Session Token Longest Unique Window

Problem Narrative

You’re on the fraud-prevention team at a large fintech app processing tens of millions of login events per day. Each event includes a session token string. Repeated characters in a short window are a strong signal of bot-generated tokens, and you need a fast way to measure “uniqueness density” in real time.

Task

Given a string s, return the length of the longest substring (contiguous segment) that contains no repeating characters.

Function Signature

• Input: s: str
• Output: int — the maximum length of a substring with all distinct characters

Formal Problem Statement

A substring is defined by two indices l and r (0 <= l <= r < len(s)) and includes s[l:r+1]. You must find the maximum possible length among all substrings where every character appears at most once.

Constraints

• 0 <= len(s) <= 10^5
• s consists of standard ASCII characters (you may assume up to 128 distinct characters)

Notes / Clarifications

1. If s is empty, return 0.
2. The solution should be efficient enough for large inputs (aim for linear time).
3. Treat characters as case-sensitive ('A' and 'a' are different).