#
# This software is Copyright 2005 by Elsevier Inc.  You may use it
# under the terms of the license at http://perl.plover.com/hop/LICENSE.txt .
#



###
### rpn_table
###

## Chapter 2 section 2

my @stack;
my $actions = {
  '+' => sub { push @stack, pop(@stack) + pop(@stack) },
  '*' => sub { push @stack, pop(@stack) * pop(@stack) },
  '-' => sub { my $s = pop(@stack); push @stack, pop(@stack) - $s },
  '/' => sub { my $s = pop(@stack); push @stack, pop(@stack) / $s },
  'NUMBER' => sub { push @stack, $_[0] },
  '_DEFAULT_' => sub { die "Unrecognized token `$_[0]'; aborting" }
};

my $result = evaluate($ARGV[0], $actions);
print "Result: $result\n";

sub evaluate {
  my ($expr, $actions) = @_;
  my @tokens = split /\s+/, $expr;
  for my $token (@tokens) {
    my $type;
    if ($token =~ /^\d+$/) {   # It's a number
      $type = 'NUMBER'; 
    }
  
    my $action = $actions->{$type} 
              || $actions->{$token} 
              || $actions->{_DEFAULT_};
    $action->($token, $type, $actions);
  }
  return pop(@stack);
}