| 1 | import random |
| 2 | import string |
| 3 | |
| 4 | |
| 1 | 5 | class Cipher: |
| 2 | 6 | def __init__(self, key=None): |
| 3 | | pass |
| 7 | if key is None: |
| 8 | # Edge Case: No key provided - generate a random key of at least 100 lowercase letters |
| 9 | self.key = ''.join(random.choices(string.ascii_lowercase, k=100)) |
| 10 | else: |
| 11 | # Edge Case: Empty key - not explicitly handled in requirements but we'll treat as invalid |
| 12 | if not key: |
| 13 | raise ValueError("Key must not be empty") |
| 14 | # Edge Case: Key with non-lowercase letters |
| 15 | if not all(c.islower() for c in key): |
| 16 | raise ValueError("Key must contain only lowercase letters") |
| 17 | self.key = key |
| 4 | 18 | |
| 5 | 19 | def encode(self, text): |
| 6 | | pass |
| 20 | # Edge Case: Empty plaintext |
| 21 | if not text: |
| 22 | return "" |
| 23 | |
| 24 | result = [] |
| 25 | key_length = len(self.key) |
| 26 | |
| 27 | for i, char in enumerate(text): |
| 28 | # Edge Case: Non-letter characters in plaintext |
| 29 | if not char.isalpha(): |
| 30 | result.append(char) |
| 31 | continue |
| 32 | |
| 33 | # Edge Case: Uppercase letters in plaintext |
| 34 | is_upper = char.isupper() |
| 35 | char = char.lower() |
| 36 | |
| 37 | key_char = self.key[i % key_length] |
| 38 | shift = ord(key_char) - ord('a') |
| 39 | char_code = ord(char) - ord('a') |
| 40 | new_code = (char_code + shift) % 26 |
| 41 | new_char = chr(new_code + ord('a')) |
| 42 | |
| 43 | # Preserve original case |
| 44 | if is_upper: |
| 45 | new_char = new_char.upper() |
| 46 | |
| 47 | result.append(new_char) |
| 48 | |
| 49 | return ''.join(result) |
| 50 | |
| 51 | # Handled Edge Cases: empty plaintext, non-letter characters in plaintext, uppercase letters in plaintext |
| 7 | 52 | |
| 8 | 53 | def decode(self, text): |
| 9 | | pass |
| 54 | # Edge Case: Empty ciphertext |
| 55 | if not text: |
| 56 | return "" |
| 57 | |
| 58 | result = [] |
| 59 | key_length = len(self.key) |
| 60 | |
| 61 | for i, char in enumerate(text): |
| 62 | # Edge Case: Non-letter characters in ciphertext |
| 63 | if not char.isalpha(): |
| 64 | result.append(char) |
| 65 | continue |
| 66 | |
| 67 | # Edge Case: Uppercase letters in ciphertext |
| 68 | is_upper = char.isupper() |
| 69 | char = char.lower() |
| 70 | |
| 71 | key_char = self.key[i % key_length] |
| 72 | shift = ord(key_char) - ord('a') |
| 73 | char_code = ord(char) - ord('a') |
| 74 | # Add 26 before modulo to handle negative results correctly |
| 75 | new_code = (char_code - shift + 26) % 26 |
| 76 | new_char = chr(new_code + ord('a')) |
| 77 | |
| 78 | # Preserve original case |
| 79 | if is_upper: |
| 80 | new_char = new_char.upper() |
| 81 | |
| 82 | result.append(new_char) |
| 83 | |
| 84 | return ''.join(result) |
| 85 | |
| 86 | # Handled Edge Cases: empty ciphertext, non-letter characters in ciphertext, uppercase letters in ciphertext |