from turtle import *

MAX_LEG_LENGTH = 50

def get_input_with_default(question, default):
    i = raw_input(question + " (default='" + str(default) + "'): ")
    if not i:
        i = default
    return i

def updaterule(s):
    return s.replace(production_rule[0], production_rule[1])

def exec_command(l, step_size = 10):
    if l == "F":
        forward(step_size)
    elif l == "+":
        left(angle)
    elif l == "-":
        right(angle)

def showrule(s, step_size = 10):
    clear()
    speed('fastest')
    color("blue")
    up()
    goto(0,0)
    setheading(0)
    down()
    for l in s:
        exec_command(l, step_size)


start_string_default = "F"
production_rule_default = "F->F+F--F+F"

start_string    = get_input_with_default("Start string", start_string_default)
production_rule_pair = get_input_with_default("Production rule", production_rule_default)
production_rule = production_rule_pair.split("->")
angle           = int(get_input_with_default("Angle", 60))

width_ratio = 1.0/3.0

current_string = start_string

for i in range(0, 6):
    current_string = updaterule(current_string)
    showrule(current_string, MAX_LEG_LENGTH * (width_ratio ** i))
    # raw_input("Generation done, press any key to continue")