com.google.javascript.jscomp.graph.LowestCommonAncestorFinder Maven / Gradle / Ivy
/*
* Copyright 2008 The Closure Compiler Authors.
*
* 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.javascript.jscomp.graph;
import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;
import com.google.common.collect.ImmutableSet;
import com.google.errorprone.annotations.Immutable;
import com.google.javascript.jscomp.graph.DiGraph.DiGraphEdge;
import com.google.javascript.jscomp.graph.DiGraph.DiGraphNode;
import java.util.ArrayDeque;
import java.util.Arrays;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Set;
/**
* Implements a lowest common ancestor search algorithm.
*
* The LCA of a set of nodes is the node that is an ancestor to all of them but is not an
* ancestor of any other common ancestor. In a non-tree, There may be multiple LCAs for a given set
* of search nodes.
*
*
In a cyclic graph, the LCAs may not be well defined. Within a cycle, all elements are both
* above and below one another, so there is no uniquely lowest element. If the set of common
* ancestors is rooted on a cycle, this implementation returns one or more elements of that cycle.
* Those elements are chosen arbitrarily but deterministically (as long as the underlying graph has
* deterministic iteration).
*/
public class LowestCommonAncestorFinder {
/**
* An abstraction for {@code LowestCommonAncestorFinder::new}.
*
* This interface allows injection in tests.
*/
@FunctionalInterface
public interface Factory {
LowestCommonAncestorFinder create(DiGraph graph);
}
/** A "color" for a node, encoded as a combination of other colors using a one-hot scheme. */
@Immutable
private static final class Color {
/**
* A set of commonly used colors to minimize allocations.
*
* An array is used to prevent bounds checking overhead.
*/
private static final Color[] COMMON_COLOR_CACHE = new Color[2 << 5];
static {
Arrays.setAll(COMMON_COLOR_CACHE, Color::new);
}
/**
* A color that when mixed with any other color {@code x} returns {@code x}.
*
*
Mixing this color is idempotent.
*/
static final Color BLANK = checkNotNull(COMMON_COLOR_CACHE[0]);
/**
* A color for common ancestors that indicates that there are lower common ancestors.
*
*
Because this color sets its MSB high, it can never equal any other color. Also notice that
* mixing this {@link Color} with any other produces this {@link Color} again; mixing it is
* nullipotent.
*/
static final Color NOT_LOWEST = new Color(-1);
static Color create(int bitset) {
if (bitset < 0) {
checkArgument(bitset == -1);
return NOT_LOWEST;
} else if (bitset < COMMON_COLOR_CACHE.length) {
return COMMON_COLOR_CACHE[bitset];
}
return new Color(bitset);
}
final int bitset;
private Color(int bitset) {
this.bitset = bitset;
}
Color mix(Color other) {
if (this.bitset == other.bitset) {
return this;
}
return create(this.bitset | other.bitset);
}
public boolean contains(Color other) {
return (this.bitset & other.bitset) == other.bitset;
}
@Override
@SuppressWarnings({"EqualsUnsafeCast"})
public boolean equals(Object other) {
return this.bitset == ((Color) other).bitset;
}
@Override
public int hashCode() {
return this.bitset;
}
}
private final DiGraph graph;
private final LinkedHashMap, Color> searchColoring = new LinkedHashMap<>();
private final ArrayDeque> searchQueue = new ArrayDeque<>();
public LowestCommonAncestorFinder(DiGraph graph) {
this.graph = graph;
}
/**
* Execute a search on all the elements of {@code roots}.
*
* This is a general-purpose, bare-bones implementation. There are lots of special case
* optimizations that could be applied.
*/
public ImmutableSet findAll(Set roots) {
// We reserved the MSB of each Color for bookkeeping.
checkArgument(roots.size() <= Integer.SIZE - 1, "Too many roots.");
checkState(this.searchColoring.isEmpty());
// In two's-complement, (2^n - 1) sets the lowest n bits high.
Color allColor = Color.create((1 << roots.size()) - 1);
/**
* Paint up from each root using the color associated with that root.
*
* When done, the set of common ancestors is the set of nodes painted `allColor`.
*/
int bitForRoot = 1;
for (N root : roots) {
DiGraphNode rootNode = this.graph.getNode(root);
checkNotNull(rootNode, "Root not present in graph: %s", root);
Color color = Color.create(bitForRoot);
this.searchColoring.merge(rootNode, color, Color::mix); // Preserve any existing colors.
this.paintAncestors(rootNode, color);
bitForRoot <<= 1;
}
/**
* For every common ancestor, paint all of its ancestors with a color indicating it is not the
* lowest.
*/
this.searchColoring.forEach(
(node, color) -> {
if (color.equals(allColor)) {
this.paintAncestors(node, Color.NOT_LOWEST);
}
});
ImmutableSet.Builder results = ImmutableSet.builder();
this.searchColoring.forEach(
(node, color) -> {
if (color.equals(allColor)) {
results.add(node.getValue());
}
});
this.searchColoring.clear();
this.searchQueue.clear();
return results.build();
}
/**
* Paint all nodes above {@code root} with {@code color}.
*
* {@code root} itself will not have its color changed. {@code color} will be mixed with all
* existing colors on ancestor nodes.
*/
private void paintAncestors(DiGraphNode root, Color color) {
checkState(this.searchQueue.isEmpty());
this.searchQueue.addLast(root);
while (!this.searchQueue.isEmpty()) {
DiGraphNode curr = this.searchQueue.removeFirst();
List> parentEdges = curr.getInEdges();
for (DiGraphEdge parentEdge : parentEdges) {
DiGraphNode parent = parentEdge.getSource();
if (parent.equals(root)) {
continue; // Don't paint `root`.
}
Color oldColor = this.searchColoring.getOrDefault(parent, Color.BLANK);
if (!oldColor.contains(color)) {
// Only explore in directions that have not yet been painted.
this.searchQueue.addLast(parent);
this.searchColoring.put(parent, oldColor.mix(color));
}
}
}
}
}