Add decrypt function to Caesar cipher

- Implement decrypt function that reverses encryption with negative
  shift
- Export decrypt in package's public API
- Update tests with basic roundtrip and edge case tests
- Update documentation to reflect encryption/decryption support

README.md

@@ -1,6 +1,6 @@
 # 🔐 Caesar Cipher
 
-A beginner-friendly Python program that encodes messages with the Caesar cipher.
+A beginner-friendly Python program that encodes and decodes messages with the Caesar cipher.
 
 ## 📚 Table of Contents
 
@@ -14,7 +14,7 @@ A beginner-friendly Python program that encodes messages with the Caesar cipher.
 
 ## 🧭 Overview
 
-The Caesar cipher shifts every letter by a fixed number of places (with a shift of 3, `E` becomes `H`). This project, from Udemy's *100 Days of Code: The Complete Python Pro Bootcamp* (Day 8), finds letter positions with `list.index` and wraps around the alphabet with the modulo operator. Part 1 covers encryption; decryption and a combined program follow in later parts. Code lives in `src/`, tests in `tests/`, docs in `docs/`.
+The Caesar cipher shifts every letter by a fixed number of places (with a shift of 3, `E` becomes `H`). This project, from Udemy's *100 Days of Code: The Complete Python Pro Bootcamp* (Day 8), finds letter positions with `list.index` and wraps around the alphabet with the modulo operator. Parts 1 and 2 cover encryption and decryption; a combined program follows in Part 3. Code lives in `src/`, tests in `tests/`, docs in `docs/`.
 
 ## 📋 Requirements
 

docs/index.md

@@ -3,7 +3,7 @@
 Each letter is moved `shift` places through the alphabet:
 `new_position = (ALPHABET.index(letter) + shift) % 26`.
 The modulo keeps positions in range for shifts past `z`.
-Non-letters are left untouched and letter case is preserved.
+Decryption is encryption with `-shift`. Non-letters are left untouched and letter case is preserved.
 
 API reference: run `doxygen Doxyfile` from the repository root and open
 `docs/doxygen/html/index.html`. The step-by-step plan is in [plan.md](plan.md).

src/caesar_cipher/__init__.py

@@ -1,6 +1,6 @@
 ## @file __init__.py
 #  @brief Public API of the caesar_cipher package.
 
-from caesar_cipher.cipher import encrypt
+from caesar_cipher.cipher import decrypt, encrypt
 
-__all__ = ["encrypt"]
+__all__ = ["encrypt", "decrypt"]

src/caesar_cipher/cipher.py

@@ -1,5 +1,5 @@
 ## @file cipher.py
-#  @brief Caesar cipher encryption.
+#  @brief Caesar cipher encryption and decryption.
 
 from caesar_cipher.constants import ALPHABET
 
@@ -19,3 +19,11 @@ def encrypt(text: str, shift: int) -> str:
         else:
             result.append(char)
     return "".join(result)
+
+
+def decrypt(text: str, shift: int) -> str:
+    ## @brief Decrypt a Caesar-cipher text.
+    #  @param text  Encrypted text.
+    #  @param shift Shift that was used to encrypt.
+    #  @return The original text.
+    return encrypt(text, -shift)

tests/test_cipher.py

@@ -1,4 +1,4 @@
-from caesar_cipher import encrypt
+from caesar_cipher import decrypt, encrypt
 from caesar_cipher.env import load_env
 
 
@@ -24,3 +24,16 @@ def test_load_env(tmp_path):
     f.write_text("# c\nA=1\nB='two'\n", encoding="utf-8")
     assert load_env(f) == {"A": "1", "B": "two"}
     assert load_env(tmp_path / "missing") == {}
+
+
+def test_decrypt_basic():
+    assert decrypt("mjqqt", 5) == "hello"
+
+
+def test_decrypt_roundtrip():
+    assert decrypt(encrypt("Hello, World!", 7), 7) == "Hello, World!"
+
+
+def test_decrypt_wraps_around_alphabet():
+    assert decrypt("abc", 3) == "xyz"
+    assert decrypt("def", 29) == "abc"