reduce alloc for hashCode (#1383)

Copy pattern used in SmallHashMap to reduce allocations
while computing the cached hash code value.

atlas-core/src/main/scala/com/netflix/atlas/core/util/SortedTagMap.scala

@@ -137,11 +137,35 @@ final class SortedTagMap private (private val data: Array[String], private val l
     */
   override def hashCode: Int = {
     if (cachedHashCode == 0) {
-      cachedHashCode = super.hashCode()
+      cachedHashCode = computeHashCode
     }
     cachedHashCode
   }
 
+  /**
+    * Compute the hash code for the map. This method is based on the
+    * [[scala.util.hashing.MurmurHash3.unorderedHash()]] method. It is more efficient
+    * for our purposes because it avoids creating tons of [[scala.runtime.IntRef]]
+    * objects as well as tuples during iteration.
+    */
+  private[util] def computeHashCode: Int = {
+    var a, b = 0
+    var c = 1
+    var i = 0
+    while (i < length) {
+      val h = data(i).hashCode
+      a += h
+      b ^= h
+      if (h != 0) c *= h
+      i += 1
+    }
+    var h = 0x3c074a61
+    h = scala.util.hashing.MurmurHash3.mix(h, a)
+    h = scala.util.hashing.MurmurHash3.mix(h, b)
+    h = scala.util.hashing.MurmurHash3.mixLast(h, c)
+    scala.util.hashing.MurmurHash3.finalizeHash(h, length)
+  }
+
   /**
     * Overridden to get better performance.
     */

atlas-core/src/test/scala/com/netflix/atlas/core/util/SortedTagMapSuite.scala

@@ -189,4 +189,44 @@ class SortedTagMapSuite extends FunSuite {
     assert(a.compareTo(b) < 0)
     assert(b.compareTo(a) > 0)
   }
+
+  test("equals contract") {
+    val a = SortedTagMap(Array("a", "1", "b", "2"))
+    val b = SortedTagMap(Array("a", "1", "b", "2", "c", "3"))
+    val c = SortedTagMap.createUnsafe(Array("a", "1", "b", "2", null, null), 4)
+    val d = SortedTagMap.createUnsafe(Array("a", "1", "b", "2", null, null, null, null), 4)
+
+    // reflexive
+    assertEquals(a, a)
+    assertEquals(b, b)
+    assertEquals(c, c)
+    assertEquals(d, d)
+
+    // symmetric
+    assert(a != b && b != a)
+    assert(a == c && c == a)
+    assert(a == d && c == d)
+
+    // transitive
+    assert(a == c && c == d && a == d)
+
+    // nulls
+    assert(a != null)
+  }
+
+  test("hashCode") {
+    val a = SortedTagMap(Array("a", "1", "b", "2"))
+    val b = SortedTagMap(Array("a", "1", "b", "2", "c", "3"))
+    val c = SortedTagMap.createUnsafe(Array("a", "1", "b", "2", null, null), 4)
+    val d = SortedTagMap.createUnsafe(Array("a", "1", "b", "2", null, null, null, null), 4)
+
+    assertEquals(a.hashCode, a.hashCode)
+    assertEquals(b.hashCode, b.hashCode)
+    assertEquals(c.hashCode, c.hashCode)
+    assertEquals(d.hashCode, d.hashCode)
+
+    assertNotEquals(a.hashCode, b.hashCode)
+    assertEquals(a.hashCode, c.hashCode)
+    assertEquals(a.hashCode, d.hashCode)
+  }
 }