"""
CS101 Class examples - Recursion
"""

import turtle as t

def drawCurve(L, level):
    """
    Recursively draws an inward turning curve
    """
    if level > 0:
        t.forward(L)
        t.left(30)
        drawCurve(L*.95, level-1)
        t.right(30)
        t.backward(L)

def drawCurveScene(L, level):
    """ 
    Moves the turtle to the left of the screen, draws a curve
    """
    # pick up the pen and move the turtle so it starts to the left 
    t.penup()
    t.goto(-t.window_width()/4 + 20, 0)
    t.pendown()    
    drawCurve(L, level)
    t.done()



# goBack illustrates pending operations after the recursive call
def goBack(n):
    """
    Prints "Go" for each recursive call; "Stop" at base case;
    then prints "Back" on the way out of the recursion.
    """
    if n == 0:
        print("Stop")
    else:
        print("Go", n)
        goBack(n-1)
        print("Back", n)
        
# What output does this call produce?
# goBack(3) 
        
def mystery(s):
    """
    Recursive function that takes a string as input and returns...
    """
    if len(s) == 1:
        return int(s)
    else:
        return int(s[0]) + mystery(s[1:])

# What output does this call produce?
# mystery('1337')

def drawTree(length, gen):
    """
    Draws recursive tree with initial trunk length
    and number of branches given by parameters.
    Ends drawing at same position where drawing started.
    """
    if gen > 0:
        t.forward(length)              # draw tree branch
        t.right(45)
        drawTree(length/2, gen-1)           # draw right subtree
        t.left(45*2)                   # undo right turn, then turn left again
        drawTree(length/2, gen-1)           # draw left subtree
        t.right(45)                    # undo left turn
        t.backward(length)             # trace back down the tree branch

def drawTreeScene():
    """ 
    Moves the turtle to the bottom of the screen, points it upward, and draws a tree.
    """
    # turn off drawing animation (too slow otherwise)
    # t.tracer(False)
        
    # pick up the pen and move the turtle so it starts at the left edge of the canvas 
    t.penup()
    t.goto(0, -t.window_height()/2 + 20)
    t.left(90)
    t.pendown()    

    # draw the tree by calling your function
    drawTree(200,4)

    # finished
    # t.update()
    t.done()