// Ulam Prime Spiral 

​

// https://youtu.be/a35KWEjRvc0

​

​

// cmd + /     = comments

​

​

​

//----------------------------------

//  CODING CHALLENGE 167 -- PRIME SPIRAL

//----------------------------------

​

​

//--- global variables

let x, y;

let px, py;

let step = 1;

let state = 0;

let numSteps = 1; // initial number

let turnCounter = 1;

​

​

// scale/resolution

let stepSize = 5

let totalSteps;

​

​

​

// function to test if number is prime

function isPrime(value) {

  if (value == 1) return false;

  for (let i = 2; i <= sqrt(value); i++) {

    if (value % i == 0) {

      return false;

    }

  }

  return true;

}

​

​

​

​

//-------------------- setup

function setup() {

  // createCanvas(700, 700);

  createCanvas(windowWidth, windowHeight);

  //background(0);

​

​

  // set up spiral

  const cols = width / stepSize;

  const rows = height / stepSize;

  totalSteps = cols * rows;

  x = width / 2;

  y = height / 2;

  px = x;

  py = y;

  background(0);

}

​

​

​

​

​

//-------------------- draw

function draw() {

​

  // if prime, draw circle

  if (isPrime(step)) {

    fill('#00ff009'); // purple 66ff99

    stroke('#00ff00');

    circle(x, y, stepSize * 0.5);

  }

​

  // connect current to previous line

  line(x, y, px, py);

  px = x;

  py = y;

​

  // switch statement to state

  switch (state) {

    case 0:

      x += stepSize;

      break;

    case 1:

        y -= stepSize;

        break;

    case 2:

        x -= stepSize;

        break;

    case 3:

        y += stepSize;

        break;

  }

​

 // change state

  // if == 0 its even

  if (step % numSteps == 0) {

    state = (state + 1) % 4;

    turnCounter++;

    if (turnCounter % 2 == 0) {

      numSteps++;

    }

  }

​

  step++;

​

  // check if done 

  if (step > totalSteps) {

    noLoop();

  }

​

}// draw

​

​

​