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/*
* Copyright 2014 The Netty Project
*
* The Netty Project 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 io.netty.util.collection;
import static io.netty.util.internal.MathUtil.safeFindNextPositivePowerOfTwo;
import java.util.AbstractCollection;
import java.util.AbstractSet;
import java.util.Arrays;
import java.util.Collection;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;
/**
* A hash map implementation of {@link ByteObjectMap} that uses open addressing for keys.
* To minimize the memory footprint, this class uses open addressing rather than chaining.
* Collisions are resolved using linear probing. Deletions implement compaction, so cost of
* remove can approach O(N) for full maps, which makes a small loadFactor recommended.
*
* @param The value type stored in the map.
*/
public class ByteObjectHashMap implements ByteObjectMap {
/** Default initial capacity. Used if not specified in the constructor */
public static final int DEFAULT_CAPACITY = 8;
/** Default load factor. Used if not specified in the constructor */
public static final float DEFAULT_LOAD_FACTOR = 0.5f;
/**
* Placeholder for null values, so we can use the actual null to mean available.
* (Better than using a placeholder for available: less references for GC processing.)
*/
private static final Object NULL_VALUE = new Object();
/** The maximum number of elements allowed without allocating more space. */
private int maxSize;
/** The load factor for the map. Used to calculate {@link #maxSize}. */
private final float loadFactor;
private byte[] keys;
private V[] values;
private int size;
private int mask;
private final Set keySet = new KeySet();
private final Set> entrySet = new EntrySet();
private final Iterable> entries = new Iterable>() {
@Override
public Iterator> iterator() {
return new PrimitiveIterator();
}
};
public ByteObjectHashMap() {
this(DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR);
}
public ByteObjectHashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
public ByteObjectHashMap(int initialCapacity, float loadFactor) {
if (loadFactor <= 0.0f || loadFactor > 1.0f) {
// Cannot exceed 1 because we can never store more than capacity elements;
// using a bigger loadFactor would trigger rehashing before the desired load is reached.
throw new IllegalArgumentException("loadFactor must be > 0 and <= 1");
}
this.loadFactor = loadFactor;
// Adjust the initial capacity if necessary.
int capacity = safeFindNextPositivePowerOfTwo(initialCapacity);
mask = capacity - 1;
// Allocate the arrays.
keys = new byte[capacity];
@SuppressWarnings({ "unchecked", "SuspiciousArrayCast" })
V[] temp = (V[]) new Object[capacity];
values = temp;
// Initialize the maximum size value.
maxSize = calcMaxSize(capacity);
}
private static T toExternal(T value) {
assert value != null : "null is not a legitimate internal value. Concurrent Modification?";
return value == NULL_VALUE ? null : value;
}
@SuppressWarnings("unchecked")
private static T toInternal(T value) {
return value == null ? (T) NULL_VALUE : value;
}
@Override
public V get(byte key) {
int index = indexOf(key);
return index == -1 ? null : toExternal(values[index]);
}
@Override
public V put(byte key, V value) {
int startIndex = hashIndex(key);
int index = startIndex;
for (;;) {
if (values[index] == null) {
// Found empty slot, use it.
keys[index] = key;
values[index] = toInternal(value);
growSize();
return null;
}
if (keys[index] == key) {
// Found existing entry with this key, just replace the value.
V previousValue = values[index];
values[index] = toInternal(value);
return toExternal(previousValue);
}
// Conflict, keep probing ...
if ((index = probeNext(index)) == startIndex) {
// Can only happen if the map was full at MAX_ARRAY_SIZE and couldn't grow.
throw new IllegalStateException("Unable to insert");
}
}
}
@Override
public void putAll(Map sourceMap) {
if (sourceMap instanceof ByteObjectHashMap) {
// Optimization - iterate through the arrays.
