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// Copyright 2017 The Bazel Authors. All rights reserved.
//
// Licensed 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 com.google.devtools.build.lib.collect;
import com.google.common.base.Objects;
import com.google.common.base.Preconditions;
import com.google.common.collect.ImmutableMap;
import com.google.common.collect.Interner;
import com.google.devtools.build.lib.concurrent.BlazeInterners;
import com.google.devtools.build.lib.skyframe.serialization.autocodec.AutoCodec.VisibleForSerialization;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import javax.annotation.concurrent.Immutable;
/**
* Provides a memory-efficient map when the key sets are likely to be shared between multiple
* instances of this class.
*
* <p>This class is appropriate where it is expected that a lot of the key sets will be the same.
* These key sets are shared and an offset table of indices is computed. Each map instance thus
* contains only a reference to the shared offset table, and a plain array of instances.
*
* <p>The map is sensitive to insertion order. Two maps with different insertion orders are *not*
* considered equal, and will not share keys.
*
* <p>This class explicitly does *not* implement the Map interface, as use of that would lead to a
* lot of GC churn.
*/
@Immutable
public class ImmutableSharedKeyMap<K, V> extends CompactImmutableMap<K, V> {
private static final Interner<OffsetTable> offsetTables = BlazeInterners.newWeakInterner();
private final OffsetTable<K> offsetTable;
@VisibleForSerialization protected final Object[] values;
private static final class OffsetTable<K> {
private final Object[] keys;
// Keep a map around to speed up get lookups for larger maps.
// We make this value lazy to avoid computing for values that end up being thrown away
// during interning anyway (the majority).
private volatile ImmutableMap<K, Integer> indexMap;
private OffsetTable(Object[] keys) {
this.keys = keys;
}
void initIndexMap() {
if (indexMap == null) {
synchronized (this) {
if (indexMap == null) {
ImmutableMap.Builder<K, Integer> builder = ImmutableMap.builder();
for (int i = 0; i < keys.length; ++i) {
@SuppressWarnings("unchecked")
K key = (K) keys[i];
builder.put(key, i);
}
this.indexMap = builder.build();
}
}
}
}
private ImmutableMap<K, Integer> getIndexMap() {
return indexMap;
}
int offsetForKey(K key) {
return getIndexMap().getOrDefault(key, -1);
}
@Override
public boolean equals(Object o) {
if (this == o) {
return true;
}
if (!(o instanceof OffsetTable)) {
return false;
}
OffsetTable that = (OffsetTable) o;
return Arrays.equals(this.keys, that.keys);
}
@Override
public int hashCode() {
return Arrays.hashCode(keys);
}
}
protected ImmutableSharedKeyMap(Object[] keys, Object[] values) {
Preconditions.checkArgument(keys.length == values.length);
this.values = values;
this.offsetTable = createOffsetTable(keys);
}
@SuppressWarnings("unchecked")
private static <K> OffsetTable<K> createOffsetTable(Object[] keys) {
OffsetTable<K> offsetTable = new OffsetTable<>(keys);
OffsetTable<K> internedTable = (OffsetTable<K>) offsetTables.intern(offsetTable);
internedTable.initIndexMap();
return internedTable;
}
@SuppressWarnings("unchecked")
@Override
public V get(K key) {
int offset = offsetTable.offsetForKey(key);
return offset != -1 ? (V) values[offset] : null;
}
@Override
public int size() {
return values.length;
}
@SuppressWarnings("unchecked")
@Override
public K keyAt(int index) {
return (K) offsetTable.keys[index];
}
@SuppressWarnings("unchecked")
@Override
public V valueAt(int index) {
return (V) values[index];
}
/** Do not use! Present only for serialization. (Annotated as @Deprecated just to prevent use.) */
@Deprecated
@VisibleForSerialization
public Object[] getKeys() {
return offsetTable.keys;
}
@Override
@SuppressWarnings("ReferenceEquality")
public boolean equals(Object o) {
if (this == o) {
return true;
}
if (o == null || getClass() != o.getClass()) {
return false;
}
ImmutableSharedKeyMap<?, ?> that = (ImmutableSharedKeyMap<?, ?>) o;
// We can use object identity for the offset table due to
// it being interned
return offsetTable == that.offsetTable && Arrays.equals(values, that.values);
}
@Override
public int hashCode() {
return Objects.hashCode(offsetTable, Arrays.hashCode(values));
}
public static <K, V> Builder<K, V> builder() {
return new Builder<>();
}
/** Builder for {@link ImmutableSharedKeyMap}. */
public static class Builder<K, V> {
private final List<Object> entries = new ArrayList<>();
private Builder() {}
public Builder<K, V> put(K key, V value) {
entries.add(key);
entries.add(value);
return this;
}
public ImmutableSharedKeyMap<K, V> build() {
int count = entries.size() / 2;
Object[] keys = new Object[count];
Object[] values = new Object[count];
int entryIndex = 0;
for (int i = 0; i < count; ++i) {
keys[i] = entries.get(entryIndex++);
values[i] = entries.get(entryIndex++);
}
return new ImmutableSharedKeyMap<>(keys, values);
}
}
}
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