A modulo operator in c

mod.c

#include <stdio.h>
#include <limits.h> 
#include <assert.h>

int mod(int a, int b) {
    int res;
    if (b < 0) {
        // because -INT_MIN == INT_MIN
        if (b > INT_MIN) {
            return -mod(-a, -b);
        } else {
            // b == INT_MIN
            if (a == INT_MIN) {
                return 0;
            } else if (a < 0) {
                return a;
            } else {
                return INT_MIN + a;
            }
        }  
    }
    assert(b > 0);
    if (a < 0) {
        res = b - 1 - (-(a + 1)) % b;
    } else {
        res = a % b;
    }
    assert(res > INT_MIN);
    return res;
}

int main (int argc, char** argv)
{
    printf("% i mod % i = % i\n", 2, 5, mod(2, 5));
    printf("% i mod % i = % i\n", -2, 5, mod(-2, 5));
    printf("% i mod % i = % i\n", 2, -5, mod(2, -5));
    printf("% i mod % i = % i\n", -2, -5, mod(-2, -5));
    printf("% i mod % i = % i\n", 9, 5, mod(9, 5));
    printf("% i mod % i = % i\n", -9, 5, mod(-9, 5));
    printf("% i mod % i = % i\n", 9, -5, mod(9, -5));
    printf("% i mod % i = % i\n", -9, -5, mod(-9, -5));
    printf("% i mod % i = % i\n", 10, 5, mod(10, 5));
    printf("% i mod % i = % i\n", -10, 5, mod(-10, 5));
    printf("% i mod % i = % i\n", 10, -5, mod(10, -5));
    printf("% i mod % i = % i\n", -10, -5, mod(-10, -5));

    printf("% i mod % i = % i\n", INT_MAX, 5, mod(INT_MAX, 5));
    printf("% i mod % i = % i\n", INT_MIN, 5, mod(INT_MIN, 5));

    printf("% i mod % i = % i\n", 5, INT_MAX, mod(5, INT_MAX));
    printf("% i mod % i = % i\n", 5, INT_MIN, mod(5, INT_MIN));

    printf("% i mod % i = % i\n", INT_MAX, INT_MIN, mod(INT_MAX, INT_MIN));
    printf("% i mod % i = % i\n", INT_MIN, INT_MAX, mod(INT_MIN, INT_MAX));

    printf("% i mod % i = % i\n", INT_MAX - 1, INT_MIN, mod(INT_MAX - 1, INT_MIN));
    printf("% i mod % i = % i\n", INT_MIN + 1, INT_MAX, mod(INT_MIN + 1, INT_MAX));

    printf("% i mod % i = % i\n", INT_MAX, INT_MIN + 1, mod(INT_MAX, INT_MIN + 1));
    printf("% i mod % i = % i\n", INT_MIN, INT_MAX - 1, mod(INT_MIN, INT_MAX - 1));

    return 0;
}