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// Copyright (C) 2009 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 com.google.gerrit.server.query;
import static com.google.common.base.Preconditions.checkState;
import com.google.common.collect.Iterables;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
/**
* An abstract predicate tree for any form of query.
*
* <p>Implementations should be immutable, such that the meaning of a predicate never changes once
* constructed. They should ensure their immutable promise by defensively copying any structures
* which might be modified externally, but was passed into the object's constructor.
*
* <p>However, implementations <i>may</i> retain non-thread-safe caches internally, to speed up
* evaluation operations within the context of one thread's evaluation of the predicate. As a
* result, callers should assume predicates are not thread-safe, but that two predicate graphs
* produce the same results given the same inputs if they are {@link #equals(Object)}.
*
* <p>Predicates should support deep inspection whenever possible, so that generic algorithms can be
* written to operate against them. Predicates which contain other predicates should override {@link
* #getChildren()} to return the list of children nested within the predicate.
*
* @param <T> type of object the predicate can evaluate in memory.
*/
public abstract class Predicate<T> {
/** A predicate that matches any input, always, with no cost. */
@SuppressWarnings("unchecked")
public static <T> Predicate<T> any() {
return (Predicate<T>) Any.INSTANCE;
}
/** Combine the passed predicates into a single AND node. */
@SafeVarargs
public static <T> Predicate<T> and(final Predicate<T>... that) {
if (that.length == 1) {
return that[0];
}
return new AndPredicate<>(that);
}
/** Combine the passed predicates into a single AND node. */
public static <T> Predicate<T> and(final Collection<? extends Predicate<T>> that) {
if (that.size() == 1) {
return Iterables.getOnlyElement(that);
}
return new AndPredicate<>(that);
}
/** Combine the passed predicates into a single OR node. */
@SafeVarargs
public static <T> Predicate<T> or(final Predicate<T>... that) {
if (that.length == 1) {
return that[0];
}
return new OrPredicate<>(that);
}
/** Combine the passed predicates into a single OR node. */
public static <T> Predicate<T> or(final Collection<? extends Predicate<T>> that) {
if (that.size() == 1) {
return Iterables.getOnlyElement(that);
}
return new OrPredicate<>(that);
}
/** Invert the passed node. */
public static <T> Predicate<T> not(final Predicate<T> that) {
if (that instanceof NotPredicate) {
// Negate of a negate is the original predicate.
//
return that.getChild(0);
}
return new NotPredicate<>(that);
}
/** Get the children of this predicate, if any. */
public List<Predicate<T>> getChildren() {
return Collections.emptyList();
}
/** Same as {@code getChildren().size()} */
public int getChildCount() {
return getChildren().size();
}
/** Same as {@code getChildren().get(i)} */
public Predicate<T> getChild(final int i) {
return getChildren().get(i);
}
/** Create a copy of this predicate, with new children. */
public abstract Predicate<T> copy(Collection<? extends Predicate<T>> children);
public boolean isMatchable() {
return this instanceof Matchable;
}
@SuppressWarnings("unchecked")
public Matchable<T> asMatchable() {
checkState(isMatchable(), "not matchable");
return (Matchable<T>) this;
}
/** @return a cost estimate to run this predicate, higher figures cost more. */
public int estimateCost() {
if (!isMatchable()) {
return 1;
}
return asMatchable().getCost();
}
@Override
public abstract int hashCode();
@Override
public abstract boolean equals(Object other);
private static class Any<T> extends Predicate<T> implements Matchable<T> {
private static final Any<Object> INSTANCE = new Any<>();
private Any() {}
@Override
public Predicate<T> copy(Collection<? extends Predicate<T>> children) {
return this;
}
@Override
public boolean match(T object) {
return true;
}
@Override
public int getCost() {
return 0;
}
@Override
public int hashCode() {
return System.identityHashCode(this);
}
@Override
public boolean equals(Object other) {
return other == this;
}
}
}
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