fix(web): cap rows of empty pieces in the highlighter's prefix step

An empty query piece (two spaces or dots in a row) is [a-z]* between two
\W+ in the prefix step, so a long row of them made the regex try every way
of splitting each separator run it crossed: 'the', 1000 spaces and 'lord'
took 40 s on D&C 132:19. Rows longer than 8 empty pieces are cut to 8;
no realistic query has nine spaces or dots in a row.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: kckern

apps/web/src/services/SearchService.js

@@ -73,10 +73,25 @@ function joinPieces(pieces) {
   return src;
 }
 
+// In the prefix step an empty piece is [a-z]*, which matches nothing inside a run of separators,
+// so a long row of them makes the regex try every way of splitting each run it crosses (a query of
+// 'the', 1000 spaces and 'lord' took 40 s on one verse). Rows of more than MAX_GAP empty pieces
+// (more than MAX_GAP + 1 spaces or dots in a row, which no realistic query has) are cut to MAX_GAP.
+const MAX_GAP = 8;
+function prefixPieces(pieces) {
+  const out = [];
+  let empties = 0;
+  for (const p of pieces) {
+    empties = p ? 0 : empties + 1;
+    if (empties <= MAX_GAP) out.push(p);
+  }
+  return out;
+}
+
 export function makeHighlighter(query) {
   const pieces = String(query).split(' ');
   const phrase = `\\b${joinPieces(pieces.map(escapeRegex))}[a-z]*`;
-  const prefixes = `\\b${pieces.map(p => `${escapeRegex(p.slice(0, 3))}[a-z]*`).join('\\W+')}[a-z]*`;
+  const prefixes = `\\b${prefixPieces(pieces).map(p => `${escapeRegex(p.slice(0, 3))}[a-z]*`).join('\\W+')}[a-z]*`;
   const stages = [phrase, prefixes].map(src => ({ test: new RegExp(src, 'i'), all: new RegExp(src, 'gi') }));
   const words = new Set(String(query).split(/[^a-z]+/i).filter(Boolean).map(q => q.slice(0, 3).toLowerCase()));
   const marked = w => {

apps/web/test/unit/searchService.test.js

@@ -203,6 +203,15 @@ describe('helpers', () => {
     expect(out).toContain("<span class='searchterm'>and</span> labour");
     expect(Date.now() - start).toBeLessThan(2000);
   });
+  test('a long row of separators in the query does not make the prefix step backtrack', () => {
+    // Runs of two separators (', ' and '; ') can each be split two ways; before the cut, 14 of these
+    // took over 20 s
+    const verse = `the ${Array(14).fill('word, word; word—word').join(' ')}`;
+    const start = Date.now();
+    const out = highlight(verse, `the${' '.repeat(1000)}lord`);
+    expect(out).not.toMatch(/<span[^>]*>[^<]*<span/);
+    expect(Date.now() - start).toBeLessThan(1000);
+  });
   test('highlight treats repeated separators as live does', () => {
     expect(highlight('faith, hope', 'faith  hope')).toBe("<span class='searchterm'>faith, hope</span>");
     expect(highlight('faith hope', 'faith  hope')).toBe("<span class='searchterm'>faith</span> <span class='searchterm'>hope</span>");