Re: your ps - see my ngram2number routine above. It does exactly that calculation.

Update: after re-reading my original message, I note that you also need to translate a list of characters into an alphabet to use the function (just in case it was not understood from the original code). You can do that with:

sub charlist2alphabet {
    my %results;
    for my $index ( 0 .. (@_ - 1) ) {
        $results{ $_[ $index ] } = $index;
    }
    \%results;
}

my $alphabet = charlist2alphabet( @charlist );
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my $alphabet = { map { ( $charlist[ $_ ] => $_ ) } ( 0 .. @charlist - 
+1 );
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