P13 - Run-length encoding of a list (direct solution).

Author: Philip Potter

We use gather as a loop modifier, to gather a list over all the loop iterations

Specification

P13 (**) Run-length encoding of a list (direct solution).
    Implement the so-called run-length encoding data compression method
    directly. I.e. don't explicitly create the sublists containing the
    duplicates, as in problem P09, but only count them. As in problem P11,
    simplify the result list by replacing the singletons [1,X] by X.

Example

> encode_direct().perl.say
([4, "a"], "b", [2, "c"], [2, "a"], "d", [4, "e"])

Source code: P13-rhebus.pl

use v6;

sub runlength (@a) {
    gather {
        last if !@a.elems;

        my $val = @a.shift;
        my $num = 1;
        while @a[0] ~~ $val { @a.shift; $num++; }
        take $num == 1 ?? $val !! [$num,$val];
    }
}

my @l = ;

say ~@l;
runlength(@l).list.perl.say;