@SuppressWarnings("unchecked")
ByteObjectHashMap source = (ByteObjectHashMap) sourceMap;
for (int i = 0; i < source.values.length; ++i) {
V sourceValue = source.values[i];
if (sourceValue != null) {
put(source.keys[i], sourceValue);
}
}
return;
}
// Otherwise, just add each entry.
for (Entry entry : sourceMap.entrySet()) {
put(entry.getKey(), entry.getValue());
}
}
@Override
public V remove(byte key) {
int index = indexOf(key);
if (index == -1) {
return null;
}
V prev = values[index];
removeAt(index);
return toExternal(prev);
}
@Override
public int size() {
return size;
}
@Override
public boolean isEmpty() {
return size == 0;
}
@Override
public void clear() {
Arrays.fill(keys, (byte) 0);
Arrays.fill(values, null);
size = 0;
}
@Override
public boolean containsKey(byte key) {
return indexOf(key) >= 0;
}
@Override
public boolean containsValue(Object value) {
@SuppressWarnings("unchecked")
V v1 = toInternal((V) value);
for (V v2 : values) {
// The map supports null values; this will be matched as NULL_VALUE.equals(NULL_VALUE).
if (v2 != null && v2.equals(v1)) {
return true;
}
}
return false;
}
@Override
public Iterable> entries() {
return entries;
}
@Override
public Collection values() {
return new AbstractCollection() {
@Override
public Iterator iterator() {
return new Iterator() {
final PrimitiveIterator iter = new PrimitiveIterator();
@Override
public boolean hasNext() {
return iter.hasNext();
}
@Override
public V next() {
return iter.next().value();
}
@Override
public void remove() {
iter.remove();
}
};
}
@Override
public int size() {
return size;
}
};
}
@Override
public int hashCode() {
// Hashcode is based on all non-zero, valid keys. We have to scan the whole keys
// array, which may have different lengths for two maps of same size(), so the
// capacity cannot be used as input for hashing but the size can.
int hash = size;
for (byte key : keys) {
// 0 can be a valid key or unused slot, but won't impact the hashcode in either case.
// This way we can use a cheap loop without conditionals, or hard-to-unroll operations,
// or the devastatingly bad memory locality of visiting value objects.
// Also, it's important to use a hash function that does not depend on the ordering
// of terms, only their values; since the map is an unordered collection and
// entries can end up in different positions in different maps that have the same
// elements, but with different history of puts/removes, due to conflicts.
hash ^= hashCode(key);
}
return hash;
}
@Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (!(obj instanceof ByteObjectMap)) {
return false;
}
@SuppressWarnings("rawtypes")
ByteObjectMap other = (ByteObjectMap) obj;
if (size != other.size()) {
return false;
}
for (int i = 0; i < values.length; ++i) {
V value = values[i];
if (value != null) {
byte key = keys[i];
Object otherValue = other.get(key);
if (value == NULL_VALUE) {
if (otherValue != null) {
return false;
}
} else if (!value.equals(otherValue)) {
return false;
}
}
}
return true;
}
@Override
public boolean containsKey(Object key) {
return containsKey(objectToKey(key));
}
@Override
public V get(Object key) {
return get(objectToKey(key));
}
@Override
public V put(Byte key, V value) {
return put(objectToKey(key), value);
}
@Override
public V remove(Object key) {
return remove(objectToKey(key));
}
@Override
public Set keySet() {
return keySet;
}
@Override
public Set> entrySet() {
return entrySet;
}
private byte objectToKey(Object key) {
return (byte) ((Byte) key).byteValue();
}
/**
* Locates the index for the given key. This method probes using double hashing.
*
* @param key the key for an entry in the map.
* @return the index where the key was found, or {@code -1} if no entry is found for that key.
*/
private int indexOf(byte key) {
int startIndex = hashIndex(key);
int index = startIndex;
for (;;) {
if (values[index] == null) {
// It's available, so no chance that this value exists anywhere in the map.
return -1;
}
if (key == keys[index]) {
return index;
}
// Conflict, keep probing ...
if ((index = probeNext(index)) == startIndex) {
return -1;
}
}
}
/**
* Returns the hashed index for the given key.
