A leap year check in three instructions

> Note that modern compilers like gcc or clang will produce something like is_leap_year2 from is_leap_year1, so there is not much point in doing this in C source, but it might be useful in other programming languages.

The optimizations that compilers can achieve kind of amaze me.

Indeed, the latest version of cal from util-linux keeps it simple in the C source:

  return ( !(year % 4) && (year % 100) ) || !(year % 400);

https://github.com/util-linux/util-linux/blob/v2.41/misc-uti...

But this is wrong and can only represent dates after the specific year when they switched from the Julian to Gregorian calendar!

For more on this, I recommend reading and implementing a function that calculates the day of the week [1]. Then you can join me in the special insanity hell of people that were trying to deal with human calendars.

And then you should implement a test case for the dates between Thursday 4 October 1582 and Friday 15 October 1582 :)

[1] https://en.m.wikipedia.org/wiki/Determination_of_the_day_of_...