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

src/caesar_cipher/main.py

@@ -1,20 +1,43 @@
-## @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()
+## @file main.py
+#  @brief Interactive command line interface.
+
+import os
+import sys
+
+from caesar_cipher import constants as c
+from caesar_cipher.cipher import caesar
+
+
+def print_title() -> None:
+    ## @brief Print the ASCII title, in colour when the terminal supports it.
+    if sys.stdout.isatty() and "NO_COLOR" not in os.environ:
+        os.system("")  # enables ANSI escape sequences on older Windows consoles
+        print(f"{c.COLOR_TITLE}{c.TITLE}{c.COLOR_RESET}")
+    else:
+        print(c.TITLE)
+
+
+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()