vigenere cipher

vigenere.ts

/**
 * alphabet order
 */
const alphabet = "abcdefghijklmnopqrstuvwxyz";
const alphabetOrderMap = new Map([...alphabet].map((c, i) => [c, i]));
const orderAlphabetMap = new Map([...alphabet].map((c, i) => [i, c]));

/**
 * utils
 */
function repeat(str: string, length: number): string {
  if (str.length <= 0 || length < 0) throw new Error("invalid argument length");

  const numStrRepeat = Math.floor(length / str.length);
  const numLeft = length % str.length;

  let ret = str.repeat(numStrRepeat);
  ret += str.slice(0, numLeft);
  return ret;
}

/**
 * tests
 */
function repeatTest() {
  assertEq(repeat("abcde", 0), "");
  assertEq(repeat("abcde", 3), "abc");
  assertEq(repeat("abcde", 5), "abcde");
  assertEq(repeat("abcde", 9), "abcdeabcd");
  assertEq(repeat("abcde", 13), "abcdeabcdeabc");
}

function integrateTest() {
  assertEq(vigenereCipher("attack at dawn", "lemon"), "lxfopv mh oeib");
}

/**
 * test utils
 */
function assertEq<T>(x: T, y: T) {
  if (x !== y) throw new Error(`${x} is not equal to ${y}`);
}

function runTests() {
  repeatTest();
  console.log("repeatTest ok");
  integrateTest();
  console.log("integrateTest ok");
}

runTests();


/**
 * code
 */
function vigenereCipher(originalStr: string, key: string): string {
  const expandedKey = repeat(key, originalStr.length);
  let ret = "";
  for (let i = 0; i < expandedKey.length; i++) {
    if (!alphabetOrderMap.has(originalStr[i])) {
      ret += originalStr[i];
      continue;
    }

    const charOrder = alphabetOrderMap.get(originalStr[i])!;
    const keyCharOrder = alphabetOrderMap.get(expandedKey[i])!;
    const destOrder = (charOrder + keyCharOrder) % 26;
    const destChar = orderAlphabetMap.get(destOrder);
    ret += destChar;
  }

  return ret;
}