177 lines
2.6 KiB
Perl
Executable File
177 lines
2.6 KiB
Perl
Executable File
#! /usr/local/bin/perl -ws
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#SHARED SYMBOL_TABLE
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my %symbol_table = ();
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package Operation;
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sub new
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{
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my ($class, %args) = @_;
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bless \%args, $class;
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}
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package Assignment_Op; @ISA = qw( Operation );
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sub eval
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{
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my ($self) = @_;
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$symbol_table{$self->{var}->{name}}
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= $self->{value}->eval();
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}
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package Addition_Op; @ISA = qw ( Operation );
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sub eval
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{
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my ($self) = @_;
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return $self->{left}->eval() + $self->{right}->eval();
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}
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package Multiplication_Op; @ISA = qw ( Operation );
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sub eval
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{
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my ($self) = @_;
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return $self->{left}->eval() * $self->{right}->eval();
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}
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package IfThenElse_Op; @ISA = qw ( Operation );
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sub eval
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{
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my ($self) = @_;
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if ($self->{condition}->eval() )
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{
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return $self->{true_expr}->eval();
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}
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else
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{
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return $self->{false_expr}->eval();
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}
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}
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package LessThan_Op; @ISA = qw ( Operation );
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sub eval
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{
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my ($self) = @_;
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return $self->{left}->eval() < $self->{right}->eval();
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}
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package Value_Op; @ISA = qw( Operation );
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sub eval
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{
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my ($self) = @_;
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return $self->{value};
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}
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package Variable_Op; @ISA = qw( Operation );
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sub eval
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{
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my ($self) = @_;
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return $symbol_table{$self->{name}};
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}
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package Sequence_Op;
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sub new
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{
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my ($class, $list_ref) = @_;
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bless $list_ref, $class;
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}
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sub eval
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{
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my ($self) = @_;
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my $last_val;
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foreach my $statement ( @$self )
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{
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$last_val = $statement->eval();
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}
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return $last_val;
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}
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package main;
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use Parse::RecDescent;
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my $grammar = q
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{
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Script: Statement(s) /^$/
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{ Sequence_Op->new( $item[1] ) }
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Statement: Assignment
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| IfThenElse
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| Expression
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| <error>
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Assignment: Variable '<-' Expression
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{ Assignment_Op->new( var => $item[1],
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value => $item[3]) }
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Expression: Product "+" Expression
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{ Addition_Op->new( left => $item[1],
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right => $item[3] ) }
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| Product
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Product: Value "*" Product
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{ Multiplication_Op->new( left => $item[1],
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right => $item[3] ) }
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| Value
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Value: /\d+/
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{ Value_Op->new( value => $item[1] ) }
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| Variable
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Variable: /(?!if)[a-z]/
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{ Variable_Op->new( name => $item[1] ); }
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IfThenElse: 'if' Condition 'then' Statement 'else' Statement
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{ IfThenElse_Op->new( condition => $item[2],
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true_expr => $item[4],
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false_expr => $item[6]) }
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Condition: Expression '<' Expression
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{ LessThan_Op->new( left => $item[1],
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right => $item[3] ) }
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};
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my $parser = Parse::RecDescent->new($grammar)
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or die "Bad grammar";
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local $/;
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my $script = <DATA>;
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my $tree = $parser->Script($script)
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or die "Bad script";
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print $tree->eval();
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__DATA__
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a <- 1
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b <- 2
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if a<b then
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c <- 3
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else
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c <- 99
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b*c+a
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