A simple solution to the "robots vs lasers" puzzle

robots.rb

#a simple solution to the "robots vs lasers" puzzle
#see http://www.puzzlenode.com/puzzles/4-robots-vs-lasers

#usage:
#simply run this program with ruby and pass the input file name as the sole argument
#example: ruby robots.rb sample-input.txt

#########################################################

#just use a structure since we only need to hold data
#could use a class but it'd be overkill for this example
FactoryLine = Struct.new :north_lasers, :conveyor_belt, :south_lasers

def read_input(filename)
  factory_floor = []

  factory_line = FactoryLine.new()

  i = 0

  #open the file and read the factory floor input
  #each factory floor is made of many factory lines
  #each factory line has 4 lines of text:
  #  1. the north lasers
  #  2. the conveyor belt
  #  3. the south lasers
  #  4. a blank line separating it from the next factory line

  # example:
  #    #|#|#|##
  #    ---X----
  #    ###||###
  File.open(filename).each_line{ |line|
    #i mod 4 since there is a new factory line after every 4 lines of input
    if (i % 4) == 0
      factory_line[:north_lasers] = line.strip!
    elsif (i % 4) == 1
      factory_line[:conveyor_belt] = line.strip!
    elsif (i % 4) == 2
      factory_line[:south_lasers] = line.strip!
      factory_floor << factory_line #append the line to the factory floor array
      factory_line = FactoryLine.new()
    end

    i += 1
  }

  return factory_floor
end

#put this check in a function so it's easier to test
def get_position(conveyor_belt)
  return conveyor_belt.index("X")
end

#calculate the total number of hits on the robot
#given a laser array (north or south), where the robot is, and if it's going east or west
def number_of_hits(laser_array, robot_position, east_west, north_south)
  number_of_hits = 0

  if east_west == :west
    click = 0

    #count from the current position down to the west-most position (aka lowest index)
    for i in robot_position.downto(0)

      # if the current index has a laser, check to see if it shoots
      if laser_array[i] == "|"
        if north_south == :north and click.even?
          #the lasers shoot from the north side on even clicks
          number_of_hits += 1

        elsif north_south == :south and click.odd?
          #the lasers shoot from the south side on odd clicks
          number_of_hits += 1
        end
      end

      click += 1
    end
  elsif east_west == :east
    click = 0

    # if the current index has a laser, check to see if it shoots
    for i in robot_position.upto(laser_array.length - 1)
      if laser_array[i] == "|"
        if north_south == :north and click.even?
          #the lasers shoot from the north side on even clicks
          number_of_hits += 1
        elsif north_south == :south and click.odd?
          #the lasers shoot from the south side on odd clicks
          number_of_hits += 1
        end
      end

      click += 1
    end
  end

  return number_of_hits
end

#calculate and print which directions the robot should take
#based on its current position in the factory line
def get_direction(factory_line)
  robot_position = get_position(factory_line[:conveyor_belt])

  hits_east = 0
  hits_west = 0
  #calculate the hits going east
  hits_east = number_of_hits(factory_line[:north_lasers], robot_position, :east, :north) + number_of_hits(factory_line[:south_lasers], robot_position, :east, :south)

  #calculate the hits going west
  hits_west = number_of_hits(factory_line[:north_lasers], robot_position, :west, :north) + number_of_hits(factory_line[:south_lasers], robot_position, :west, :south)

  #return EAST if number of hits going east
  #is larger than the number of hits going west
  if hits_east < hits_west
    return "GO EAST"
  end

  #otherwise return WEST if hits_west is greater
  #or the 2 are equal
  return "GO WEST"
end


#actually run the program here
#read the input file and print the directions
factory_floor = read_input(ARGV[0])
factory_floor.each do |factory_line|
  puts get_direction(factory_line)
end

#test methods
raise "get_position test 1 failed" unless (get_position("--X---") == 2)
raise "get_position test 2 failed" unless (get_position("X------") == 0)
raise "get_position test 3 failed" unless (get_position("----X") == 4)

raise "number_of_hits test 1 failed" unless (number_of_hits("#|#|#|##", 3, :west, :north) == 2)
raise "number_of_hits test 2 failed" unless (number_of_hits("###||###", 3, :west, :north) == 1)
raise "number_of_hits test 3 failed" unless (number_of_hits("#|#|#|##", 2, :east, :south) == 2)