// pi.c  2010-11-14  Hatada  
// πの値を指定桁まで算出

#include <stdio.h>
#include <stdlib.h>

int series(int *data, int keta, int maxItem,
           int bunsi, int bunbo, int plusMinus) {
    int bunbo2;
    int *item;
    int *itemDiv;
    int  len;
    int  i, j, k;
    int  x;

    len = (keta-1)/4 + 2 + 1;
    if ((item = malloc(len * sizeof(int))) == 0) return -1;
    for (i = 0; i < len; i++) item[i] = 0;

    if ((itemDiv = malloc(len * sizeof(int))) == 0) {
        free(item);
        return -1;
    }

    item[0] = bunsi * bunbo * 4;
    bunbo2  = bunbo * bunbo;

    for (j = 1; maxItem--; j += 2) {
        x = 0;
        for (i = 0; i < len; i++) {
            x = x * 10000 + item[i];
            item[i] = x / bunbo2;
            x = x % bunbo2;
        }
        
        if (j == 1) {
	    for (i = 0; i < len; i++) itemDiv[i] = item[i];
        } else {
            x = 0;
            for (i = 0; i < len; i++) {
                x = x * 10000 + item[i];
                itemDiv[i] = x / j;
                x = x % j;
            }
        }
        k = 0;
        if (plusMinus & 1) {
            for (i = len; --i >= 0; ) {
		data[i] += itemDiv[i] + k;
                if (data[i] < 10000) k = 0;
                else { data[i] -= 10000; k = 1; }
            }
        } else {
            for (i = len; --i >= 0; ) {
		data[i] -= itemDiv[i] + k;
                if (data[i] >= 0) k = 0;
                else { data[i] += 10000; k = 1; }
            }
        }
        plusMinus++;
    }
    free(itemDiv);
    free(item);
    return 0;
}

int pi(int *data, int keta) {
    int maxItem_5   = keta * 500 /  699 + 2;
    int maxItem_239 = keta * 500 / 2378 + 2;
    int i;

    for (i = 0; i < (keta-1)/4+2; i++) data[i] = 0;
    if (series(data, keta, maxItem_5, 4, 5, 1) != 0)
        return -1;
    if (series(data, keta, maxItem_239, 1, 239, 0) != 0)
        return -1;
    return 0;
}

void main(int argc, char *argv[]) {
    char* pai = "3.1415 9265 3589 7932 3846 2643 3832 7950 2884 1971 "
		"6939 9375 1058 2097 4944 5923 0781 6406 2862 0899 "
		"8628 0348 2534 2117 0679 8214 8086 5132 8230 6647";
    int k;
    int* data;
    int n;

    if (argc > 1) {
	n = atoi(argv[1]);
    } else {
        printf("桁数(4の倍数)＝"); 
        scanf("%d", &n);
    }
    printf("n=%d\n", n);
    data = calloc(n*10, 1);	// いくら必要か吟味していない
    pi(data, n);
    for (k = 0; k < (n-1)/4 + 2; k++) {
        printf("%04d ", data[k]);
    }
    printf("\n解:%s\n", pai);
} 
// http://bal4u.dip.jp/mt/program/2004/08/pi.html