, "%03u.tmp", i);
		if (touch(procs[i].filename, O_WRONLY | O_CREAT)) {
			log("Error creating file '%s': %s\n",
					procs[i].filename, strerror(errno));
			return EXIT_FAILURE;
		}
	}

	next = &procs[0];
	for (i = 0; i < NPROC; i++) {
		if (spawn_watcher(procs[i].filename) < 0)
			goto error;

		procs[i].deadline = now() + random_delay();
		if (procs[i].deadline < next->deadline)
			next = &procs[i];
	}

	while (1) {
		uint64_t n = now();

		if (next->deadline > n) {
			usleep(next->deadline - n);
			continue;
		}

		if (touch(next->filename, O_WRONLY))
			goto error;

		if (spawn_watcher(next->filename) < 0)
			goto error;

		next->deadline = now() + random_delay();
		for (i = 0; i < NPROC; i++) {
			if (procs[i].deadline < next->deadline)
				next = &procs[i];
		}
	}

error:
	for (i = 0; i < NPROC; i++)
		(void)touch(procs[i].filename, O_WRONLY);

	return EXIT_FAILURE;
}