Add Concept Exercise for 'randomness': Captain's Log

exercises/concept/captains-log/.meta/config.json

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+{
+  "authors": [
+    "SneakyMallard"
+  ],
+  "contributors": [],
+  "forked-from": ["java/captains-log"],
+  "concept": [
+    {
+      "uuid": "need-to-generate",
+      "slug": "captains-log",
+      "name": "Captain's Log",
+      "concepts": ["randomness"],
+      "prerequisites": ["numbers", "arithmetic-operators"]
+    }
+  ],
+  "files": {
+    "solution": ["captains-log.js"],
+    "test": ["captains-log.spec.js"],
+    "exemplar": [".meta/exemplar.js"]
+  },
+  "blurb": "Learn about randomness and the Math.random() function while helping Mary generate stardates and starship registry numbers for her Star Trek roleplay."
+}

exercises/concept/captains-log/.meta/design.md

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+# Design
+
+## Goal
+
+The goal of this exercise is to teach the student how to generate psuedorandom numbers in JavaScript.
+
+## Learning objectives
+
+- Know how to generate a random number with `Math.random()`
+- Know how to generate a random number in a range.
+- Know how to generate a random integer.
+
+## Out of scope
+
+- Details of pseudorandom number generation in general.
+- Different algorithms for pseudorandom number generation.
+
+
+## Concepts
+
+The Concepts this exercise unlocks are:
+
+- `randomness`: Know of the `Math.random()` function and know how to use it to generate random numbers.
+
+## Prerequisites
+
+- `numbers`: Know how numbers work in JavaScript. Know some number methods.

exercises/concept/captains-log/.meta/exemplar.js

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+/// <reference path="./global.d.ts" />
+// @ts-check
+
+/**
+ * Generates a random starship registry number.
+ *
+ * @returns {string} the generated registry number.
+ */
+export function randomShipRegistryNumber() {
+  return "NCC-"+Math.floor(1000 + Math.random()*9000)
+}
+
+/**
+ * Generates a random stardate.
+ *
+ * @returns {number} a stardate between 41000 (inclusive) and 42000 (exclusive).
+ */
+export function randomStardate() {
+  return 41000 + Math.random() * 1000
+}
+
+/**
+ * Generates a random planet class.
+ *
+ * @returns {string} a one-letter planet class.
+ */
+export function randomPlanetClass() {
+  const planetClasses = ['D', 'H', 'J', 'K', 'L', 'M', 'N', 'R', 'T', 'Y']
+  return planetClasses[Math.floor(Math.random() * 10)]
+}

exercises/concept/captains-log/captains-log.js

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+/// <reference path="./global.d.ts" />
+// @ts-check
+
+/**
+ * Generates a random starship registry number.
+ *
+ * @returns {string} the generated registry number.
+ */
+export function randomShipRegistryNumber() {
+  throw new Error("Please remove this line and implement the randomShipRegistryNumber() function");
+}
+
+/**
+ * Generates a random stardate.
+ *
+ * @returns {number} a stardate between 41000 (inclusive) and 42000 (exclusive).
+ */
+export function randomStardate() {
+  throw new Error("Please remove this line and implement the randomStardate() function");
+}
+
+
+/**
+ * Generates a random planet class.
+ *
+ * @returns {string} a one-letter planet class.
+ */
+export function randomPlanetClass() {
+  throw new Error("Please remove this line and implement the randomStardate() function");
+}

exercises/concept/captains-log/captains-log.spec.js

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+import { describe, expect, test } from '@jest/globals';
+import { randomShipRegistryNumber, randomStardate, randomPlanetClass } from './captains-log';
+describe('randomShipRegistryNumber',() => {
+  test('registry numbers are valid',() => {
+    for (let i=0;i<4; i++){
+      expect(randomShipRegistryNumber()).toMatch(/NCC-[1-9][0-9]{3}/))
+    }
+  });
+  test('returns a random registry number',() => {
+    expect(randomShipRegistryNumber()).not.toEqual(randomShipRegistryNumber()) 
+  });
+});
+function loadDie(...values) {
+  const originalRandom = Math.random()
+
+  Math.random = function loadedDie { 
+    if (values.length === 0) {
+      return originalRandom();
+    }
+    return values.shift();
+  }
+  return () => {
+    Math.random = originalRandom;
+  }
+}
+describe('randomStardate',() => {
+  test('stardate is between 41000 and 42000',() => {
+    const restore = loadDie(
+      0, 0, 0, 0, 0, 0,
+      1, 1, 1, 1, 1, 1,
+      0.5, 0.5, 0.5, 0.5, 0.5, 0.5
+    );
+
+    // 6 * 0, 6 * 1, 6 * 0.5 and everything else random
+
+    for (let i = 0; i < 10_000; i++) {
+      const starDate = randomStardate()
+      expect(starDate).toBeGreaterThanOrEqual(41_000);
+      expect(starDate).toBeLessThan(42_000);
+    }
+    restore();
+  }
+}
+describe('randomPlanetClass', () => {
+  test('planet classes are valid',  () => {
+    const planetClasses = ['D', 'H', 'J', 'K', 'L', 'M', 'N', 'R', 'T', 'Y'];
+    for (let i=0;i<4; i++){
+      expect(planetClasses).toContain(randomPlanetClass())
+    }
+  });
+  test('returns a random planet class',() => {
+    expect(randomPlanetClass()).not.toEqual(randomPlanetClass())
+  });
+});

