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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 com.google.gwtorm.client.OrmException;
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.
*
* @type <T> type of object the predicate can evaluate in memory.
*/
public abstract class Predicate<T> {
/** Combine the passed predicates into a single AND node. */
public static <T> Predicate<T> and(final Predicate<T>... that) {
return new AndPredicate<T>(that);
}
/** Combine the passed predicates into a single AND node. */
public static <T> Predicate<T> and(
final Collection<? extends Predicate<T>> that) {
return new AndPredicate<T>(that);
}
/** Combine the passed predicates into a single OR node. */
public static <T> Predicate<T> or(final Predicate<T>... that) {
return new OrPredicate<T>(that);
}
/** Combine the passed predicates into a single OR node. */
public static <T> Predicate<T> or(
final Collection<? extends Predicate<T>> that) {
return new OrPredicate<T>(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<T>(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);
/**
* Does this predicate match this object?
*
* @throws OrmException
*/
public abstract boolean match(T object) throws OrmException;
/** @return a cost estimate to run this predicate, higher figures cost more. */
public abstract int getCost();
@Override
public abstract int hashCode();
@Override
public abstract boolean equals(Object other);
}
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