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86d333ee94
* chore: Switch to Node 20 + Vitest * chore: migrate to vitest mock functions * chore: code style (switch to prettier) * test: re-enable long-running test Seems the switch to Node 20 and Vitest has vastly improved the code's and / or the test's runtime! see #1193 * chore: code style * chore: fix failing tests * Updated Documentation in README.md * Update contribution guidelines to state usage of Prettier * fix: set prettier printWidth back to 80 * chore: apply updated code style automatically * fix: set prettier line endings to lf again * chore: apply updated code style automatically --------- Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com> Co-authored-by: Lars Müller <34514239+appgurueu@users.noreply.github.com>
106 lines
2.8 KiB
JavaScript
106 lines
2.8 KiB
JavaScript
/**
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* @description - The affine cipher is a type of monoalphabetic substitution cipher, where each letter in an alphabet is mapped to its numeric equivalent, encrypted using a simple mathematical function, and converted back to a letter
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* @see - [wiki](https://en.wikipedia.org/wiki/Affine_cipher)
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*/
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import { CoPrimeCheck } from '../Maths/CoPrimeCheck'
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// Default key for Affine Cipher
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const key = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
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/**
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* Fix result for negative value in modulas equation
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* @param {Number} n - Constant number
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* @param {Number} m - Modulos value
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* @return {Number} Return n mod m
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*/
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function mod(n, m) {
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return ((n % m) + m) % m
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}
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/**
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* Modular multiplicative inverse
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* @param {Number} a - A coefficient
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* @param {Number} m - Modulos value
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* @return {Number} Return modular multiplicative inverse of coefficient a and modulos m
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*/
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function inverseMod(a, m) {
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for (let x = 1; x < m; x++) {
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if (mod(a * x, m) === 1) return x
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}
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}
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/**
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* Argument validation
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* @param {String} str - String to be checked
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* @param {Number} a - A coefficient to be checked
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* @param {Number} b - B coefficient to be checked
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* @return {Boolean} Result of the checking
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*/
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function isCorrectFormat(str, a, b) {
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if (typeof a !== 'number' || typeof b !== 'number') {
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throw new TypeError('Coefficient a, b should be number')
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}
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if (typeof str !== 'string') {
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throw new TypeError('Argument str should be String')
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}
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if (!CoPrimeCheck(a, 26)) {
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throw new Error(a + ' is not coprime of 26')
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}
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return true
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}
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/**
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* Find character index based on ASCII order
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* @param {String} char - Character index to be found
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* @return {Boolean} Character index
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*/
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function findCharIndex(char) {
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return char.toUpperCase().charCodeAt(0) - 'A'.charCodeAt(0)
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}
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/**
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* Encrypt a Affine Cipher
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* @param {String} str - String to be encrypted
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* @param {Number} a - A coefficient
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* @param {Number} b - B coefficient
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* @return {String} result - Encrypted string
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*/
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function encrypt(str, a, b) {
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let result = ''
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if (isCorrectFormat(str, a, b)) {
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for (let x = 0; x < str.length; x++) {
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const charIndex = findCharIndex(str[x])
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if (charIndex < 0) result += '-1' + ' '
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else result += key.charAt(mod(a * charIndex + b, 26)) + ' '
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}
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}
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return result.trim()
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}
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/**
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* Decrypt a Affine Cipher
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* @param {String} str - String to be decrypted
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* @param {Number} a - A coefficient
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* @param {Number} b - B coefficient
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* @return {String} result - Decrypted string
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*/
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function decrypt(str, a, b) {
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let result = ''
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if (isCorrectFormat(str, a, b)) {
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str = str.split(' ')
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for (let x = 0; x < str.length; x++) {
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if (str[x] === '-1') result += ' '
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else {
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const charIndex = findCharIndex(str[x])
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result += key[mod(inverseMod(a, 26) * (charIndex - b), 26)]
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}
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}
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return result
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}
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}
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export { encrypt, decrypt }
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