SmallSortedMap.java
// Protocol Buffers - Google's data interchange format
// Copyright 2008 Google Inc. All rights reserved.
//
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file or at
// https://developers.google.com/open-source/licenses/bsd
package com.google.protobuf;
import java.util.AbstractMap;
import java.util.AbstractSet;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.SortedMap;
import java.util.TreeMap;
/**
* A custom map implementation from FieldDescriptor to Object optimized to minimize the number of
* memory allocations for instances with a small number of mappings. The implementation stores the
* first {@code k} mappings in an array for a configurable value of {@code k}, allowing direct
* access to the corresponding {@code Entry}s without the need to create an Iterator. The remaining
* entries are stored in an overflow map. Iteration over the entries in the map should be done as
* follows:
*
* <pre>{@code
* for (int i = 0; i < fieldMap.getNumArrayEntries(); i++) {
* process(fieldMap.getArrayEntryAt(i));
* }
* for (Map.Entry<K, V> entry : fieldMap.getOverflowEntries()) {
* process(entry);
* }
* }</pre>
*
* The resulting iteration is in order of ascending field tag number. The object returned by {@link
* #entrySet()} adheres to the same contract but is less efficient as it necessarily involves
* creating an object for iteration.
*
* <p>The tradeoff for this memory efficiency is that the worst case running time of the {@code
* put()} operation is {@code O(k + lg n)}, which happens when entries are added in descending
* order. {@code k} should be chosen such that it covers enough common cases without adversely
* affecting larger maps. In practice, the worst case scenario does not happen for extensions
* because extension fields are serialized and deserialized in order of ascending tag number, but
* the worst case scenario can happen for DynamicMessages.
*
* <p>The running time for all other operations is similar to that of {@code TreeMap}.
*
* <p>Instances are not thread-safe until {@link #makeImmutable()} is called, after which any
* modifying operation will result in an {@link UnsupportedOperationException}.
*
* @author darick@google.com Darick Tong
*/
// This class is final for all intents and purposes because the constructor is
// private. However, the FieldDescriptor-specific logic is encapsulated in
// a subclass to aid testability of the core logic.
class SmallSortedMap<K extends Comparable<K>, V> extends AbstractMap<K, V> {
static final int DEFAULT_FIELD_MAP_ARRAY_SIZE = 16;
/**
* Creates a new instance for mapping FieldDescriptors to their values. The {@link
* #makeImmutable()} implementation will convert the List values of any repeated fields to
* unmodifiable lists.
*/
static <FieldDescriptorT extends FieldSet.FieldDescriptorLite<FieldDescriptorT>>
SmallSortedMap<FieldDescriptorT, Object> newFieldMap() {
return new SmallSortedMap<FieldDescriptorT, Object>() {
@Override
public void makeImmutable() {
if (!isImmutable()) {
for (int i = 0; i < getNumArrayEntries(); i++) {
final Map.Entry<FieldDescriptorT, Object> entry = getArrayEntryAt(i);
if (entry.getKey().isRepeated()) {
final List<?> value = (List) entry.getValue();
entry.setValue(Collections.unmodifiableList(value));
}
}
for (Map.Entry<FieldDescriptorT, Object> entry : getOverflowEntries()) {
if (entry.getKey().isRepeated()) {
final List<?> value = (List) entry.getValue();
entry.setValue(Collections.unmodifiableList(value));
}
}
}
super.makeImmutable();
}
};
}
/** Creates a new instance for testing. */
static <K extends Comparable<K>, V> SmallSortedMap<K, V> newInstanceForTest() {
return new SmallSortedMap<>();
}
// Only has Entry elements inside.
// Can't declare this as Entry[] because Entry is generic, so you get "generic array creation"
// error. Instead, use an Object[], and cast to Entry on read.
// null Object[] means 'empty'.
private Object[] entries;
// Number of elements in entries that are valid, like ArrayList.size.
private int entriesSize;
private Map<K, V> overflowEntries;
private boolean isImmutable;
// The EntrySet is a stateless view of the Map. It's initialized the first
// time it is requested and reused henceforth.
private volatile EntrySet lazyEntrySet;
private Map<K, V> overflowEntriesDescending;
private volatile DescendingEntrySet lazyDescendingEntrySet;
private SmallSortedMap() {
this.overflowEntries = Collections.emptyMap();
this.overflowEntriesDescending = Collections.emptyMap();
}
/** Make this map immutable from this point forward. */
public void makeImmutable() {
if (!isImmutable) {
// Note: There's no need to wrap the entries in an unmodifiableList
// because none of the array's accessors are exposed. The iterator() of
// overflowEntries, on the other hand, is exposed so it must be made
// unmodifiable.
