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This iterator is my 'naive' approach to producing derangements via an iterator (at japhy's prompting so he can produce his "Secret Santa" lists). A derangement is a permutation where none of the elements remained in their starting positions.

#!/usr/bin/perl -w use strict; sub derange { my @set= @_; # items (strings) to be deranged my $last= $#set; # last index into our list my @stack= # lists of indices to be tried at each locatio +n [ reverse 0 .. $last ]; my @redo; # lists of indices already tried at each locat +ion my @ret; # offsets to each selected item my $i= 0; # which slot we are trying to fill my $left= $stack[$i]; # indices to consider for the current slot return sub { while( 1 ) { do { if( ! @$left ) { return if --$i < 0; $left= $stack[$i]; push @{$redo[$i]}, $ret[$i]; } if( @$left && $i == $left->[-1] ) { # skip this index as it'd not make a derangement push @{$redo[$i]}, pop @$left; } } while( ! @$left ); $ret[$i]= pop @$left; if( $i == $last ) { return @set[@ret]; } $left= [ @$left, @{$redo[$i]} ]; $redo[++$i]= []; $stack[$i]= $left; } }; } @ARGV= 1..5 if ! @ARGV; my $iter= derange( @ARGV ); my @list; while( @list= $iter->() ) { print "@list\n"; }

- tye        

In reply to Derangements iterator by tye

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