Find the PDF filename in a URL hash in two linear steps

Searching for a name followed by ".pdf" is quadratic on a hash which
doesn't contain one, so locate the last ".pdf" first and then extend
it to the left.

src/display/display_utils.js

@@ -188,10 +188,22 @@ function getPdfFilenameFromUrl(url, defaultFilename = "document.pdf") {
   }
 
   if (newURL.hash) {
-    const reFilename = /[^/?#=]+\.pdf\b(?!.*\.pdf\b)/i;
-    const hashFilename = reFilename.exec(newURL.hash);
-    if (hashFilename) {
-      return decode(hashFilename[0]);
+    // Locate the last ".pdf" and then extend it to the left, up to the closest
+    // separator. Both steps are linear, whereas a single pattern starting with
+    // `[^/?#=]+` is quadratic on a hash which contains no ".pdf" at all.
+    const { hash } = newURL;
+    let extensionStart = -1;
+    for (const { index } of hash.matchAll(/\.pdf\b/gi)) {
+      extensionStart = index;
+    }
+    if (extensionStart > 0) {
+      let filenameStart = extensionStart;
+      while (filenameStart > 0 && !"/?#=".includes(hash[filenameStart - 1])) {
+        filenameStart--;
+      }
+      if (filenameStart < extensionStart) {
+        return decode(hash.slice(filenameStart, extensionStart + 4));
+      }
     }
   }
 

test/unit/display_utils_spec.js

@@ -122,6 +122,30 @@ describe("display_utils", function () {
       expect(
         getPdfFilenameFromUrl("http://www.example.com/pdfs/pdf.html#file2.pdf")
       ).toEqual("file2.pdf");
+      // Only the last ".pdf" of the hash is used.
+      expect(getPdfFilenameFromUrl("/pdfs/pdfs.html#a.pdf/b.pdf")).toEqual(
+        "b.pdf"
+      );
+      // A ".pdf" which isn't preceded by a name is ignored.
+      expect(getPdfFilenameFromUrl("/pdfs/pdfs.html#=.pdf")).toEqual(
+        "document.pdf"
+      );
+      // An invalid last ".pdf" prevents an earlier valid one from being used.
+      expect(getPdfFilenameFromUrl("/pdfs/pdfs.html#a.pdf/=.pdf")).toEqual(
+        "document.pdf"
+      );
+    });
+
+    it("gets PDF filename from a long hash string efficiently", function () {
+      // Scanning the hash for a name is quadratic when it contains no ".pdf".
+      const url = `/pdfs/pdfs.html#${"a".repeat(200000)}`;
+
+      const startTime = performance.now();
+      const filename = getPdfFilenameFromUrl(url);
+      const duration = performance.now() - startTime;
+
+      expect(filename).toEqual("document.pdf");
+      expect(duration).toBeLessThan(1000);
     });
 
     it("gets correct PDF filename when multiple ones are present", function () {