Simple yes, but not quick.

Ok, here is the index-based algorithm:

use strict;
use warnings;

sub first_idx {
    my( $N, $M ) = @_;
    return ( $N-$M .. $N-1 );
}

sub next_idx {
    my( $N, $M, @idx ) = @_;
    return () if $idx[ -1 ] == $M-1;
    my $i = $M-1;
    $i-- while $i>0 and $idx[$i]-$idx[$i-1]==1;
    return ( @idx[0..$i-1], $idx[ $i ]-1, 
         map { $idx[ $i ] + $N - $idx[ $M - $_+ $i ] } $i+1 .. $M-1);
}

my $N = 5;
my $M = 3;
my @indices = first_idx( $N, $M );
1 while print( "@indices\n"), @indices = next_idx( $N, $M, @indices );
[download]

Cool! It even runs in the same direction as my original example.

Though I suspect that once the slice and map operations have been unwound in C, it'll look a lot more complicated.

---

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