// 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();
let N = 64; //min=1 max=900 step=1 
let scale = 2.7;  //min=0.1 max=5 step=0.1 
let x = -90; //min=-200 max=0 step=1
let sides = 2; //min=2 max=30 step=1
let seen = new Set();
let a = 0;

let y = 0;
seen.add(a);

// Improved Recamán's Sequence
turtle.pendown();

// Move to start
turtle.penup();
turtle.goto(x, y);
turtle.pendown();

for (let n = 1; n <= N; n++) {
  let candidate = a - n;
  let b = (candidate > 0 && !seen.has(candidate))
    ? candidate
    : a + n;

  let x2 = x + (b - a) * scale;
  let r = Math.abs(x2 - x) / 2;
  let cx = (x + x2) / 2;

  drawSemicircle(cx, y, r, n % 2 === 0);

  x = x2;
  a = b;
  seen.add(a);
}

// ---- helpers ----

function drawSemicircle(cx, cy, r, up) {
  
  let start = up ? 0 : 180;
  let dir = up ? 1 : -1;

  turtle.penup();
  turtle.goto(cx - r, cy);
  turtle.setheading(0);
  for (let i = 0; i <= sides; i++) {
    let t = Math.PI * i / sides;
    let px = cx + r * Math.cos(t);
    let py = cy + dir * r * Math.sin(t);
    
    turtle.goto(px, py);
    turtle.pendown();

  }

}