Mars Rover

rover.rb

=begin

### PROMPT ###
 
A squad of robotic rovers are to be landed by NASA on a plateau on Mars. 
This plateau, which is curiously rectangular, must be navigated by 
the rovers so that their on-board cameras can get a complete view of the 
surrounding terrain to send back to Earth.
 
A rover's position and location is represented by a combination 
of x and y co-ordinates and a letter representing one of the four 
cardinal compass points. The plateau is divided up into a grid to simplify 
navigation. An example position might be 0, 0, N, which means the rover 
is in the bottom left corner and facing North.
 
In order to control a rover, NASA sends a simple string of letters. 
The possible letters are 'L', 'R' and 'M'. 'L' and 'R' makes the rover 
spin 90 degrees left or right respectively, without moving from its 
current spot. 'M' means move forward one grid point, and maintain the 
same heading.
 
Assume that the square directly North from (x, y) is (x, y+1).
 
INPUT:
The first line of input is the upper-right coordinates of the 
plateau, the lower-left coordinates are assumed to be 0,0.
 
The rest of the input is information pertaining to the rovers that 
have been deployed. Each rover has two lines of input. The first 
line gives the rover's position, and the second line is a series of 
instructions telling the rover how to explore the plateau.
 
The position is made up of two integers and a letter separated by 
spaces, corresponding to the x and y co-ordinates and the 
rover's orientation.
 
Each rover will be finished sequentially, which means that the 
second rover won't start to move until the first one has finished moving.

OUTPUT:
The output for each rover should be its final co-ordinates and heading.
 
Sample IO
Test Input:
5 5
1 2 N
LMLMLMLMM
3 3 E
MMRMMRMRRM
 
Expected Output:
1 3 N
5 1 E

### NOTES ###

To run:
  Run `ruby rover.rb` with input.txt in the same directory

Assumptions:
  Input file is formatted correctly and is present
  The units are measure in kilomters, even if rovers occupy the same grid
    space, they won't collide.

Notes:
  If the rover drives off the side of the plateau, the program will notify
    you so (and report drop location in the interest of salvaging the rover).
    This wasn't mentioned in the spec, but it's the only reason I could think
    of to keep track of the dimensions of the plateau. 

=end

class Rover
  def initialize(height, width)
    @height = height
    @width = width
    @x = 0
    @y = 0
    @cur_dir = 0
    @directions = ['N','E','S','W']
  end

  def act(i)
    if i == 'R' or i == 'L'
      rotate(i)
    else
      step
    end
  end

  def step
    if @cur_dir == 0
      @y += 1
    elsif @cur_dir == 1
      @x += 1
    elsif @cur_dir == 2
      @y -= 1
    elsif @cur_dir == 3
      @x -= 1
    end
    if @x < 0 or @x > @width or @y < 0 or @y > @height
      throw :side_error
    end
  end

  def rotate(d)
    # decides which direction to turn and adjusts the int accordingly
    c = d == 'L' ? -1 : 1
    @cur_dir = (@cur_dir + c) % 4
  end

  def get_location
    return "#{@x} #{@y} #{@directions[@cur_dir]}"
  end

  def set_location(x, y, d)
    @x = x
    @y = y
    @cur_dir = @directions.index(d)
  end
end

# file input
f = open('input.txt')
a = f.first.split(' ')
height = a[0].to_i
width = a[1].to_i

# initialize variables
r = Rover.new(height,width)
instructions = false
good = true

f.each do |line|
  # location line
  if !instructions 
    l = line.chomp.split(' ')
    r.set_location(l[0].to_i,l[1].to_i,l[2])
  # action line
  else
    l = line.chomp.split("")
    l.each do |action|
      begin
        r.act(action)
      rescue Exception => e
        puts "Rover fell off at #{r.get_location}!"
        good = false
        break
      end
    end
    # prints if the rover never fell off
    puts r.get_location if good
  end
  # the next line is the other option
  instructions = !instructions
  good = true
end