"""Generates the L-System for the Dragon Fractal, using
the turtle module."""

import re, turtle
from math import sin, pi

"""The default L-System rules for the dragon fractal are:
    Angle 45 degrees
    Starting Axiom FX
    F =
    Y = +FX--FY+
    X = -FX++FY-

I've optimised them a little bit the following ways:
    Take out all occurances of F.
    Replace Y with +FN--FY+
    Replace X with -FX++FY-
    Replace N with X
    Take out all occurances of -+
    Take out all occurances of +- """

def rules(instring):
    """I originally had this as a series of nested re.sub expressions.
    Once the rule list got long though, I split it up to make it more
    "pythonic". """
    newstring, subs = re.subn("F", "", instring)
    # Use N as a placeholder for X
    newstring, subs = re.subn("Y", "+FN--FY+", newstring)
    # So that we don't get double substitution.
    newstring, subs = re.subn("X", "-FX++FY-", newstring)
    # Now we replace the placeholder with X
    newstring, subs = re.subn("N", "X", newstring)
    # And optimise the string in regard to left/right turns.
    subs = 1
    while subs:
        newstring, first = re.subn("-\+", "", newstring)
        newstring, second = re.subn("\+-", "", newstring)
        subs = first + second
    return newstring

def colorator(value):
    # Makes the colour of the cursor cycle.
    range, fract = divmod(value*6, 1)
    if range == 0:
        red = 1.0
        green = fract
        blue = 0.0
    elif range == 1:
        red = 1.0 - fract
        green = 1.0
        blue = 0.0
    elif range == 2:
        red = 0.0
        green = 1.0
        blue = fract
    elif range == 3:
        red = 0.0
        green = 1.0 - fract
        blue = 1.0
    elif range == 4:
        red = fract
        green = 0.0
        blue = 1.0
    elif range >= 5:
        red = 1.0
        green = 0.0
        blue = 1.0 - fract
    return red, green, blue

# The default is that the turtle will only move one pixel
def parser(parsestring, distance=1, angle=45):
    # First we clean up the parsestring
    newstring = re.sub("X", "", parsestring)
    newstring = re.sub("Y", "", newstring)
    # Clear the screen
    turtle.clear()
    strlen = len(newstring)
    colorinc = 1.0 / float(strlen)
    turtle.color(colorator(0))
    for i in range(strlen):
        value = newstring[i]
        turtle.color(colorator(float(i) * colorinc))
        if value == "+":
            turtle.right(angle)
        elif value == "-":
            turtle.left(angle)
        elif value == "F":
            turtle.forward(distance)
    # Hide the cursor
    turtle.color(1.0,1.0,1.0)

def run(count=15, distance=1, angle=45, width=1):
    string = "FX"
    while count > 0:
        string = rules(string)
        count -= 1
    # "Hide" the cursor while we are moving it.
##    print string
    turtle.width(width)
    turtle.color(1.0,1.0,1.0)
    # Move the cursor so the turtle won't go off the screen.
    # You might want to resize the turtle screen while the program is doing this
    turtle.setx(100)
    turtle.sety(-200)
    parser(string, distance=distance, angle=angle)
        

if __name__ == "__main__":
    run(15)