import random

XSIZE = 15
YSIZE = 12

_world = []

_x_pos = -1
_y_pos = -1


def init_world():
    """
    Initializes the world to a rectangle of empty cells and adds some
    obstacles.
    """
    # make rectangle of empty cells
    for y in range(YSIZE):
        _world[y:] = [[' '] * XSIZE]
    # add some obstacles
    # obstacle 1
    _world[YSIZE-2][XSIZE-2] = 'w'
    _world[YSIZE-1][XSIZE-2] = 'w'
    _world[YSIZE-2][XSIZE-1] = 'w'
    _world[YSIZE-1][XSIZE-1] = 'w'
    # obstacle 2
    for y in [3,4]:
        for x in range(2,9):
            _world[y][x] = 'w'
    for y in [5,6,7]:
        _world[y][2] = 'w'
        _world[y][3] = 'w'
    _world[5][7] = 'w'
    _world[5][8] = 'w'


def init_robot():
    """
    Chooses a random position in the world and places the robot there, if
    the position is free. Otherwise it tries again.
    """
    placed = False
    while not placed:
        global _x_pos
        _x_pos = random.randint(0,XSIZE-1)
        global _y_pos
        _y_pos = random.randint(0,YSIZE-1)
        if _world[_y_pos][_x_pos] == ' ':
            _world[_y_pos][_x_pos] = 'R'
            placed = True


def display_world():
    """
    Print out a representation of the world on the screen.
    """
    print "= " * (XSIZE + 2)
    for row in _world:
        print "|",
        for cell in row:
            print cell,
        print"|\n",
    print "= " * (XSIZE + 2)


def get_cell(x,y):
    """
    Returns the 'content' of the grid cell at position x and y. Returns
    'w' if either x or y are beyond the boundaries of the grid.
    """
    if x < 0 or y < 0 or x >= XSIZE or y >= YSIZE:
        return 'w'
    else:
        return _world[y][x]


def move(where):
    """
    Tries to move the robot into the direction specified by the parameter where.
    Nothing happens if it is not possible to move the robot into that direction
    because of a wall.
    """
    global _y_pos
    global _x_pos
    if where == 'n' and get_cell(_x_pos,_y_pos-1)!='w':
        _world[_y_pos][_x_pos] = ' '
        _y_pos = _y_pos - 1
        _world[_y_pos][_x_pos] = 'R'
    elif where == 'e' and get_cell(_x_pos+1,_y_pos)!='w':
        _world[_y_pos][_x_pos] = ' '
        _x_pos = _x_pos + 1
        _world[_y_pos][_x_pos] = 'R'
    elif where == 's' and get_cell(_x_pos,_y_pos+1)!='w':
        _world[_y_pos][_x_pos] = ' '
        _y_pos = _y_pos + 1
        _world[_y_pos][_x_pos] = 'R'
    elif where == 'w' and get_cell(_x_pos-1,_y_pos)!='w':
        _world[_y_pos][_x_pos] = ' '
        _x_pos = _x_pos - 1
        _world[_y_pos][_x_pos] = 'R'
        

def agent_function():
    """
    Specifies the rules according to which the robot moves.
    """
    where = random.choice(['n','e','s','w'])
    move(where)


def main():
    # initialize the grid world
    init_world()

    # place the robot in the grid world
    init_robot()
    display_world()

    # make the robot move one step at a time as long as the user keeps on
    # pressing return
    more = raw_input("Press return for the next step; 'q' and return to stop\n")
    while more != 'q':
        agent_function()
        display_world()
        more = raw_input("Press return for the next step; 'q' and return to stop\n")



main()