add some scala collections tricks, fix #6

src/main/scala/com/spotify/bdrc/scala/Collections.scala

@@ -0,0 +1,70 @@
+/*
+ * Copyright 2017 Spotify AB.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package com.spotify.bdrc.scala
+
+/**
+ * Examples for working with Scala collections.
+ */
+object Collections {
+
+  def mapValues: Unit = {
+    val m = Map("a" -> 1, "b" -> 2, "c" -> 3)
+
+    // Native approach, inefficient  since .toList, .map, .toMap each creates a copy
+    m.toList.map(t => (t._1, t._2 + 1)).toMap
+
+    // Better, one copy
+    m.map(kv => (kv._1, kv._2 + 1))
+
+    // Lazy version, no copy
+    m.mapValues(_ + 1)
+  }
+
+  def mergeMaps: Unit = {
+    val m1 = Map("a" -> 1.0, "b" -> 2.0, "c" -> 3.0)
+    val m2 = Map("a" -> 1.5, "b" -> 2.5, "d" -> 3.5)
+
+    // Native approach, inefficient since it creates many copies
+    val i = m1.keySet intersect m2.keySet
+    val m = i.map(k => k -> (m1(k) + m2(k))) // sum values of common keys
+    (m1 -- i) ++ (m2 -- i) ++ m // inefficient, creates 2 more temporary maps
+    m1 ++ m2 ++ m // slightly better, values from RHS overwrites those from LHS
+
+    // Slightly better but still creates a temporary set
+    (m1.keySet ++ m2.keySet).map(k => k -> (m1.getOrElse(k, 0.0) + m2.getOrElse(k, 0.0)))
+
+    // Better but slightly cryptic
+    m1 ++ m2.map { case (k, v) => k -> (v + m1.getOrElse(k, 0.0)) }
+  }
+
+  def listToMap: Unit = {
+    val l = List(1, 2, 3, 4, 5)
+
+    // Native approach, creates a temporary copy
+    l.map(x => "key" + x -> x).toMap
+
+    // Slightly better, using a mutable builder
+    val b = Map.newBuilder[String, Int]
+    l.foreach(x => b += "key" + x -> x)
+    b.result()
+
+    // Use implicits to automatically build for the target collection type Map[String, Int]
+    val m: Map[String, Int] = l.map(x => "key" + x -> x)(scala.collection.breakOut)
+  }
+
+}