高精度加法

/*******************************************************************************
 **
 ** Copyright (c) 20010-2012 UESTC.
 ** All rights reserved.
 **
 ** File name: big_int_add.c
 ** Description:
 **
 ** Current Version: 0.0
 ** Revision: 0.0.0.0
 ** Author: panhong
 ** Date: 2012-03-06
 ** Email:[e-mail]
 ******************************************************************************/
#include<stdio.h>
#include<string.h>
#include<assert.h>
#define MAX(a, b) (((a) < (b)) ? (b) : (a))
#define MAXBIT 100

int pos[MAXBIT];

char* str_reverse(char *s)
{
    char* p = s;
    size_t len = strlen(p);
    char tmp;
    size_t i;
    for(i = 0; i < len / 2; ++i)
    {
        tmp = *(p + i);
       *(p + i) = *(p + len -1- i);
       *(p + len -1 - i) = tmp;
    }
    return s;
}

char* big_int_add(const char *lhs, const char *rhs, char *result_buf, size_t buf_size)
{
   assert(lhs != NULL && rhs != NULL);

    size_t llen = strlen(lhs);
    size_t rlen = strlen(rhs);

    if(MAX(llen, rlen) + 1 > buf_size || MAX(llen, rlen) > MAXBIT)
        return NULL;

    if(llen < rlen)
    {
        const char *p = lhs;
        lhs = rhs;
        rhs = p;
        llen ^= rlen;
        rlen ^= llen;
        llen ^= rlen;
    }

    memset(pos, 0, sizeof(pos));
    memset(result_buf, 0, buf_size);

    int bit_sum = 0;
    size_t i = 0;
    lhs += llen;
    rhs += rlen;
    for(; i < rlen; ++i)
    {
        bit_sum = (*(--lhs) - '0') + (*(--rhs) - '0') + pos[i];
       *(result_buf + i) = (char)((bit_sum) % 10 + '0');
       if(bit_sum > 9)
           pos[i + 1] = 1;//进位
    }

    for(; i < llen; ++i)
    {
        bit_sum = *(--lhs) - '0' + pos[i];
       *(result_buf + i) = (char)((bit_sum % 10) + '0');
       if(bit_sum > 9)
           pos[i + 1] = 1;
    }

    if(pos[i] == 1)
        *(result_buf + i) = (char)(pos[i] + '0');

    return str_reverse(result_buf);
}

int main()
{
    char n1[100];
    char n2[100];

    scanf("%s", n1);
    scanf("%s", n2);
    char result[100];
    printf("%s is the sum\n", big_int_add(n1, n2, result, 100));
    return 0;
}