order of permutations about roast HOT 3 OPEN

👍 on the sorting. also, there really ought to be more tests. tests for 0 or negative numbers, tests for other stuff in lists, tests for other kinds of list (ranges for example. Buf/Blob perhaps?)

from roast.

I'm not sure how permutations is supposed to behave for zero or negative numbers. I don't think such values should be allowed.

Also permuting lists only uses slices made from permutations(Int $), so it really is the only thing that really needs testing.

We should test for larger value than 3 though, especially now that it is getting faster.

sub slow-permutations($n) {
    $n == 1 ?? ((0,),) !!
    gather for ^$n -> $i {
        my @omitted = grep none($i), ^$n;
        take ($i, |@omitted[@$_]) for slow-permutations($n - 1);
    }
}

use Test;

my $n = 4;
is permutations($n).sort(:by({ .join })).perl, slow-permutations($n).sort(:by({.join})).perl;

from roast.

There should be 1 permutation of 0 things, as there is exactly one way to order it.

This should hold true for all $n >= 0

+permutations($n) == factorial($n)

factorial 0 is defined as returning 1

I think the only thing that makes sense is for it to return a sequence containing a single empty list. ((),).Seq

from roast.