Port of MurmurHash to 2.8.x

Author: Antonio Cunei

src/library/scala/util/MurmurHash.scala

@@ -0,0 +1,196 @@
+/*                     __                                               *\
+**     ________ ___   / /  ___     Scala API                            **
+**    / __/ __// _ | / /  / _ |    (c) 2003-2011, LAMP/EPFL             **
+**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
+** /____/\___/_/ |_/____/_/ | |                                         **
+**                          |/                                          **
+\*                                                                      */
+
+package scala.util
+
+/** An implementation of Austin Appleby's MurmurHash 3.0 algorithm
+ *  (32 bit version); reference: http://code.google.com/p/smhasher
+ *
+ *  This is the hash used by collections and case classes (including
+ *  tuples).
+ *
+ *  @author  Rex Kerr
+ *  @version 2.9
+ *  @since   2.9
+ */
+
+import java.lang.Integer.{ rotateLeft => rotl }
+import scala.collection.Iterator
+
+/** A class designed to generate well-distributed non-cryptographic
+ *  hashes.  It is designed to be passed to a collection's foreach method,
+ *  or can take individual hash values with append.  Its own hash code is
+ *  set equal to the hash code of whatever it is hashing.
+ */
+class MurmurHash[@specialized(Int,Long,Float,Double) T](seed: Int) extends (T => Unit) {
+  import MurmurHash._
+
+  private var h = startHash(seed)
+  private var c = hiddenMagicA
+  private var k = hiddenMagicB
+  private var hashed = false
+  private var hashvalue = h
+
+  /** Begin a new hash using the same seed. */
+  def reset() {
+    h = startHash(seed)
+    c = hiddenMagicA
+    k = hiddenMagicB
+    hashed = false
+  }
+
+  /** Incorporate the hash value of one item. */
+  def apply(t: T) {
+    h = extendHash(h,t.##,c,k)
+    c = nextMagicA(c)
+    k = nextMagicB(k)
+    hashed = false
+  }
+
+  /** Incorporate a known hash value. */
+  def append(i: Int) {
+    h = extendHash(h,i,c,k)
+    c = nextMagicA(c)
+    k = nextMagicB(k)
+    hashed = false
+  }
+
+  /** Retrieve the hash value */
+  def hash = {
+    if (!hashed) {
+      hashvalue = finalizeHash(h)
+      hashed = true
+    }
+    hashvalue
+  }
+  override def hashCode = hash
+}
+
+/** An object designed to generate well-distributed non-cryptographic
+ *  hashes.  It is designed to hash a collection of integers; along with
+ *  the integers to hash, it generates two magic streams of integers to
+ *  increase the distribution of repetitive input sequences.  Thus,
+ *  three methods need to be called at each step (to start and to
+ *  incorporate a new integer) to update the values.  Only one method
+ *  needs to be called to finalize the hash.
+ */
+
+object MurmurHash {
+  // Magic values used for MurmurHash's 32 bit hash.
+  // Don't change these without consulting a hashing expert!
+  final private val visibleMagic = 0x971e137b
+  final private val hiddenMagicA = 0x95543787
+  final private val hiddenMagicB = 0x2ad7eb25
+  final private val visibleMixer = 0x52dce729
+  final private val hiddenMixerA = 0x7b7d159c
+  final private val hiddenMixerB = 0x6bce6396
+  final private val finalMixer1 = 0x85ebca6b
+  final private val finalMixer2 = 0xc2b2ae35
+
+  // Arbitrary values used for hashing certain classes
+  final private val seedString = 0xf7ca7fd2
+  final private val seedArray = 0x3c074a61
+
+  /** The first 23 magic integers from the first stream are stored here */
+  val storedMagicA =
+    Iterator.iterate(hiddenMagicA)(nextMagicA).take(23).toArray
+
+  /** The first 23 magic integers from the second stream are stored here */
+  val storedMagicB =
+    Iterator.iterate(hiddenMagicB)(nextMagicB).take(23).toArray
+
+  /** Begin a new hash with a seed value. */
+  def startHash(seed: Int) = seed ^ visibleMagic
+
+  /** The initial magic integers in the first stream. */
+  def startMagicA = hiddenMagicA
+
+  /** The initial magic integer in the second stream. */
+  def startMagicB = hiddenMagicB
+
+  /** Incorporates a new value into an existing hash.
+   *
+   *  @param   hash    the prior hash value
+   *  @param  value    the new value to incorporate
+   *  @param magicA    a magic integer from the stream
+   *  @param magicB    a magic integer from a different stream
+   *  @return          the updated hash value
+   */
+  def extendHash(hash: Int, value: Int, magicA: Int, magicB: Int) = {
+    (hash ^ rotl(value*magicA,11)*magicB)*3 + visibleMixer
+  }
+
+  /** Given a magic integer from the first stream, compute the next */
+  def nextMagicA(magicA: Int) = magicA*5 + hiddenMixerA
+
+  /** Given a magic integer from the second stream, compute the next */
+  def nextMagicB(magicB: Int) = magicB*5 + hiddenMixerB
+
+  /** Once all hashes have been incorporated, this performs a final mixing */
+  def finalizeHash(hash: Int) = {
+    var i = (hash ^ (hash>>>16))
+    i *= finalMixer1
+    i ^= (i >>> 13)
+    i *= finalMixer2
+    i ^= (i >>> 16)
+    i
+  }
+
+  /** Compute a high-quality hash of an array */
+  def arrayHash[T](a: Array[T]) = {
+    var h = startHash(a.length * seedArray)
+    var c = hiddenMagicA
+    var k = hiddenMagicB
+    var j = 0
+    while (j < a.length) {
+      h = extendHash(h, a(j).##, c, k)
+      c = nextMagicA(c)
+      k = nextMagicB(k)
+      j += 1
+    }
+    finalizeHash(h)
+  }
+
+  /** Compute a high-quality hash of a string */
+  def stringHash(s: String) = {
+    var h = startHash(s.length * seedString)
+    var c = hiddenMagicA
+    var k = hiddenMagicB
+    var j = 0
+    while (j+1 < s.length) {
+      val i = (s.charAt(j)<<16) + s.charAt(j+1);
+      h = extendHash(h,i,c,k)
+      c = nextMagicA(c)
+      k = nextMagicB(k)
+      j += 2
+    }
+    if (j < s.length) h = extendHash(h,s.charAt(j),c,k)
+    finalizeHash(h)
+  }
+
+  /** Compute a hash that is symmetric in its arguments--that is,
+   *  where the order of appearance of elements does not matter.
+   *  This is useful for hashing sets, for example.
+   */
+  def symmetricHash[T](xs: collection.TraversableOnce[T], seed: Int) = {
+    var a,b,n = 0
+    var c = 1
+    xs.foreach(i => {
+      val h = i.##
+      a += h
+      b ^= h
+      if (h != 0) c *= h
+      n += 1
+    })
+    var h = startHash(seed * n)
+    h = extendHash(h, a, storedMagicA(0), storedMagicB(0))
+    h = extendHash(h, b, storedMagicA(1), storedMagicB(1))
+    h = extendHash(h, c, storedMagicA(2), storedMagicB(2))
+    finalizeHash(h)
+  }
+}