"""
we use the following conventions:
directions 0: up, 1: left, 2: down: 3: right

pixel white: True, black: False

turn right: True, left: False
"""

# number of iteration steps per frame
# set this to 1 to see a slow animation of each
# step or to 10 or 100 for a faster animation

STEP = 100
count = 0

def setup():
    global x, y, direction

    # 100x100 is large enough to show the
    # corridor after about 10000 cycles
    size(100, 100, P2D)

    background(255)
    x = width / 2
    y = height / 2
    direction = 0

def draw():
    global count
    for i in range(STEP):
        count += 1
        pix = get(x, y) != -1 # white =-1
        setBool(x, y, pix)

        turn(pix)
        move()

        if (x < 0 or y < 0 or x >= width or y >= height):
            println("finished")
            noLoop()
            break

    if count % 1000 == 0:
        println("iteration {}".format(count))

def move():
    global x, y
    if direction == 0:
        y -= 1
    elif direction == 1:
        x -= 1
    elif direction == 2:
        y += 1
    elif direction == 3:
        x += 1

def turn(rightleft):
    global direction
    direction += 1 if rightleft else -1
    if direction == -1:
        direction = 3
    if direction == 4:
        direction = 0

def setBool(x, y, white):
    set(x, y, -1 if white else 0)