pi.c

#include <stdio.h>
#define L 100000
#define N L/4+1
// L 为位数，N是array长度
void add(int *, int *, int*);
void sub(int *, int *, int*);
void div(int *, int, int*);
int main(void)
{
    int s[N + 3] =
    {
        0
    };
    int w[N + 3] =
    {
        0
    };
    int v[N + 3] =
    {
        0
    };
    int q[N + 3] =
    {
        0
    };
    int n = (int)(L / 1.39793 + 1);
    int k;
    w[0] = 16 * 5;
    v[0] = 4 * 239;
    for (k = 1; k <= n; k++)
    {
        // 套用公式
        div(w, 25, w);
        div(v, 239, v);
        div(v, 239, v);
        sub(w, v, q);
        div(q, 2 *k - 1, q);
        if (k % 2)
            // 奇数项
            add(s, q, s);
        else
            // 偶数项
            sub(s, q, s);
    }
    printf("%d.", s[0]);
    for (k = 1; k < N; k++)
        printf("%04d", s[k]);
    printf("\n");
    return 0;
}

void add(int *a, int *b, int *c)
{
    int i, carry = 0;
    for (i = N + 1; i >= 0; i--)
    {
        c[i] = a[i] + b[i] + carry;
        if (c[i] < 10000)
            carry = 0;
        else
        {
            // 进位
            c[i] = c[i] - 10000;
            carry = 1;
        }
    }
}

void sub(int *a, int *b, int *c)
{
    int i, borrow = 0;
    for (i = N + 1; i >= 0; i--)
    {
        c[i] = a[i] - b[i] - borrow;
        if (c[i] >= 0)
            borrow = 0;
        else
        {
            // 借位
            c[i] = c[i] + 10000;
            borrow = 1;
        }
    }
}

void div(int *a, int b, int *c)
{
    // b 为除数
    int i, tmp, remain = 0;
    for (i = 0; i <= N + 1; i++)
    {
        tmp = a[i] + remain;
        c[i] = tmp / b;
        remain = (tmp % b) *10000;
    }
}