有向无环图，所有的树都是有向无环图
/*
* Copyright 2018 The Android Open Source Project
*
* 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 androidx.coordinatorlayout.widget;
import static androidx.annotation.RestrictTo.Scope.LIBRARY;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.annotation.RestrictTo;
import androidx.collection.SimpleArrayMap;
import androidx.core.util.Pools;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
/**
* A class which represents a simple directed acyclic graph.
*
* @param <T> Class for the data objects of this graph.
*
* @hide
*/
@RestrictTo(LIBRARY)
public final class DirectedAcyclicGraph<T> {
private final Pools.Pool<ArrayList<T>> mListPool = new Pools.SimplePool<>(10);
private final SimpleArrayMap<T, ArrayList<T>> mGraph = new SimpleArrayMap<>();
private final ArrayList<T> mSortResult = new ArrayList<>();
private final HashSet<T> mSortTmpMarked = new HashSet<>();
/**
* Add a node to the graph.
*
* <p>If the node already exists in the graph then this method is a no-op.</p>
*
* @param node the node to add
*/
public void addNode(@NonNull T node) {
if (!mGraph.containsKey(node)) {
mGraph.put(node, null);
}
}
/**
* Returns true if the node is already present in the graph, false otherwise.
*/
public boolean contains(@NonNull T node) {
return mGraph.containsKey(node);
}
/**
* Add an edge to the graph.
*
* <p>Both the given nodes should already have been added to the graph through
* {@link #addNode(Object)}.</p>
*
* @param node the parent node
* @param incomingEdge the node which has is an incoming edge to {@code node}
*/
public void addEdge(@NonNull T node, @NonNull T incomingEdge) {
if (!mGraph.containsKey(node) || !mGraph.containsKey(incomingEdge)) {
throw new IllegalArgumentException("All nodes must be present in the graph before"
+ " being added as an edge");
}
ArrayList<T> edges = mGraph.get(node);
if (edges == null) {
// If edges is null, we should try and get one from the pool and add it to the graph
edges = getEmptyList();
mGraph.put(node, edges);
}
// Finally add the edge to the list
edges.add(incomingEdge);
}
/**
* Get any incoming edges from the given node.
*
* @return a list containing any incoming edges, or null if there are none.
*/
@Nullable
public List getIncomingEdges(@NonNull T node) {
return mGraph.get(node);
}
/**
* Get any outgoing edges for the given node (i.e. nodes which have an incoming edge
* from the given node).
*
* @return a list containing any outgoing edges, or null if there are none.
*/
@Nullable
public List<T> getOutgoingEdges(@NonNull T node) {
ArrayList<T> result = null;
for (int i = 0, size = mGraph.size(); i < size; i++) {
ArrayList<T> edges = mGraph.valueAt(i);
if (edges != null && edges.contains(node)) {
if (result == null) {
result = new ArrayList<>();
}
result.add(mGraph.keyAt(i));
}
}
return result;
}
/**
* Checks whether we have any outgoing edges for the given node (i.e. nodes which have
* an incoming edge from the given node).
*
* @return <code>true</code> if the node has any outgoing edges, <code>false</code>
* otherwise.
*/
public boolean hasOutgoingEdges(@NonNull T node) {
for (int i = 0, size = mGraph.size(); i < size; i++) {
ArrayList<T> edges = mGraph.valueAt(i);
if (edges != null && edges.contains(node)) {
return true;
}
}
return false;
}
/**
* Clears the internal graph, and releases resources to pools.
*/
public void clear() {
for (int i = 0, size = mGraph.size(); i < size; i++) {
ArrayList<T> edges = mGraph.valueAt(i);
if (edges != null) {
poolList(edges);
}
}
mGraph.clear();
}
/**
* Returns a topologically sorted list of the nodes in this graph. This uses the DFS algorithm
* as described by Cormen et al. (2001). If this graph contains cyclic dependencies then this
* method will throw a {@link RuntimeException}.
*
* <p>The resulting list will be ordered such that index 0 will contain the node at the bottom
* of the graph. The node at the end of the list will have no dependencies on other nodes.</p>
*/
@NonNull
public ArrayList<T> getSortedList() {
mSortResult.clear();
mSortTmpMarked.clear();
// Start a DFS from each node in the graph
for (int i = 0, size = mGraph.size(); i < size; i++) {
dfs(mGraph.keyAt(i), mSortResult, mSortTmpMarked);
}
return mSortResult;
}
private void dfs(final T node, final ArrayList<T> result, final HashSet<T> tmpMarked) {
if (result.contains(node)) {
// We've already seen and added the node to the result list, skip...
return;
}
if (tmpMarked.contains(node)) {
throw new RuntimeException("This graph contains cyclic dependencies");
}
// Temporarily mark the node
tmpMarked.add(node);
// Recursively dfs all of the node's edges
final ArrayList<T> edges = mGraph.get(node);
if (edges != null) {
for (int i = 0, size = edges.size(); i < size; i++) {
dfs(edges.get(i), result, tmpMarked);
}
}
// Unmark the node from the temporary list
tmpMarked.remove(node);
// Finally add it to the result list
result.add(node);
}
/**
* Returns the size of the graph
*/
int size() {
return mGraph.size();
}
@NonNull
private ArrayList<T> getEmptyList() {
ArrayList<T> list = mListPool.acquire();
if (list == null) {
list = new ArrayList<>();
}
return list;
}
private void poolList(@NonNull ArrayList<T> list) {
list.clear();
mListPool.release(list);
}
}