From 1cd9a28c8126e8461b7fe8c8a253d6d7ee328236 Mon Sep 17 00:00:00 2001
From: Jonas Jenwald <jonas.jenwald@gmail.com>
Date: Sat, 17 Aug 2019 11:44:51 +0200
Subject: [PATCH] Replace the `XRef.cache` Array with a Map instead

Given that the different types of `Stream`s will never be cached, this thus implies that the `XRef.cache` Array will *always* be more-or-less sparse.
Generally speaking, the longer the document the more sparse the `XRef.cache` will thus become. For example, looking at the `pdf.pdf` file from the test-suite: The length of the `XRef.cache` Array will be a few hundred thousand elements, with approximately 95% of them being empty.

Hence it seems pretty clear that an Array isn't really the best data-structure for this kind of cache, and this patch thus changes it to a Map instead.

This patch-series was tested using the PDF file from issue 2618, i.e. http://bugzilla-attachments.gnome.org/attachment.cgi?id=226471, with the following manifest file:
```
[
    {  "id": "issue2618",
       "file": "../web/pdfs/issue2618.pdf",
       "md5": "",
       "rounds": 200,
       "type": "eq"
    }
]
```

which gave the following results when comparing this patch-series against the `master` branch:
```
-- Grouped By browser, stat --
browser | stat         | Count | Baseline(ms) | Current(ms) | +/- |    %  | Result(P<.05)
------- | ------------ | ----- | ------------ | ----------- | --- | ----- | -------------
Firefox | Overall      |   200 |         2736 |        2736 |   1 |  0.02 |
Firefox | Page Request |   200 |            2 |           2 |   0 | -8.26 |        faster
Firefox | Rendering    |   200 |         2733 |        2734 |   1 |  0.03 |
```
---
 src/core/obj.js | 23 +++++++++++------------
 1 file changed, 11 insertions(+), 12 deletions(-)

diff --git a/src/core/obj.js b/src/core/obj.js
index 3446f8821..9b74c7b1b 100644
--- a/src/core/obj.js
+++ b/src/core/obj.js
@@ -1043,8 +1043,7 @@ var XRef = (function XRefClosure() {
     this.pdfManager = pdfManager;
     this.entries = [];
     this.xrefstms = Object.create(null);
-    // prepare the XRef cache
-    this.cache = [];
+    this._cacheMap = new Map(); // Prepare the XRef cache.
     this.stats = {
       streamTypes: Object.create(null),
       fontTypes: Object.create(null),
@@ -1636,9 +1635,10 @@ var XRef = (function XRefClosure() {
       if (!(ref instanceof Ref)) {
         throw new Error('ref object is not a reference');
       }
-      var num = ref.num;
-      if (num in this.cache) {
-        var cacheEntry = this.cache[num];
+      const num = ref.num;
+
+      if (this._cacheMap.has(num)) {
+        const cacheEntry = this._cacheMap.get(num);
         // In documents with Object Streams, it's possible that cached `Dict`s
         // have not been assigned an `objId` yet (see e.g. issue3115r.pdf).
         if (cacheEntry instanceof Dict && !cacheEntry.objId) {
@@ -1646,12 +1646,11 @@ var XRef = (function XRefClosure() {
         }
         return cacheEntry;
       }
+      let xrefEntry = this.getEntry(num);
 
-      var xrefEntry = this.getEntry(num);
-
-      // the referenced entry can be free
-      if (xrefEntry === null) {
-        return (this.cache[num] = null);
+      if (xrefEntry === null) { // The referenced entry can be free.
+        this._cacheMap.set(num, xrefEntry);
+        return xrefEntry;
       }
 
       if (xrefEntry.uncompressed) {
@@ -1709,7 +1708,7 @@ var XRef = (function XRefClosure() {
         xrefEntry = parser.getObj();
       }
       if (!isStream(xrefEntry)) {
-        this.cache[num] = xrefEntry;
+        this._cacheMap.set(num, xrefEntry);
       }
       return xrefEntry;
     },
@@ -1757,7 +1756,7 @@ var XRef = (function XRefClosure() {
         num = nums[i];
         var entry = this.entries[num];
         if (entry && entry.offset === tableOffset && entry.gen === i) {
-          this.cache[num] = entries[i];
+          this._cacheMap.set(num, entries[i]);
         }
       }
       xrefEntry = entries[xrefEntry.gen];