Explaining Python Code

Look at this Python code and tell me what it is doing.

Explain the algorithm, its output, and its complexity. Then implement an equivalent function that accepts a string and returns the length of its longest substring without repeating characters.

def analyze(text):
    last_seen = {}
    start = best = 0
    for end, char in enumerate(text):
        if char in last_seen and last_seen[char] >= start:
            start = last_seen[char] + 1
        last_seen[char] = end
        best = max(best, end - start + 1)
    return best

Input: a string. Output: an integer length. Treat every character, including spaces and symbols, as distinct.