Scala example source code file (HashTable.scala)
The Scala HashTable.scala source code/* __ *\
** ________ ___ / / ___ Scala API **
** / __/ __// _ | / / / _ | (c) 2003-2011, LAMP/EPFL **
** __\ \/ /__/ __ |/ /__/ __ | http://www.scala-lang.org/ **
** /____/\___/_/ |_/____/_/ | | **
** |/ **
\* */
package scala.collection
package mutable
/** This class can be used to construct data structures that are based
* on hashtables. Class `HashTable[A]` implements a hashtable
* that maps keys of type `A` to values of the fully abstract
* member type `Entry`. Classes that make use of `HashTable`
* have to provide an implementation for `Entry`.
*
* There are mainly two parameters that affect the performance of a hashtable:
* the <i>initial size and the load factor. The size
* refers to the number of <i>buckets in the hashtable, and the load
* factor</i> is a measure of how full the hashtable is allowed to get before
* its size is automatically doubled. Both parameters may be changed by
* overriding the corresponding values in class `HashTable`.
*
* @author Matthias Zenger
* @author Martin Odersky
* @version 2.0, 31/12/2006
* @since 1
*
* @tparam A type of the elements contained in this hash table.
*/
trait HashTable[A, Entry >: Null <: HashEntry[A, Entry]] extends HashTable.HashUtils[A] {
import HashTable._
@transient protected var _loadFactor = defaultLoadFactor
/** The actual hash table.
*/
@transient protected var table: Array[HashEntry[A, Entry]] = new Array(initialCapacity)
/** The number of mappings contained in this hash table.
*/
@transient protected var tableSize: Int = 0
/** The next size value at which to resize (capacity * load factor).
*/
@transient protected var threshold: Int = initialThreshold(_loadFactor)
/** The array keeping track of the number of elements in 32 element blocks.
*/
@transient protected var sizemap: Array[Int] = null
protected def initialSize: Int = HashTable.initialSize
/**
* Initializes the collection from the input stream. `f` will be called for each key/value pair
* read from the input stream in the order determined by the stream. This is useful for
* structures where iteration order is important (e.g. LinkedHashMap).
*/
private[collection] def init[B](in: java.io.ObjectInputStream, f: (A, B) => Entry) {
in.defaultReadObject
_loadFactor = in.readInt
assert(_loadFactor > 0)
val size = in.readInt
tableSize = 0
assert(size >= 0)
val smDefined = in.readBoolean
table = new Array(capacity(sizeForThreshold(_loadFactor, size)))
threshold = newThreshold(_loadFactor, table.size)
if (smDefined) sizeMapInit(table.length) else sizemap = null
var index = 0
while (index < size) {
addEntry(f(in.readObject.asInstanceOf[A], in.readObject.asInstanceOf[B]))
index += 1
}
}
/**
* Serializes the collection to the output stream by saving the load factor, collection
* size, collection keys and collection values. `value` is responsible for providing a value
* from an entry.
*
* `foreach` determines the order in which the key/value pairs are saved to the stream. To
* deserialize, `init` should be used.
*/
private[collection] def serializeTo[B](out: java.io.ObjectOutputStream, value: Entry => B) {
out.defaultWriteObject
out.writeInt(_loadFactor)
out.writeInt(tableSize)
out.writeBoolean(isSizeMapDefined)
foreachEntry { entry =>
out.writeObject(entry.key)
out.writeObject(value(entry))
}
}
/** Find entry with given key in table, null if not found.
*/
protected def findEntry(key: A): Entry = {
val h = index(elemHashCode(key))
var e = table(h).asInstanceOf[Entry]
while (e != null && !elemEquals(e.key, key)) e = e.next
e
}
/** Add entry to table
* pre: no entry with same key exists
*/
protected def addEntry(e: Entry) {
val h = index(elemHashCode(e.key))
e.next = table(h).asInstanceOf[Entry]
table(h) = e
tableSize = tableSize + 1
nnSizeMapAdd(h)
if (tableSize > threshold)
resize(2 * table.length)
}
/** Remove entry from table if present.
