From: Ed Brannin <edbrannin@...>
Date: 2006-11-06T09:32:36+09:00
Subject: Re: [QUIZ] Bytecode Compiler (#100)

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This is my first RubyQuiz submission.  Like Cameron Pope, I decdied to let
Ruby's parser do the heavy lifting by monkey-patching a to_expr onto Integer
and running a regex on Compiler::compile's input.

This differes from his entry by using method_missing to handle all
operators, and I didn't bother with separate number/expression classes.

Also, his byte conversion looks much shorter -- I'll have to see how it
works and wrap my head around Array#pack.

I've dabbled in Ruby for a while, but I still feel like I'm at the
hello_world stage; constructive criticism is *quite* welcome.

# Compiler.rb

class Integer
  # Much easier than properly parsing a string for Exper.new(???)
  # If I *really* wanted to avoid monkey-patching, I could define
  # Expr#-@ and expr#+@ instead.
  def to_expr
    Compiler::Expr.new self
  end
end

module Compiler
  CONST  = 2**15
  LCONST = 2**31

  def Compiler.compile input
    # I initially tried Expr.new(\1), but this way lets me use Ruby's
    # sign-parsing.
    m = input. gsub /(\d+)(\D*)/, '\1.to_expr()\2'
    exp = eval m
    exp.compile
  end

  # The meat of this module...
  # Rather than do any parsing, I'm just converting all the expression's
numbers
  # into Expr objects, whose +-/*, etc. methods (all method_missing) just
build
  # a parse-tree when I run eval.
  class Expr
    attr_reader :val

    OPERATORS = { :+ => 0x0a,
                :- => 0x0b,
                :* => 0x0c,
                :** => 0x0d,
                :/ => 0x0e,
                :% => 0x0f,
                :swap => 0xa0 } # Swap doesn't have an operator, but
whatever.

    def initialize *v
      @val = v
    end

    # Take care of all those operators
    def method_missing sym, *args
      if OPERATORS.include? sym
        Expr.new [val, args.first, sym]
      else
        raise "Unknown operator: #{sym}, #{args.inspect}"
      end
    end

    def to_s
      "Expr: <#{flatten.join ' '}>"
    end

    def flatten
      # Flatten the array as much as we can, then tackle any Expr objects.
      # Finally, make sure the result is also flat
      # (because the map turns each Expr into an array)
      val.flatten.map do |i|
        if i.respond_to? :flatten
          i.flatten
        else
          i
        end
      end.flatten
    end

    def compile
      # Get a flat copy of our value, then encode each number and symbol.
      # Finally, flatten all the encoded numbers into our answer.
      arr = flatten
      arr.map do |i|
        if i.is_a? Integer
          bytes_for(i)
        elsif OPERATORS.include? i
          OPERATORS[i]
        else
          # What's the preferred method of dealing with this?
          # I could raise a different exception, or attempt to call the same
          # method in my superclass...
          raise "Unknown operator: #{i.inspect}, #{i.class}"
        end
      end.flatten
    end

    # Convert a number to bytes
    def bytes_for number
      type = size = 0
      values = []
      if number < CONST and number >= -CONST
        type, size = 1, 2
      elsif number < LCONST and number >= -LCONST
        type, size = 2, 4
      else
        raise "#{number} is too big to encode!"
      end

      size.times do |s|
        number, byte = number.divmod 256
        # I could use << here, but then I'd need to reverse values.
        values.unshift byte
      end
      [type, *values]
    end

  end #expr

end #Compiler

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