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Comment by sebastialonso | Hacker News Reader

Wow the way this is presented is super confusing, to the point that your description includes a contradiction. Specifically, of a year is divisible by 400 it's also divisible by 100, so it's both a leap year and not leap year!

Instead of showing rules as a linear list, I'd suggest it's easier to visualise it as a tree, or simpler still, as a series of nested ifs.

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I mean, that contradiction is also in the first two rules. It's a leap year if it's divisible by 4, but not if it's divisible by 100, so what about 200?

Best to describe it as a flowchart

* Is the year divisible by 400? YES -> Leap year

* NO -> Is the year divisible by 100? YES -> Not leap year

* NO -> Is the year divisible by 4? YES -> Leap year

* NO -> Not leap year

Pedantic but this drives me nuts - this algorithm has to do 3 divisions for almost every year. If you flip it upside down, it's significantly faster.

* Is the year divisible by 4? NO -> not leap year

* YES -> Is the year divisible by 100? NO -> leap year

* YES -> Is the year divisible by 400? NO -> not leap year

* YES -> leap year

This way you do only 1 division for 75% of years.

Fortunately, it's not a very big N. ;)

> Instead of showing rules as a linear list, I'd suggest it's easier to visualise it as a tree, or simpler still, as a series of nested ifs.

Feel free to post a comment with ASCII art or (pseudo-)code. :)

(gets 1978 copy of The C Programming Language off the shelf)

   if (year % 4 == 0 && year % 100 != 0 || year % 400 == 0)
      [it's a leap year]
   else
      [it's not]

I remember this because it is where I first learned the extra rules, beyond just every 4th year.

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