From dda7a5d1b7888287cf7bb82995190d5f2cbbff7b Mon Sep 17 00:00:00 2001
From: Jonas Jenwald <jonas.jenwald@gmail.com>
Date: Tue, 9 Jun 2020 16:06:26 +0200
Subject: [PATCH] Convert the `ChunkedStreamManager.requestsByChunk` property
 to a `Map`

Compared to regular `Object`s, `Map`s have a number of advantageous properties: Of particular importance in this case is the built-in iteration support, and that determining if the structure is empty is easy.
---
 src/core/chunked_stream.js | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/src/core/chunked_stream.js b/src/core/chunked_stream.js
index dfc8920d9..7437ae391 100644
--- a/src/core/chunked_stream.js
+++ b/src/core/chunked_stream.js
@@ -320,7 +320,7 @@ class ChunkedStreamManager {
     this.currRequestId = 0;
 
     this._chunksNeededByRequest = new Map();
-    this.requestsByChunk = Object.create(null);
+    this._requestsByChunk = new Map();
     this.promisesByRequest = Object.create(null);
     this.progressiveDataLength = 0;
     this.aborted = false;
@@ -401,11 +401,14 @@ class ChunkedStreamManager {
 
     const chunksToRequest = [];
     for (const chunk of chunksNeeded) {
-      if (!(chunk in this.requestsByChunk)) {
-        this.requestsByChunk[chunk] = [];
+      let requestIds = this._requestsByChunk.get(chunk);
+      if (!requestIds) {
+        requestIds = [];
+        this._requestsByChunk.set(chunk, requestIds);
+
         chunksToRequest.push(chunk);
       }
-      this.requestsByChunk[chunk].push(requestId);
+      requestIds.push(requestId);
     }
 
     if (!chunksToRequest.length) {
@@ -521,8 +524,11 @@ class ChunkedStreamManager {
     const loadedRequests = [];
     for (let curChunk = beginChunk; curChunk < endChunk; ++curChunk) {
       // The server might return more chunks than requested.
-      const requestIds = this.requestsByChunk[curChunk] || [];
-      delete this.requestsByChunk[curChunk];
+      const requestIds = this._requestsByChunk.get(curChunk);
+      if (!requestIds) {
+        continue;
+      }
+      this._requestsByChunk.delete(curChunk);
 
       for (const requestId of requestIds) {
         const chunksNeeded = this._chunksNeededByRequest.get(requestId);
@@ -539,7 +545,7 @@ class ChunkedStreamManager {
 
     // If there are no pending requests, automatically fetch the next
     // unfetched chunk of the PDF file.
-    if (!this.disableAutoFetch && isEmptyObj(this.requestsByChunk)) {
+    if (!this.disableAutoFetch && this._requestsByChunk.size === 0) {
       let nextEmptyChunk;
       if (this.stream.numChunksLoaded === 1) {
         // This is a special optimization so that after fetching the first