// Global code will be evaluated once.
Canvas.setpenopacity(.5);

const turtle = new Turtle(-90,100);
turtle.left(40);

// l-system
function createLSystem(numIters, axiom) {
    let s = axiom;
    for (let i=0; i<numIters; i++) {
        s = processString(s);
    }
    return s;
}

function processString(oldStr) {
    let newstr = "";
    for (let i=0; i<oldStr.length; i++) {
        newstr += applyRules(oldStr[i]);
    }
    return newstr;
}

function applyRules(ch) {
    switch (ch) {
        case "X": return "F+[[X]-X]-F[-FX]+X"; //   # Rule 1
        case "F": return "FF"; //   # Rule 2
        default: return ch;
    }
}

const inst = createLSystem(7, "X"); // number of iterations and axiom
const distance = .6;
const angle = 25;
const states = [];

// The walk function will be called until it returns false.
function walk(i) {
    const cmd = inst[i];
    
    switch (cmd) {
        case "F":   turtle.forward(distance);
                    break;
        case "B":   turtle.backward(distance);
                    break;
        case "+":   turtle.right(angle);
                    break;
        case "-":   turtle.left(angle);
                    break;
        case "[":   states.push({
                        x: turtle.xcor(),
                        y: turtle.ycor(),
                        h: turtle.heading()
                    });
                    break;
        case "]":   const state = states.pop();
                    turtle.penup();
                    turtle.goto(state.x, state.y);
                    turtle.setheading(state.h);
                    turtle.pendown();
                    break;
        default:
    }
      
    return i < inst.length - 1;
}