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# jpleboeuf's solution

## to Leap in the Emacs Lisp Track

Published at Apr 10 2021 · 0 comments
Instructions
Test suite
Solution

Given a year, report if it is a leap year.

The tricky thing here is that a leap year in the Gregorian calendar occurs:

``````on every year that is evenly divisible by 4
except every year that is evenly divisible by 100
unless the year is also evenly divisible by 400
``````

For example, 1997 is not a leap year, but 1996 is. 1900 is not a leap year, but 2000 is.

If your language provides a method in the standard library that does this look-up, pretend it doesn't exist and implement it yourself.

## Notes

Though our exercise adopts some very simple rules, there is more to learn!

For a delightful, four minute explanation of the whole leap year phenomenon, go watch this youtube video.

## Source

JavaRanch Cattle Drive, exercise 3 http://www.javaranch.com/leap.jsp

## Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

### leap-test.el

``````;;; leap-test.el --- Tests for Leap exercise (exercism)

;;; Commentary:

;;; Code:

(ert-deftest vanilla-leap-year ()
(should (leap-year-p 1996)))

(ert-deftest any-old-year ()
(should-not (leap-year-p 1997)))

(ert-deftest non-leap-even-year ()
(should-not (leap-year-p 1997)))

(ert-deftest century ()
(should-not (leap-year-p 1900)))

(ert-deftest exceptional-century ()
(should (leap-year-p 2000)))

(provide 'leap-test)
;;; leap-test.el ends here``````
``````;;; leap.el --- Leap exercise (exercism)

;;; Commentary:

;;; Code:

(defsubst divp (a b)
(zerop (% a b))
)

(defsubst !divp (a b)
(not (divp a b))
)

(defun leap-year-p (y)
(and
(divp y 4)
(or
(!divp y 100)
(divp y 400)
)
)
)

(provide 'leap-year-p)

;;; leap.el ends here``````