Refactor to unified caesar() function with encode/decode support

- Replace separate encrypt/decrypt with shared caesar() function
- Add direction constants (DIRECTION_ENCODE, DIRECTION_DECODE) and
  prompts
- Update main.py to support interactive encode/decode loop
- Keep encrypt/decrypt as wrappers for backward compatibility
- Add tests for caesar() with both directions and error handling
- Update tests for main.py with interactive scenario tests

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. The interactive program lets you encode or decode as many messages as you like. 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.
+Decoding shifts by `-shift`; one shared `caesar(text, shift, direction)` function does both. 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 caesar, decrypt, encrypt
 
-__all__ = ["encrypt"]
+__all__ = ["caesar", "encrypt", "decrypt"]

src/caesar_cipher/cipher.py

@@ -1,14 +1,21 @@
 ## @file cipher.py
-#  @brief Caesar cipher encryption.
+#  @brief Caesar cipher: one shared function for encoding and decoding.
 
-from caesar_cipher.constants import ALPHABET
+from caesar_cipher.constants import ALPHABET, DIRECTION_DECODE, DIRECTION_ENCODE
 
 
-def encrypt(text: str, shift: int) -> str:
+def caesar(text: str, shift: int, direction: str) -> str:
-    ## @brief Encrypt a text with the Caesar cipher.
+    ## @brief Encode or decode a text with the Caesar cipher.
-    #  @param text  Original text.
+    #  @param text      Text to transform.
-    #  @param shift Positions to move each letter (may be negative or > 26).
+    #  @param shift     Positions to move each letter (may be negative or > 26).
-    #  @return The encrypted text; non-letters are kept, case is preserved.
+    #  @param direction DIRECTION_ENCODE or DIRECTION_DECODE.
+    #  @return The transformed text; non-letters are kept, case is preserved.
+    #  @throws ValueError If direction is not a known direction.
+    if direction == DIRECTION_DECODE:
+        shift = -shift
+    elif direction != DIRECTION_ENCODE:
+        raise ValueError(f"Unknown direction: {direction!r}")
+
     result = []
     for char in text:
         lower = char.lower()
@@ -19,3 +26,19 @@ def encrypt(text: str, shift: int) -> str:
         else:
             result.append(char)
     return "".join(result)
+
+
+def encrypt(text: str, shift: int) -> str:
+    ## @brief Encrypt a text with the Caesar cipher.
+    #  @param text  Original text.
+    #  @param shift Number of positions to shift forward.
+    #  @return The encrypted text.
+    return caesar(text, shift, DIRECTION_ENCODE)
+
+
+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 caesar(text, shift, DIRECTION_DECODE)

src/caesar_cipher/constants.py

@@ -4,10 +4,38 @@
 ## Lowercase alphabet used to look up letter positions.
 ALPHABET = list("abcdefghijklmnopqrstuvwxyz")
 
+## Direction keyword for encoding.
+DIRECTION_ENCODE = "encode"
+## Direction keyword for decoding.
+DIRECTION_DECODE = "decode"
+
+## Prompt for the direction.
+PROMPT_DIRECTION = "Type 'encode' to encrypt, type 'decode' to decrypt:\n"
 ## Prompt for the message.
 PROMPT_TEXT = "Type your message:\n"
 ## Prompt for the shift number.
 PROMPT_SHIFT = "Type the shift number:\n"
+## Prompt to continue.
+PROMPT_AGAIN = "Type 'yes' if you want to go again. Otherwise type 'no'.\n"
 
 ## Name of the environment file holding tokens.
 ENV_FILE = ".env"
+
+## ASCII art title shown at start-up.
+TITLE = r"""
+  ____    _    _____ ____    _    ____
+ / ___|  / \  | ____/ ___|  / \  |  _ \
+| |     / _ \ |  _| \___ \ / _ \ | |_) |
+| |___ / ___ \| |___ ___) / ___ \|  _ <
+ \____/_/   \_\_____|____/_/   \_\_| \_\
+  ____ ___ ____  _   _ _____ ____
+ / ___|_ _|  _ \| | | | ____|  _ \
+| |    | || |_) | |_| |  _| | |_) |
+| |___ | ||  __/|  _  | |___|  _ <
+ \____|___|_|   |_| |_|_____|_| \_\
+"""
+
+## ANSI escape code for bright cyan text.
+COLOR_TITLE = "\033[96m"
+## ANSI escape code that resets text colour.
+COLOR_RESET = "\033[0m"

