116 lines
3.2 KiB
Markdown
116 lines
3.2 KiB
Markdown
---
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id: 5900f3bb1000cf542c50fece
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title: 'Problem 79: Passcode derivation'
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challengeType: 5
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forumTopicId: 302192
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dashedName: problem-79-passcode-derivation
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---
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# --description--
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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`.
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The arrays, `keylog1`, `keylog2`, and `keylog3`, contains fifty successful login attempts.
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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.
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# --hints--
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`passcodeDerivation(keylog1)` should return a number.
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```js
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assert(typeof passcodeDerivation(_keylog1) === 'number');
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```
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`passcodeDerivation(keylog1)` should return `531278`.
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```js
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assert.strictEqual(passcodeDerivation(_keylog1), 531278);
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```
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`passcodeDerivation(keylog2)` should return `1230567`.
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```js
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assert.strictEqual(passcodeDerivation(_keylog2), 1230567);
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```
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`passcodeDerivation(keylog3)` should return `73162890`.
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```js
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assert.strictEqual(passcodeDerivation(_keylog3), 73162890);
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```
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# --seed--
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## --after-user-code--
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```js
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const _keylog1 = [
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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
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];
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const _keylog2 = [
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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
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];
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const _keylog3 = [
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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,
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];
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```
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## --seed-contents--
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```js
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function passcodeDerivation(arr) {
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return true;
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}
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// Only change code above this line
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const keylog1 = [
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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,
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];
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passcodeDerivation(keylog1);
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```
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# --solutions--
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```js
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function passcodeDerivation(arr) {
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const numbersInPasscode = [];
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const relativePositions = new Array(10)
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.fill()
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.map(() => new Array(10).fill(0));
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for (let i = 0; i < arr.length; i++) {
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const curAttempt = arr[i]
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.toString()
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.split('')
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.map(key => parseInt(key, 10));
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for (let j = 0; j < curAttempt.length; j++) {
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if (numbersInPasscode.indexOf(curAttempt[j]) === -1) {
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numbersInPasscode.push(curAttempt[j]);
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}
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for (let k = j + 1; k < curAttempt.length; k++) {
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relativePositions[curAttempt[j]][curAttempt[k]] += 1;
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}
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}
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}
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const ranks = {};
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for (let i = 0; i < numbersInPasscode.length; i++) {
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const curNumber = numbersInPasscode[i];
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ranks[curNumber] = relativePositions[curNumber].filter(
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count => count > 0
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).length;
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}
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const passcode = numbersInPasscode
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.sort((i, j) => ranks[i] - ranks[j])
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.reverse()
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.join('');
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return parseInt(passcode, 10);
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}
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```
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