[util-core] Allocate a hash table of sufficient size in MapUtil.newHashMap

Differential Revision: https://phabricator.twitter.biz/D1175381

Author: mbezoyan

util-core/src/main/scala-2.12-/com/twitter/util/MapUtil.scala

@@ -1,13 +1,20 @@
 package com.twitter.util
 
+import java.lang.Integer.numberOfLeadingZeros
 import scala.collection.mutable
 
 object MapUtil {
 
-  def newHashMap[K, V](initialCapacity: Int, loadFactor: Double = 0.75): mutable.HashMap[K, V] = {
+  def newHashMap[K, V](expectedSize: Int, loadFactor: Double = 0.75): mutable.HashMap[K, V] = {
     new mutable.HashMap[K, V]() {
       this._loadFactor = (loadFactor * 1000).toInt
-      override protected val initialSize: Int = (size.toLong / loadFactor).toInt
+      override protected def initialSize: Int = {
+        roundUpToNextPowerOfTwo((expectedSize / loadFactor).toInt max 4)
+      }
     }
   }
+
+  private[this] def roundUpToNextPowerOfTwo(target: Int): Int = {
+    1 << -numberOfLeadingZeros(target - 1)
+  }
 }

util-core/src/main/scala-2.13+/com/twitter/util/MapUtil.scala

@@ -4,7 +4,7 @@ import scala.collection.mutable
 
 object MapUtil {
 
-  def newHashMap[K, V](initialCapacity: Int, loadFactor: Double = 0.75): mutable.HashMap[K, V] = {
-    new mutable.HashMap[K, V](initialCapacity, loadFactor)
+  def newHashMap[K, V](expectedSize: Int, loadFactor: Double = 0.75): mutable.HashMap[K, V] = {
+    new mutable.HashMap[K, V]((expectedSize / loadFactor).toInt, loadFactor)
   }
 }