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

.gitignore

@@ -174,3 +174,6 @@ cython_debug/
 # PyPI configuration file
 .pypirc
 
+
+# Doxygen output
+docs/doxygen/

Doxyfile

@@ -0,0 +1,12 @@
+PROJECT_NAME           = "Caesar Cipher"
+PROJECT_BRIEF          = "Udemy 100 Days of Code - Day 8"
+OUTPUT_DIRECTORY       = docs/doxygen
+INPUT                  = src README.md docs/index.md
+USE_MDFILE_AS_MAINPAGE = README.md
+FILE_PATTERNS          = *.py *.md
+RECURSIVE              = YES
+EXTRACT_ALL            = YES
+OPTIMIZE_OUTPUT_JAVA   = YES
+GENERATE_HTML          = YES
+GENERATE_LATEX         = NO
+QUIET                  = YES

README.md

@@ -1,2 +1,76 @@
-# 008-caesar_cipher
+# 🔐 Caesar Cipher
 
+A beginner-friendly Python program that encodes and decodes messages with the Caesar cipher.
+
+## 📚 Table of Contents
+
+- [Overview](#-overview)
+- [Requirements](#-requirements)
+- [Setup](#-setup)
+- [Run](#-run)
+- [Tests](#-tests)
+- [License](#-license)
+- [Links](#-links)
+
+## 🧭 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. 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
+
+- Python 3.13 or newer
+- `pytest` (development only; no runtime dependencies)
+- Optional: [Doxygen](https://www.doxygen.nl/) to build the API docs
+
+## 🛠️ Setup
+
+Create and activate a local virtual environment, then upgrade pip and install the project:
+
+```bash
+python -m venv .venv
+```
+
+Activate it:
+
+```bash
+# Linux / macOS
+source .venv/bin/activate
+# Git Bash on Windows
+source .venv/Scripts/activate
+```
+
+```powershell
+# Windows PowerShell
+.venv\Scripts\Activate.ps1
+```
+
+```bash
+python -m pip install --upgrade pip
+python -m pip install -e ".[dev]"
+```
+
+## ▶️ Run
+
+```bash
+python -m caesar_cipher
+```
+
+or, after installation, `caesar-cipher`.
+
+## 🧪 Tests
+
+```bash
+python -m pytest
+```
+
+Build the API docs with `doxygen Doxyfile` (output in `docs/doxygen/html`).
+
+## 📄 License
+
+GNU AGPL-3.0 – see [LICENSE](LICENSE).
+
+## 🔗 Links
+
+- [Repository](https://git.tirsystem.com/Tirsvad-Udemy-100_days_of_code)
+- [Documentation](docs/index.md)
+- [Issue tracker](https://git.tirsystem.com/Tirsvad-Udemy-100_days_of_code/008-caesar_cipher/issues)

docs/index.md

@@ -0,0 +1,9 @@
+# Caesar Cipher – documentation
+
+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`.
+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).

pyproject.toml

@@ -0,0 +1,26 @@
+[build-system]
+requires = ["setuptools>=77"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "caesar-cipher"
+version = "0.1.0"
+description = "A beginner-friendly Caesar cipher encoder/decoder (Udemy 100 Days of Code, Day 8)."
+readme = "README.md"
+requires-python = ">=3.13"
+license = "AGPL-3.0-or-later"
+license-files = ["LICENSE"]
+dependencies = []
+
+[project.optional-dependencies]
+dev = ["pytest>=8"]
+
+[project.scripts]
+caesar-cipher = "caesar_cipher.main:main"
+
+[tool.setuptools.packages.find]
+where = ["src"]
+
+[tool.pytest.ini_options]
+pythonpath = ["src"]
+testpaths = ["tests"]

src/caesar_cipher/__init__.py

@@ -0,0 +1,6 @@
+## @file __init__.py
+#  @brief Public API of the caesar_cipher package.
+
+from caesar_cipher.cipher import decrypt, encrypt
+
+__all__ = ["encrypt", "decrypt"]

src/caesar_cipher/__main__.py

@@ -0,0 +1,6 @@
+## @file __main__.py
+#  @brief Enables `python -m caesar_cipher`.
+
+from caesar_cipher.main import main
+
+main()

