[Sync Iteration] python/rotational-cipher/1

solutions/python/rotational-cipher/1/rotational_cipher.py

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+"""
+The rotational cipher, also sometimes called the Caesar cipher.
+
+The Caesar cipher is a simple shift cipher that relies on transposing
+all the letters in the alphabet using an integer key between 0 and 26.
+Using a key of 0 or 26 will always yield the same output due to modular
+arithmetic. The letter is shifted for as many values as the value of the key.
+"""
+
+import string
+
+LETTERS_LOWER: str = string.ascii_lowercase
+LETTERS_UPPER: str = string.ascii_uppercase
+
+
+def rotate(text: str, key: int) -> str:
+    """
+    Rotate each letter in the text by the specified key using Caesar cipher.
+
+    Non-alphabetic characters remain unchanged. The rotation wraps around
+    the alphabet (a->z, A->Z) using modular arithmetic.
+
+    :param text: The input string to be rotated
+    :param key: The number of positions to shift each letter (0-25)
+    :return: The rotated string with letters shifted by key positions
+
+    Examples:
+        >>> rotate("abc", 1)
+        "bcd"
+        >>> rotate("Hello, World!", 13)
+        "Uryyb, Jbeyq!"
+    """
+    new_str: list[str] = []
+    for char in text:
+        # Not a letter
+        if not char.isalpha():
+            new_str.append(char)
+        else:
+            new_str.append(replace_char(char, key))
+    # Convert to a string
+    return "".join(new_str)
+
+
+def replace_char(char: str, key: int) -> str:
+    """
+    Shifts the character by the specified key positions within its alphabet
+    (uppercase or lowercase). Uses modular arithmetic to wrap around the alphabet.
+
+    :param char: The alphabetic character to rotate
+    :param key: The number of positions to shift the character
+    :return: The rotated character in the same case as the input
+    """
+    if char in LETTERS_UPPER:
+        new_index: int = LETTERS_UPPER.index(char) + key
+        if new_index < 26:
+            return LETTERS_UPPER[new_index]
+        return LETTERS_UPPER[new_index - 26]
+
+    new_index = LETTERS_LOWER.index(char) + key
+    if new_index < 26:
+        return LETTERS_LOWER[new_index]
+    return LETTERS_LOWER[new_index - 26]