perf: harden chunking against quadratic scans

src/auto-reply/chunk.test.ts

@@ -1,4 +1,5 @@
-import { describe, expect, it } from "vitest";
+import { describe, expect, it, vi } from "vitest";
+import * as fences from "../markdown/fences.js";
 import { hasBalancedFences } from "../test-utils/chunk-test-helpers.js";
 import {
   chunkByNewline,
@@ -217,6 +218,17 @@ describe("chunkMarkdownText", () => {
     expect(chunks[0]?.length).toBe(20);
     expect(chunks.join("")).toBe(text);
   });
+
+  it("parses fence spans once for long fenced payloads", () => {
+    const parseSpy = vi.spyOn(fences, "parseFenceSpans");
+    const text = `\`\`\`txt\n${"line\n".repeat(600)}\`\`\``;
+
+    const chunks = chunkMarkdownText(text, 80);
+
+    expect(chunks.length).toBeGreaterThan(2);
+    expect(parseSpy).toHaveBeenCalledTimes(1);
+    parseSpy.mockRestore();
+  });
 });
 
 describe("chunkByNewline", () => {

src/auto-reply/chunk.ts

@@ -306,7 +306,7 @@ export function chunkText(text: string, limit: number): string[] {
   }
   return chunkTextByBreakResolver(text, limit, (window) => {
     // 1) Prefer a newline break inside the window (outside parentheses).
-    const { lastNewline, lastWhitespace } = scanParenAwareBreakpoints(window);
+    const { lastNewline, lastWhitespace } = scanParenAwareBreakpoints(window, 0, window.length);
     // 2) Otherwise prefer the last whitespace (word boundary) inside the window.
     return lastNewline > 0 ? lastNewline : lastWhitespace;
   });
@@ -319,14 +319,24 @@ export function chunkMarkdownText(text: string, limit: number): string[] {
   }
 
   const chunks: string[] = [];
-  let remaining = text;
+  const spans = parseFenceSpans(text);
+  let start = 0;
+  let reopenFence: ReturnType<typeof findFenceSpanAt> | undefined;
 
-  while (remaining.length > limit) {
-    const spans = parseFenceSpans(remaining);
-    const window = remaining.slice(0, limit);
+  while (start < text.length) {
+    const reopenPrefix = reopenFence ? `${reopenFence.openLine}\n` : "";
+    const contentLimit = Math.max(1, limit - reopenPrefix.length);
+    if (text.length - start <= contentLimit) {
+      const finalChunk = `${reopenPrefix}${text.slice(start)}`;
+      if (finalChunk.length > 0) {
+        chunks.push(finalChunk);
+      }
+      break;
+    }
 
-    const softBreak = pickSafeBreakIndex(window, spans);
-    let breakIdx = softBreak > 0 ? softBreak : limit;
+    const windowEnd = Math.min(text.length, start + contentLimit);
+    const softBreak = pickSafeBreakIndex(text, start, windowEnd, spans);
+    let breakIdx = softBreak > start ? softBreak : windowEnd;
 
     const initialFence = isSafeFenceBreak(spans, breakIdx)
       ? undefined
@@ -335,38 +345,38 @@ export function chunkMarkdownText(text: string, limit: number): string[] {
     let fenceToSplit = initialFence;
     if (initialFence) {
       const closeLine = `${initialFence.indent}${initialFence.marker}`;
-      const maxIdxIfNeedNewline = limit - (closeLine.length + 1);
+      const maxIdxIfNeedNewline = start + (contentLimit - (closeLine.length + 1));
 
-      if (maxIdxIfNeedNewline <= 0) {
+      if (maxIdxIfNeedNewline <= start) {
         fenceToSplit = undefined;
-        breakIdx = limit;
+        breakIdx = windowEnd;
       } else {
         const minProgressIdx = Math.min(
-          remaining.length,
-          initialFence.start + initialFence.openLine.length + 2,
+          text.length,
+          Math.max(start + 1, initialFence.start + initialFence.openLine.length + 2),
         );
-        const maxIdxIfAlreadyNewline = limit - closeLine.length;
+        const maxIdxIfAlreadyNewline = start + (contentLimit - closeLine.length);
 
