unique_substring.py

import unittest


def find_longest_unique_substring(string: str) -> int:
    """
    Returns length of the longest substring of unique characters
    """
    max_length = 0
    current_substr_chars = set()
    start_index = leading_index = 0

    str_length = len(string)
    while start_index < str_length and leading_index < str_length:
        char = string[leading_index]
        if char not in current_substr_chars:
            current_substr_chars.add(char)
            leading_index += 1
            current_length = len(string[start_index:leading_index])
            max_length = max(max_length, current_length)
        else:
            # counting from a new start, so we need to exclude first seen char
            # as it not a part of current substring anymore and can be repeated
            current_substr_chars.remove(string[start_index])
            start_index += 1

    return max_length


class TestFundSubstring(unittest.TestCase):
    in_out = {
        '1234567AS9999999LLLLLL': 10,  # 1234567AS9
        'ABCDEEFGDKLMNOOO': 9,  # EFGDKLMNO
        '///AD1;AC;C;;C;C': 5,  # /AD1;
        '111111111111': 1,  # 1
        '111111222222': 2,  # 12
        'awesome': 6,  # awesom
        'ACDECOPGQAZM': 10,  # DECOPGQAZM
        'AAAAAAAAAAAA' * 10000: 1,  # A
        '': 0,
    }

    def test_find_longest_unique_substring(self):
        for string, expected_length in self.in_out.items():
            found_length = find_longest_unique_substring(string)
            self.assertEqual(
                expected_length,
                found_length,
                msg=f'Expected substring of {expected_length} unique chars, got {found_length} in {string}'
            )


if __name__ == '__main__':
    unittest.main()