| 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 | ||
| 21 | # Group into chunks of 5 characters | |
| 22 | grouped_result = [] | |
| 23 | for i in range(0, len(result), 5): | |
| 24 | grouped_result.append(''.join(result[i:i+5])) | |
| 25 | ||
| 26 | return ' '.join(grouped_result) | |
| 3 | 27 | |
| 4 | 28 | |
| 5 | 29 | def decode(ciphered_text: str, a: int, b: int) -> str: |
| 6 | pass | |
| 30 | m = 26 | |
| 31 | ||
| 32 | # Edge Case: Check if a and m are coprime | |
| 33 | if gcd(a, m) != 1: | |
| 34 | raise ValueError("a and m must be coprime.") | |
| 35 | ||
| 36 | # Find modular multiplicative inverse of a | |
| 37 | a_inv = mod_inverse(a, m) | |
| 38 | ||
| 39 | result = [] | |
| 40 | for char in ciphered_text: | |
| 41 | if char.isalpha(): | |
| 42 | # Convert to index (0-25) | |
| 43 | y = ord(char) - ord('a') | |
| 44 | # Apply decryption formula | |
| 45 | decrypted_index = (a_inv * (y - b)) % m | |
| 46 | # Convert back to character | |
| 47 | result.append(chr(decrypted_index + ord('a'))) | |
| 48 | elif char.isdigit(): | |
| 49 | # Edge Case: Keep digits as they are | |
| 50 | result.append(char) | |
| 51 | # Edge Case: Ignore spaces and punctuation | |
| 52 | return ''.join(result) | |
| 53 | ||
| 54 | ||
| 55 | def gcd(a: int, b: int) -> int: | |
| 56 | # Edge Case: Handle negative numbers by taking absolute values | |
| 57 | a, b = abs(a), abs(b) | |
| 58 | while b: | |
| 59 | a, b = b, a % b | |
| 60 | return a | |
| 61 | ||
| 62 | ||
| 63 | def mod_inverse(a: int, m: int) -> int: | |
| 64 | # Edge Case: Handle negative a by converting to positive equivalent | |
| 65 | a = a % m | |
| 66 | for x in range(1, m): | |
| 67 | if (a * x) % m == 1: | |
| 68 | return x | |
| 69 | # Edge Case: This should not happen if gcd(a, m) == 1, but included for completeness | |
| 70 | raise ValueError("Modular inverse does not exist") | |
| 71 | # Handled Edge Cases: encode function handles non-coprime a and m, non-alphabetic characters, grouping; decode function handles non-coprime a and m, non-alphabetic characters, spaces and punctuation; gcd handles negative numbers; mod_inverse handles cases where inverse doesn't exist |
| 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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