大数加法、乘法

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

// x1 + x2 = yy
int add_1(char *x1, char *x2, char *yy)
{
    int x1_len, x2_len, x_max, yy_len;
    int i, j, carry, tt;
    char *y1;
    
    x1_len = strlen(x1);
    x2_len = strlen(x2);
    
    //printf("x1_len:%d, x2_len:%d
", x1_len, x2_len);
    
    x_max = x1_len > x2_len ? x1_len : x2_len;
    // yy_len的最大长度为: x_max+1
    // yy_len的最小长度为: x_max
    if((y1 = malloc(sizeof(char) * (x_max + 2))) == NULL)
    {
        return 0;    
    }
    memset(y1, 0x00, x_max + 2);
    
    carry  = 0;
    yy_len = 0;
    for(i=x1_len, j=x2_len; i > 0 && j > 0; i--, j--, yy_len++)
    {
        tt = (x1[i-1]-'0') + (x2[j-1]-'0') + carry;
        y1[yy_len] = (tt % 10) + '0';
        carry = tt / 10;
    }
    for(; i > 0; i--, yy_len++)
    {
        tt = (x1[i-1]-'0') + carry;
        y1[yy_len] = (tt % 10) + '0';
        carry = tt / 10;            
    }
    for(; j > 0; j--, yy_len++)
    {
        tt = (x2[j-1]-'0') + carry;
        y1[yy_len] = (tt % 10) + '0';
        carry = tt / 10;            
    }
    if(carry != 0)
    {
        y1[yy_len++] = carry + '0';
    }
    
    //printf("yy_len: %d
", yy_len);
    
    for(i = 0; i < yy_len; i++)
    {
        yy[i] = y1[yy_len-i-1];
    }
    yy[yy_len] = '';
    
    free(y1);

    return yy_len;
}

// x1 + x2 = yy 
int add_2(char *x1, char *x2, char *yy)
{
    int x1_len, x2_len, x_max, yy_len;
    int i, tmp, carry;
    char *y1;
    
    x1_len = strlen(x1);
    x2_len = strlen(x2);
    //printf("x1_len:%d, x2_len:%d
", x1_len, x2_len);

    x_max = x1_len > x2_len ? x1_len : x2_len;

    if((y1 = malloc(sizeof(char) * (x_max + 2))) == NULL)
    {
        return 0;    
    } 

    carry = 0;
    yy_len = 0;
    
    while(x1_len > 0 && x2_len > 0)
    {
        tmp = x1[--x1_len] + x2[--x2_len] - 2 * '0' + carry;
        y1[yy_len++] = tmp % 10 + '0';
        carry  = tmp / 10;
    }
    while(x1_len > 0)
    {
        tmp = x1[--x1_len] - '0' + carry;
        y1[yy_len++] = tmp % 10 + '0';
        carry  = tmp / 10;
    }
    while(x2_len > 0)
    {
        tmp = x2[--x2_len] - '0' + carry;
        y1[yy_len++] = tmp % 10 + '0';
        carry  = tmp / 10;     
    }
    if(carry != 0)
    {
        y1[yy_len++] = carry + '0';
    }

    //printf("yy_len: %d
", yy_len);
    
    for(i = 0; i < yy_len; i++)
    {
        yy[yy_len-i-1] = y1[i];
    }
    yy[yy_len] = '';

    free(y1);

    return yy_len;
}

// x1 * x2 = yy
int mult(char *x1, char *x2, char *yy)
{
    int x1_len, x2_len, yy_len;
    int i, carry, pos, tt;
    char *pp;
    char *y1;

    x1_len = strlen(x1);
    x2_len = strlen(x2);
    
    //printf("x1_len: %d, x2_len:%d
", x1_len, x2_len);
    
    // yy_len的最大长度为: x1_len+x2_len;
    // yy_len的最小长度为: x1_len+x2_len-1;
    if((y1 = malloc(sizeof(char) * (x1_len+x2_len+1))) == NULL)
    {
        return 0;    
    }
    memset(y1, 0x00, x1_len+x2_len+1);
        
    for(pos = 0; x2_len > 0; x2_len--, pos++)
    {
        carry = 0;
        pp = &y1[pos];
        
        for(i = x1_len; i > 0; i--)
        {
            // 当前位的乘积+进位位+原有位
            tt    = (x2[x2_len-1]-'0') * (x1[i-1]-'0') + carry + (*pp);
            *pp++ = tt % 10;
            carry = tt / 10;
        }
        
        if(carry != 0)
        {
            *pp++ = carry;
        }
    }
    
    yy_len = pp - y1;
    //printf("yy_len: %d
", yy_len);

    while(y1[yy_len-1] == 0 && yy_len > 0)
    {
        yy_len--;
    }
    if(yy_len == 0)
    {
        yy_len = 1;
    }

    for(i = 0; i < yy_len; i++)
    {
        yy[yy_len-i-1] = y1[i] + '0';
    }
    yy[yy_len] = '';

    free(y1);

    return yy_len;
}

int main(void)
{
#define MAX_LEN     (100)
    char *x1 = "999";
    char *x2 = "999";
    char *yy;

    yy = malloc(sizeof(char) * MAX_LEN);

    memset(yy, 0x00, sizeof(char)*MAX_LEN); 
    add_1(x1, x2, yy);
    printf("%s + %s = %s
", x1, x2, yy);

    memset(yy, 0x00, sizeof(char)*MAX_LEN); 
    add_2(x1, x2, yy);
    printf("%s + %s = %s
", x1, x2, yy);

    memset(yy, 0x00, sizeof(char)*MAX_LEN); 
    mult(x1, x2, yy);
    printf("%s * %s = %s
", x1, x2, yy);

    return 0;
}