## This is a combination of Listings 9.6, 9.7 (with a bug fix), and
## 9.8 (with minor improvements) from Python Programming in Context by
## Miller and Ranum to serve as a starting point for an MCS-177
## project.

from cTurtle import Turtle

## Listing 9.6
def applyProduction(axiom,rules,n):
    for i in range(n):
        newString = ""
        for ch in axiom:
            newString = newString + rules.get(ch,ch)
        axiom = newString
    return axiom

## Listing 9.7
def drawLS(aTurtle,instructions,angle,distance):
    stateSaver = []
    for cmd in instructions:
        if cmd == 'F':
            aTurtle.forward(distance)
        elif cmd == 'B':
            aTurtle.backward(distance)
        elif cmd == '+':
            aTurtle.right(angle)
        elif cmd == '-':
            aTurtle.left(angle)
        elif cmd == '[':
            pos = aTurtle.position()
            head = aTurtle.heading()
            stateSaver.append((pos,head))
        elif cmd == ']':
            pos,head = stateSaver.pop()
            aTurtle.up()   # this line was not in Listing 9.7 (a bug)
            aTurtle.setposition(pos)
            aTurtle.setheading(head)
            aTurtle.down() # this line was not in Listing 9.7 (a bug)

## Listing 9.8
def lsystem(axiom,rules,depth,initialPosition,heading,angle,length):
    aTurtle = Turtle()
    aTurtle.speed(0)       # this line improves on Listing 9.8
    aTurtle.shape('blank') # this line improves on Listing 9.8
    aTurtle.up()
    aTurtle.setposition(initialPosition)
    aTurtle.down()
    aTurtle.setheading(heading)
    ## The variable "instructions" was called "newRules" in Listing 9.8,
    ## which was misleading regarding what kind of thing it names.
    instructions = applyProduction(axiom,rules,depth)
    drawLS(aTurtle,instructions,angle,length)
    aTurtle.exitOnClick()