#!/usr/bin/perl -w

use strict;
use Parse::RecDescent;
use Data::Dumper;

use vars qw(%VARIABLE);

# Enable warnings within the Parse::RecDescent module.

$::RD_ERRORS = 1; # Make sure the parser dies when it encounters an error
$::RD_WARN   = 1; # Enable warnings. This will warn on unused rules &c.
$::RD_HINT   = 1; # Give out hints to help fix problems.

my $grammar = <<'_EOGRAMMAR_';

  # Terminals (macros that can't expand further)
  #

  OP       : m([-+*/%])      # Mathematical operators
  INTEGER  : /[-+]?\d+/      # Signed integers
  VARIABLE : /\w[a-z0-9_]*/i # Variable

  expression : INTEGER OP expression
             { return main::expression(@item) }
             | VARIABLE OP expression
             { return main::expression(@item) }
             | INTEGER
             | VARIABLE
             { return $main::VARIABLE{$item{VARIABLE}} }

  print_instruction  : /print/i expression
                     { print $item{expression}."\n" }
  assign_instruction : VARIABLE "=" expression
                     { $main::VARIABLE{$item{VARIABLE}} = $item{expression} }

  instruction : print_instruction
              | assign_instruction

  startrule: instruction(s /;/)

_EOGRAMMAR_

sub expression {
  shift;
  my ($lhs,$op,$rhs) = @_;
  $lhs = $VARIABLE{$lhs} if $lhs=~/[^-+0-9]/;
  return eval "$lhs $op $rhs";
}

my $parser = Parse::RecDescent->new($grammar);

#These Work as Expected
print "a=2\n";             $parser->startrule("a=2");
print "a=1+3\n";           $parser->startrule("a=1+3");
print "print 5*7\n";       $parser->startrule("print 5*7");
print "print 2/4\n";       $parser->startrule("print 2/4");
print "print 2+2/4\n";     $parser->startrule("print 2+2/4");
print "print 2+-2/4\n";    $parser->startrule("print 2+-2/4");
print "a = 5 ; print a\n"; $parser->startrule("a = 5 ; print a");

#This works too...
print "a = 5; b = 2; print a+b\n"; $parser->startrule("a = 5; b=2; print a+2");

#These Do not.
#print bug
print "a = 5; b = 2; print a; print 1+1\n"; $parser->startrule("a = 5; b=2; print a; print 1+1");
#assign bug
print "a = 5+1 ; print a\n"; $parser->startrule("a = 5+1 ; print a");