Merge Sort Modified - Sort array in place

mergeSort.c

void merge_sort(int size, int (*comparator)(int, int), int arr[]);
int increasing(int a, int b);
int decreasing(int a, int b);

int main(void) {
//    int test[7] = {5, 23, 7, 26, 17, 2, 9};
    int test[21] = {21, 20, 15, 17, 1, 7, 3, 1, 56, 48,
	832, 38, 20, 19, 39, 40, 7, 38, 19, 28, 38};
    merge_sort(21, increasing, test);
    for(int i = 0; i < 21; i++) {
	printf("%i, ", test[i]);
    }
    printf("\n");
}

int increasing(int a, int b) {
    return a - b;
}

int decreasing (int a, int b) {
    return b - a;
}

// Sorts based on what the comparator returns.
// < 0, > 0, or 0 for equality
void merge_sort(int size, int (*comparator)(int, int), int arr[]) {
    // Thid defines sorting in increasing or decreasing order
    if(size <= 1)
	return;
    
    int mid = size / 2;

    int *lhs = &arr[0];
    int *rhs = &arr[mid];

    merge_sort(rhs - lhs, comparator, lhs); // left half
    merge_sort(&arr[size] - rhs, comparator, rhs); // right half

    int placeholder = 0;

    while(lhs < &arr[mid] && rhs < &arr[size]) {
	int comp = comparator(*lhs, *rhs);
	if(comp == 0) {
	    if(lhs < &arr[mid])
		lhs++;
	    else if(rhs < &arr[size])
		rhs++;
	    else {
		// Since both pointers are at the end of their own half
		// We are done
		break;
	    }
	}
	else if(comp < 0) {
	    lhs++;
	    while(lhs < &arr[mid] && comparator(*lhs, *rhs) < 0) {
		lhs++;
	    }
	}
	else if(comp > 0) {
	    placeholder = *lhs;
	    *lhs = *rhs;
	    *rhs = placeholder;
	    lhs++;
	    int *temp_r = rhs;
	    while(temp_r < &arr[size - 1] && comparator(*temp_r, *(temp_r + 1)) > 0) {
		placeholder = *temp_r;
		*temp_r = *(temp_r + 1);
		*(temp_r + 1) = placeholder;
		temp_r++;
	    }
	}
    }
}