Hash map using String values as keys mapped to primitive int values.
/*
* Copyright (c) 2000-2005, Dennis M. Sosnoski. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* Redistributions of source code must retain the above copyright notice, this
* list of conditions and the following disclaimer. Redistributions in binary
* form must reproduce the above copyright notice, this list of conditions and
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*/
/**
* Hash map using <code>String</code> values as keys mapped to primitive
* <code>int</code> values. This implementation is unsynchronized in order to
* provide the best possible performance for typical usage scenarios, so
* explicit synchronization must be implemented by a wrapper class or directly
* by the application in cases where instances are modified in a multithreaded
* environment. The map implementation is not very efficient when resizing, but
* works well when the size of the map is known in advance.
*
* @author Dennis M. Sosnoski
* @version 1.1
*/
public class StringIntHashMap
{
/** Default value returned when key not found in table. */
public static final int DEFAULT_NOT_FOUND = Integer.MIN_VALUE;
/** Default fill fraction allowed before growing table. */
protected static final double DEFAULT_FILL = 0.3d;
/** Minimum size used for hash table. */
protected static final int MINIMUM_SIZE = 31;
/** Fill fraction allowed for this hash table. */
protected final double m_fillFraction;
/** Number of entries present in table. */
protected int m_entryCount;
/** Entries allowed before growing table. */
protected int m_entryLimit;
/** Size of array used for keys. */
protected int m_arraySize;
/** Offset added (modulo table size) to slot number on collision. */
protected int m_hitOffset;
/** Array of key table slots. */
protected String[] m_keyTable;
/** Array of value table slots. */
protected int[] m_valueTable;
/** Value returned when key not found in table. */
protected int m_notFoundValue;
/**
* Constructor with full specification.
*
* @param count number of values to assume in initial sizing of table
* @param fill fraction full allowed for table before growing
* @param miss value returned when key not found in table
*/
public StringIntHashMap(int count, double fill, int miss) {
// check the passed in fill fraction
if (fill <= 0.0d || fill >= 1.0d) {
throw new IllegalArgumentException("fill value out of range");
}
m_fillFraction = fill;
// compute initial table size (ensuring odd)
m_arraySize = Math.max((int)(count / m_fillFraction), MINIMUM_SIZE);
m_arraySize += (m_arraySize + 1) % 2;
// initialize the table information
m_entryLimit = (int)(m_arraySize * m_fillFraction);
m_hitOffset = m_arraySize / 2;
m_keyTable = new String[m_arraySize];
m_valueTable = new int[m_arraySize];
m_notFoundValue = miss;
}
/**
* Constructor with size and fill fraction specified. Uses default hash
* technique and value returned when key not found in table.
*
* @param count number of values to assume in initial sizing of table
* @param fill fraction full allowed for table before growing
*/
public StringIntHashMap(int count, double fill) {
this(count, fill, DEFAULT_NOT_FOUND);
}
/**
* Constructor with only size supplied. Uses default hash technique and
* values for fill fraction and value returned when key not found in table.
*
* @param count number of values to assume in initial sizing of table
*/
public StringIntHashMap(int count) {
this(count, DEFAULT_FILL);
}
/**
* Default constructor.
*/
public StringIntHashMap() {
this(0, DEFAULT_FILL);
}
/**
* Copy (clone) constructor.
*
* @param base instance being copied
*/
public StringIntHashMap(StringIntHashMap base) {
// copy the basic occupancy information
m_fillFraction = base.m_fillFraction;
m_entryCount = base.m_entryCount;
m_entryLimit = base.m_entryLimit;
m_arraySize = base.m_arraySize;
m_hitOffset = base.m_hitOffset;
m_notFoundValue = base.m_notFoundValue;
// copy table of items
m_keyTable = new String[m_arraySize];
System.arraycopy(base.m_keyTable, 0, m_keyTable, 0, m_arraySize);
m_valueTable = new int[m_arraySize];
System.arraycopy(base.m_valueTable, 0, m_valueTable, 0, m_arraySize);
}
/**
* Step the slot number for an entry. Adds the collision offset (modulo
* the table size) to the slot number.
*
* @param slot slot number to be stepped
* @return stepped slot number
*/
private final int stepSlot(int slot) {
return (slot + m_hitOffset) % m_arraySize;
}
/**
* Find free slot number for entry. Starts at the slot based directly
* on the hashed key value. If this slot is already occupied, it adds
* the collision offset (modulo the table size) to the slot number and
* checks that slot, repeating until an unused slot is found.
*
* @param slot initial slot computed from key
* @return slot at which entry was added
*/
private final int freeSlot(int slot) {
while (m_keyTable[slot] != null) {
slot = stepSlot(slot);
}
return slot;
}
/**
* Standard base slot computation for a key.
*
* @param key key value to be computed
* @return base slot for key
*/
private final int standardSlot(Object key) {
return (key.hashCode() & Integer.MAX_VALUE) % m_arraySize;
}
/**
* Standard find key in table. This method may be used directly for key
* lookup using either the <code>hashCode()</code> method defined for the
* key objects or the <code>System.identityHashCode()</code> method, and
* either the <code>equals()</code> method defined for the key objects or
* the <code>==</code> operator, as selected by the hash technique
* constructor parameter. To implement a hash class based on some other
* methods of hashing and/or equality testing, define a separate method in
* the subclass with a different name and use that method instead. This
* avoids the overhead caused by overrides of a very heavily used method.
