#include <stdio.h>
#include <signal.h>
#include <fcntl.h>

void forkprocess(int, char**, int);
void setNonBlocking(int);
int numToSpawn;
int numChildren = 0;
int count = -1;
int pipesin[1000];
int pipesout[1000];

void int_handler(int sig)
{
	signal(SIGUSR1, int_handler);
	/* parent */
	char outbuf[1024];
	int a, b, c, i, n, length;
	
	for (i = 0; i < count; i++)
	{
		n = read(pipesin[i], outbuf, 1024);
		if (n != 0)
			break;
	}
	sscanf(outbuf, "%d %d", &a, &b);
	c = a - b;
	while (c >= b)
		c = c - b;
	sprintf(outbuf, "%d", c);
	length = strlen(outbuf);
	if (write(pipesout[count], outbuf, length) != length)
	   	perror("write to pipe");
}

int parent_pid;
main(int argc, char **argv)
{
	// check the command line input to make sure it's pairs of numbers
	if (argc % 2 == 0 || argc == 1)
	{
		printf("input must be a set of pairs of numbers");
	        exit(0);
	}
	int i = 1;
	int j;
	
	while(i < argc)
	{
		for(j = 0; j < strlen(argv[i]); j++)
		{
			if (!isdigit(argv[i][j]))		
			{
				printf("input must be a set of pairs of numbers");
	        		exit(0);
			}	
		}
		i++;
	}
	
	signal(SIGUSR1, int_handler);
	numToSpawn = (argc - 1) / 2;
	parent_pid = getpid();
	forkprocess(argc, argv, 1);
}

void forkprocess(int argc, char **argv, int position)
{	
	int pipev[2];
	int pipev2[2];
	int length;
	int i;
	char buf[1024];
	char buf2[1024];
	
	for(i  = 0; i < numToSpawn; i++)
	{
		// open one pipe to read from child
       		if (pipe(pipev) == -1)
			perror("open pipe");
		//open second pipe to write to child
		if (pipe(pipev2) == -1)
		   	perror("open pipe");
	
		count++;
		pipesin[count] = pipev[0];
		pipesout[count] = pipev2[1];
		setNonBlocking(pipesin[count]);

		numChildren++;
		int pid = fork();
		/* error */
		if (pid < 0)
			perror("fork");
		/* child */
		else if (pid == 0)
		{
			close(pipev[0]);
			close(pipev2[1]);	
			int last, next, m, n;
			sscanf(argv[position + 1], "%d", &last);
			sscanf(argv[position], "%d", &next);
			m = next;
			n = last;
			sprintf(buf, "%s %s", argv[position], argv[position + 1]);
			length = strlen(buf);
			if (write(pipev[1], buf, length) != length)
	   			perror("write to pipe");
			while(1)
			{
				next = last;
				read(pipev2[0], buf2, 1024);
				sscanf(buf2, "%d", &last);
				if (last == 0) 
				{
					printf("GCD(%d, %d) = %d ", m, n, next);
					exit(0);
// for child, this does nothing!
				// numChildren--;
				}
				sprintf(buf, "%d %d", next, last);
// may need to clean buffer
	       			if (write(pipev[1], buf, length) != length)
	   				perror("write to pipe");
				//signal(SIGUSR1, int_handler);
				kill(parent_pid, SIGUSR1);
			}
		}
		position = position + 2;
	}
	while (1){
		wait(); 
		if (--numChildren == 0) exit(0);
	}
}

void setNonBlocking(int fd)
{
	int val;
	if ((val = fcntl(fd, F_GETFL, 0)) < 0)
		perror("fcntl F_GETFL error");
	val |= O_NONBLOCK;	// turn on flags
	if (fcntl(fd, F_SETFL, val)<0)
		perror("fcntl F_SETFL error");
}