*/
private int hashIndex(byte key) {
// The array lengths are always a power of two, so we can use a bitmask to stay inside the array bounds.
return hashCode(key) & mask;
}
/**
* Returns the hash code for the key.
*/
private static int hashCode(byte key) {
return (int) key;
}
/**
* Get the next sequential index after {@code index} and wraps if necessary.
*/
private int probeNext(int index) {
// The array lengths are always a power of two, so we can use a bitmask to stay inside the array bounds.
return (index + 1) & mask;
}
/**
* Grows the map size after an insertion. If necessary, performs a rehash of the map.
*/
private void growSize() {
size++;
if (size > maxSize) {
if(keys.length == Integer.MAX_VALUE) {
throw new IllegalStateException("Max capacity reached at size=" + size);
}
// Double the capacity.
rehash(keys.length << 1);
}
}
/**
* Removes entry at the given index position. Also performs opportunistic, incremental rehashing
* if necessary to not break conflict chains.
*
* @param index the index position of the element to remove.
* @return {@code true} if the next item was moved back. {@code false} otherwise.
*/
private boolean removeAt(final int index) {
--size;
// Clearing the key is not strictly necessary (for GC like in a regular collection),
// but recommended for security. The memory location is still fresh in the cache anyway.
keys[index] = 0;
values[index] = null;
// In the interval from index to the next available entry, the arrays may have entries
// that are displaced from their base position due to prior conflicts. Iterate these
// entries and move them back if possible, optimizing future lookups.
// Knuth Section 6.4 Algorithm R, also used by the JDK's IdentityHashMap.
int nextFree = index;
int i = probeNext(index);
for (V value = values[i]; value != null; value = values[i = probeNext(i)]) {
byte key = keys[i];
int bucket = hashIndex(key);
if (i < bucket && (bucket <= nextFree || nextFree <= i) ||
bucket <= nextFree && nextFree <= i) {
// Move the displaced entry "back" to the first available position.
keys[nextFree] = key;
values[nextFree] = value;
// Put the first entry after the displaced entry
keys[i] = 0;
values[i] = null;
nextFree = i;
}
}
return nextFree != index;
}
/**
* Calculates the maximum size allowed before rehashing.
*/
private int calcMaxSize(int capacity) {
// Clip the upper bound so that there will always be at least one available slot.
int upperBound = capacity - 1;
return Math.min(upperBound, (int) (capacity * loadFactor));
}
/**
* Rehashes the map for the given capacity.
*
* @param newCapacity the new capacity for the map.
*/
private void rehash(int newCapacity) {
byte[] oldKeys = keys;
V[] oldVals = values;
keys = new byte[newCapacity];
@SuppressWarnings({ "unchecked", "SuspiciousArrayCast" })
V[] temp = (V[]) new Object[newCapacity];
values = temp;
maxSize = calcMaxSize(newCapacity);
mask = newCapacity - 1;
// Insert to the new arrays.
for (int i = 0; i < oldVals.length; ++i) {
V oldVal = oldVals[i];
if (oldVal != null) {
// Inlined put(), but much simpler: we don't need to worry about
// duplicated keys, growing/rehashing, or failing to insert.
byte oldKey = oldKeys[i];
int index = hashIndex(oldKey);
for (;;) {
if (values[index] == null) {
keys[index] = oldKey;
values[index] = oldVal;
break;
}
// Conflict, keep probing. Can wrap around, but never reaches startIndex again.
index = probeNext(index);
}
}
}
}
@Override
public String toString() {
if (isEmpty()) {
return "{}";
}
StringBuilder sb = new StringBuilder(4 * size);
sb.append('{');
boolean first = true;
for (int i = 0; i < values.length; ++i) {
V value = values[i];
if (value != null) {
if (!first) {
sb.append(", ");
}
sb.append(keyToString(keys[i])).append('=').append(value == this ? "(this Map)" :
toExternal(value));
first = false;
}
}
return sb.append('}').toString();
}
/**
* Helper method called by {@link #toString()} in order to convert a single map key into a string.
