Problem Statement

Given a symbol table of a zero-indexed array of all possible input symbols this algorithm reversibly transforms a sequence of input symbols into an array of output numbers (indices). The transform in many cases acts to give frequently repeated input symbols lower indices which is useful in some compression algorithms.

Encoding the string of character symbols 'broood' using a symbol table of the lowercase characters   a-to-z

Decoding the indices back to the original symbol order:

The strings are:

(Note the misspellings in the above strings.)

Let's start with the solution:

sub encode ( Str $word ) {
    my @sym = 'a' .. 'z';
    gather for $word.comb -> $c {
	die "Symbol '$c' not found in @sym" if $c eq @sym.none;
        @sym[0 .. take (@sym ... $c).end] .= rotate(-1);
    }
}
 
sub decode ( @enc ) {
    my @sym = 'a' .. 'z';
    [~] gather for @enc -> $pos {
        take @sym[$pos];
        @sym[0..$pos] .= rotate(-1);
    }
}
 
use Test;
plan 3;
for  -> $word {
    my $enc = encode($word);
    my $dec = decode($enc);
    is $word, $dec, "$word.fmt('%-12s') ($enc[])";
}