http://www.reddit.com/r/dailyprogrammer/comments/32vlg8/20150417_challenge_210_hard_loopy_robots/

loopy.jl

# http://www.reddit.com/r/dailyprogrammer/comments/32vlg8/20150417_challenge_210_hard_loopy_robots/
type Point{T <: Real}
	x :: T
	y :: T
end

const NORTH = uint8(0)
const EAST  = uint8(1)
const SOUTH = uint8(2)
const WEST  = uint8(3)
const RIGHT = uint8(4)
const LEFT  = uint8(5)

typealias Direction Uint8

type Robot
	pt :: Point{Int}
	dir :: Direction
	instrcount :: Uint
end
Robot(pt, dir) = Robot(pt, dir, 0)

immutable RobotState
	pt :: Point{Int}
	dir :: Direction
end
RobotState(robot :: Robot) = RobotState(deepcopy(robot.pt), robot.dir)

# ENTRYPOINT
function main(args)
	command = args[1]
	
	robot = Robot(Point(0, 0), NORTH)
	initialstate = RobotState(robot)
	checkloop!(robot, command, initialstate)
end

function checkloop!(robot :: Robot, command :: String, initialstate :: RobotState)
	robotexecute!(robot, command, initialstate)
	cycles = if initialstate.pt.x == robot.pt.x &&
			initialstate.pt.y == robot.pt.y &&
			initialstate.dir == robot.dir
			1
		elseif initialstate.dir == robot.dir
			0
		elseif initialstate.dir == (robot.dir + uint8(2)) % 4
			2
		else
			4
		end
	if cycles == 0
		println("No loop detected.")
	else
		println("Entered in a loop after $cycles instructions!")
	end
end

function robotexecute!(robot :: Robot, command :: String, initialstate :: RobotState)
	for i = 1:length(command)
		if command[i] == 'S'
			robotstep!(robot)
		elseif command[i] == 'R'
			robotturn!(robot, RIGHT)
		elseif command[i] == 'L'
			robotturn!(robot, LEFT)
		else
			error("Invalid command: $(command[i])")
		end
		robot.instrcount += 1
	end
end

function robotstep!(robot :: Robot)
	if robot.dir == NORTH
		robot.pt.y += 1
	elseif robot.dir == EAST
		robot.pt.x += 1
	elseif robot.dir == SOUTH
		robot.pt.y -= 1
	elseif robot.dir == WEST
		robot.pt.x -= 1
	else
		error("Invalid robot direction: $(robot.dir)")
	end
end

function robotturn!(robot :: Robot, dir :: Direction)
	if dir == RIGHT
		robot.dir = (robot.dir + uint8(1)) % 4
	elseif dir == LEFT
		robot.dir = (robot.dir - uint8(1)) % 4
	else
		error("Invalid turn direction: $dir")
	end
end

main(ARGS)