overflowEntries =
overflowEntries.isEmpty()
? Collections.<K, V>emptyMap()
: Collections.unmodifiableMap(overflowEntries);
overflowEntriesDescending =
overflowEntriesDescending.isEmpty()
? Collections.<K, V>emptyMap()
: Collections.unmodifiableMap(overflowEntriesDescending);
isImmutable = true;
}
}
/** @return Whether {@link #makeImmutable()} has been called. */
public boolean isImmutable() {
return isImmutable;
}
/** @return The number of entries in the entry array. */
public int getNumArrayEntries() {
return entriesSize;
}
/** @return The array entry at the given {@code index}. */
public Map.Entry<K, V> getArrayEntryAt(int index) {
if (index >= entriesSize) {
throw new ArrayIndexOutOfBoundsException(index);
}
@SuppressWarnings("unchecked")
Entry e = (Entry) entries[index];
return e;
}
/** @return There number of overflow entries. */
public int getNumOverflowEntries() {
return overflowEntries.size();
}
/** @return An iterable over the overflow entries. */
public Iterable<Map.Entry<K, V>> getOverflowEntries() {
return overflowEntries.isEmpty()
? Collections.emptySet()
: overflowEntries.entrySet();
}
@Override
public int size() {
return entriesSize + overflowEntries.size();
}
/**
* The implementation throws a {@code ClassCastException} if o is not an object of type {@code K}.
*
* <p>{@inheritDoc}
*/
@Override
public boolean containsKey(Object o) {
@SuppressWarnings("unchecked")
final K key = (K) o;
return binarySearchInArray(key) >= 0 || overflowEntries.containsKey(key);
}
/**
* The implementation throws a {@code ClassCastException} if o is not an object of type {@code K}.
*
* <p>{@inheritDoc}
*/
@Override
public V get(Object o) {
@SuppressWarnings("unchecked")
final K key = (K) o;
final int index = binarySearchInArray(key);
if (index >= 0) {
@SuppressWarnings("unchecked")
Entry e = (Entry) entries[index];
return e.getValue();
}
return overflowEntries.get(key);
}
@Override
public V put(K key, V value) {
checkMutable();
final int index = binarySearchInArray(key);
if (index >= 0) {
// Replace existing array entry.
@SuppressWarnings("unchecked")
Entry e = (Entry) entries[index];
return e.setValue(value);
}
ensureEntryArrayMutable();
final int insertionPoint = -(index + 1);
if (insertionPoint >= DEFAULT_FIELD_MAP_ARRAY_SIZE) {
// Put directly in overflow.
return getOverflowEntriesMutable().put(key, value);
}
// Insert new Entry in array.
if (entriesSize == DEFAULT_FIELD_MAP_ARRAY_SIZE) {
// Shift the last array entry into overflow.
@SuppressWarnings("unchecked")
final Entry lastEntryInArray = (Entry) entries[DEFAULT_FIELD_MAP_ARRAY_SIZE - 1];
entriesSize--;
getOverflowEntriesMutable().put(lastEntryInArray.getKey(), lastEntryInArray.getValue());
}
System.arraycopy(
entries, insertionPoint, entries, insertionPoint + 1, entries.length - insertionPoint - 1);
entries[insertionPoint] = new Entry(key, value);
entriesSize++;
return null;
}
@Override
public void clear() {
checkMutable();
if (entriesSize != 0) {
entries = null;
entriesSize = 0;
}
if (!overflowEntries.isEmpty()) {
overflowEntries.clear();
}
}
/**
* The implementation throws a {@code ClassCastException} if o is not an object of type {@code K}.
*
* <p>{@inheritDoc}
*/
@Override
public V remove(Object o) {
checkMutable();
@SuppressWarnings("unchecked")
final K key = (K) o;
final int index = binarySearchInArray(key);
if (index >= 0) {
return removeArrayEntryAt(index);
}
// overflowEntries might be Collections.unmodifiableMap(), so only
// call remove() if it is non-empty.
if (overflowEntries.isEmpty()) {
return null;
} else {
return overflowEntries.remove(key);
}
}
private V removeArrayEntryAt(int index) {
checkMutable();
@SuppressWarnings("unchecked")
final V removed = ((Entry) entries[index]).getValue();
// shift items across
System.arraycopy(entries, index + 1, entries, index, entriesSize - index - 1);
entriesSize--;
if (!overflowEntries.isEmpty()) {
// Shift the first entry in the overflow to be the last entry in the
// array.
final Iterator<Map.Entry<K, V>> iterator = getOverflowEntriesMutable().entrySet().iterator();
entries[entriesSize] = new Entry(iterator.next());
entriesSize++;
iterator.remove();
}
return removed;
}
/**
* @param key The key to find in the entry array.
