def longest_substring(word):
    longest = current = 0
    seen = {}
    for i in range(len(word)):
        character = word[i]
        if character in seen:
            longest = max(longest, (i - seen[character]))
            current = 0
        current += 1
        seen[word[i]] = i
    return max(longest, current)


assert longest_substring('abcabcbb') == 3
assert longest_substring('pwwkew') == 3
assert longest_substring('abcdefgh') == 8
assert longest_substring('abadefgh') == 6
assert longest_substring('aaaaa') == 1
assert longest_substring('') == 0