#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <assert.h>

int round_down(int n, int multiple_of) {
    // int n_rounded_down = (n / multiple_of) * multiple_of; // CORRECT
    
    int n_rounded_down = (n * multiple_of) / multiple_of; // BUG!!!
    
    // Sanity check: 'n_rounded_down' should be a multiple of 'multiple_of'
    assert(n_rounded_down % multiple_of == 0);

    // Q: Will the assert(…) slow down execuation of my program?
    // A: Yes, but only in normal testing.  If you compile with
    //    gcc … -DNDEBUG then all asserts will go away.  Essentially,
    //    they become just a semicolon on its own which does nothing.

    return n_rounded_down;
}

int main(int argc, char* argv[]) {
    
    printf("round_down(7, 3) == %d  (expected: 6)\n",
            round_down(7, 3));

    printf("round_down(8, 3) == %d  (expected: 6)\n",
            round_down(8, 3));

    return EXIT_SUCCESS;
}
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