add affine-cipher

config.json

@@ -1408,6 +1408,14 @@
           "lists"
         ]
       },
+      {
+        "slug": "affine-cipher",
+        "name": "Affine Cipher",
+        "uuid": "2326f3c2-4e92-4111-b2f9-6d6a3bcbdfca",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 5
+      },
       {
         "slug": "all-your-base",
         "name": "All Your Base",

exercises/practice/affine-cipher/.docs/instructions.md

@@ -0,0 +1,74 @@
+# Instructions
+
+Create an implementation of the affine cipher, an ancient encryption system created in the Middle East.
+
+The affine cipher is a type of monoalphabetic substitution cipher.
+Each character is mapped to its numeric equivalent, encrypted with a mathematical function and then converted to the letter relating to its new numeric value.
+Although all monoalphabetic ciphers are weak, the affine cipher is much stronger than the Atbash cipher, because it has many more keys.
+
+[//]: # " monoalphabetic as spelled by Merriam-Webster, compare to polyalphabetic "
+
+## Encryption
+
+The encryption function is:
+
+```text
+E(x) = (ai + b) mod m
+```
+
+Where:
+
+- `i` is the letter's index from `0` to the length of the alphabet - 1.
+- `m` is the length of the alphabet.
+  For the Latin alphabet `m` is `26`.
+- `a` and `b` are integers which make up the encryption key.
+
+Values `a` and `m` must be _coprime_ (or, _relatively prime_) for automatic decryption to succeed, i.e., they have number `1` as their only common factor (more information can be found in the [Wikipedia article about coprime integers][coprime-integers]).
+In case `a` is not coprime to `m`, your program should indicate that this is an error.
+Otherwise it should encrypt or decrypt with the provided key.
+
+For the purpose of this exercise, digits are valid input but they are not encrypted.
+Spaces and punctuation characters are excluded.
+Ciphertext is written out in groups of fixed length separated by space, the traditional group size being `5` letters.
+This is to make it harder to guess encrypted text based on word boundaries.
+
+## Decryption
+
+The decryption function is:
+
+```text
+D(y) = (a^-1)(y - b) mod m
+```
+
+Where:
+
+- `y` is the numeric value of an encrypted letter, i.e., `y = E(x)`
+- it is important to note that `a^-1` is the modular multiplicative inverse (MMI) of `a mod m`
+- the modular multiplicative inverse only exists if `a` and `m` are coprime.
+
+The MMI of `a` is `x` such that the remainder after dividing `ax` by `m` is `1`:
+
+```text
+ax mod m = 1
+```
+
+More information regarding how to find a Modular Multiplicative Inverse and what it means can be found in the [related Wikipedia article][mmi].
+
+## General Examples
+
+- Encrypting `"test"` gives `"ybty"` with the key `a = 5`, `b = 7`
+- Decrypting `"ybty"` gives `"test"` with the key `a = 5`, `b = 7`
+- Decrypting `"ybty"` gives `"lqul"` with the wrong key `a = 11`, `b = 7`
+- Decrypting `"kqlfd jzvgy tpaet icdhm rtwly kqlon ubstx"` gives `"thequickbrownfoxjumpsoverthelazydog"` with the key `a = 19`, `b = 13`
+- Encrypting `"test"` with the key `a = 18`, `b = 13` is an error because `18` and `26` are not coprime
+
+## Example of finding a Modular Multiplicative Inverse (MMI)
+
+Finding MMI for `a = 15`:
+
+- `(15 * x) mod 26 = 1`
+- `(15 * 7) mod 26 = 1`, ie. `105 mod 26 = 1`
+- `7` is the MMI of `15 mod 26`
+
+[mmi]: https://en.wikipedia.org/wiki/Modular_multiplicative_inverse
+[coprime-integers]: https://en.wikipedia.org/wiki/Coprime_integers

exercises/practice/affine-cipher/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "BNAndras"
+  ],
+  "files": {
+    "solution": [
+      "affine-cipher.ts"
+    ],
+    "test": [
+      "affine-cipher.test.ts"
+    ],
+    "example": [
+      ".meta/proof.ci.ts"
+    ]
+  },
+  "blurb": "Create an implementation of the Affine cipher, an ancient encryption algorithm from the Middle East.",
+  "source": "Wikipedia",
+  "source_url": "https://en.wikipedia.org/wiki/Affine_cipher"
+}

