Mars Rover Kata

gistfile1.txt

#Mars Rover Kata

#Develop an api that moves a rover around on a grid.
#You are given the initial starting point (x,y) of a rover and the direction (N,S,E,W) it is facing.
#The rover receives a character array of commands.
#Implement commands that move the rover forward/backward (f,b).
#Implement commands that turn the rover left/right (l,r).
#
#Implement wrapping from one edge of the grid to another. (planets are spheres after all)
#Implement obstacle detection before each move to a new square. If a given sequence of commands encounters an obstacle, the rover moves up to the last possible point and reports the obstacle.

require 'rspec'

describe "Mars Rover" do
  context "starts at 0,0 and facing north" do
    subject do
      MarsRover.new(0,0,"N")
    end

    {'f' => [0,1,'N'],
      'b' => [0,-1,'N'],
      'fb' => [0,0,'N'],
      'l' => [0,0,'W'],
      'r' => [0,0,'E'],
      'rl' => [0,0,'N'],
      'rf' => [1,0,'E'],
      'rrf' => [0,-1,'S'],
      'rb' => [-1,0,'E'],
      'rrbbllffbbbb' => [0,0,'N'],
      '2r' => [0,0,'S'],
      '2r2b2l2f4b' => [0,0,'N']
    }.each do |move, expected|
      it "move #{move} should result in #{expected.inspect}" do
        subject.move(move).should == expected
      end
    end
  end
end

class MarsRover
  INST =  {
      'N' => {'l' => 'W', 'r' => 'E', :multiplier => 1, :dir => :@y},
      'W' => {'l' => 'S', 'r' => 'N', :multiplier => -1, :dir => :@x},
      'S' => {'l' => 'E', 'r' => 'W', :multiplier => -1, :dir => :@y},
      'E' => {'l' => 'N', 'r' => 'S', :multiplier => 1, :dir => :@x}
    }

  def initialize(x,y,heading)
    @x, @y, @heading = x,y,heading
  end

  def move(commands)
    commands.each_char {|c| command(c)}
    [@x, @y, @heading]
  end

  private

  def command(cmd)
    case(cmd)
    when 'f'
      movement(:advance, 1)
    when 'b'
      movement(:advance, -1)
    when /l|r/
      movement(:change_dir, cmd)
    else
      @count = cmd.to_i
    end
  end

  def movement(underlying_cmd, arg)
    (@count || 1).times {send(underlying_cmd, arg)}
    @count = nil
  end

  def change_dir(cmd)
    @heading = INST[@heading][cmd]
  end

  def advance(posneg)
    instructions = INST[@heading]
    addend = posneg * instructions[:multiplier]
    ivar = instructions[:dir]
    instance_variable_set(ivar, instance_variable_get(ivar) + addend)
  end

end