* This is protected to allow subclasses to override the appearance of a given key.
*/
protected String keyToString(byte key) {
return Byte.toString(key);
}
/**
* Set implementation for iterating over the entries of the map.
*/
private final class EntrySet extends AbstractSet> {
@Override
public Iterator> iterator() {
return new MapIterator();
}
@Override
public int size() {
return ByteObjectHashMap.this.size();
}
}
/**
* Set implementation for iterating over the keys.
*/
private final class KeySet extends AbstractSet {
@Override
public int size() {
return ByteObjectHashMap.this.size();
}
@Override
public boolean contains(Object o) {
return ByteObjectHashMap.this.containsKey(o);
}
@Override
public boolean remove(Object o) {
return ByteObjectHashMap.this.remove(o) != null;
}
@Override
public boolean retainAll(Collection retainedKeys) {
boolean changed = false;
for(Iterator> iter = entries().iterator(); iter.hasNext(); ) {
PrimitiveEntry entry = iter.next();
if (!retainedKeys.contains(entry.key())) {
changed = true;
iter.remove();
}
}
return changed;
}
@Override
public void clear() {
ByteObjectHashMap.this.clear();
}
@Override
public Iterator iterator() {
return new Iterator() {
private final Iterator> iter = entrySet.iterator();
@Override
public boolean hasNext() {
return iter.hasNext();
}
@Override
public Byte next() {
return iter.next().getKey();
}
@Override
public void remove() {
iter.remove();
}
};
}
}
/**
* Iterator over primitive entries. Entry key/values are overwritten by each call to {@link #next()}.
*/
private final class PrimitiveIterator implements Iterator>, PrimitiveEntry {
private int prevIndex = -1;
private int nextIndex = -1;
private int entryIndex = -1;
private void scanNext() {
while (++nextIndex != values.length && values[nextIndex] == null) {
}
}
@Override
public boolean hasNext() {
if (nextIndex == -1) {
scanNext();
}
return nextIndex != values.length;
}
@Override
public PrimitiveEntry next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
prevIndex = nextIndex;
scanNext();
// Always return the same Entry object, just change its index each time.
entryIndex = prevIndex;
return this;
}
@Override
public void remove() {
if (prevIndex == -1) {
throw new IllegalStateException("next must be called before each remove.");
}
if (removeAt(prevIndex)) {
// removeAt may move elements "back" in the array if they have been displaced because their spot in the
// array was occupied when they were inserted. If this occurs then the nextIndex is now invalid and
// should instead point to the prevIndex which now holds an element which was "moved back".
nextIndex = prevIndex;
}
prevIndex = -1;
}
// Entry implementation. Since this implementation uses a single Entry, we coalesce that
// into the Iterator object (potentially making loop optimization much easier).
@Override
public byte key() {
return keys[entryIndex];
}
@Override
public V value() {
return toExternal(values[entryIndex]);
}
@Override
public void setValue(V value) {
values[entryIndex] = toInternal(value);
}
}
/**
* Iterator used by the {@link Map} interface.
*/
private final class MapIterator implements Iterator> {
private final PrimitiveIterator iter = new PrimitiveIterator();
@Override
public boolean hasNext() {
return iter.hasNext();
}
@Override
public Entry next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
iter.next();
return new MapEntry(iter.entryIndex);
}
@Override
public void remove() {
iter.remove();
}
}
/**
* A single entry in the map.
*/
final class MapEntry implements Entry {
private final int entryIndex;
MapEntry(int entryIndex) {
this.entryIndex = entryIndex;
}
@Override
public Byte getKey() {
verifyExists();
return keys[entryIndex];
}
@Override
public V getValue() {
verifyExists();
return toExternal(values[entryIndex]);
}
@Override
public V setValue(V value) {
verifyExists();
V prevValue = toExternal(values[entryIndex]);
values[entryIndex] = toInternal(value);
return prevValue;
}
private void verifyExists() {
if (values[entryIndex] == null) {
throw new IllegalStateException("The map entry has been removed");
}
}
}
}