Fully functioning Brainf*ck interpreter in Ruby

brainfrog

#!/usr/bin/env ruby

$DEBUG = false

EOF = "\0"
String.class_eval { define_method(:arg?) { ARGV.include? self } }

ALLOC = '--alloc'.arg? ? ARGV[(ARGV.index '--alloc') + 1].to_i : 100
PROGRAM = (File.read ARGV.select { |arg| File.file? arg }.first) + EOF

module BrainFrog
  MEMORY = Array.new ALLOC, 0
  @pointer = 0

  def self.run
    i = 0
    until PROGRAM[i] == EOF
      if PROGRAM[i] == '['
        i = loop i
      else
        execute PROGRAM[i]
      end
      i += 1
    end
  end

  private def self.loop index
    i = index + 1
    loop_end = nil

    until MEMORY[@pointer].zero?
      until PROGRAM[i] == ']'
        if PROGRAM[i] == '['
          i = loop i
        else
          execute PROGRAM[i]
        end
        i += 1
      end
      loop_end = i
      i = index + 1
    end

    if loop_end.nil?  # Find end of loop if we don't run it.
      until PROGRAM[i] == ']'
        if PROGRAM[i] == '['
          i = loop i
        end
        i += 1
      end
      loop_end = i
    end
    
    return loop_end
  end
  
  def self.execute index
    case PROGRAM[index]
      when '>'; @pointer += 1
      when '<'; @pointer -= 1
      when '+'; MEMORY[@pointer] += 1
      when '-'; MEMORY[@pointer] -= 1
      when '.'; 
      if $DEBUG
        puts "\nCHR: #{MEMORY[@pointer].chr.inspect} | ORD: #{MEMORY[@pointer]}"
      else 
        print MEMORY[@pointer].chr
      end
      when ','; MEMORY[@pointer] = gets[0].ord
      else; return
    end

    debug index
  end

  def self.debug i
    puts "'#{PROGRAM[i]}' => #{MEMORY} :: *POINTER = #{MEMORY[@pointer]}" if $DEBUG
  end
end

BrainFrog.run