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

//#define ENABLE_DEBUGGING_CODE  // define a symbol with no value
// OR compile with:
// $ gcc dbg_log.c -o dbg_log   -DENABLE_DEBUGGING_CODE
//                              └─────────────────────┘
//                     equivalent to #define ENABLE_DEBUGGING_CODE
// 
// // $ is just to remind you that this is something you type in bash.

#ifdef ENABLE_DEBUGGING_CODE
#   define dbg_log_int(n)   (printf("# %s == %d\n", (#n), (n)))
#   define dbg_printf(...)  (printf(__VA_ARGS__))
#else
#   define dbg_log_int(n)
#   define dbg_printf(...)
#endif

// Reminder:  #n means the text of the expression passed to the function-like macro.
// ... and __VA_ARGS__ are for a variadic macro (different from variadic function in HW05).
// They pass any/all arguments to the rhs of the macro.

// #define dbg_log_int(n)
// #define dbg_printf(...)
//
// Invocations to the function-like macro disappear.
// ∙ dbg_log_int(divisor);  ⇒   ;

bool is_prime(int n) {
    bool n_looks_prime = (n % 2) != 0;  // Assume n is prime unless we find that it is divisible
                                        // by something other than 1 or itself.

    // Try all ODD numbers from 3 to n/2
    if(n_looks_prime) {
        for(int divisor = 3; divisor < n / 2; divisor += 2) {

            // printf("# divisor == %d\n", divisor);  // debugging output
            dbg_log_int(divisor);
            dbg_printf("n == %d    divisor == %d\n", n, divisor);

            assert(divisor % 2 == 1);  // "I claim that divisor must be odd."
            if(n % divisor == 0) {
                n_looks_prime = false;
                break;
            }
        }
    }

    assert(n == 2 || n % 2 == 1 || n_looks_prime == false);  // "I claim that n cannot be prime and even unless n is 2."
    assert(n == 2 || n % 2 == 1 || ! n_looks_prime);         // "I claim that n cannot be prime and even unless n is 2."

    return n_looks_prime;
}

int main(int argc, char* argv[]) {
    int n = 13;
    if( is_prime(n) ) {
        printf("%d is prime", n);  // REQUIRED OUTPUT (by specification for program)
    }
    return EXIT_SUCCESS;
}
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