Caesar Cipher 凱撒密碼

// return a new string
function caesarCipher (string, num) {
    ...
}

const caesarCipher = (str, num) => {
    // % 的計算不管正負，最後
    // 結果的正負號會與被除數一樣
    num = num % 26;
    const alphabet = [...`abcdefghijklmnopqrstuvwxyz`];

    const _getNewIndex = (currentIndex, shiftNum) => {
        const newIndex = (currentIndex + shiftNum) % 26;
        return newIndex < 0 ? newIndex + 26 : newIndex;
    };

    const _getLetterInCipher = (letter, i) => {
        const currentLetterIndex = alphabet.indexOf(letter);
        if (currentLetterIndex < 0) {
            // 如果找不到， indexOf 的結果會是 -1，代表該字非英文字母
            return letter;
        } else {
            const newIndex = _getNewIndex(
                currentLetterIndex,
                num
            );
            const newLetter = alphabet[newIndex];
            // 判斷該字母在原始 str 中是否為大寫
            const isUpperCase =
                str[i] === letter.toUpperCase();
            return isUpperCase
                ? newLetter.toUpperCase()
                : newLetter;
        }
    };

    // 將 str 一個個字母經過位移後，再組合成新 string
    let newStr = ``;
    str.toLowerCase()
        .split(``)
        .forEach(
            (letter, i) =>
                (newStr += _getLetterInCipher(letter, i))
        );

    return newStr;
};

console.log(caesarCipher('A*-*BcDe', 25));
console.log(caesarCipher('apple', 2));
console.log(caesarCipher('Bee!!', -28));

function caesarCipher(str,num) {
  num = num % 26;
  var lowerCaseString = str.toLowerCase();
  var alphabet = 'abcdefghijklmnopqrstuvwxyz'.split('');
  var newString = '';

  for (var i = 0; i < lowerCaseString.length; i++) {
    var currentLetter = lowerCaseString[i];
    if (currentLetter === ' ') {
      newString += currentLetter;
      continue;
    }
    var currentIndex = alphabet.indexOf(currentLetter);
    var newIndex = currentIndex + num;
    if (newIndex > 25) newIndex = newIndex - 26;
    if (newIndex < 0) newIndex = 26 + newIndex;
    if (str[i] === str[i].toUpperCase()) {
      newString += alphabet[newIndex].toUpperCase();
    }
    else newString += alphabet[newIndex];
  };

  return newString;
}
caesarCipher('Zoo Keeper', 2);