Perl Weekly Challenge 103: Chinese Calendar and streaming players

One way to let me improve my knowledge about Raku (aka Perl 6) is to implement programs in it. Unluckily, I don’t have any production code to implement in Raku yet (sob!). So, why not try solving the Perl Weekly Challenge tasks?

In the following, the assigned tasks for Challenge 103.

Seems I missed a couple of PWCs

Yes, and that was on purpose because I was really busy with a professional PostgreSQL training course, so I did not have enough time to get some extra stuff.

PWC 103 - Task 1

The first task was about producing a Chinese Calendar script, something that could determine the Chinese year. The hardest part was about understanding how the Chinese Calendar works, and I’m not really sure I got the point but it seems to work.

multi sub MAIN( Int $year where { $year > 1900 } ) {
    my @animals = qw/ Rat Ox Tiger Rabbit Dragon Snake Horse Goat Monkey Rooster Dog Pig /;
    my @elements = qw/ Wood Fire Earth Metal Water /;

    say "Year $year is %s %s".sprintf:
        @elements[ ( $year - 4 ) % 10 / 2 ],
        @animals[ ( $year - 4  ) % 12 ];

multi sub MAIN() {
    MAIN( );

The idea is quite simple: compute a modulo on the year and display the animal and the element of the corresponding computation.
The only interesting part is that I decided to produce a multi MAIN so that you can let the script desume the Chinese year depending on the current date and time. It is interesting to note that the Chinese Calendar starts on February, but I assume this level of detail is not required since the input requested is just the year and not the exact day of the year.

A class-ish implementation

Why not produce a stand-alone class to interact with the Chinese Calendar? That would be something as:

class ChineseCalendar {
    has DateTime $!now;
    has Str @!animals;
    has Str @!elements;

    submethod BUILD( :$year = Nil ) {
        @!animals  = qw/ Rat Ox Tiger Rabbit Dragon Snake Horse Goat Monkey Rooster Dog Pig /;
        @!elements = qw/ Wood Fire Earth Metal Water /;
        $!now      = $year ?? year => $year !!;

    method element() {
        return @!elements[ ( $!now.year - 4 ) % 10 / 2 ];

    method animal() {
        return @!animals[ ( $!now.year - 4 ) % 12 ];

Probably there is no need to keep the DateTime object, but my idea is that this can help in handling the exact year change related to February.
The program therefore changes as:

multi sub MAIN( Int $year where { $year > 1900 } ) {
    my $cc = year => $year;

    say "Year $year is %s %s".sprintf:
        $cc.element, $cc.animal;

multi sub MAIN() {
    MAIN( );

PWC 103 - Task 2

The second task at glance frightnened me: the explaination was too much longer than the required task, at least this is what I can understand. You have a playback player that loops thru songs, and given the start time, the current time and the list of songs, you have to understand what has been played right now.
My idea was to lookup tracks into a @tracks array, where each entry is an hash with the duration time and title. Then I iteratively add the length of every song to the start time and see if it reaches the now time, or better I see when the difference between the current time and the start time reaches zero: this means I’m playing the current song.

sub MAIN( Int $start-ms where { $start-ms > 0 },
          Int $now-ms where { $now-ms >= $start-ms },
          Str $filename ) {

    # lookup file name and tracks
    my @tracks;
    for $filename.IO.lines {
        my ( $ms, $track ) = .split: ',';
         @tracks.push: { time => $ms,
                         track => $track };

    # try to reach the end of the time
    my $diff = $now-ms - $start-ms;
    my $last-track = 0;
    while ( $diff > 0 ) {
        $last-track = 0;
        for @tracks.kv -> $index, $track {
            $diff -= $track< time >;
            last if ( $diff <= 0 );
            $last-track = $index;

    "Now playing { @tracks[ $last-track ]<track> } ".say;

There are different ways this can be improved, for example computing the total amount of the playlist duration and see if that is lower than the difference between times: this can speedup the process by reducing the amount of iterations. However, for a quick and dirt solution, a nested loop does suffice.

The article Perl Weekly Challenge 103: Chinese Calendar and streaming players has been posted by Luca Ferrari on March 8, 2021