My basic idea is to map the array indices to bits in a binary number. If a bit is on, you take that element out of the source array. For example: 
 0 = 0b0000 --> {nothing}
 3 = 0b0011 --> 'horse:cow'
13 = 0b1101 --> 'horse:dog:cat'

[download]
The algorithm then simply becomes a loop over 1 .. 2**@kw -1, testing the bits for each number.

Here's my first implementation of it. It's probably not as efficient as possible yet. 

sub rhesa {
    # initial source in sequence order
    my $source = 'horse:cow:dog:cat';

    my @kw = split /:/, $source;
    my @res;
    
    for my $i( 1 .. 2**@kw - 1 ) {
        my @ar; my $t;
        while( $i > 0 ) {
            push @ar, $kw[$t] if $i & 1;
            $i >>= 1; $t++;
        }
        push @res, join ':', @ar;
    }

    return @res;
}

[download]
I'm a bit irritated with the number of temporary variables, but I can't think of anything prettier just now. Hope it helps :)

BTW, a simple Benchmark comparison showed a 200% speed increase over your version.

In reply to Re: possible combinations in sequence by rhesa
in thread possible combinations in sequence by ruzam

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