Streaming Stop Token: Return Prefix Before First Occurrence (Stop Token May Split Across Chunks)

Problem: Find a Stop Token in a Streaming String and Return the Prefix ASAP You are given an ordered stream of string chunks and a stop_token (a string). Starting from the beginning of the stream, you conceptually concatenate incoming chunks. As soon as the concatenated data contains the first occurrence of stop_token, you must immediately return all characters before that occurrence (excluding the stop_token). Key requirements: The stop_token may be split across chunk boundaries. You must be able to return quickly once the token is detected; you cannot wait to buffer the entire stream and then search. Return the prefix before the first occurrence. Input (you may define an appropriate interface) chunks: an ordered sequence of strings (e.g., List[str] or an iterator), each element is a chunk. stop_token: a string. Output A string: all characters from the start up to (but not including) the first occurrence of stop_token. If the stream ends without the token, return the full concatenation. Constraints len(stop_token) >= 1 Chunk lengths vary. Examples / Tests Input: chunks=["hello wor","ldENDxxx"], stop_token="END" Output: "hello world" (token split across chunks) Input: chunks=["ab","c","def"], stop_token="bcd" Output: "a" (token at boundary) Input: chunks=["foo","bar"], stop_token="bar" Output: "foo" (token never appears) Input: chunks=["aa","bb"], stop_token="cc" Output: "aabb" (multiple occurrences; take the first) Input: chunks=["xxSTOPyySTOPzz"], stop_token="STOP" Output: "xx" Example Input hello wor ldENDxxx END Output hello world