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/* __ *\
** ________ ___ / / ___ Scala API **
** / __/ __// _ | / / / _ | (c) 2003-2011, LAMP/EPFL **
** __\ \/ /__/ __ |/ /__/ __ | http://scala-lang.org/ **
** /____/\___/_/ |_/____/_/ | | **
** |/ **
\* */
/**
* 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
*/
package akka.routing
import java.lang.Integer.{ rotateLeft ⇒ rotl }
/**
* 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: Int = 0x971e137b
final private val hiddenMagicA: Int = 0x95543787
final private val hiddenMagicB: Int = 0x2ad7eb25
final private val visibleMixer: Int = 0x52dce729
final private val hiddenMixerA: Int = 0x7b7d159c
final private val hiddenMixerB: Int = 0x6bce6396
final private val finalMixer1: Int = 0x85ebca6b
final private val finalMixer2: Int = 0xc2b2ae35
// Arbitrary values used for hashing certain classes
final private val seedString: Int = 0xf7ca7fd2
final private val seedArray: Int = 0x3c074a61
/** The first 23 magic integers from the first stream are stored here */
private val storedMagicA: Array[Int] =
Iterator.iterate(hiddenMagicA)(nextMagicA).take(23).toArray
/** The first 23 magic integers from the second stream are stored here */
private val storedMagicB: Array[Int] =
Iterator.iterate(hiddenMagicB)(nextMagicB).take(23).toArray
/** Begin a new hash with a seed value. */
def startHash(seed: Int): Int = seed ^ visibleMagic
/** The initial magic integers in the first stream. */
def startMagicA: Int = hiddenMagicA
/** The initial magic integer in the second stream. */
def startMagicB: Int = 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): Int =
(hash ^ rotl(value * magicA, 11) * magicB) * 3 + visibleMixer
/** Given a magic integer from the first stream, compute the next */
def nextMagicA(magicA: Int): Int = magicA * 5 + hiddenMixerA
/** Given a magic integer from the second stream, compute the next */
def nextMagicB(magicB: Int): Int = magicB * 5 + hiddenMixerB
/** Once all hashes have been incorporated, this performs a final mixing */
def finalizeHash(hash: Int): 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]): Int = {
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): Int = {
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: TraversableOnce[T], seed: Int): 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)
}
}
