From b0bcd78e38086f95dbc0efaeaf4a5d134b9c8e5d Mon Sep 17 00:00:00 2001
From: Ilya Gorbunov <ilya.gorbunov@jetbrains.com>
Date: Thu, 23 Aug 2018 23:56:48 +0300
Subject: [PATCH] Optimize copying for typed arrays in JS

---
 libraries/stdlib/js/src/kotlin/collections.kt | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/libraries/stdlib/js/src/kotlin/collections.kt b/libraries/stdlib/js/src/kotlin/collections.kt
index fa9c69c59cd..33178bcae3f 100644
--- a/libraries/stdlib/js/src/kotlin/collections.kt
+++ b/libraries/stdlib/js/src/kotlin/collections.kt
@@ -136,13 +136,18 @@ internal fun <T> arrayCopy(source: Array<out T>, destination: Array<in T>, desti
     val rangeSize = endIndex - startIndex
     AbstractList.checkRangeIndexes(destinationOffset, destinationOffset + rangeSize, destination.size)
 
-    if (source !== destination || destinationOffset <= startIndex) {
-        for (index in 0 until rangeSize) {
-            destination[destinationOffset + index] = source[startIndex + index]
-        }
+    if (js("ArrayBuffer").isView(destination) && js("ArrayBuffer").isView(source)) {
+        val subrange = source.asDynamic().subarray(startIndex, endIndex)
+        destination.asDynamic().set(subrange, destinationOffset)
     } else {
-        for (index in rangeSize - 1 downTo 0) {
-            destination[destinationOffset + index] = source[startIndex + index]
+        if (source !== destination || destinationOffset <= startIndex) {
+            for (index in 0 until rangeSize) {
+                destination[destinationOffset + index] = source[startIndex + index]
+            }
+        } else {
+            for (index in rangeSize - 1 downTo 0) {
+                destination[destinationOffset + index] = source[startIndex + index]
+            }
         }
     }
 }