*/
protected def removeEntry(key: A) : Entry = {
val h = index(elemHashCode(key))
var e = table(h).asInstanceOf[Entry]
if (e != null) {
if (elemEquals(e.key, key)) {
table(h) = e.next
tableSize = tableSize - 1
nnSizeMapRemove(h)
return e
} else {
var e1 = e.next
while (e1 != null && !elemEquals(e1.key, key)) {
e = e1
e1 = e1.next
}
if (e1 != null) {
e.next = e1.next
tableSize = tableSize - 1
nnSizeMapRemove(h)
return e1
}
}
}
null
}
/** An iterator returning all entries.
*/
protected def entriesIterator: Iterator[Entry] = new Iterator[Entry] {
val iterTable = table
var idx = table.length - 1
var es = iterTable(idx).asInstanceOf[Entry]
scan()
def hasNext = es != null
def next() = {
val res = es
es = es.next
scan()
res
}
def scan() {
while (es == null && idx > 0) {
idx = idx - 1
es = iterTable(idx).asInstanceOf[Entry]
}
}
}
/*
* We should implement this as a primitive operation over the underlying array, but it can
* cause a behaviour change in edge cases where:
* - Someone modifies a map during iteration
* - The insertion point is close to the iteration point.
*
* The reason this happens is that the iterator prefetches the following element before
* returning from next (to simplify the implementation of hasNext) while the natural
* implementation of foreach does not.
*
* It should be mentioned that modifying a map during iteration leads to unpredictable
* results with either implementation.
*/
protected final def foreachEntry[C](f: Entry => C) { entriesIterator.foreach(f) }
/** An iterator returning all entries */
@deprecated("use entriesIterator instead", "2.8.0")
protected def entries: Iterator[Entry] = entriesIterator
/** Remove all entries from table
*/
protected def clearTable() {
var i = table.length - 1
while (i >= 0) { table(i) = null; i = i - 1 }
tableSize = 0
nnSizeMapReset(0)
}
private def resize(newSize: Int) {
val oldTable = table
table = new Array(newSize)
nnSizeMapReset(table.length)
var i = oldTable.length - 1
while (i >= 0) {
var e = oldTable(i)
while (e != null) {
val h = index(elemHashCode(e.key))
val e1 = e.next
e.next = table(h).asInstanceOf[Entry]
table(h) = e
e = e1
nnSizeMapAdd(h)
}
i = i - 1
}
threshold = newThreshold(_loadFactor, newSize)
}
/* Size map handling code */
/*
* The following three sizeMap* functions (Add, Remove, Reset)
* are used to update the size map of the hash table.
*
* The size map logically divides the hash table into `sizeMapBucketSize` element buckets
* by keeping an integer entry for each such bucket. Each integer entry simply denotes
* the number of elements in the corresponding bucket.
* Best understood through an example, see:
* table = [/, 1, /, 6, 90, /, -3, 5] (8 entries)
* sizemap = [ 2 | 3 ] (2 entries)
* where sizeMapBucketSize == 4.
*
* By default the size map is not initialized, so these methods don't do anything, thus,
* their impact on hash table performance is negligible. However, if the hash table
* is converted into a parallel hash table, the size map is initialized, as it will be needed
* there.
*/
protected def nnSizeMapAdd(h: Int) = if (sizemap ne null) {
sizemap(h >> sizeMapBucketBitSize) += 1
}
protected def nnSizeMapRemove(h: Int) = if (sizemap ne null) {
sizemap(h >> sizeMapBucketBitSize) -= 1
}
protected def nnSizeMapReset(tableLength: Int) = if (sizemap ne null) {
val nsize = calcSizeMapSize(tableLength)
if (sizemap.length != nsize) sizemap = new Array[Int](nsize)
else java.util.Arrays.fill(sizemap, 0)
}
private[collection] final def totalSizeMapBuckets = if (sizeMapBucketSize < table.length) 1 else table.length / sizeMapBucketSize
protected def calcSizeMapSize(tableLength: Int) = (tableLength >> sizeMapBucketBitSize) + 1
// discards the previous sizemap and only allocates a new one
protected def sizeMapInit(tableLength: Int) {
sizemap = new Array[Int](calcSizeMapSize(tableLength))
}
// discards the previous sizemap and populates the new one
protected def sizeMapInitAndRebuild() {
sizeMapInit(table.length)
// go through the buckets, count elements
var tableidx = 0
var bucketidx = 0
