org.apache.xerces.util.SymbolTable Maven / Gradle / Ivy
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* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.xerces.util;
/**
* This class is a symbol table implementation that guarantees that
* strings used as identifiers are unique references. Multiple calls
* to addSymbol will always return the same string
* reference.
*
* The symbol table performs the same task as String.intern()
* with the following differences:
*
* -
* A new string object does not need to be created in order to
* retrieve a unique reference. Symbols can be added by using
* a series of characters in a character array.
*
* -
* Users of the symbol table can provide their own symbol hashing
* implementation. For example, a simple string hashing algorithm
* may fail to produce a balanced set of hashcodes for symbols
* that are mostly unique. Strings with similar leading
* characters are especially prone to this poor hashing behavior.
*
*
*
* An instance of SymbolTable has two parameters that affect its
* performance: initial capacity and load factor. The
* capacity is the number of buckets in the SymbolTable, and the
* initial capacity is simply the capacity at the time the SymbolTable
* is created. Note that the SymbolTable is open: in the case of a "hash
* collision", a single bucket stores multiple entries, which must be searched
* sequentially. The load factor is a measure of how full the SymbolTable
* is allowed to get before its capacity is automatically increased.
* When the number of entries in the SymbolTable exceeds the product of the load
* factor and the current capacity, the capacity is increased by calling the
* rehash method.
*
* Generally, the default load factor (.75) offers a good tradeoff between
* time and space costs. Higher values decrease the space overhead but
* increase the time cost to look up an entry (which is reflected in most
* SymbolTable operations, including addSymbol and containsSymbol).
*
* The initial capacity controls a tradeoff between wasted space and the
* need for rehash operations, which are time-consuming.
* No rehash operations will ever occur if the initial
* capacity is greater than the maximum number of entries the
* Hashtable will contain divided by its load factor. However,
* setting the initial capacity too high can waste space.
*
* If many entries are to be made into a SymbolTable,
* creating it with a sufficiently large capacity may allow the
* entries to be inserted more efficiently than letting it perform
* automatic rehashing as needed to grow the table.
* @see SymbolHash
*
* @author Andy Clark
* @author John Kim, IBM
*
* @version $Id: SymbolTable.java 1358350 2012-07-06 19:04:35Z mrglavas $
*/
public class SymbolTable {
//
// Constants
//
/** Default table size. */
protected static final int TABLE_SIZE = 101;
/** Maximum hash collisions per bucket for a table with load factor == 1. */
protected static final int MAX_HASH_COLLISIONS = 40;
protected static final int MULTIPLIERS_SIZE = 1 << 5;
protected static final int MULTIPLIERS_MASK = MULTIPLIERS_SIZE - 1;
//
// Data
//
/** Buckets. */
protected Entry[] fBuckets = null;
/** actual table size **/
protected int fTableSize;
/** The total number of entries in the hash table. */
protected transient int fCount;
/** The table is rehashed when its size exceeds this threshold. (The
* value of this field is (int)(capacity * loadFactor).) */
protected int fThreshold;
/** The load factor for the SymbolTable. */
protected float fLoadFactor;
/**
* A new hash function is selected and the table is rehashed when
* the number of keys in the bucket exceeds this threshold.
*/
protected final int fCollisionThreshold;
/**
* Array of randomly selected hash function multipliers or null
* if the default String.hashCode() function should be used.
*/
protected int[] fHashMultipliers;
//
// Constructors
//
/**
* Constructs a new, empty SymbolTable with the specified initial
* capacity and the specified load factor.
*
* @param initialCapacity the initial capacity of the SymbolTable.
* @param loadFactor the load factor of the SymbolTable.
* @throws IllegalArgumentException if the initial capacity is less
* than zero, or if the load factor is nonpositive.
*/
public SymbolTable(int initialCapacity, float loadFactor) {
if (initialCapacity < 0) {
throw new IllegalArgumentException("Illegal Capacity: " + initialCapacity);
}
if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
throw new IllegalArgumentException("Illegal Load: " + loadFactor);
}
if (initialCapacity == 0) {
initialCapacity = 1;
}
fLoadFactor = loadFactor;
fTableSize = initialCapacity;
fBuckets = new Entry[fTableSize];
fThreshold = (int)(fTableSize * loadFactor);
fCollisionThreshold = (int)(MAX_HASH_COLLISIONS * loadFactor);
fCount = 0;
}
/**
* Constructs a new, empty SymbolTable with the specified initial capacity
* and default load factor, which is 0.75.
