Changed modulo to bitwise AND in hash calculation

(cherry picked from commit 7952525)
scala · adriaanm · Jan 6, 2017 · Jan 5, 2017 · Jul 21, 2016 · Nov 22, 2016
commit 26ad9cc73c88bad0140adb7cec583a4d05db498d
diff --git a/src/library/scala/collection/mutable/HashTable.scala b/src/library/scala/collection/mutable/HashTable.scala
@@ -411,58 +411,23 @@ private[collection] object HashTable {
 
     protected def elemHashCode(key: KeyType) = key.##
 
-    protected final def improve(hcode: Int, seed: Int) = {
-      /* Murmur hash
-       *  m = 0x5bd1e995
-       *  r = 24
-       *  note: h = seed = 0 in mmix
-       *  mmix(h,k) = k *= m; k ^= k >> r; k *= m; h *= m; h ^= k; */
-      // var k = hcode * 0x5bd1e995
-      // k ^= k >> 24
-      // k *= 0x5bd1e995
-      // k
-
-      /* Another fast multiplicative hash
-       * by Phil Bagwell
-       *
-       * Comment:
-       * Multiplication doesn't affect all the bits in the same way, so we want to
-       * multiply twice, "once from each side".
-       * It would be ideal to reverse all the bits after the first multiplication,
-       * however, this is more costly. We therefore restrict ourselves only to
-       * reversing the bytes before final multiplication. This yields a slightly
-       * worse entropy in the lower 8 bits, but that can be improved by adding:
-       *
-       * `i ^= i >> 6`
-       *
-       * For performance reasons, we avoid this improvement.
-       * */
-      val i= scala.util.hashing.byteswap32(hcode)
-
-      /* Jenkins hash
-       * for range 0-10000, output has the msb set to zero */
-      // var h = hcode + (hcode << 12)
-      // h ^= (h >> 22)
-      // h += (h << 4)
-      // h ^= (h >> 9)
-      // h += (h << 10)
-      // h ^= (h >> 2)
-      // h += (h << 7)
-      // h ^= (h >> 12)
-      // h
-
-      /* OLD VERSION
-       * quick, but bad for sequence 0-10000 - little entropy in higher bits
-       * since 2003 */
-      // var h: Int = hcode + ~(hcode << 9)
-      // h = h ^ (h >>> 14)
-      // h = h + (h << 4)
-      // h ^ (h >>> 10)
-
-      // the rest of the computation is due to SI-5293
-      val rotation = seed % 32
-      val rotated = (i >>> rotation) | (i << (32 - rotation))
-      rotated
+    /**
+      * Defer to a high-quality hash in [[scala.util.hashing]].
+      * The goal is to distribute across bins as well as possible even if a hash code has low entropy at some bits.
+      * <p/>
+      * OLD VERSION - quick, but bad for sequence 0-10000 - little entropy in higher bits - since 2003
+      * {{{
+      * var h: Int = hcode + ~(hcode << 9)
+      * h = h ^ (h >>> 14)
+      * h = h + (h << 4)
+      * h ^ (h >>> 10)
+      * }}}
+      * the rest of the computation is due to SI-5293
+      */
+    protected final def improve(hcode: Int, seed: Int): Int = {
+      val hash = scala.util.hashing.byteswap32(hcode)
+      val shift = seed & ((1 << 5) - 1)
+      (hash >>> shift) | (hash << (32 - shift))
     }
   }