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From: Ralph Corderoy <ralph@inputplus.co.uk>
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Hi Bakul,

> For permutations I just use this k program:
>
> $ cat perm.k
> p:{:[1<x;,/(>:'(x,x)#1,x#0)[;0,'1+_f x-1];,!x]}
> perm:{x@p[#x]}

I was thinking it looks like APL and it turns out to be a descendant.
https://en.wikipedia.org/wiki/K_(programming_language)

That pushed me into writing an awk-permutation generator.  It's
inefficient as it keeps altering $0 which cascades into re-working $1,
etc., even if that's lazily done.  Still, it sufficies for the small
cases I typically need.

    $ cat perm
    #! /bin/awk -f

    function perm(s,   f, l, t) {
        if (!NF) {
            print substr(s, 2)
            return
        }
        l = $0
        for (f = 1; f <= NF; f++) {
            t = $f
            $f = ""; $0 = $0
            perm(s " " t)
            $0 = l
        }
    }

    { perm("") }
    $ 
    $ ./perm <<<''

    $ ./perm <<<1
    1
    $ ./perm <<<'1 2'
    1 2
    2 1
    $ ./perm <<<'1 2 3'
    1 2 3
    1 3 2
    2 1 3
    2 3 1
    3 1 2
    3 2 1
    $ 
    $ time ./perm <<<'1 2 3 4 5 6 7 8 9' | wc -l
    362880

    real    0m5.421s
    user    0m5.406s
    sys     0m0.082s
    $
    $ dc <<<'9 8*7*6*5*4*3*2*p'
    362880
    $

-- 
Cheers, Ralph.