/*                     __                                               *\
**     ________ ___   / /  ___     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[@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),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.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)
  }
}