example.c

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

#define SIZE(x) (sizeof x / sizeof x[0])

int64_t mdiv(int64_t *x, size_t n, int64_t y);

typedef struct {
    size_t const n; // Size of the dividend
    int64_t const *x; // Dividend
    int64_t const y; // Divisor
} data_t;

static const data_t data[] = {
    {1, (int64_t[1]){13}, 5}, // Dividend is `13`.
    {2, (int64_t[2]){0, 13}, 5}, // Dividend is `13 << 64` (239807672958224171008).
    {3, (int64_t[3]){1, 1, 1}, 2}, // Dividend is `(1 << 128) + (1 << 64) + 1` (340282366920938463481821351505477763073).
};

int main() {
    for (size_t i = 0; i < SIZE(data); ++i) {
        size_t n = data[i].n;
        int64_t *work_space = malloc(n * sizeof(int64_t));
        assert(work_space);
        memcpy(work_space, data[i].x, n * sizeof(int64_t));

        printf("Test case %zu:\n", i);
        printf("Dividend:  ");
        for (size_t j = 0; j < data[i].n; j++) {
            printf("0x%016" PRIx64 " ", data[i].x[j]);
        }
        printf("\nDivisor:   0x%016" PRIx64 "\n", data[i].y);

        int64_t r = mdiv(work_space, n, data[i].y);

        printf("Quotient:  ");
        for (size_t j = 0; j < n; ++j) {
            printf("0x%016" PRIx64 " ", work_space[j]);
        }
        printf("\nRemainder: 0x%016" PRIx64 "\n\n", r);

        free(work_space);
    }
}