integer_overflows.c

#include <stdio.h>
#include <stdint.h>
#define u32 uint32_t

// raw difference wrap around from left -> right
// on a number line, if I can go in one direction (towards positive), how many steps from left -> right
uint32_t dif32(uint32_t  left, uint32_t  right) {
    uint32_t max32 = ((uint32_t )~0U);
    if ( right > left ) {
        return (right-left);
    }
    return (max32 - left) + right;
}

/* must handle integer wraparound*/
uint64_t mymax64_wrap(uint64_t a, uint64_t b) {
    uint32_t a32 = (uint32_t)a;
    uint32_t b32 = (uint32_t)b;
    uint32_t left_to_right = dif32(a32, b32); // raw dif from a to b
    uint32_t right_to_left = dif32(b32, a32); // raw dif from b to a
    // less steps from b to a then a to b -> so order must be (b,a)
    if ( right_to_left < left_to_right ) { 
        return (uint64_t)a32;
    }
    // less steps from a to b then b to a -> so order must be (a,b)
    return (uint64_t)b32;
}

int main()
{   
    uint64_t a;
    uint64_t b;
    uint64_t max32 = (uint64_t)((uint32_t )~0U);
    uint64_t max_sub_4 = max32 - 4;
    
    printf("Hello World\n");
    // test 1:
    a = 10;
    b = 6;
    printf("TEST 1\n");
    printf("a: %llu, b:%llu, max: %llu\n", a, b, mymax64_wrap(a, b));
    
    a = 6;
    b = 10;
    printf("TEST 2\n");
    printf("a: %llu, b:%llu, max: %llu\n", a, b, mymax64_wrap(a, b));
    
    a = max_sub_4;
    b = 10;
    printf("TEST 3\n");
    printf("a: %llu, b:%llu, max: %llu\n", a, b, mymax64_wrap(a, b));

    b = max_sub_4;
    a = 10;
    printf("TEST 3\n");
    printf("a: %llu, b:%llu, max: %llu\n", a, b, mymax64_wrap(a, b));
    return 0;
}