         let pickedNewline = false;
-        let lastNewline = remaining.lastIndexOf("\n", Math.max(0, maxIdxIfAlreadyNewline - 1));
-        while (lastNewline !== -1) {
+        let lastNewline = text.lastIndexOf("\n", Math.max(start, maxIdxIfAlreadyNewline - 1));
+        while (lastNewline >= start) {
           const candidateBreak = lastNewline + 1;
           if (candidateBreak < minProgressIdx) {
             break;
           }
           const candidateFence = findFenceSpanAt(spans, candidateBreak);
           if (candidateFence && candidateFence.start === initialFence.start) {
-            breakIdx = Math.max(1, candidateBreak);
+            breakIdx = candidateBreak;
             pickedNewline = true;
             break;
           }
-          lastNewline = remaining.lastIndexOf("\n", lastNewline - 1);
+          lastNewline = text.lastIndexOf("\n", lastNewline - 1);
         }
 
         if (!pickedNewline) {
           if (minProgressIdx > maxIdxIfAlreadyNewline) {
             fenceToSplit = undefined;
-            breakIdx = limit;
+            breakIdx = windowEnd;
           } else {
             breakIdx = Math.max(minProgressIdx, maxIdxIfNeedNewline);
           }
@@ -378,68 +388,72 @@ export function chunkMarkdownText(text: string, limit: number): string[] {
         fenceAtBreak && fenceAtBreak.start === initialFence.start ? fenceAtBreak : undefined;
     }
 
-    let rawChunk = remaining.slice(0, breakIdx);
-    if (!rawChunk) {
+    const rawContent = text.slice(start, breakIdx);
+    if (!rawContent) {
       break;
     }
 
-    const brokeOnSeparator = breakIdx < remaining.length && /\s/.test(remaining[breakIdx]);
-    const nextStart = Math.min(remaining.length, breakIdx + (brokeOnSeparator ? 1 : 0));
-    let next = remaining.slice(nextStart);
+    let rawChunk = `${reopenPrefix}${rawContent}`;
+    const brokeOnSeparator = breakIdx < text.length && /\s/.test(text[breakIdx]);
+    let nextStart = Math.min(text.length, breakIdx + (brokeOnSeparator ? 1 : 0));
 
     if (fenceToSplit) {
       const closeLine = `${fenceToSplit.indent}${fenceToSplit.marker}`;
       rawChunk = rawChunk.endsWith("\n") ? `${rawChunk}${closeLine}` : `${rawChunk}\n${closeLine}`;
-      next = `${fenceToSplit.openLine}\n${next}`;
+      reopenFence = fenceToSplit;
     } else {
-      next = stripLeadingNewlines(next);
+      nextStart = skipLeadingNewlines(text, nextStart);
+      reopenFence = undefined;
     }
 
     chunks.push(rawChunk);
-    remaining = next;
-  }
-
-  if (remaining.length) {
-    chunks.push(remaining);
+    start = nextStart;
   }
   return chunks;
 }
 
-function stripLeadingNewlines(value: string): string {
-  let i = 0;
+function skipLeadingNewlines(value: string, start = 0): number {
+  let i = start;
   while (i < value.length && value[i] === "\n") {
     i++;
   }
-  return i > 0 ? value.slice(i) : value;
+  return i;
 }
 
-function pickSafeBreakIndex(window: string, spans: ReturnType<typeof parseFenceSpans>): number {
-  const { lastNewline, lastWhitespace } = scanParenAwareBreakpoints(window, (index) =>
+function pickSafeBreakIndex(
+  text: string,
+  start: number,
+  end: number,
+  spans: ReturnType<typeof parseFenceSpans>,
+): number {
+  const { lastNewline, lastWhitespace } = scanParenAwareBreakpoints(text, start, end, (index) =>
     isSafeFenceBreak(spans, index),
   );
 
-  if (lastNewline > 0) {
+  if (lastNewline > start) {
     return lastNewline;
   }
-  if (lastWhitespace > 0) {
+  if (lastWhitespace > start) {
     return lastWhitespace;
   }
   return -1;
 }
 
 function scanParenAwareBreakpoints(
-  window: string,
+  text: string,
+  start: number,
+  end: number,
   isAllowed: (index: number) => boolean = () => true,
 ): { lastNewline: number; lastWhitespace: number } {
   let lastNewline = -1;
   let lastWhitespace = -1;
   let depth = 0;
 
-  for (let i = 0; i < window.length; i++) {
+  for (let i = start; i < end; i++) {
     if (!isAllowed(i)) {
       continue;
     }
-    const char = window[i];
+    const char = text[i];
     if (char === "(") {
       depth += 1;
       continue;