大整数乘法

 

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

#define MAX 10000
#define BASE 1000000000
#define MAX_DIGIT 5010
#define BASE_DIGIT 9

char str1[MAX_DIGIT], str2[MAX_DIGIT];
unsigned long long num1[MAX], num2[MAX];
unsigned long long result[MAX];
int num1_dig, num2_dig, res_dig;

void convert(char *, unsigned long long *, int *);
void calculate();
void print_value(unsigned long long *, int);
void clear_result();

int main(void) {
    while (scanf("%s %s", str1, str2) != EOF) {
        clear_result();
        convert(str1, num1, &num1_dig);
        convert(str2, num2, &num2_dig);
        calculate();
        print_value(result, res_dig);
    }
    return 0;
}

void convert(char *str, unsigned long long *num, int *dig) {
    int i, j, len;
    unsigned long long val;

    len = strlen(str);
    *dig = 0;
    for(i = len - 1; i >= 0; i -= BASE_DIGIT) {
        j = i - BASE_DIGIT + 1;
        if(j < 0) {
            j = 0;
        }
        val = 0;
        for(; j <= i; ++j) {
            val *= 10;
            val += str[j] - '0';
        }
        num[*dig] = val;
        (*dig)++;
    }
}

void print_value(unsigned long long *num, int dig) {
    int i = dig - 1;
    printf("%d", num[i]);
    for (--i; i >= 0; --i) {
        printf("%09d", num[i]);
    }
    printf("\n");
}

void calculate() {
    int i, j, k, l;
    unsigned long long val, tmp;

    if((num1_dig == 1 && num1[0] == 0)
        || (num2_dig == 1 && num2[0] == 0))
    {
            res_dig = 0;
　　　　　　　result[0] = 0;
            goto end;
    }
    for(i = 0; i < num2_dig; ++i) {
        val = 0;
        k = i;
        for(j = 0; j < num1_dig; ++j, ++k) {
            val += num2[i] * num1[j];
            result[k] += val % BASE;
            val /= BASE;
            l = k;
            while(result[l] >= BASE) {
                tmp = result[l];
                result[l] = tmp % BASE;
                result[l + 1] += tmp / BASE;
                ++l;
            }
            if(l > res_dig) {
                res_dig = l;
            }
        }
        if(val > 0) {
            result[k] += val;
            l = k;
            while(result[l] >= BASE) {
                tmp = result[l];
                result[l] = tmp % BASE;
                result[l + 1] += tmp / BASE;
                ++l;
            }
        }
        if(l > res_dig) {
            res_dig = l;
        }
    }
end:
    res_dig++;

}
void clear_result() {
    memset(result, 0, sizeof(result));
    res_dig = 0;
}