From 8baa6474a4b78af335d1fc7ae6309da5c461ea1d Mon Sep 17 00:00:00 2001
From: "Manu Mtz.-Almeida" <manu.valladolid@gmail.com>
Date: Sun, 23 Apr 2017 16:00:04 +0200
Subject: [PATCH] perf(virtual-scroll): fast path for removing

---
 .../virtual-scroll/test/basic/app.module.ts   |  7 +++--
 .../virtual-scroll/virtual-scroll.ts          | 31 +++++++++++--------
 2 files changed, 22 insertions(+), 16 deletions(-)

diff --git a/src/components/virtual-scroll/test/basic/app.module.ts b/src/components/virtual-scroll/test/basic/app.module.ts
index 1607a51826..9faa4cbee5 100644
--- a/src/components/virtual-scroll/test/basic/app.module.ts
+++ b/src/components/virtual-scroll/test/basic/app.module.ts
@@ -44,18 +44,19 @@ export class E2EPage {
 
   addRandomItem() {
     const index = Math.floor(Math.random() * this.items.length);
+    console.log('Adding to index: ', index);
     this.items.splice( index, 0, {
       value: Math.floor(Math.random() * 10000),
       someMethod: function() {
         return '!!';
       }
-    }
-    );
+    });
   }
 
   changeItem() {
     const index = Math.floor(Math.random() * this.items.length);
-    this.items[index].value = Math.floor(Math.random() * 10000);
+    console.log('Change to index: ', index);
+    this.items[index] = { value: Math.floor(Math.random() * 10000), someMethod: () => '!!' };
   }
 
   trackByFn(index: number, item: any) {
diff --git a/src/components/virtual-scroll/virtual-scroll.ts b/src/components/virtual-scroll/virtual-scroll.ts
index de7c1a622b..18829b5e54 100644
--- a/src/components/virtual-scroll/virtual-scroll.ts
+++ b/src/components/virtual-scroll/virtual-scroll.ts
@@ -233,7 +233,8 @@ export class VirtualScroll implements DoCheck, AfterContentInit, OnDestroy {
     scrollTop: 0,
   };
   _queue: number = SCROLL_QUEUE_NO_CHANGES;
-  _recordSize: number = 0;
+
+
   _virtualTrackBy: TrackByFn;
 
   @ContentChild(VirtualItem) _itmTmp: VirtualItem;
@@ -416,10 +417,7 @@ export class VirtualScroll implements DoCheck, AfterContentInit, OnDestroy {
    */
   firstRecord(): number {
     const cells = this._cells;
-    if (cells.length > 0) {
-      return cells[0].record;
-    }
-    return 0;
+    return (cells.length > 0) ? cells[0].record : 0;
   }
 
   /**
@@ -427,10 +425,7 @@ export class VirtualScroll implements DoCheck, AfterContentInit, OnDestroy {
    */
   lastRecord(): number {
     const cells = this._cells;
-    if (cells.length > 0) {
-      return cells[cells.length - 1].record;
-    }
-    return 0;
+    return (cells.length > 0) ? cells[cells.length - 1].record : 0;
   }
 
   /**
@@ -451,15 +446,25 @@ export class VirtualScroll implements DoCheck, AfterContentInit, OnDestroy {
     let needClean = false;
     if (changes) {
       var lastRecord = this.lastRecord() + 1;
-      changes.forEachOperation((item, _, cindex) => {
-        if (item.previousIndex != null || (cindex < lastRecord)) {
+
+      changes.forEachOperation((_, pindex, cindex) => {
+
+        // add new record after current position
+        if (pindex === null && (cindex < lastRecord)) {
+          console.debug('adding record before current position, slow path');
           needClean = true;
+          return;
+        }
+        // remove record after current position
+        if (pindex < lastRecord && cindex === null) {
+          console.debug('removing record before current position, slow path');
+          needClean = true;
+          return;
         }
       });
     } else {
       needClean = true;
     }
-    this._recordSize = this._records.length;
 
     this.readUpdate(needClean);
     this.writeUpdate(needClean);
@@ -555,7 +560,7 @@ export class VirtualScroll implements DoCheck, AfterContentInit, OnDestroy {
           this._itmTmp.viewContainer,
           this._itmTmp.templateRef,
           this._hdrTmp && this._hdrTmp.templateRef,
-          this._ftrTmp && this._ftrTmp.templateRef, needClean,
+          this._ftrTmp && this._ftrTmp.templateRef, needClean
         );
       });