[Solution] [Quiz] Turtle Graphics (#104)
 

Here is my quick solution.
( Quick, so, it will fail the unit test, since it does not check the
parameters and throw the expected exception; however, it shows the
example graphics correctly )



# Created by Morton Goldberg on November 02, 2006.
# Modified on November 14, 2006
# turtle.rb

# An implementation of Turtle Procedure Notation (TPN) as described in
# H. Abelson & A. diSessa, "Turtle Geometry", MIT Press, 1981.
#
# Turtles navigate by traditional geographic coordinates: X-axis pointing
# east, Y-axis pointing north, and angles measured clockwise from the
# Y-axis (north) in degrees.

class Turtle
include Math # turtles understand math methods
DEG = Math::PI / 180.0

attr_accessor :track
alias run instance_eval

def initialize
clear
end

attr_reader :xy, :heading

# Place the turtle at [x, y]. The turtle does not draw when it changes
# position.
def xy=(coords)
@xy = coords.dup
end

# Set the turtle's heading to <degrees>.
def heading=(degrees)
@heading = degrees
end

# Raise the turtle's pen. If the pen is up, the turtle will not draw;
# i.e., it will cease to lay a track until a pen_down command is given.
def pen_up
@pen = false
end

# Lower the turtle's pen. If the pen is down, the turtle will draw;
# i.e., it will lay a track until a pen_up command is given.
def pen_down
@pen = true
end

# Is the pen up?
def pen_up?
!pen_down?
end

# Is the pen down?
def pen_down?
@pen
end

# Places the turtle at the origin, facing north, with its pen up.
# The turtle does not draw when it goes home.
def home
@xy = [0.0, 0.0]
@heading = 0.0
pen_up
end

# Homes the turtle and empties out it's track.
def clear
home
@track = []
end

# Turn right through the angle <degrees>.
def right(degrees)
@heading += degrees
end

# Turn left through the angle <degrees>.
def left(degrees)
right(-degrees)
end

# Move forward by <steps> turtle steps.
def forward(steps)
x = @xy[0] + steps * cos((90 - @heading) * DEG)
y = @xy[1] + steps * sin((90 - @heading) * DEG)
go([x,y])
end

# Move backward by <steps> turtle steps.
def back(steps)
forward(-steps)
end

# Move to the given point.
def go(pt)
if @pen
if !@track.empty? && @track.last.last == @xy
@track.last << pt.dup
else
@track << [@xy.dup, pt.dup]
end
end
@xy = pt.dup
end

# Turn to face the given point.
def toward(pt)
@heading = 90 - acos((pt[0] - @xy[0]) / distance(pt)) / DEG
end

# Return the distance between the turtle and the given point.
def distance(pt)
sqrt((@xy[0] - pt[0]) ** 2 + (@xy[1] - pt[1]) ** 2)
end

# Traditional abbreviations for turtle commands.
alias fd forward
alias bk back
alias rt right
alias lt left
alias pu pen_up
alias pd pen_down
alias pu? pen_up?
alias pd? pen_down?
alias set_h heading=
alias set_xy xy=
alias face toward
alias dist distance
end