First, this is not about the standard answer about inverting a hash using 'reverse' and how non-unique values clobber each other in the new hash. I'm writing about efficiency. I frequently find myself using code that looks like this: 
sub invert {
 my $x = shift;
 my $y = {};
 foreach my $xk1 (keys %$x) {
  foreach my $xk2 (keys %{$x->{$xk1}}) {
   $y->{$xk2}{$xk1} = $x->{$xk1}{$xk2};
  }
 }
 return $y;
}

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Basically, I'm just rearranging a hash, for example, if I have a hash of 'days' of 'hours' of 'something', and I want to rearrange into 'hours' of 'days' of something. Is there a better/more perly way to do this? A function call to a c-compiled call? Something that does not require foreaches to essentially claw through every element in perl. I'm doing this a lot, on some very large data structures. It's not particularly slow, but still... Also, I'm trying to minimize unecessary copying. The code above at least only copies references, if that's what exist past the first two levels, but no copying at all would be best.

In reply to Invert a hash... not a FAQ (I hope) by djacobow

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