###
### memoize-normalize4
###

## Chapter 3 section 7

sub memoize {
  my ($func, $keygen) = @_;
  my $keyfunc;
  if ($keygen eq '') {
    $keygen = q{join ',', @_} 
  } elsif (UNIVERSAL::isa($keygen, 'CODE')) {
    $keyfunc = $keygen;
    $keygen = q{$keyfunc->(@_)};
  }
  my %cache;
  my $newcode = q{ 
    sub { my $key = do { KEYGEN };
          $cache{$key} = $func->(@_) unless exists $cache{$key};
          return $cache{$key};
        }
  };
  $newcode =~ s/KEYGEN/$keygen/g;
  return eval $newcode;
}