Re-factor the find helper function, in src/core/document.js, to search through the raw bytes rather than a string

During initial parsing of every PDF document we're currently creating a few `1 kB` strings, in order to find certain commands needed for initialization.
This seems inefficient, not to mention completely unnecessary, since we can just as well search through the raw bytes directly instead (similar to other parts of the code-base). One small complication here is the need to support backwards search, which does add some amount of "duplication" to this function.

The main benefits here are:
 - No longer necessary to allocate *temporary* `1 kB` strings during initial parsing, thus saving some memory.
 - In practice, for well-formed PDF documents, the number of iterations required to find the commands are usually very low. (For the `tracemonkey.pdf` file, there's a *total* of only 30 loop iterations.)

src/core/document.js

@@ -15,8 +15,8 @@
 /* eslint no-var: error */
 
 import {
-  assert, bytesToString, FormatError, info, InvalidPDFException, isArrayBuffer,
-  isArrayEqual, isBool, isNum, isSpace, isString, OPS, shadow, stringToBytes,
+  assert, FormatError, info, InvalidPDFException, isArrayBuffer, isArrayEqual,
+  isBool, isNum, isSpace, isString, OPS, shadow, stringToBytes,
   stringToPDFString, Util, warn
 } from '../shared/util';
 import { Catalog, ObjectLoader, XRef } from './obj';
@@ -344,21 +344,59 @@ class Page {
   }
 }
 
+const PDF_HEADER_SIGNATURE = new Uint8Array([0x25, 0x50, 0x44, 0x46, 0x2D]);
+const STARTXREF_SIGNATURE = new Uint8Array([
+  0x73, 0x74, 0x61, 0x72, 0x74, 0x78, 0x72, 0x65, 0x66]);
+const ENDOBJ_SIGNATURE = new Uint8Array([0x65, 0x6E, 0x64, 0x6F, 0x62, 0x6A]);
+
 const FINGERPRINT_FIRST_BYTES = 1024;
-const EMPTY_FINGERPRINT = '\x00\x00\x00\x00\x00\x00\x00' +
-                          '\x00\x00\x00\x00\x00\x00\x00\x00\x00';
+const EMPTY_FINGERPRINT =
+  '\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00';
 
-function find(stream, needle, limit, backwards = false) {
-  assert(limit > 0, 'The "limit" must be a positive integer.');
+function find(stream, signature, limit = 1024, backwards = false) {
+  if (typeof PDFJSDev === 'undefined' ||
+      PDFJSDev.test('!PRODUCTION || TESTING')) {
+    assert(limit > 0, 'The "limit" must be a positive integer.');
+  }
+  const signatureLength = signature.length;
 
-  const str = bytesToString(stream.peekBytes(limit));
+  const scanBytes = stream.peekBytes(limit);
+  const scanLength = scanBytes.length - signatureLength;
 
-  const index = backwards ? str.lastIndexOf(needle) : str.indexOf(needle);
-  if (index === -1) {
+  if (scanLength <= 0) {
     return false;
   }
-  stream.pos += index;
-  return true;
+  if (backwards) {
+    const signatureEnd = signatureLength - 1;
+
+    let pos = scanBytes.length - 1;
+    while (pos >= signatureEnd) {
+      let j = 0;
+      while (j < signatureLength &&
+             scanBytes[pos - j] === signature[signatureEnd - j]) {
+        j++;
+      }
+      if (j >= signatureLength) { // `signature` found.
+        stream.pos += (pos - signatureEnd);
+        return true;
+      }
+      pos--;
+    }
+  } else { // forwards
+    let pos = 0;
+    while (pos <= scanLength) {
+      let j = 0;
+      while (j < signatureLength && scanBytes[pos + j] === signature[j]) {
+        j++;
+      }
+      if (j >= signatureLength) { // `signature` found.
+        stream.pos += pos;
+        return true;
+      }
+      pos++;
+    }
+  }
+  return false;
 }
 
 /**
@@ -450,13 +488,13 @@ class PDFDocument {
     if (this.linearization) {
       // Find the end of the first object.
       stream.reset();
-      if (find(stream, 'endobj', 1024)) {
+      if (find(stream, ENDOBJ_SIGNATURE)) {
         startXRef = (stream.pos + 6) - stream.start;
       }
     } else {
       // Find `startxref` by checking backwards from the end of the file.
       const step = 1024;
-      const startXRefLength = 'startxref'.length;
+      const startXRefLength = STARTXREF_SIGNATURE.length;
       let found = false, pos = stream.end;
 
       while (!found && pos > 0) {
@@ -465,7 +503,7 @@ class PDFDocument {
           pos = 0;
         }
         stream.pos = pos;
-        found = find(stream, 'startxref', step, true);
+        found = find(stream, STARTXREF_SIGNATURE, step, true);
       }
 
       if (found) {
@@ -494,7 +532,7 @@ class PDFDocument {
     const stream = this.stream;
     stream.reset();
 
-    if (!find(stream, '%PDF-', 1024)) {
+    if (!find(stream, PDF_HEADER_SIGNATURE)) {
       // May not be a PDF file, but don't throw an error and let
       // parsing continue.
       return;