const zoom = .7; //min=.2 max=10 step=.1

// You can find the Turtle API reference here: https://turtletoy.net/syntax
Canvas.setpenopacity(-1);

// Global code will be evaluated once.
const turtle = new Turtle();

function* step(s, steps = 0) {
    while(++steps) {
        for(let i = 0; i < 2; i++) {
            for(let j = 0; j < steps; j++) yield turtle.forward(s);
            turtle.left(90);
        }
    }
}
const isPrime = num => {
    for(let i = 2, s = Math.sqrt(num); i <= s; i++) if(num % i === 0) return false;
    return num > 1;
}
const point = (radius) => { radius = Math.max(.2, radius); [px, py] = turtle.pos(); turtle.jump(px+radius,py); for(let i = 0, max = Math.ceil(radius*2*Math.PI); i <= max; i++) { turtle.goto(px+Math.cos(2*Math.PI*i/max)*radius, py-Math.sin(2*Math.PI*i/max)*radius); } turtle.jump(px, py); }
const stepIterator = step(zoom);

// The walk function will be called until it returns false.
function walk(i) {
    if(isPrime(i + 1)) {point(zoom/3);point(zoom/2);}
    turtle.up();
    stepIterator.next();
    turtle.down();
    return turtle.x() <= 101;
}