AffineCypherjs

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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
      * @see - [wiki](https://en.wikipedia.org/wiki/Affine_cipher)
      */
    
    import { CoPrimeCheck } from '../Maths/CoPrimeCheck'
    // Default key for Affine Cipher
    const key = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
    
    /**
      * Fix result for negative value in modulas equation
      * @param {Number} n - Constant number
      * @param {Number} m - Modulos value
      * @return {Number} Return n mod m
      */
    function mod (n, m) {
      return ((n % m) + m) % m
    }
    
    /**
      * Modular multiplicative inverse
      * @param {Number} a - A coefficient
      * @param {Number} m - Modulos value
      * @return {Number} Return modular multiplicative inverse of coefficient a and modulos m
      */
    function inverseMod (a, m) {
      for (let x = 1; x < m; x++) {
        if (mod(a * x, m) === 1) return x
      }
    }
    
    /**
      * Argument validation
      * @param {String} str - String to be checked
      * @param {Number} a - A coefficient to be checked
      * @param {Number} b - B coefficient to be checked
      * @return {Boolean} Result of the checking
      */
    function isCorrectFormat (str, a, b) {
      if (typeof a !== 'number' || typeof b !== 'number') {
        throw new TypeError('Coefficient a, b should be number')
      }
    
      if (typeof str !== 'string') {
        throw new TypeError('Argument str should be String')
      }
    
      if (!CoPrimeCheck(a, 26)) {
        throw new Error(a + ' is not coprime of 26')
      }
    
      return true
    }
    
    /**
      * Find character index based on ASCII order
      * @param {String} char - Character index to be found
      * @return {Boolean} Character index
      */
    function findCharIndex (char) {
      return char.toUpperCase().charCodeAt(0) - ('A').charCodeAt(0)
    }
    
    /**
      * Encrypt a Affine Cipher
      * @param {String} str - String to be encrypted
      * @param {Number} a - A coefficient
      * @param {Number} b - B coefficient
      * @return {String} result - Encrypted string
      */
    
    function encrypt (str, a, b) {
      let result = ''
      if (isCorrectFormat(str, a, b)) {
        for (let x = 0; x < str.length; x++) {
          const charIndex = findCharIndex(str[x])
          if (charIndex < 0) result += '-1' + ' '
          else result += key.charAt(mod(a * charIndex + b, 26)) + ' '
        }
      }
      return result.trim()
    }
    
    /**
      * Decrypt a Affine Cipher
      * @param {String} str - String to be decrypted
      * @param {Number} a - A coefficient
      * @param {Number} b - B coefficient
      * @return {String} result - Decrypted string
      */
    function decrypt (str, a, b) {
      let result = ''
      if (isCorrectFormat(str, a, b)) {
        str = str.split(' ')
        for (let x = 0; x < str.length; x++) {
          if (str[x] === '-1') result += ' '
          else {
            const charIndex = findCharIndex(str[x])
            result += key[mod(inverseMod(a, 26) * (charIndex - b), 26)]
          }
        }
        return result
      }
    }
    export { encrypt, decrypt }
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