Solution to PuzzleNode problem #4 - Robots vs. Lasers

robot.rb

#!/usr/bin/env ruby

class Enumerator
  def count_with_index
    count = 0
    each_with_index do |item, index|
      count += yield(item, index) ? 1 : 0
    end
    count
  end
end

class Robot
  def initialize(text)
    @north, @conveyor, @south = text.split(/\n/)
  end

  def run
    pos = @conveyor.index('X')
    east_hits = @north[pos..-1].chars.count_with_index { |p, i| i.even? && p == '|' } +
                @south[pos..-1].chars.count_with_index { |p, i| i.odd?  && p == '|' }
    west_hits = @north[0..pos].reverse.chars.count_with_index { |p, i| i.even? && p == '|' } +
                @south[0..pos].reverse.chars.count_with_index { |p, i| i.odd?  && p == '|' }
    east_hits < west_hits ? 'GO EAST' : 'GO WEST'
  end
end

if $0 == __FILE__
  ARGF.read.split(/\n\n/).each do |robot|
    puts Robot.new(robot).run
  end
end

robot_test.rb

require 'test/unit'
require './robot'

class OutputTest < Test::Unit::TestCase
  def test_west
    assert_equal 'GO WEST', Robot.new("#|#|#|##\n---X----\n###||###").run
  end

  def test_east
    assert_equal 'GO EAST', Robot.new("##|#|#|#\n----X---\n###||###").run
  end

  def test_tie
    assert_equal 'GO WEST', Robot.new("##|#|#|#\n----X---\n###|||##").run
  end
end