exercises/practice/affine-cipher/.meta/proof.ci.ts

@@ -0,0 +1,65 @@
+export type Key = {
+  a: number
+  b: number
+}
+
+const alphabetLength = 26
+
+const mod = (value: number, modulus: number): number =>
+  ((value % modulus) + modulus) % modulus
+
+const gcd = (a: number, b: number): number => {
+  while (b !== 0) {
+    const remainder = mod(a, b)
+    a = b
+    b = remainder
+  }
+
+  return Math.abs(a)
+}
+
+const assertValidKey = (key: Key): void => {
+  if (gcd(key.a, alphabetLength) !== 1) {
+    throw new Error('a and m must be coprime.')
+  }
+}
+
+const modularInverse = (value: number): number => {
+  for (let candidate = 1; candidate < alphabetLength; candidate += 1) {
+    if (mod(value * candidate, alphabetLength) === 1) {
+      return candidate
+    }
+  }
+
+  throw new Error('a and m must be coprime.')
+}
+
+const normalize = (phrase: string): string =>
+  phrase.toLowerCase().replace(/[^a-z0-9]/g, '')
+
+const chunk = (text: string): string => text.match(/.{1,5}/g)?.join(' ') ?? ''
+
+const letterIndex = (char: string): number =>
+  char.charCodeAt(0) - 'a'.charCodeAt(0)
+
+const letterFor = (index: number): string =>
+  String.fromCharCode(mod(index, alphabetLength) + 'a'.charCodeAt(0))
+
+const translate = (
+  text: string,
+  transformIndex: (index: number) => number
+): string =>
+  text.replace(/[a-z]/g, (char) => letterFor(transformIndex(letterIndex(char))))
+
+export const encode = (phrase: string, key: Key): string => {
+  assertValidKey(key)
+
+  return chunk(translate(normalize(phrase), (index) => key.a * index + key.b))
+}
+
+export const decode = (phrase: string, key: Key): string => {
+  assertValidKey(key)
+
+  const inverse = modularInverse(key.a)
+  return translate(normalize(phrase), (index) => inverse * (index - key.b))
+}

exercises/practice/affine-cipher/.meta/tests.toml

@@ -0,0 +1,58 @@
+# This is an auto-generated file.
+#
+# Regenerating this file via `configlet sync` will:
+# - Recreate every `description` key/value pair
+# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
+# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
+# - Preserve any other key/value pair
+#
+# As user-added comments (using the # character) will be removed when this file
+# is regenerated, comments can be added via a `comment` key.
+
+[2ee1d9af-1c43-416c-b41b-cefd7d4d2b2a]
+description = "encode -> encode yes"
+
+[785bade9-e98b-4d4f-a5b0-087ba3d7de4b]
+description = "encode -> encode no"
+
+[2854851c-48fb-40d8-9bf6-8f192ed25054]
+description = "encode -> encode OMG"
+
+[bc0c1244-b544-49dd-9777-13a770be1bad]
+description = "encode -> encode O M G"
+
+[381a1a20-b74a-46ce-9277-3778625c9e27]
+description = "encode -> encode mindblowingly"
+
+[6686f4e2-753b-47d4-9715-876fdc59029d]
+description = "encode -> encode numbers"
+
+[ae23d5bd-30a8-44b6-afbe-23c8c0c7faa3]
+description = "encode -> encode deep thought"
+
+[c93a8a4d-426c-42ef-9610-76ded6f7ef57]
+description = "encode -> encode all the letters"
+
+[0673638a-4375-40bd-871c-fb6a2c28effb]
+description = "encode -> encode with a not coprime to m"
+
+[3f0ac7e2-ec0e-4a79-949e-95e414953438]
+description = "decode -> decode exercism"
+
+[241ee64d-5a47-4092-a5d7-7939d259e077]
+description = "decode -> decode a sentence"
+
+[33fb16a1-765a-496f-907f-12e644837f5e]
+description = "decode -> decode numbers"
+
+[20bc9dce-c5ec-4db6-a3f1-845c776bcbf7]
+description = "decode -> decode all the letters"
+
+[623e78c0-922d-49c5-8702-227a3e8eaf81]
+description = "decode -> decode with no spaces in input"
+
+[58fd5c2a-1fd9-4563-a80a-71cff200f26f]
+description = "decode -> decode with too many spaces"
+
+[b004626f-c186-4af9-a3f4-58f74cdb86d5]
+description = "decode -> decode with a not coprime to m"

exercises/practice/affine-cipher/.vscode/extensions.json

@@ -0,0 +1,7 @@
+{
+  "recommendations": [
+    "arcanis.vscode-zipfs",
+    "dbaeumer.vscode-eslint",
+    "esbenp.prettier-vscode"
+  ]
+}

exercises/practice/affine-cipher/.vscode/settings.json

@@ -0,0 +1,7 @@
+{
+  "cSpell.words": ["exercism"],
+  "search.exclude": {
+    "**/.yarn": true,
+    "**/.pnp.*": true
+  }
+}

