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efim's solution

to Leap in the Emacs Lisp Track

Published at Apr 27 2021 · 0 comments
Test suite

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.


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.


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 --- Tests for Leap exercise (exercism)

;;; Commentary:

;;; Code:
(load-file "leap.el")

(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:
(defun leap-year-p (year)
  (let* ((basic-leap-p (= 0 (% year 4)))
         (century-p (= 0 (% year 100)))
         (quad-tury-fix-p (= 0 (% year 400)))
         (not-exceptional-leap-p (and century-p (not quad-tury-fix-p))))
    (and basic-leap-p (not not-exceptional-leap-p))

(provide 'leap-year-p)
;;; leap.el ends here

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efim's Reflection

Not most beautiful and not most efficient way.
Learned about let and let* functions