*
* @param initialCapacity the initial capacity of the hashtable.
* @throws IllegalArgumentException if the initial capacity is less
* than zero.
*/
public SymbolTable(int initialCapacity) {
this(initialCapacity, 0.75f);
}
/**
* Constructs a new, empty SymbolTable with a default initial capacity (101)
* and load factor, which is 0.75.
*/
public SymbolTable() {
this(TABLE_SIZE, 0.75f);
}
//
// Public methods
//
/**
* Adds the specified symbol to the symbol table and returns a
* reference to the unique symbol. If the symbol already exists,
* the previous symbol reference is returned instead, in order
* guarantee that symbol references remain unique.
*
* @param symbol The new symbol.
*/
public String addSymbol(String symbol) {
// search for identical symbol
int collisionCount = 0;
int bucket = hash(symbol) % fTableSize;
for (Entry entry = fBuckets[bucket]; entry != null; entry = entry.next) {
if (entry.symbol.equals(symbol)) {
return entry.symbol;
}
++collisionCount;
}
return addSymbol0(symbol, bucket, collisionCount);
} // addSymbol(String):String
private String addSymbol0(String symbol, int bucket, int collisionCount) {
if (fCount >= fThreshold) {
// Rehash the table if the threshold is exceeded
rehash();
bucket = hash(symbol) % fTableSize;
}
else if (collisionCount >= fCollisionThreshold) {
// Select a new hash function and rehash the table if
// the collision threshold is exceeded.
rebalance();
bucket = hash(symbol) % fTableSize;
}
// create new entry
Entry entry = new Entry(symbol, fBuckets[bucket]);
fBuckets[bucket] = entry;
++fCount;
return entry.symbol;
} // addSymbol0(String,int,int):String
/**
* Adds the specified symbol to the symbol table and returns a
* reference to the unique symbol. If the symbol already exists,
* the previous symbol reference is returned instead, in order
* guarantee that symbol references remain unique.
*
* @param buffer The buffer containing the new symbol.
* @param offset The offset into the buffer of the new symbol.
* @param length The length of the new symbol in the buffer.
*/
public String addSymbol(char[] buffer, int offset, int length) {
// search for identical symbol
int collisionCount = 0;
int bucket = hash(buffer, offset, length) % fTableSize;
OUTER: for (Entry entry = fBuckets[bucket]; entry != null; entry = entry.next) {
if (length == entry.characters.length) {
for (int i = 0; i < length; i++) {
if (buffer[offset + i] != entry.characters[i]) {
++collisionCount;
continue OUTER;
}
}
return entry.symbol;
}
++collisionCount;
}
return addSymbol0(buffer, offset, length, bucket, collisionCount);
} // addSymbol(char[],int,int):String
private String addSymbol0(char[] buffer, int offset, int length, int bucket, int collisionCount) {
if (fCount >= fThreshold) {
// Rehash the table if the threshold is exceeded
rehash();
bucket = hash(buffer, offset, length) % fTableSize;
}
else if (collisionCount >= fCollisionThreshold) {
// Select a new hash function and rehash the table if
// the collision threshold is exceeded.
rebalance();
bucket = hash(buffer, offset, length) % fTableSize;
}
// add new entry
Entry entry = new Entry(buffer, offset, length, fBuckets[bucket]);
fBuckets[bucket] = entry;
++fCount;
return entry.symbol;
} // addSymbol0(char[],int,int,int,int):String
/**
* Returns a hashcode value for the specified symbol. The value
* returned by this method must be identical to the value returned
* by the hash(char[],int,int) method when called
* with the character array that comprises the symbol string.
*
* @param symbol The symbol to hash.
*/
public int hash(String symbol) {
if (fHashMultipliers == null) {
return symbol.hashCode() & 0x7FFFFFFF;
}
return hash0(symbol);
} // hash(String):int
private int hash0(String symbol) {
int code = 0;
final int length = symbol.length();
final int[] multipliers = fHashMultipliers;
for (int i = 0; i < length; ++i) {
code = code * multipliers[i & MULTIPLIERS_MASK] + symbol.charAt(i);
}
return code & 0x7FFFFFFF;
} // hash0(String):int
/**
* Returns a hashcode value for the specified symbol information.
* The value returned by this method must be identical to the value
* returned by the hash(String) method when called
* with the string object created from the symbol information.
*
* @param buffer The character buffer containing the symbol.