src/caesar_cipher/main.py

@@ -1,20 +1,43 @@
-## @file main.py
+## @file main.py
-#  @brief Interactive command line interface.
+#  @brief Interactive command line interface.
-
+
-from caesar_cipher import constants as c
+import os
-from caesar_cipher.cipher import encrypt
+import sys
-
+
-
+from caesar_cipher import constants as c
-def main() -> None:
+from caesar_cipher.cipher import caesar
-    ## @brief Ask for a message and a shift, then print the encrypted text.
+
-    text = input(c.PROMPT_TEXT)
+
-    try:
+def print_title() -> None:
-        shift = int(input(c.PROMPT_SHIFT))
+    ## @brief Print the ASCII title, in colour when the terminal supports it.
-    except ValueError:
+    if sys.stdout.isatty() and "NO_COLOR" not in os.environ:
-        print("The shift must be a whole number.")
+        os.system("")  # enables ANSI escape sequences on older Windows consoles
-        return
+        print(f"{c.COLOR_TITLE}{c.TITLE}{c.COLOR_RESET}")
-    print(f"Here's the encoded result: {encrypt(text, shift)}")
+    else:
-
+        print(c.TITLE)
-
+
-if __name__ == "__main__":
+
-    main()
+def main() -> None:
+    ## @brief Run the interactive encode/decode loop until the user stops.
+    print_title()
+    while True:
+        direction = input(c.PROMPT_DIRECTION).strip().lower()
+        if direction not in (c.DIRECTION_ENCODE, c.DIRECTION_DECODE):
+            print("Please type 'encode' or 'decode'.")
+            continue
+        text = input(c.PROMPT_TEXT)
+        try:
+            shift = int(input(c.PROMPT_SHIFT))
+        except ValueError:
+            print("The shift must be a whole number.")
+            continue
+
+        print(f"Here's the {direction}d result: {caesar(text, shift, direction)}")
+
+        if input(c.PROMPT_AGAIN).strip().lower() != "yes":
+            print("Goodbye")
+            break
+
+
+if __name__ == "__main__":
+    main()

tests/test_cipher.py

@@ -1,4 +1,6 @@
-from caesar_cipher import encrypt
+import pytest
+
+from caesar_cipher import caesar, decrypt, encrypt
 from caesar_cipher.env import load_env
 
 
@@ -24,3 +26,26 @@ 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"
+
+
+def test_caesar_encode_and_decode():
+    assert caesar("hello", 5, "encode") == "mjqqt"
+    assert caesar("mjqqt", 5, "decode") == "hello"
+
+
+def test_caesar_unknown_direction():
+    with pytest.raises(ValueError):
+        caesar("hello", 5, "sideways")

tests/test_main.py

@@ -0,0 +1,28 @@
+from caesar_cipher.main import main
+
+
+def run(monkeypatch, capsys, answers):
+    it = iter(answers)
+    monkeypatch.setattr("builtins.input", lambda _="": next(it))
+    main()
+    return capsys.readouterr().out
+
+
+def test_encode_then_stop(monkeypatch, capsys):
+    out = run(monkeypatch, capsys, ["encode", "hello", "5", "no"])
+    assert "mjqqt" in out and "Goodbye" in out
+
+
+def test_decode_then_go_again(monkeypatch, capsys):
+    out = run(monkeypatch, capsys, ["decode", "mjqqt", "5", "yes", "encode", "a", "1", "no"])
+    assert "hello" in out and "b" in out
+
+
+def test_invalid_input_reprompts(monkeypatch, capsys):
+    out = run(monkeypatch, capsys, ["nope", "decode", "x", "abc", "encode", "a", "1", "no"])
+    assert "encode' or 'decode" in out and "whole number" in out
+
+
+def test_title_printed_plain_when_not_a_tty(monkeypatch, capsys):
+    out = run(monkeypatch, capsys, ["encode", "a", "1", "no"])
+    assert "____" in out and "\033[" not in out