// You can find the Turtle API reference here: https://turtletoy.net/syntax
Canvas.setpenopacity(0.5);

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

class Noise {
	// http://mrl.nyu.edu/~perlin/noise/
	constructor(setup) {
		this.p = new Uint8Array(512);
		this.octaves = setup.octaves || 1;
		const p = new Uint8Array(256);
		for (let i = 0; i < 256; i++) p[i] = i;
		for (let i = 255; i > 0; i--) {
			const n = Math.floor((i + 1) * Math.random());
			const q = p[i];
			p[i] = p[n];
			p[n] = q;
		}
		for (let i = 0; i < 512; i++) {
			this.p[i] = p[i & 255];
		}
	}
	lerp(t, a, b) {
		return a + t * (b - a);
	}
	grad2d(i, x, y) {
		const v = (i & 1) === 0 ? x : y;
		return (i & 2) === 0 ? -v : v;
	}
	noise2d(x2d, y2d) {
		const X = Math.floor(x2d) & 255;
		const Y = Math.floor(y2d) & 255;
		const x = x2d - Math.floor(x2d);
		const y = y2d - Math.floor(y2d);
		const fx = (3 - 2 * x) * x * x;
		const fy = (3 - 2 * y) * y * y;
		const p0 = this.p[X] + Y;
		const p1 = this.p[X + 1] + Y;
		return this.lerp(
			fy,
			this.lerp(
				fx,
				this.grad2d(this.p[p0], x, y),
				this.grad2d(this.p[p1], x - 1, y)
			),
			this.lerp(
				fx,
				this.grad2d(this.p[p0 + 1], x, y - 1),
				this.grad2d(this.p[p1 + 1], x - 1, y - 1)
			)
		);
	}
	noise(x, y) {
		let e = 1,
			k = 1,
			s = 0;
		for (let i = 0; i < this.octaves; ++i) {
			e *= 0.5;
			s += e * (1 + this.noise2d(k * x, k * y)) / 2;
			k *= 2;
		}
		return s;
	}
}
const perlin = new Noise({ octaves: 2 });
let y = 100;
const line = Array.from({ length: 200 }, () => 100);

// The walk function will be called until it returns false.
function walk(i) {
    y -= 0.3;
	turtle.goto(-100, y);
	turtle.down();
	let v = false;
	for (let x = -100; x < 100; x++) {
		let z = y +	0.25 * (
		    (2048 * perlin.noise(x * 0.02, 6 + y * 0.04)) & (16 + 32 + 0 + 128 + 256)
		);
		if (z > -100) v = true;
		if (z > line[x]) z = line[x] - 0.01;
		line[x] = z;
		turtle.goto(x, z);
	}
	turtle.up();
	return v;
}