exercises/concept/captains-log/docs/hints.md

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+# Hints
+
+## 1. Generate a random starship registry number
+  - To generate a random number in the range _min_ (inclusive) to _max_ (exclusive) you can use the snippet `min + Math.random()*(max - min)`.
+  - There is a [built in function][floor] for turning a floating point number into an integer.
+##  2.Generate a random stardate
+ - To generate a random number in the range _min_ (inclusive) to _max_ (exclusive) you can use the snippet `min + Math.random()*(max - min)`.
+## 3. Generate a random planet
+- You can use a randomly generated integer as an array index.
+
+[floor]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/floor

exercises/concept/captains-log/docs/instructions.md

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+## Instructions
+
+Mary is a big fan of the TV series Star Trek: The Next Generation. She often plays pen-and-paper role playing games, where she and her friends pretend to be the crew of the Starship Enterprise. Mary's character is Captain Picard, which means she has to keep the captain's log. She loves the creative part of the game, but doesn't like to generate random data on the spot.
+
+Help Mary by creating random generators for data commonly appearing in the captain's log.
+### 1. Generate a random starship registry number
+
+Enterprise (registry number NCC-1701) is not the only starship flying around! When it rendezvous with another starship, Mary needs to log the registry number of that starship.
+
+Registry numbers start with the prefix "NCC-" and then use a number from 1000 to 9999 (both inclusive).
+
+Implement the randomShipRegistryNumber() function that returns a random starship registry number.
+```javascript
+randomShipRegistryNumber()
+// => "NCC-1947"
+```
+### 2. Generate a random stardate
+
+What's the use of a log if it doesn't include dates?
+
+A stardate is a floating point number. The adventures of the Starship Enterprise from the first season of The Next Generation take place between the stardates 41000.0 and 42000.0. The "4" stands for the 24th century, the "1" for the first season.
+
+Implement the function randomStardate that returns a floating point number between 41000.0 (inclusive) and 42000.0 (exclusive).
+
+```javascript
+randomStardate()
+// => 41458.15721310934
+```
+### 3. Generate a random planet
+
+The Starship Enterprise encounters many planets in its travels. Planets in the Star Trek universe are split into categories based on their properties. For example, Earth is a class M planet. All possible planetary classes are: D, H, J, K, L, M, N, R, T, and Y.
+
+Implement the randomPlanetClassfunction. It should return one of the planetary classes at random.
+```javascript
+randomPlanetClass()
+// => "K"
+```

exercises/concept/captains-log/docs/introduction.md

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+# Introduction
+
+Many programs need (apparently) random values to simulate real-world events.
+
+Common, familiar examples include:
+
+  - A coin toss: a random value from ('H', 'T').
+  - The roll of a die: a random integer from 1 to 6.
+  - Shuffling a deck of cards: a random ordering of a card list.
+  - The creation of trees and bushes in a 3-D graphics simulation.
+
+Generating truly random values with a computer is a [surprisingly difficult technical challenge][why-randomness-is-hard], so you may see these results referred to as "pseudorandom".
+## Generating random numbers
+In Javascript, you can generate psuedorandom numbers using the [`Math.random()`][Math.random] function.
+It will return a psuedorandom floating-point number between 0 (inclusive), and 1 (exclusive).
+
+To get a random number between _min_ (inclusive) and _max_ (exclusive) you can use a function something like this: 
+```javascript
+function getRandomInRange(min, max) {
+  return min + Math.random() * (max - min) ;
+}
+getRandomInRange(4, 10)
+// => 5.72
+```
+## Generating random integers
+To generate a random integer, you can use `Math.floor()` or `Math.ceil()` to turn a randomly generated number into an integer.
+~~~~exercism/caution
+
+The `Math.random()` function should NOT be used for security and cryptographic applications!!
+
+Instead, you can use the Web Crypto API, which provides various cryptographic functions.
+~~~~
+
+[why-randomness-is-hard]: https://www.malwarebytes.com/blog/news/2013/09/in-computers-are-random-numbers-really-random
+[Math.random]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/random