src/caesar_cipher/cipher.py

@@ -0,0 +1,29 @@
+## @file cipher.py
+#  @brief Caesar cipher encryption and decryption.
+
+from caesar_cipher.constants import ALPHABET
+
+
+def encrypt(text: str, shift: int) -> str:
+    ## @brief Encrypt a text with the Caesar cipher.
+    #  @param text  Original text.
+    #  @param shift Positions to move each letter (may be negative or > 26).
+    #  @return The encrypted text; non-letters are kept, case is preserved.
+    result = []
+    for char in text:
+        lower = char.lower()
+        if lower in ALPHABET:
+            new_position = (ALPHABET.index(lower) + shift) % len(ALPHABET)
+            new_char = ALPHABET[new_position]
+            result.append(new_char.upper() if char.isupper() else new_char)
+        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)

src/caesar_cipher/constants.py

@@ -0,0 +1,13 @@
+## @file constants.py
+#  @brief Constants used by the Caesar cipher.
+
+## Lowercase alphabet used to look up letter positions.
+ALPHABET = list("abcdefghijklmnopqrstuvwxyz")
+
+## Prompt for the message.
+PROMPT_TEXT = "Type your message:\n"
+## Prompt for the shift number.
+PROMPT_SHIFT = "Type the shift number:\n"
+
+## Name of the environment file holding tokens.
+ENV_FILE = ".env"

src/caesar_cipher/env.py

@@ -0,0 +1,31 @@
+## @file env.py
+#  @brief Minimal stdlib `.env` reader (no third-party dependency).
+
+import os
+from pathlib import Path
+
+from caesar_cipher.constants import ENV_FILE
+
+
+def load_env(path: str | Path = ENV_FILE) -> dict[str, str]:
+    ## @brief Read KEY=VALUE pairs from an env file.
+    #  @param path Path of the env file.
+    #  @return Mapping of variables; empty if the file does not exist.
+    values: dict[str, str] = {}
+    env_path = Path(path)
+    if not env_path.is_file():
+        return values
+    for line in env_path.read_text(encoding="utf-8").splitlines():
+        line = line.strip()
+        if not line or line.startswith("#") or "=" not in line:
+            continue
+        key, _, value = line.partition("=")
+        values[key.strip()] = value.strip().strip("\"'")
+    return values
+
+
+def get_token(name: str, path: str | Path = ENV_FILE) -> str | None:
+    ## @brief Get a token from the process environment or the env file.
+    #  @param name Variable name, e.g. GITEA_TOKEN.
+    #  @return The token or None when not set.
+    return os.environ.get(name) or load_env(path).get(name)

src/caesar_cipher/main.py

@@ -0,0 +1,20 @@
+## @file main.py
+#  @brief Interactive command line interface.
+
+from caesar_cipher import constants as c
+from caesar_cipher.cipher import encrypt
+
+
+def main() -> None:
+    ## @brief Ask for a message and a shift, then print the encrypted text.
+    text = input(c.PROMPT_TEXT)
+    try:
+        shift = int(input(c.PROMPT_SHIFT))
+    except ValueError:
+        print("The shift must be a whole number.")
+        return
+    print(f"Here's the encoded result: {encrypt(text, shift)}")
+
+
+if __name__ == "__main__":
+    main()

tests/test_cipher.py

@@ -0,0 +1,39 @@
+from caesar_cipher import decrypt, encrypt
+from caesar_cipher.env import load_env
+
+
+def test_encrypt_basic():
+    assert encrypt("hello", 5) == "mjqqt"
+
+
+def test_wraps_around_alphabet():
+    assert encrypt("xyz", 3) == "abc"
+    assert encrypt("abc", 29) == "def"
+
+
+def test_non_letters_and_case_preserved():
+    assert encrypt("Hi there, 42!", 1) == "Ij uifsf, 42!"
+
+
+def test_negative_shift():
+    assert encrypt("def", -3) == "abc"
+
+
+def test_load_env(tmp_path):
+    f = tmp_path / ".env"
+    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"