#include <pthread.h>
#include <stdio.h>
#define INTERVAL 1000000
#define THNUMS	3

//thread params
struct ThreadParams
{
	int id; //id
	int low; //start
	int high; //end
	double ysum; //return partial sum
}

#define BLOCK_LOW(id, p, n) ((id)*(n)/(p))
#define BLOCK_HIGH(id, p, n) (BLOCK_LOW((id)+1,p,n)-1)
#define BLOCK_SIZE(id, p, n) (BLOCK_HIGH(id,p,n)-BLOCK_LOW(id,p,n)+1)

//calculate pi partial sum
void *calcpi(void *arg)
{
	double ysum;
	double xi;
	int i;

	struct ThreadParams *pm = (struct ThreadParams*)arg;

	ysum = 0.0;

	for(i = pm->low; i <= pm->high; i++)
	{
		xi=((1.0/INTERVALS)*(i+0.5));
		ysum=ysum+4.0/(1.0+xi*xi);
	}

	pm->ysum = ysum;
}

int main(int argc, char* argv[])
{
	struct ThreadParams ptharg[THNUMS];
	double area;
	double ysum;
	int i;

	pthread_t tid;
	void *status;

	//create multithreads to calculate partial sum
	ysum = 0.0;

	for(i=0; i<THNUMS; i++)
	{
		ptharg[i].id = i;
		ptharg[i].low = BLOCK_LOW(i,THNUMS,INTERVALS);
		ptharg[i].high = BLOCK_HIGH(i,THNUMS,INTERVALS);

		if(pthread_create(&tid, NULL, calcpi, (void*)&ptharg[i])!=0)
		{
			perror("error creating child");
			return -1;
		}

		pthread_join(tid, &status);
	}

	//calculate total area
	area = 0;

	for(i=0; i<THNUMS;i++)
	{
		area = area + ptharg[i].ysum;
	}

	area = area * (1.0/INTERVALS);
	printf("Area is %13.11f\n",area);
	return 0;
}