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Published at Apr 06 2021
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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.

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

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)
"return t if leap year, nil otherwise"
(setq is_leap nil)
(if (eq (mod year 4) 0)
(setq is_leap t)
)
(if (eq (mod year 100) 0)
(setq is_leap nil)
)
(if (and (eq (mod year 4) 0)
(not is_leap)
(eq (mod year 400) 0))
(setq is_leap t))
is_leap
)
(defun leap-year-p (year)
"Steal from python"
(and (eq (mod year 4) 0)
(or (not (eq (mod year 100) 0))
(eq (mod year 400) 0)))
)
(leap-year-p 1999)
(provide 'leap-year-p)
;;; leap.el ends here
```

A huge amount can be learned from reading other peopleâ€™s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

- What compromises have been made?
- Are there new concepts here that you could read more about to improve your understanding?

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