*
* @param key to be found in table
* @return index of matching key, or <code>-index-1</code> of slot to be
* used for inserting key in table if not already present (always negative)
*/
private int standardFind(Object key) {
// find the starting point for searching table
int slot = standardSlot(key);
// scan through table to find target key
while (m_keyTable[slot] != null) {
// check if we have a match on target key
if (m_keyTable[slot].equals(key)) {
return slot;
} else {
slot = stepSlot(slot);
}
}
return -slot-1;
}
/**
* Reinsert an entry into the hash map. This is used when the table is being
* directly modified, and does not adjust the count present or check the
* table capacity.
*
* @param slot position of entry to be reinserted into hash map
* @return <code>true</code> if the slot number used by the entry has has
* changed, <code>false</code> if not
*/
private boolean reinsert(int slot) {
String key = m_keyTable[slot];
m_keyTable[slot] = null;
return assignSlot(key, m_valueTable[slot]) != slot;
}
/**
* Internal remove pair from the table. Removes the pair from the table
* by setting the key entry to <code>null</code> and adjusting the count
* present, then chains through the table to reinsert any other pairs
* which may have collided with the removed pair. If the associated value
* is an object reference, it should be set to <code>null</code> before
* this method is called.
*
* @param slot index number of pair to be removed
*/
protected void internalRemove(int slot) {
// delete pair from table
m_keyTable[slot] = null;
m_entryCount--;
while (m_keyTable[(slot = stepSlot(slot))] != null) {
// reinsert current entry in table to fill holes
reinsert(slot);
}
}
/**
* Restructure the table. This is used when the table is increasing or
* decreasing in size, and works directly with the old table representation
* arrays. It inserts pairs from the old arrays directly into the table
* without adjusting the count present or checking the table size.
*
* @param keys array of keys
* @param values array of values
*/
private void restructure(String[] keys, int[] values) {
for (int i = 0; i < keys.length; i++) {
if (keys[i] != null) {
assignSlot(keys[i], values[i]);
}
}
}
/**
* Assign slot for entry. Starts at the slot found by the hashed key value.
* If this slot is already occupied, it steps the slot number and checks the
* resulting slot, repeating until an unused slot is found. This method does
* not check for duplicate keys, so it should only be used for internal
* reordering of the tables.
*
* @param key to be added to table
* @param value associated value for key
* @return slot at which entry was added
*/
private int assignSlot(String key, int value) {
int offset = freeSlot(standardSlot(key));
m_keyTable[offset] = key;
m_valueTable[offset] = value;
return offset;
}
/**
* Add an entry to the table. If the key is already present in the table,
* this replaces the existing value associated with the key.
*
* @param key key to be added to table (non- <code>null</code>)
* @param value associated value for key
* @return value previously associated with key, or reserved not found value
* if key not previously present in table
*/
public int add(String key, int value) {
// first validate the parameters
if (key == null) {
throw new IllegalArgumentException("null key not supported");
} else if (value == m_notFoundValue) {
throw new IllegalArgumentException(
"value matching not found return not supported");
} else {
// check space available
int min = m_entryCount + 1;
if (min > m_entryLimit) {
// find the array size required
int size = m_arraySize;
int limit = m_entryLimit;
while (limit < min) {
size = size * 2 + 1;
limit = (int) (size * m_fillFraction);
}
// set parameters for new array size
m_arraySize = size;
m_entryLimit = limit;
m_hitOffset = size / 2;
// restructure for larger arrays
String[] keys = m_keyTable;
m_keyTable = new String[m_arraySize];
int[] values = m_valueTable;
m_valueTable = new int[m_arraySize];
restructure(keys, values);
}
// find slot of table
int offset = standardFind(key);
if (offset >= 0) {
// replace existing value for key
int prior = m_valueTable[offset];
m_valueTable[offset] = value;
return prior;
} else {
// add new pair to table
m_entryCount++;
offset = -offset - 1;
m_keyTable[offset] = key;
m_valueTable[offset] = value;
return m_notFoundValue;
}
}
}
/**
* Check if an entry is present in the table. This method is supplied to
* support the use of values matching the reserved not found value.
*
* @param key key for entry to be found
* @return <code>true</code> if key found in table, <code>false</code>
* if not
*/
public final boolean containsKey(String key) {
return standardFind(key) >= 0;
}
/**
* Find an entry in the table.
*
* @param key key for entry to be returned
* @return value for key, or reserved not found value if key not found
*/
public final int get(String key) {
int slot = standardFind(key);
if (slot >= 0) {
return m_valueTable[slot];
} else {
return m_notFoundValue;
}
}
/**
* Remove an entry from the table. If multiple entries are present with the
* same key value, only the first one found will be removed.
*
* @param key key to be removed from table
* @return value associated with removed key, or reserved not found value if
* key not found in table
*/
public int remove(String key) {
int slot = standardFind(key);
if (slot >= 0) {
int value = m_valueTable[slot];
internalRemove(slot);
return value;
} else {
return m_notFoundValue;
}
}
/**
* Construct a copy of the table.
*
* @return shallow copy of table
*/
public Object clone() {
return new StringIntHashMap(this);
}
}
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