Stop-Sequence Detection (Streaming)

Background

LLM streaming generation needs to detect when a stop sequence appears so it can cut off mid-token-batch. The complication: a stop sequence like "</answer>" may straddle two streamed chunks. The streaming buffer keeps the last $|s|-1$ characters from the previous chunk so cross-chunk matches are caught.

Problem statement

Implement detect_stop(streamed_chunks, stop). Walk the chunks in order, return the index of the chunk in which the stop sequence first completes, OR -1 if it never appears.

Input

• streamed_chunks - list[str], concatenation-order text chunks.
• stop - str, the stop sequence.

Output

• int, the chunk index where stop completes (the last char of stop lies in this chunk), or -1.

Examples

Input: chunks=["The ans", "wer is ", "42</an", "swer>"], stop="</answer>"
Output: 3    # completes in the last chunk; straddles chunks 2-3

Constraints

• Empty stop raises ValueError.
• Length 1 string (or matching only within one chunk) returns the chunk it appears in.

Notes

• Why buffer. Concatenating all of history is wasteful; keeping just the last $|stop|-1$ chars from the running buffer is sufficient.