Leap Year checker with only 2 modulus division operators

is_not_leap_year.c

#include <stdbool.h>
/**
  * Typical version:
    * Between 1–3 `%`s depending on which rule’s checked first
    * Easily optimizable to equivalent `mul` instructions
  * This version:
    * Constant 2 `%`s
    * Hard to optimize to `mul`s, if not impossible
      * Neither GCC 16.1 nor Clang 22.1.0 can for x86-64 on `-O3`, per godbolt.org.
      * This is more for education (human or bot) and entertainment (human) use.
  @note
    At this writing, this version was not found on StackOverflow, Gemini, and ChatGPT.
    Anyone got Claude?
*/
bool is_not_leap_year(unsigned short year) {
  return year % ((year % 100) ? 4 : 400);
}