val tbl = table
var tableuntil = 0
if (tbl.length < sizeMapBucketSize) tableuntil = tbl.length else tableuntil = sizeMapBucketSize
val totalbuckets = totalSizeMapBuckets
while (bucketidx < totalbuckets) {
var currbucketsize = 0
while (tableidx < tableuntil) {
var e = tbl(tableidx)
while (e ne null) {
currbucketsize += 1
e = e.next
}
tableidx += 1
}
sizemap(bucketidx) = currbucketsize
tableuntil += sizeMapBucketSize
bucketidx += 1
}
}
private[collection] def printSizeMap() {
println(sizemap.toList)
}
protected def sizeMapDisable() = sizemap = null
protected def isSizeMapDefined = sizemap ne null
// override to automatically initialize the size map
protected def alwaysInitSizeMap = false
/* End of size map handling code */
protected def elemEquals(key1: A, key2: A): Boolean = (key1 == key2)
// Note:
// we take the most significant bits of the hashcode, not the lower ones
// this is of crucial importance when populating the table in parallel
protected final def index(hcode: Int) = {
val ones = table.length - 1
val improved = improve(hcode)
val shifted = (improved >> (32 - java.lang.Integer.bitCount(ones))) & ones
shifted
}
protected def initWithContents(c: HashTable.Contents[A, Entry]) = {
if (c != null) {
_loadFactor = c.loadFactor
table = c.table
tableSize = c.tableSize
threshold = c.threshold
sizemap = c.sizemap
}
if (alwaysInitSizeMap && sizemap == null) sizeMapInitAndRebuild
}
private[collection] def hashTableContents = new HashTable.Contents(
_loadFactor,
table,
tableSize,
threshold,
sizemap
)
}
private[collection] object HashTable {
/** The load factor for the hash table (in 0.001 step).
*/
private[collection] final def defaultLoadFactor: Int = 750 // corresponds to 75%
private[collection] final def loadFactorDenum = 1000;
/** The initial size of the hash table.
*/
private[collection] final def initialSize: Int = 16
/** The initial threshold.
*/
private[collection] final def initialThreshold(_loadFactor: Int): Int = newThreshold(_loadFactor, initialCapacity)
private[collection] final def initialCapacity = capacity(initialSize)
private[collection] final def newThreshold(_loadFactor: Int, size: Int) = ((size.toLong * _loadFactor) / loadFactorDenum).toInt
private[collection] final def sizeForThreshold(_loadFactor: Int, thr: Int) = thr * loadFactorDenum / _loadFactor
private[collection] final def capacity(expectedSize: Int) = if (expectedSize == 0) 1 else powerOfTwo(expectedSize)
trait HashUtils[KeyType] {
protected final def sizeMapBucketBitSize = 5
// so that:
protected final def sizeMapBucketSize = 1 << sizeMapBucketBitSize
protected def elemHashCode(key: KeyType) = key.##
protected final def improve(hcode: 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.
* */
var i = hcode * 0x9e3775cd
i = java.lang.Integer.reverseBytes(i)
i * 0x9e3775cd
// a slower alternative for byte reversal:
// i = (i << 16) | (i >> 16)
// i = ((i >> 8) & 0x00ff00ff) | ((i << 8) & 0xff00ff00)
/* 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 enthropy in higher bits
* since 2003 */
// var h: Int = hcode + ~(hcode << 9)
// h = h ^ (h >>> 14)
// h = h + (h << 4)
// h ^ (h >>> 10)
}
}
/**
* Returns a power of two >= `target`.
*/
private[collection] def powerOfTwo(target: Int): Int = {
/* See http://bits.stephan-brumme.com/roundUpToNextPowerOfTwo.html */
var c = target - 1;
c |= c >>> 1;
c |= c >>> 2;
c |= c >>> 4;
c |= c >>> 8;
c |= c >>> 16;
c + 1;
}
class Contents[A, Entry >: Null <: HashEntry[A, Entry]](
val loadFactor: Int,
val table: Array[HashEntry[A, Entry]],
val tableSize: Int,
val threshold: Int,
val sizemap: Array[Int]
) {
import collection.DebugUtils._
private[collection] def debugInformation = buildString {
append =>
append("Hash table contents")
append("-------------------")
append("Table: [" + arrayString(table, 0, table.length) + "]")
append("Table size: " + tableSize)
append("Load factor: " + loadFactor)
append("Threshold: " + threshold)
append("Sizemap: [" + arrayString(sizemap, 0, sizemap.length) + "]")
}
}
}
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