index.js

/**
 * Most simplest encryption scheme. Read more: [http://practicalcryptography.com/ciphers/caesar-cipher/]
 * @param {String} str
 * @param {Number} num
 */

function caesarCipher(str, num) {
  if (!num) throw new Error('Missing argument: num');

  const lowerCaseString = str.toLowerCase();
  const alphabets = 'abcdefghijklmnopqrstuvwxyz'.split('');
  const totalAlphabets = alphabets.length;
  let result = '';

  // handle large number, like 300 or -300
  num %= totalAlphabets;

  const alphabetsMap = new Map();

  for (const index in alphabets) {
    if (index) {
      alphabetsMap[alphabets[index]] = index;
    }
  }

  for (const index in lowerCaseString) {
    if (index) {
      // get the current character
      const currentCharacter = lowerCaseString[index];

      // if character is space, add it to the result and continue to next
      if (currentCharacter === ' ') {
        result += currentCharacter;
        continue;
      }

      // determine the new index
      /**
       * const currentIndex = alphabets.indexOf(currentCharacter);
       *
       * With indexOf complexity will be O(n*26)
       * With Map complexity will be O(n).
       */
      const currentIndex = Number(alphabetsMap[currentCharacter]);
      let newIndex = currentIndex + num;

      // if the index passes 25, restart from 0
      if (newIndex > totalAlphabets - 1) {
        newIndex -= totalAlphabets;
      }

      if (newIndex < 0) {
        newIndex = totalAlphabets + newIndex;
      }

      // check if the character in original string was upper case
      if (str[index] === str[index].toUpperCase()) {
        result += alphabets[newIndex].toUpperCase();
      } else {
        result += alphabets[newIndex];
      }
    }
  }
  return result;
}

module.exports = {
  caesarCipher,
};