exercises/practice/affine-cipher/.yarnrc.yml

@@ -0,0 +1,3 @@
+compressionLevel: mixed
+
+enableGlobalCache: true

exercises/practice/affine-cipher/affine-cipher.test.ts

@@ -0,0 +1,102 @@
+import { describe, expect, it, xit } from '@jest/globals'
+import { encode, decode } from './affine-cipher.ts'
+
+describe('Affine Cipher', () => {
+  describe('encode', () => {
+    it('encode yes', () => {
+      const key = { a: 5, b: 7 }
+      expect(encode('yes', key)).toEqual('xbt')
+    })
+
+    xit('encode no', () => {
+      const key = { a: 15, b: 18 }
+      expect(encode('no', key)).toEqual('fu')
+    })
+
+    xit('encode OMG', () => {
+      const key = { a: 21, b: 3 }
+      expect(encode('OMG', key)).toEqual('lvz')
+    })
+
+    xit('encode O M G', () => {
+      const key = { a: 25, b: 47 }
+      expect(encode('O M G', key)).toEqual('hjp')
+    })
+
+    xit('encode mindblowingly', () => {
+      const key = { a: 11, b: 15 }
+      expect(encode('mindblowingly', key)).toEqual('rzcwa gnxzc dgt')
+    })
+
+    xit('encode numbers', () => {
+      const key = { a: 3, b: 4 }
+      expect(encode('Testing,1 2 3, testing.', key)).toEqual(
+        'jqgjc rw123 jqgjc rw'
+      )
+    })
+
+    xit('encode deep thought', () => {
+      const key = { a: 5, b: 17 }
+      expect(encode('Truth is fiction.', key)).toEqual('iynia fdqfb ifje')
+    })
+
+    xit('encode all the letters', () => {
+      const key = { a: 17, b: 33 }
+      const phrase = 'The quick brown fox jumps over the lazy dog.'
+      const expected = 'swxtj npvyk lruol iejdc blaxk swxmh qzglf'
+      expect(encode(phrase, key)).toEqual(expected)
+    })
+
+    xit('encode with a not coprime to m', () => {
+      const key = { a: 6, b: 17 }
+      expect(() => encode('This is a test.', key)).toThrow(
+        'a and m must be coprime.'
+      )
+    })
+  })
+
+  describe('decode', () => {
+    xit('decode exercism', () => {
+      const key = { a: 3, b: 7 }
+      expect(decode('tytgn fjr', key)).toEqual('exercism')
+    })
+
+    xit('decode a sentence', () => {
+      const key = { a: 19, b: 16 }
+      const phrase = 'qdwju nqcro muwhn odqun oppmd aunwd o'
+      const expected = 'anobstacleisoftenasteppingstone'
+      expect(decode(phrase, key)).toEqual(expected)
+    })
+
+    xit('decode numbers', () => {
+      const key = { a: 25, b: 7 }
+      expect(decode('odpoz ub123 odpoz ub', key)).toEqual('testing123testing')
+    })
+
+    xit('decode all the letters', () => {
+      const key = { a: 17, b: 33 }
+      const phrase = 'swxtj npvyk lruol iejdc blaxk swxmh qzglf'
+      const expected = 'thequickbrownfoxjumpsoverthelazydog'
+      expect(decode(phrase, key)).toEqual(expected)
+    })
+
+    xit('decode with no spaces in input', () => {
+      const key = { a: 17, b: 33 }
+      const phrase = 'swxtjnpvyklruoliejdcblaxkswxmhqzglf'
+      const expected = 'thequickbrownfoxjumpsoverthelazydog'
+      expect(decode(phrase, key)).toEqual(expected)
+    })
+
+    xit('decode with too many spaces', () => {
+      const key = { a: 15, b: 16 }
+      expect(decode('vszzm    cly   yd cg    qdp', key)).toEqual(
+        'jollygreengiant'
+      )
+    })
+
+    xit('decode with a not coprime to m', () => {
+      const key = { a: 13, b: 5 }
+      expect(() => decode('Test', key)).toThrow('a and m must be coprime.')
+    })
+  })
+})

exercises/practice/affine-cipher/affine-cipher.ts

@@ -0,0 +1,7 @@
+export const encode = (phrase: unknown, key: unknown): unknown => {
+  throw new Error('Remove this line and implement the function')
+}
+
+export const decode = (phrase: unknown, key: unknown): unknown => {
+  throw new Error('Remove this line and implement the function')
+}

exercises/practice/affine-cipher/babel.config.cjs

@@ -0,0 +1,5 @@
+module.exports = {
+  // eslint-disable-next-line @typescript-eslint/no-require-imports
+  presets: [[require('@exercism/babel-preset-typescript'), { corejs: '3.38' }]],
+  plugins: [],
+}

exercises/practice/affine-cipher/eslint.config.mjs

@@ -0,0 +1,26 @@
+// @ts-check
+
+import tsEslint from 'typescript-eslint'
+import config from '@exercism/eslint-config-typescript'
+import maintainersConfig from '@exercism/eslint-config-typescript/maintainers.mjs'
+
+export default [
+  ...tsEslint.config(...config, {
+    files: ['.meta/proof.ci.ts', '.meta/exemplar.ts', '*.test.ts'],
+    extends: maintainersConfig,
+  }),
+  {
+    ignores: [
+      // # Protected or generated
+      '.git/**/*',
+      '.vscode/**/*',
+
+      //# When using npm
+      'node_modules/**/*',
+
+      // # Configuration files
+      'babel.config.cjs',
+      'jest.config.cjs',
+    ],
+  },
+]