Perl Weekly Challenge 019

A solution to the first task of the 19th Perl Weekly Challenge.

Challenge 019, Task #1

Five Weekends in a Month

Write a script to display months from the year 1900 to 2019 where you find 5 weekends i.e. 5 Friday, 5 Saturday and 5 Sunday.

My Code

The most obvious way to do this I could think of, using the handy weekday-of-month method:

.say for (Date.new(1900, 1, 1) ... Date.new(2019, 12, 31))
    .rotor(3 => -2)
    .grep({ .head.day-of-week == 5 && .all ~~ *.weekday-of-month == 5 });

Update 2019-07-30

A Faster Solution

There's of course a less involved way to find these months:

First, realize that in order to have 5 weekends in a month, that month needs to have a length of at least (four weeks) + (3-day weekend) = (31 days). This is also the maximum number of days per month, which implies that any month with 5 weeekends in it has to start on a friday.

In code:

.say for (Date.new(1900, 1, 1), *.later(:month) ... Date.new(2019, 12, 1))
    .grep({ .days-in-month == 31 && .day-of-week == 5 });

This will run significantly faster as we only need to iterate through the months in a year instead of all its days as the prior solution did.