Perl Weekly Challenge : Week 48

scimon profile image Simon Proctor ・3 min read

I've decided I'm going to try blogging about my attempts at the Perl Weekly Challenge which I've been doing since it started. Generally I do my challenge in Raku the language formally known as Perl6.

This weeks challenge has a couple of fun parts. If you've not tried it yet you might want to come back after you have.

Challenge 1

So I'd seen a video on this before it's called the Josephus Problem so I knew there was a mathematical solution to it.... but I couldn't remember it.

As is often the way I went a bit above and beyond and worked out a function to give the survivor number for any number of swordsmen. My thinking about it was simple. Get a list of number from 1 to 50, take the first two from the front. The first survives the second is dead. Put the survivor at the end of the list. Repeat until you have 1 survivor :

#| Calculate the survivor of the swordsmen suicide pact
multi sub MAIN(
    UInt $swords = 50, #= Number of swordsmen (default 50)
) {
    my @men = [1..$swords];
    while ( @men.elems > 1 ) {
        my ( $alive, $dead ) = @men.splice(0,2);

    say "Survivor of $swords is number {@men[0]}";

With that I was able to run the test for lots of different value and worked out the mathematical solution. Given a number of swordsmen (s) we find p where p < s and p is a power of 2. Then the number of the survivor is the nth odd number where n = s - p.

For example for s = 50, p = 32 and n is the 18th odd number (indexed on 0) which is 37.

The code for this :

#| Calculate mathematically
multi sub MAIN(
    UInt $swords = 50, #= Number of swordsmen
) {
    my $low-power = (1,* * 2...*).first(* > $swords) div 2;
    say "Survivor of $swords is number {(1,3,5...*)[$swords - $low-power]}";

Couple of fun uses of Raku sequences one to generate the powers of 2 and the second to generate the list of odd numbers.

Challenge 2

For challenge 2 I initially worked on the theory I'll look at every date between 2020-01-01 and 2999-12-31 and look for palindromes. Turns out... that's a lot of days (357937 to be exact) and it take a while. Even when you use .hyper to thread the tests over multiple cores. But then I had a thought.

If I went through every month and day combination that's 12 x 31 (we'll worry about out of range values in a second) and that's only 372 to check. If we take the month and day and flip it to get a year if we can check it's valid and it's between our start and end dates.

#| Find the palendromic numbers (writen mmddyyy) between 2000-01-01 and 2999-01-01
sub MAIN() {
    my sub df( Date $d) {
        # Bleh American dates
        sprintf "%02d%02d%04d", .month, .day, .year given $d;

    constant START = Date.new(2000,1,1,formatter => &df);
    constant END = Date.new(2999,12,31, formatter => &df);

    my @out;

    for (1..12) -> $month {
        for (1..31) -> $day {
            my $date;
            my $year = sprintf( "%02d%02d", $month, $day ).flip;
            try {
                $date = Date.new($year,$month,$day,formatter => &df);
            next unless $date;
            next unless START <= $date <= END;

    .say for @out.sort;

I hope that all makes sense. Feel free to drop a comment if you've got questions or thoughts.

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scimon profile

Simon Proctor


Was an Englishman in Scotland but now back in England. Gamer, coder and voracious devourer of information. Occasionally writes stuff.


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