C++ Insights

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///usr/bin/env ccache gcc -Wall -Wextra -Werror -O3 -std=gnu17 "$0" -o /tmp/a -lm && /tmp/a "$@"; exit
// For the line just above, see my answer here: https://stackoverflow.com/a/75491834/4561887

#include <limits.h>
#include <stdbool.h> // For `true` (`1`) and `false` (`0`) macros in C
#include <stdint.h>  // For `uint8_t`, `int8_t`, etc.
#include <stdio.h>   // For `printf()`

#define TEST_EQ(func, num1, num2, equals) \
    printf("%s\n", func((num1), (num2)) == (equals) ? "Passed" : "FAILED")

/// Safely and efficiently return `abs((int8_t)num1 - (int8_t)num2)`
uint8_t abs_num1_minus_num2_int8(int8_t num1, int8_t num2)
{
    // Note: due to implicit type promotion rules, rule 3 in my answer here
    // (https://stackoverflow.com/a/72654668/4561887) applies, and the `>` comparison, as well as
    // subtraction, takes place below as `int` types. While signed `int` overflow and underflow is
    // undefined behavior, none of that occurs here. It's just useful to understand that even
    // though we are doing `(uint8_t)num1 - (uint8_t)num2`, the compiler really sees it as this:
    // `(int)(uint8_t)num1 - (int)(uint8_t)num2`.
    uint8_t abs_diff = num1 > num2 ?
        (uint8_t)num1 - (uint8_t)num2 :
        (uint8_t)num2 - (uint8_t)num1;

// debugging
    printf("num1 = %4i (%3u); num2 = %4i (%3u); num1-num2=%3u;  ",
        num1, (uint8_t)num1, num2, (uint8_t)num2, abs_diff);

return abs_diff;
}

/// Safely and efficiently return `abs((int)num1 - (int)num2)`
unsigned int abs_num1_minus_num2_int(int num1, int num2)
{
    unsigned int abs_diff = num1 > num2 ?
        (unsigned int)num1 - (unsigned int)num2 :
        (unsigned int)num2 - (unsigned int)num1;

// debugging
    printf("num1 = %11i (%10u); num2 = %11i (%10u); num1-num2=%10u;  ",
        num1, (unsigned int)num1, num2, (unsigned int)num2, abs_diff);

return abs_diff;
}

int main()
{
    printf("Absolute difference tests.\n");

// ---------------
    // int8_t types
    // ---------------

int8_t num1_8;
    int8_t num2_8;

printf("\n");
    printf("INT8_MIN = %i, INT8_MAX = %i\n", INT8_MIN, INT8_MAX); // -128, 127

num1_8 = -7;
    num2_8 = -4;
    TEST_EQ(abs_num1_minus_num2_int8, num1_8, num2_8, 3);

num1_8 = INT8_MIN;
    num2_8 = INT8_MAX;
    TEST_EQ(abs_num1_minus_num2_int8, num1_8, num2_8, UINT8_MAX);

num1_8 = INT8_MAX;
    num2_8 = INT8_MIN;
    TEST_EQ(abs_num1_minus_num2_int8, num1_8, num2_8, UINT8_MAX);

num1_8 = 100;
    num2_8 = 10;
    TEST_EQ(abs_num1_minus_num2_int8, num1_8, num2_8, 90);

num1_8 = 10;
    num2_8 = 100;
    TEST_EQ(abs_num1_minus_num2_int8, num1_8, num2_8, 90);

num1_8 = 10;
    num2_8 = 10;
    TEST_EQ(abs_num1_minus_num2_int8, num1_8, num2_8, 0);

num1_8 = INT8_MAX;
    num2_8 = 1;
    TEST_EQ(abs_num1_minus_num2_int8, num1_8, num2_8, 126);

num1_8 = 1;
    num2_8 = INT8_MAX;
    TEST_EQ(abs_num1_minus_num2_int8, num1_8, num2_8, 126);

// ---------------
    // int types
    // ---------------

int num1;
    int num2;

printf("\n");
    printf("INT_MIN = %i, INT_MAX = %i\n", INT_MIN, INT_MAX); // -2147483648, 2147483647

num1 = -7;
    num2 = -4;
    TEST_EQ(abs_num1_minus_num2_int, num1, num2, 3);

num1 = INT_MIN;
    num2 = INT_MAX;
    TEST_EQ(abs_num1_minus_num2_int, num1, num2, UINT_MAX);

num1 = INT_MAX;
    num2 = INT_MIN;
    TEST_EQ(abs_num1_minus_num2_int, num1, num2, UINT_MAX);

num1 = 100;
    num2 = 10;
    TEST_EQ(abs_num1_minus_num2_int, num1, num2, 90);

num1 = 10;
    num2 = 100;
    TEST_EQ(abs_num1_minus_num2_int, num1, num2, 90);

num1 = 10;
    num2 = 10;
    TEST_EQ(abs_num1_minus_num2_int, num1, num2, 0);

num1 = INT_MAX;
    num2 = 1;
    TEST_EQ(abs_num1_minus_num2_int, num1, num2, 2147483646);

num1 = 1;
    num2 = INT_MAX;
    TEST_EQ(abs_num1_minus_num2_int, num1, num2, 2147483646);

return 0;
}

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