* @return The returned integer position follows the same semantics as the value returned by
* {@link java.util.Arrays#binarySearch()}.
*/
private int binarySearchInArray(K key) {
int left = 0;
int right = entriesSize - 1;
// Optimization: For the common case in which entries are added in
// ascending tag order, check the largest element in the array before
// doing a full binary search.
if (right >= 0) {
@SuppressWarnings("unchecked")
int cmp = key.compareTo(((Entry) entries[right]).getKey());
if (cmp > 0) {
return -(right + 2); // Insert point is after "right".
} else if (cmp == 0) {
return right;
}
}
while (left <= right) {
int mid = (left + right) / 2;
@SuppressWarnings("unchecked")
int cmp = key.compareTo(((Entry) entries[mid]).getKey());
if (cmp < 0) {
right = mid - 1;
} else if (cmp > 0) {
left = mid + 1;
} else {
return mid;
}
}
return -(left + 1);
}
/**
* Similar to the AbstractMap implementation of {@code keySet()} and {@code values()}, the entry
* set is created the first time this method is called, and returned in response to all subsequent
* calls.
*
* <p>{@inheritDoc}
*/
@Override
public Set<Map.Entry<K, V>> entrySet() {
if (lazyEntrySet == null) {
lazyEntrySet = new EntrySet();
}
return lazyEntrySet;
}
Set<Map.Entry<K, V>> descendingEntrySet() {
if (lazyDescendingEntrySet == null) {
lazyDescendingEntrySet = new DescendingEntrySet();
}
return lazyDescendingEntrySet;
}
/** @throws UnsupportedOperationException if {@link #makeImmutable()} has has been called. */
private void checkMutable() {
if (isImmutable) {
throw new UnsupportedOperationException();
}
}
/**
* @return a {@link SortedMap} to which overflow entries mappings can be added or removed.
* @throws UnsupportedOperationException if {@link #makeImmutable()} has been called.
*/
private SortedMap<K, V> getOverflowEntriesMutable() {
checkMutable();
if (overflowEntries.isEmpty() && !(overflowEntries instanceof TreeMap)) {
overflowEntries = new TreeMap<K, V>();
overflowEntriesDescending = ((TreeMap<K, V>) overflowEntries).descendingMap();
}
return (SortedMap<K, V>) overflowEntries;
}
/**
* Lazily creates the entry array. Any code that adds to the array must first call this method.
*/
private void ensureEntryArrayMutable() {
checkMutable();
if (entries == null) {
entries = new Object[DEFAULT_FIELD_MAP_ARRAY_SIZE];
}
}
/**
* Entry implementation that implements Comparable in order to support binary search within the
* entry array. Also checks mutability in {@link #setValue()}.
*/
private class Entry implements Map.Entry<K, V>, Comparable<Entry> {
private final K key;
private V value;
Entry(Map.Entry<K, V> copy) {
this(copy.getKey(), copy.getValue());
}
Entry(K key, V value) {
this.key = key;
this.value = value;
}
@Override
public K getKey() {
return key;
}
@Override
public V getValue() {
return value;
}
@Override
public int compareTo(Entry other) {
return getKey().compareTo(other.getKey());
}
@Override
public V setValue(V newValue) {
checkMutable();
final V oldValue = this.value;
this.value = newValue;
return oldValue;
}
@Override
public boolean equals(Object o) {
if (o == this) {
return true;
}
if (!(o instanceof Map.Entry)) {
return false;
}
Map.Entry<?, ?> other = (Map.Entry<?, ?>) o;
return equals(key, other.getKey()) && equals(value, other.getValue());
}
@Override
public int hashCode() {
return (key == null ? 0 : key.hashCode()) ^ (value == null ? 0 : value.hashCode());
}
@Override
public String toString() {
return key + "=" + value;
}
/** equals() that handles null values. */
private boolean equals(Object o1, Object o2) {
return o1 == null ? o2 == null : o1.equals(o2);
}
}
/** Stateless view of the entries in the field map. */
private class EntrySet extends AbstractSet<Map.Entry<K, V>> {
@Override
public Iterator<Map.Entry<K, V>> iterator() {
return new EntryIterator();
}
@Override
public int size() {
return SmallSortedMap.this.size();
}
/**
* Throws a {@link ClassCastException} if o is not of the expected type.
*
* <p>{@inheritDoc}
*/
@Override
public boolean contains(Object o) {
@SuppressWarnings("unchecked")
final Map.Entry<K, V> entry = (Map.Entry<K, V>) o;
final V existing = get(entry.getKey());
final V value = entry.getValue();
return existing == value || (existing != null && existing.equals(value));
}
@Override
public boolean add(Map.Entry<K, V> entry) {
if (!contains(entry)) {
put(entry.getKey(), entry.getValue());
return true;
}
return false;
}
/**
* Throws a {@link ClassCastException} if o is not of the expected type.
