From e1cceb5c58f693b925b7dc845d0cd81db4b01efc Mon Sep 17 00:00:00 2001
From: Paul Marrapese <paul.marrapese@gmail.com>
Date: Sun, 5 Apr 2020 19:06:08 -0700
Subject: [PATCH] improved performance of addLayers by preallocating array

Cherry picked from https://github.com/Leaflet/Leaflet.markercluster/pull/988
---
 src/MarkerClusterGroup.js | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/src/MarkerClusterGroup.js b/src/MarkerClusterGroup.js
index 99c56e7d..db5c3d7c 100644
--- a/src/MarkerClusterGroup.js
+++ b/src/MarkerClusterGroup.js
@@ -291,7 +291,8 @@ export var MarkerClusterGroup = L.MarkerClusterGroup = L.FeatureGroup.extend({
 
 			process();
 		} else {
-			var needsClustering = this._needsClustering;
+			var needsClustering = new Array(l - offset);	// improve performance by preallocating the maximum size of our array
+			var tail = 0;
 
 			for (; offset < l; offset++) {
 				m = layersArray[offset];
@@ -317,8 +318,11 @@ export var MarkerClusterGroup = L.MarkerClusterGroup = L.FeatureGroup.extend({
 					continue;
 				}
 
-				needsClustering.push(m);
+				needsClustering[tail++] = m;
 			}
+
+			needsClustering = needsClustering.slice(0, tail);	// truncate empty elements
+			this._needsClustering.push.apply(this._needsClustering, needsClustering);
 		}
 		return this;
 	},