Perl Weekly Challenge 97: flipping and swapping

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 97.

My eyes…

I’m waiting and hoping for another consultancy, the next week, by another specialied center.

PWC 97 - Task 1

The first task was about to implement a Caesar Cipher, that is to translated every single letter in a phrase with a swapped one in the alphabet.
The idea was to have a program that with a line like the following could do the trick:

print %cipher{ $_ }:exists ?? %cipher{ $_ } !! $_ for $S.comb;

where %cipher is a shifted alphabet where every single letter of an original alphabet is transposed to a different letter. For example, assuming you are ciphing with 3 the letter A is translated to X, B to Y and so on. In the above, the assumption is that the %cipher hash contains the key as the unencrypted letter and the value as the encrypted one, such as %cipher{ 'A' } = 'X'.
Now, the main part is to build the %cipher hash. In the beginning I took a quite simple and straightforward approach like:

my $index = @alphabet.elems - $N;
for @alphabet {
    %cipher{ $_ } = @alphabet[ $index ];
    $index = $index + 1 < @alphabet.elems ?? $index + 1 !! 0 ;

Having @alphabet being the standard alphabet, and $N the cypher offset, every element in the %cipher hash is keyed to the current alphabet letter and the modulo index of the alphabet itself.
Then, I remembered about the rotate function in an array, that does exactly what the above code does. Therefore, the whole program becomes:

          Int $N where { $N > 0 && $N < ( 'A' .. 'Z' ).elems } = 3 ) {
    my @alphabet = 'A' .. 'Z';
    my %cipher;

    %cipher{ @alphabet[ $_ ] } = @alphabet.rotate( $N * -1  )[ $_ ] for ^@alphabet.elems;

    say "Encoding $S";
    print %cipher{ $_ }:exists ?? %cipher{ $_ } !! $_ for $S.comb;
    say "\ndone";

With a single line I can initialize the %cipher hash using the rotate method and extracing the current element corresponding to a letter. Of course, invoking @alphabet.rotate( $N * -1 )[ $_ ] on every single initialization is expensive, therefore materializing the rotation could improve the script performances.

PWC 97 - Task 2

The second task was about computing the number of flips required to obtain a binary string, previously splitted. The main idea, as far as I understand from the examples, is to split a binary string into chuks, then take the first chunk and see how many bit changes are required to produce all subsequent chunks.
I implemented it as follows:

sub MAIN( Str $B = "101100101",
          Int $S = 3 ) {

    my @splits = $B.comb: $S.Int;
    my $flips = 0;

    my ( $a, $b ) = @splits[ 0 ], Nil;

    for 1 ..^ @splits.elems  {
        $b = @splits[ $_ ];

        # find out how many chars are different
        $flips++ if ( $a.comb[ $_ ] != $b.comb[ $_ ] ) for ^@$a.comb.elems;

    say $flips;

The idea is to create first the array of chunks, named @splits. Then $a is the first chunk and $b is the next chunk to analyze. For every character that is not the same (on the same position) between the two chunks, I increment the $flips counter.

The article Perl Weekly Challenge 97: flipping and swapping has been posted by Luca Ferrari on January 25, 2021