Caesar cipher demo

caesar-cipher-demo.ts

/**
 * My kid shared his fascination with Caesar ciphers yesterday. He
 * challenged me to decrypt one of his messages without the cipher
 * offset. I tried to do it by hand, but it was surprisingly difficult
 * for his short message. He made me promise not to use frequency
 * analysis, but he never said anything about using a computer to brute
 * force it!
 */

const validCharacters = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z'] as const;
type ValidCharacters = typeof validCharacters;
type ValidCharacter = ValidCharacters[number];

const offsetCharacter = (inputCharacter: ValidCharacter, offset: number): ValidCharacter => {
  const indexOfInput = validCharacters.indexOf(inputCharacter);
  const indexOfOffset = (indexOfInput + offset) % validCharacters.length;
  return validCharacters[indexOfOffset];
};

const offsetText = (inputText: string, offset: number): string => {
  const capitalizedInputText = inputText.toUpperCase();
  const characters = capitalizedInputText.split('');
  const offsetCharacters = characters.map(character =>
    validCharacters.includes(character as ValidCharacter)
      ? offsetCharacter(character as ValidCharacter, offset)
      : character
  );
  const offsetText = offsetCharacters.join('');
  return offsetText;
};

const encryptedText = 'can you understand me';
for (let offset = 0; offset <= validCharacters.length; offset += 1) {
  console.log(`${ offset }: ${ offsetText(encryptedText, offset) }`);
}