Caesar Cipher / ROT13

A Caesar cipher shifts every letter of a message a fixed number of places along the alphabet — shift by 3 and a becomes d, b becomes e — wrapping past z back to a, keeping each letter's case, and leaving spaces, digits, and punctuation untouched. It is one of the oldest known ciphers, named for Julius Caesar, and a common warm-up in coding interviews (background on Wikipedia). You will implement three functions on one object: encode to shift a message, decode to shift it back, and rot13, the shift-by-13 case that is its own inverse.

Signature

caesarCipher.encode(text, shift)  // shift each letter forward by `shift`, wrapping z -> a
caesarCipher.decode(text, shift)  // undo an encode that used the same shift
caesarCipher.rot13(text)          // encode with shift 13 — its own inverse

All three live on one object: caesarCipher = { encode, decode, rot13 }. shift is any integer.

Examples

caesarCipher.encode('abc', 3);            // 'def'
caesarCipher.encode('xyz', 3);            // 'abc'   — wraps past z
caesarCipher.encode('Hello, World!', 3);  // 'Khoor, Zruog!'
caesarCipher.encode('abc', 29);           // 'def'   — 29 mod 26 is 3
caesarCipher.encode('def', -3);           // 'abc'   — negative shift

caesarCipher.decode('Khoor, Zruog!', 3);           // 'Hello, World!'
caesarCipher.rot13('abc');                          // 'nop'
caesarCipher.rot13(caesarCipher.rot13('Secret!'));  // 'Secret!'  — self-inverse

Notes

• Preserve case — A-Z stay uppercase and a-z stay lowercase. Anchor each letter to the start of its own case.
• Wrap around — after z comes a again; the shift is taken modulo 26.
• Pass non-letters through — digits, spaces, and punctuation are copied unchanged, never shifted.
• Any integer shift — shift may be 0, larger than 26, or negative; normalize it with modulo 26. Watch that a negative shift % 26 is still negative in JavaScript.
• decode undoes encode — decode(encode(text, s), s) returns the original text for every shift s.
• Not encryption — the Caesar cipher is a toy anyone can undo; do not use it to protect real secrets.