string_wk.h

#include <linux/string.h>
#define XOR_SWAP(a, b, type) \
    do {                     \
        type *__c = (a);     \
        type *__d = (b);     \
        *__c ^= *__d;        \
        *__d ^= *__c;        \
        *__c ^= *__d;        \
    } while (0)

static void __swap(void *a, void *b, size_t size)
{
    if (a == b)
        return;

    switch (size) {
    case 1:
        XOR_SWAP(a, b, char);
        break;
    case 2:
        XOR_SWAP(a, b, short);
        break;
    case 4:
        XOR_SWAP(a, b, unsigned int);
        break;
    case 8:
        XOR_SWAP(a, b, unsigned long);
        break;
    default:
        /* Do nothing */
        break;
    }
}

typedef struct str {
    char numberStr[128];
} str_t;

static void add_str(char *a, char *b, char *out)
{
    size_t size_a = strlen(a), size_b = strlen(b);
    int i, sum, carry = 0;
    if (size_a >= size_b) {
        for (i = 0; i < size_b; i++) {
            sum = (a[i] - '0') + (b[i] - '0') + carry;
            out[i] = '0' + sum % 10;
            carry = sum / 10;
        }

        for (i = size_b; i < size_a; i++) {
            sum = (a[i] - '0') + carry;
            out[i] = '0' + sum % 10;
            carry = sum / 10;
        }
    } else {
        for (i = 0; i < size_a; i++) {
            sum = (a[i] - '0') + (b[i] - '0') + carry;
            out[i] = '0' + sum % 10;
            carry = sum / 10;
        }

        for (i = size_a; i < size_b; i++) {
            sum = (b[i] - '0') + carry;
            out[i] = '0' + sum % 10;
            carry = sum / 10;
        }
    }

    if (carry)
        out[i++] = '0' + carry;
    out[i] = '\0';
}

static void reverse_str(char *str, size_t n)
{
    for (int i = 0; i < (n >> 1); i++)
        __swap(&str[i], &str[n - i - 1], sizeof(char));
}