freeCodeCamp/curriculum/challenges/chinese/10-coding-interview-prep/project-euler/problem-79-passcode-derivation.md

---
id: 5900f3bb1000cf542c50fece
title: 'Problem 79: Passcode derivation'
challengeType: 5
forumTopicId: 302192
dashedName: problem-79-passcode-derivation
---

# --description--

A common security method used for online banking is to ask the user for three random characters from a passcode. For example, if the passcode was `531278`, they may ask for the 2nd, 3rd, and 5th characters; the expected reply would be: `317`.

The arrays, `keylog1`, `keylog2`, and `keylog3`, contains fifty successful login attempts.

Given that the three characters are always asked for in order, analyze the array so as to determine the shortest possible secret passcode of unknown length.

# --hints--

`passcodeDerivation(keylog1)` should return a number.

```js
assert(typeof passcodeDerivation(_keylog1) === 'number');
```

`passcodeDerivation(keylog1)` should return `531278`.

```js
assert.strictEqual(passcodeDerivation(_keylog1), 531278);
```

`passcodeDerivation(keylog2)` should return `1230567`.

```js
assert.strictEqual(passcodeDerivation(_keylog2), 1230567);
```

`passcodeDerivation(keylog3)` should return `73162890`.

```js
assert.strictEqual(passcodeDerivation(_keylog3), 73162890);
```

# --seed--

## --after-user-code--

```js
const _keylog1 = [
  127,327,178,528,537,538,532,328,127,178,537,127,317,328,512,278,328,327,178,327,578,317,527,178,128,328,517,312,531,128,128,317,527,312,328,532,512,518,317,127,537,528,537,527,327,278,532,128, 318,517
];
const _keylog2 = [
  305,367,256,123,357,120,125,307,236,256,356,267,357,256,356,136,257,107,126,567,567,105,120,237,367,120,367,135,207,167,367,367,307,125,120,130,367,230,106,356,126,106,130,123,307,127,306,167,136,126
];
const _keylog3 = [
  319,680,180,690,129,620,762,689,762,318,368,710,720,710,629,168,160,689,716,731,736,729,316,729,729,710,769,290,719,680,318,389,162,289,162,718,729,319,790,680,890,362,319,760,316,729,380,319,728,716,
];
```

## --seed-contents--

```js
function passcodeDerivation(arr) {

  return true;
}

// Only change code above this line

const keylog1 = [
  319,680,180,690,129,620,762,689,762,318,368,710,720,710,629,168,160,689,716,731,736,729,316,729,729,710,769,290,719,680,318,389,162,289,162,718,729,319,790,680,890,362,319,760,316,729,380,319,728,716,
];

passcodeDerivation(keylog1);
```

# --solutions--

```js
function passcodeDerivation(arr) {
  const numbersInPasscode = [];
  const relativePositions = new Array(10)
    .fill()
    .map(() => new Array(10).fill(0));

  for (let i = 0; i < arr.length; i++) {
    const curAttempt = arr[i]
      .toString()
      .split('')
      .map(key => parseInt(key, 10));
    for (let j = 0; j < curAttempt.length; j++) {
      if (numbersInPasscode.indexOf(curAttempt[j]) === -1) {
        numbersInPasscode.push(curAttempt[j]);
      }
      for (let k = j + 1; k < curAttempt.length; k++) {
        relativePositions[curAttempt[j]][curAttempt[k]] += 1;
      }
    }
  }

  const ranks = {};
  for (let i = 0; i < numbersInPasscode.length; i++) {
    const curNumber = numbersInPasscode[i];
    ranks[curNumber] = relativePositions[curNumber].filter(
      count => count > 0
    ).length;
  }

  const passcode = numbersInPasscode
    .sort((i, j) => ranks[i] - ranks[j])
    .reverse()
    .join('');

  return parseInt(passcode, 10);
}
```