* @param offset The offset into the character buffer of the start
* of the symbol.
* @param length The length of the symbol.
*/
public int hash(char[] buffer, int offset, int length) {
if (fHashMultipliers == null) {
int code = 0;
for (int i = 0; i < length; ++i) {
code = code * 31 + buffer[offset + i];
}
return code & 0x7FFFFFFF;
}
return hash0(buffer, offset, length);
} // hash(char[],int,int):int
private int hash0(char[] buffer, int offset, int length) {
int code = 0;
final int[] multipliers = fHashMultipliers;
for (int i = 0; i < length; ++i) {
code = code * multipliers[i & MULTIPLIERS_MASK] + buffer[offset + i];
}
return code & 0x7FFFFFFF;
} // hash0(char[],int,int):int
/**
* Increases the capacity of and internally reorganizes this
* SymbolTable, in order to accommodate and access its entries more
* efficiently. This method is called automatically when the
* number of keys in the SymbolTable exceeds this hashtable's capacity
* and load factor.
*/
protected void rehash() {
rehashCommon(fBuckets.length * 2 + 1);
}
/**
* Randomly selects a new hash function and reorganizes this SymbolTable
* in order to more evenly distribute its entries across the table. This
* method is called automatically when the number keys in one of the
* SymbolTable's buckets exceeds the given collision threshold.
*/
protected void rebalance() {
if (fHashMultipliers == null) {
fHashMultipliers = new int[MULTIPLIERS_SIZE];
}
PrimeNumberSequenceGenerator.generateSequence(fHashMultipliers);
rehashCommon(fBuckets.length);
}
private void rehashCommon(final int newCapacity) {
int oldCapacity = fBuckets.length;
Entry[] oldTable = fBuckets;
Entry[] newTable = new Entry[newCapacity];
fThreshold = (int)(newCapacity * fLoadFactor);
fBuckets = newTable;
fTableSize = fBuckets.length;
for (int i = oldCapacity ; i-- > 0 ;) {
for (Entry old = oldTable[i] ; old != null ; ) {
Entry e = old;
old = old.next;
int index = hash(e.symbol) % newCapacity;
e.next = newTable[index];
newTable[index] = e;
}
}
}
/**
* Returns true if the symbol table already contains the specified
* symbol.
*
* @param symbol The symbol to look for.
*/
public boolean containsSymbol(String symbol) {
// search for identical symbol
int bucket = hash(symbol) % fTableSize;
int length = symbol.length();
OUTER: for (Entry entry = fBuckets[bucket]; entry != null; entry = entry.next) {
if (length == entry.characters.length) {
for (int i = 0; i < length; i++) {
if (symbol.charAt(i) != entry.characters[i]) {
continue OUTER;
}
}
return true;
}
}
return false;
} // containsSymbol(String):boolean
/**
* Returns true if the symbol table already contains the specified
* symbol.
*
* @param buffer The buffer containing the symbol to look for.
* @param offset The offset into the buffer.
* @param length The length of the symbol in the buffer.
*/
public boolean containsSymbol(char[] buffer, int offset, int length) {
// search for identical symbol
int bucket = hash(buffer, offset, length) % fTableSize;
OUTER: for (Entry entry = fBuckets[bucket]; entry != null; entry = entry.next) {
if (length == entry.characters.length) {
for (int i = 0; i < length; i++) {
if (buffer[offset + i] != entry.characters[i]) {
continue OUTER;
}
}
return true;
}
}
return false;
} // containsSymbol(char[],int,int):boolean
//
// Classes
//
/**
* This class is a symbol table entry. Each entry acts as a node
* in a linked list.
*/
protected static final class Entry {
//
// Data
//
/** Symbol. */
public final String symbol;
/**
* Symbol characters. This information is duplicated here for
* comparison performance.
*/
public final char[] characters;
/** The next entry. */
public Entry next;
//
// Constructors
//
/**
* Constructs a new entry from the specified symbol and next entry
* reference.
*/
public Entry(String symbol, Entry next) {
this.symbol = symbol.intern();
characters = new char[symbol.length()];
symbol.getChars(0, characters.length, characters, 0);
this.next = next;
}
/**
* Constructs a new entry from the specified symbol information and
* next entry reference.
*/
public Entry(char[] ch, int offset, int length, Entry next) {
characters = new char[length];
System.arraycopy(ch, offset, characters, 0, length);
symbol = new String(characters).intern();
this.next = next;
}
} // class Entry
} // class SymbolTable