*
* <p>{@inheritDoc}
*/
@Override
public boolean remove(Object o) {
@SuppressWarnings("unchecked")
final Map.Entry<K, V> entry = (Map.Entry<K, V>) o;
if (contains(entry)) {
SmallSortedMap.this.remove(entry.getKey());
return true;
}
return false;
}
@Override
public void clear() {
SmallSortedMap.this.clear();
}
}
private class DescendingEntrySet extends EntrySet {
@Override
public Iterator<java.util.Map.Entry<K, V>> iterator() {
return new DescendingEntryIterator();
}
}
/**
* Iterator implementation that switches from the entry array to the overflow entries
* appropriately.
*/
private class EntryIterator implements Iterator<Map.Entry<K, V>> {
private int pos = -1;
private boolean nextCalledBeforeRemove;
private Iterator<Map.Entry<K, V>> lazyOverflowIterator;
@Override
public boolean hasNext() {
return (pos + 1) < entriesSize
|| (!overflowEntries.isEmpty() && getOverflowIterator().hasNext());
}
@Override
public Map.Entry<K, V> next() {
nextCalledBeforeRemove = true;
// Always increment pos so that we know whether the last returned value
// was from the array or from overflow.
if (++pos < entriesSize) {
@SuppressWarnings("unchecked")
Entry e = (Entry) entries[pos];
return e;
}
return getOverflowIterator().next();
}
@Override
public void remove() {
if (!nextCalledBeforeRemove) {
throw new IllegalStateException("remove() was called before next()");
}
nextCalledBeforeRemove = false;
checkMutable();
if (pos < entriesSize) {
removeArrayEntryAt(pos--);
} else {
getOverflowIterator().remove();
}
}
/**
* It is important to create the overflow iterator only after the array entries have been
* iterated over because the overflow entry set changes when the client calls remove() on the
* array entries, which invalidates any existing iterators.
*/
private Iterator<Map.Entry<K, V>> getOverflowIterator() {
if (lazyOverflowIterator == null) {
lazyOverflowIterator = overflowEntries.entrySet().iterator();
}
return lazyOverflowIterator;
}
}
/**
* Reverse Iterator implementation that switches from the entry array to the overflow entries
* appropriately.
*/
private class DescendingEntryIterator implements Iterator<Map.Entry<K, V>> {
private int pos = entriesSize;
private Iterator<Map.Entry<K, V>> lazyOverflowIterator;
@Override
public boolean hasNext() {
return (pos > 0 && pos <= entriesSize) || getOverflowIterator().hasNext();
}
@Override
public Map.Entry<K, V> next() {
if (getOverflowIterator().hasNext()) {
return getOverflowIterator().next();
}
@SuppressWarnings("unchecked")
Entry e = (Entry) entries[--pos];
return e;
}
@Override
public void remove() {
throw new UnsupportedOperationException();
}
/**
* It is important to create the overflow iterator only after the array entries have been
* iterated over because the overflow entry set changes when the client calls remove() on the
* array entries, which invalidates any existing iterators.
*/
private Iterator<Map.Entry<K, V>> getOverflowIterator() {
if (lazyOverflowIterator == null) {
lazyOverflowIterator = overflowEntriesDescending.entrySet().iterator();
}
return lazyOverflowIterator;
}
}
@Override
public boolean equals(Object o) {
if (this == o) {
return true;
}
if (!(o instanceof SmallSortedMap)) {
return super.equals(o);
}
SmallSortedMap<?, ?> other = (SmallSortedMap<?, ?>) o;
final int size = size();
if (size != other.size()) {
return false;
}
// Best effort try to avoid allocating an entry set.
final int numArrayEntries = getNumArrayEntries();
if (numArrayEntries != other.getNumArrayEntries()) {
return entrySet().equals(other.entrySet());
}
for (int i = 0; i < numArrayEntries; i++) {
if (!getArrayEntryAt(i).equals(other.getArrayEntryAt(i))) {
return false;
}
}
if (numArrayEntries != size) {
return overflowEntries.equals(other.overflowEntries);
}
return true;
}
@Override
public int hashCode() {
int h = 0;
final int listSize = getNumArrayEntries();
for (int i = 0; i < listSize; i++) {
h += entries[i].hashCode();
}
// Avoid the iterator allocation if possible.
if (getNumOverflowEntries() > 0) {
h += overflowEntries.hashCode();
}
return h;
}
}