# paulfioravanti's solution

## to Leap in the Elixir Track

Published at Jun 20 2019 · 0 comments
Instructions
Test suite
Solution

#### Note:

This exercise has changed since this solution was written.

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.

## Running tests

Execute the tests with:

``````\$ elixir leap_test.exs
``````

### Pending tests

In the test suites, all but the first test have been skipped.

Once you get a test passing, you can unskip the next one by commenting out the relevant `@tag :pending` with a `#` symbol.

For example:

``````# @tag :pending
test "shouting" do
assert Bob.hey("WATCH OUT!") == "Whoa, chill out!"
end
``````

Or, you can enable all the tests by commenting out the `ExUnit.configure` line in the test suite.

``````# ExUnit.configure exclude: :pending, trace: true
``````

If you're stuck on something, it may help to look at some of the available resources out there where answers might be found.

## 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.exs

``````if !System.get_env("EXERCISM_TEST_EXAMPLES") do
end

ExUnit.start()
ExUnit.configure(exclude: :pending, trace: true)

defmodule LeapTest do
use ExUnit.Case

# @tag :pending
test "vanilla leap year" do
assert Year.leap_year?(1996)
end

@tag :pending
test "any old year" do
refute Year.leap_year?(1997), "1997 is not a leap year."
end

@tag :pending
test "century" do
refute Year.leap_year?(1900), "1900 is not a leap year."
end

@tag :pending
test "exceptional century" do
assert Year.leap_year?(2400)
end
end``````
``````defmodule Year do
@leap_year 4
@centurial_year 100
@leap_cycle_length @leap_year * @centurial_year

defguardp leap?(year)
when rem(year, @leap_year) == 0 and
(rem(year, @centurial_year) != 0 or
rem(year, @leap_cycle_length) == 0)

@doc """
Returns whether 'year' is a leap year.

A leap year 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
"""
@spec leap_year?(non_neg_integer) :: boolean
def leap_year?(year) when leap?(year), do: true
def leap_year?(_year), do: false
end``````