| 1 | 1 | def encode(plain_text: str, a: int, b: int) -> str: |
| 2 | pass | |
| 2 | m = 26 | |
| 3 | ||
| 4 | # Edge Case: Check if a and m are coprime | |
| 5 | if gcd(a, m) != 1: | |
| 6 | raise ValueError("a and m must be coprime.") | |
| 7 | ||
| 8 | result = [] | |
| 9 | for char in plain_text: | |
| 10 | if char.isalpha(): | |
| 11 | # Convert to lowercase and then to index (0-25) | |
| 12 | i = ord(char.lower()) - ord('a') | |
| 13 | # Apply encryption formula | |
| 14 | encrypted_index = (a * i + b) % m | |
| 15 | # Convert back to character | |
| 16 | result.append(chr(encrypted_index + ord('a'))) | |
| 17 | elif char.isdigit(): | |
| 18 | # Edge Case: Keep digits as they are | |
| 19 | result.append(char) | |
| 20 | # Edge Case: Ignore spaces and punctuation | |
| 21 | ||
| 22 | # Group into chunks of 5 characters | |
| 23 | grouped_result = [] | |
| 24 | for i in range(0, len(result), 5): | |
| 25 | grouped_result.append(''.join(result[i:i+5])) | |
| 26 | ||
| 27 | return ' '.join(grouped_result) | |
| 3 | 28 | |
| 4 | 29 | |
| 5 | 30 | def decode(ciphered_text: str, a: int, b: int) -> str: |
| 6 | pass | |
| 31 | m = 26 | |
| 32 | ||
| 33 | # Edge Case: Check if a and m are coprime | |
| 34 | if gcd(a, m) != 1: | |
| 35 | raise ValueError("a and m must be coprime.") | |
| 36 | ||
| 37 | # Find modular multiplicative inverse of a | |
| 38 | a_inv = mod_inverse(a, m) | |
| 39 | ||
| 40 | result = [] | |
| 41 | for char in ciphered_text: | |
| 42 | if char.isalpha(): | |
| 43 | # Convert to index (0-25) | |
| 44 | y = ord(char) - ord('a') | |
| 45 | # Apply decryption formula | |
| 46 | decrypted_index = (a_inv * (y - b)) % m | |
| 47 | # Convert back to character | |
| 48 | result.append(chr(decrypted_index + ord('a'))) | |
| 49 | elif char.isdigit(): | |
| 50 | # Edge Case: Keep digits as they are | |
| 51 | result.append(char) | |
| 52 | # Edge Case: Ignore spaces and punctuation | |
| 53 | ||
| 54 | return ''.join(result) | |
| 55 | ||
| 56 | ||
| 57 | def gcd(a: int, b: int) -> int: | |
| 58 | # Edge Case: Handle negative numbers by taking absolute values | |
| 59 | a, b = abs(a), abs(b) | |
| 60 | while b: | |
| 61 | a, b = b, a % b | |
| 62 | return a | |
| 63 | ||
| 64 | ||
| 65 | def mod_inverse(a: int, m: int) -> int: | |
| 66 | # Extended Euclidean Algorithm to find modular inverse | |
| 67 | # Edge Case: Handle negative a | |
| 68 | a = a % m | |
| 69 | # Add iteration limit to prevent potential infinite loop | |
| 70 | for x in range(1, m + 1): | |
| 71 | if (a * x) % m == 1: | |
| 72 | return x | |
| 73 | # Edge Case: This should not happen if gcd(a, m) == 1 | |
| 74 | raise ValueError("Modular inverse does not exist") | |
| 75 | ||
| 76 | # Handled Edge Cases: | |
| 77 | # - Check if a and m are coprime in both encode and decode | |
| 78 | # - Ignore spaces and punctuation in both encode and decode | |
| 79 | # - Keep digits unchanged in both encode and decode | |
| 80 | # - Convert all letters to lowercase in encode | |
| 81 | # - Group encoded text into chunks of 5 characters separated by spaces | |
| 82 | # - Handle negative numbers in gcd function | |
| 83 | # - Handle negative a in mod_inverse function | |
| 84 | # - Raise ValueError when modular inverse does not exist (should not happen if gcd check passes) |
| Test Name | Status |
|---|---|
test_decode_a_sentence | Pass |
test_decode_all_the_letters | Pass |
test_decode_exercism | Pass |
test_decode_numbers | Pass |
test_decode_with_a_not_coprime_to_m | Pass |
test_decode_with_no_spaces_in_input | Pass |
test_decode_with_too_many_spaces | Pass |
test_encode_all_the_letters | Pass |
test_encode_deep_thought | Pass |
test_encode_mindblowingly | Pass |
test_encode_no | Pass |
test_encode_numbers | Pass |
test_encode_o_m_g | Pass |
test_encode_omg | Pass |
test_encode_with_a_not_coprime_to_m | Pass |
test_encode_yes | Pass |
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