---
id: 5900f3a61000cf542c50feb9
challengeType: 5
title: 'Problem 58: Spiral primes'
forumTopicId: 302169
---

## Description
<section id='description'>

Starting with 1 and spiralling anticlockwise in the following way, a square spiral with side length 7 is formed.

<div style='text-align: center;'>
  <strong><span style='color: red;'>37</span></strong> 36 35 34 33 32 <strong><span style='color: red;'>31</span></strong><br>
  38 <strong><span style='color: red;'>17</span></strong> 16 15 14 <strong><span style='color: red;'>13</span></strong> 30<br>
  39 18  <strong><span style='color: red;'>5</span></strong>  4  <strong><span style='color: red;'>3</span></strong> 12 29<br>
  40 19  6  1  2 11 28<br>
  41 20  <strong><span style='color: red;'>7</span></strong>  8  9 10 27<br>
  42 21 22 23 24 25 26<br>
  <strong><span style='color: red;'>43</span></strong> 44 45 46 47 48 49<br>
</div>

It is interesting to note that the odd squares lie along the bottom right diagonal, but what is more interesting is that 8 out of the 13 numbers lying along both diagonals are prime; that is, a ratio of 8/13 ≈ 62%.

If one complete new layer is wrapped around the spiral above, a square spiral with side length 9 will be formed. If this process is continued, what is the side length of the square spiral for which the ratio of primes along both diagonals first falls below 10%?

</section>

## Instructions
<section id='instructions'>

</section>

## Tests
<section id='tests'>

```yml
tests:
  - text: <code>spiralPrimes()</code> should return a number.
    testString: assert(typeof spiralPrimes() === 'number');
  - text: <code>spiralPrimes()</code> should return 26241.
    testString: assert.strictEqual(spiralPrimes(), 26241);

```

</section>

## Challenge Seed
<section id='challengeSeed'>

<div id='js-seed'>

```js
function spiralPrimes() {

  return true;
}

spiralPrimes();
```

</div>



</section>

## Solution
<section id='solution'>

```js
// solution required
```

</section>