Scala example source code file (MurmurHash.scala)

This example Scala source code file (MurmurHash.scala) is included in my "Source Code Warehouse" project. The intent of this project is to help you more easily find Scala source code examples by using tags.

All credit for the original source code belongs to scala-lang.org; I'm just trying to make examples easier to find. (For my Scala work, see my Scala examples and tutorials.)

Scala tags/keywords

array, collection, int, murmurhash, murmurhash3, string, t, unit, use

The MurmurHash.scala Scala example source code

/*                     __                                               *\
**     ________ ___   / /  ___     Scala API                            **
**    / __/ __// _ | / /  / _ |    (c) 2003-2013, LAMP/EPFL             **
**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
** /____/\___/_/ |_/____/_/ | |                                         **
**                          |/                                          **
\*                                                                      */

package scala
package 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.
 */
@deprecated("Use the object MurmurHash3 instead.", "2.10.0")
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.
 */
@deprecated("Use the object MurmurHash3 instead.", "2.10.0")
// NOTE: Used by SBT 0.13.0-M2 and below
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[@specialized 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).toInt,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: scala.collection.TraversableOnce[T], seed: Int) = {
    var a,b,n = 0
    var c = 1
    xs.seq.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)
  }
}