push @{ $h{$_} }, shift @B for (@A, reverse @A) x (@B / @A / 2), (@A, reverse @A)[ 0 .. @B % (2 * @A) - 1 ]; #### for my $i (0 .. $#A) { $h{ $A[$i] } = [ @B[ grep { my $m = $_ % (2 * @A); grep $_ == $m, $i, 2 * @A - $i - 1 } 0 .. $#B ] ]; } #### my $iter = do { my ($pos, $step) = ('0E0', 1); sub { my $r = $H{ $A[$pos] } ||= []; if ($pos % $#A || $pos =~ /E/) { $pos += $step; } else { $pos .= 'E0'; $step *= -1; } return $r } }; push @{ $iter->() }, $_ for @B;