Get unique elements of an array using a lookup table

unique.c

// Count elements of an array by using a lookup table.
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <time.h>

int main(int argc, char *argv[])
{
    // Generate random array of integers, with
    // size given by args.
    int max_int = atoi(argv[1]);
    int array_size = atoi(argv[2]);
    bool do_counting = atoi(argv[3]);

    // Create array of integers:
    int *array = malloc(array_size * sizeof(int));
    for (int i = 0; i < array_size; i++)
    {
        array[i] = rand() % max_int;
    }

    // Start timer:
    clock_t start = clock();

    int *unique;
    int *unique_counts;

    if (do_counting)
    {
        // Create array to count elements:
        int *count = calloc(max_int, sizeof(int));

        // Count elements:
        for (int i = 0; i < array_size; i++)
        {
            count[array[i]]++;
        }

        // Count number nonzero:
        int nonzero = 0;
        for (int i = 0; i < max_int; i++)
        {
            if (count[i] != 0)
            {
                nonzero++;
            }
        }

        // Create array containing unique elements of array,
        // and another array with their count:
        unique = malloc(nonzero * sizeof(int));
        unique_counts = calloc(nonzero, sizeof(int));
        int j = 0;
        for (int i = 0; i < max_int; i++)
        {
            if (count[i] != 0)
            {
                unique[j] = i;
                unique_counts[j] = count[i];
                j++;
            }
        }
    }
    else // don't do counting
    {
        // Create array to mask elements
        bool *count = calloc(max_int, sizeof(bool));

        // Mask existing elements:
        for (int i = 0; i < array_size; i++)
        {
            count[array[i]] = true;
        }

        // Count number nonzero:
        int nonzero = 0;
        for (int i = 0; i < max_int; i++)
        {
            if (count[i] != 0)
            {
                nonzero++;
            }
        }

        // Create array containing unique elements of array,
        // and another array with their count:
        unique = malloc(nonzero * sizeof(int));
        unique_counts = malloc(nonzero * sizeof(int));
        // ^ We don't actually do anything with this.
        int j = 0;
        for (int i = 0; i < max_int; i++)
        {
            if (count[i])
            {
                unique[j] = i;
                j++;
            }
        }
    }
    // Stop timer:
    clock_t end = clock();
    // Prevent C from cheating, but writing first 10 elements to file:
    FILE *file = fopen("output.txt", "w");
    for (int i = 0; i < 10; i++)
    {
        fprintf(file, "%d\n", unique[i]);
        fprintf(file, "%d\n", unique_counts[i]);
    }
    // Print time:
    printf("Time: %f\n", (double)(end - start) / CLOCKS_PER_SEC);
}