// 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.index.query;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkState;

import com.google.common.collect.Iterables;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;

/**
 * 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> {
  /** Query String that was used to create this predicate. Only set from the Antlr query parser. */
  private String predicateString = null;

  /**
   * Boolean indicating if this predicate is a leaf predicate in a composite expression. Only set
   * from the Antlr query parser.
   */
  private boolean isLeaf = false;

  /** Sets the {@link #predicateString} field. This can only be set once. */
  void setPredicateString(String predicateString) {
    this.predicateString = this.predicateString == null ? predicateString : this.predicateString;
  }

  public String getPredicateString() {
    return predicateString;
  }

  void setLeaf(boolean isLeaf) {
    this.isLeaf = isLeaf;
  }

  public boolean isLeaf() {
    return isLeaf;
  }

  /** 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(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(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(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(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(Predicate<T> that) {
    checkArgument(
        !(that instanceof Any), "negating any() is unsafe because it post-filters all results");
    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(int i) {
    return getChildren().get(i);
  }

  /** Get the number of leaf terms in this predicate. */
  public int getLeafCount() {
    int leafCount = 0;
    for (Predicate<?> childPredicate : getChildren()) {
      if (childPredicate instanceof IndexPredicate) {
        leafCount++;
      }
      leafCount += childPredicate.getLeafCount();
    }
    return leafCount;
  }

  /** Returns a list of this predicate and all its descendants. */
  public List<Predicate<T>> getFlattenedPredicateList() {
    List<Predicate<T>> result = new ArrayList<>();
    Queue<Predicate<T>> queue = new LinkedList<>();
    queue.add(this);
    while (!queue.isEmpty()) {
      Predicate<T> current = queue.poll();
      result.add(current);
      current.getChildren().forEach(p -> queue.add(p));
    }
    return result;
  }

  /** 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;
  }

  /** Whether this predicate can be used in search queries. */
  public boolean supportedForQueries() {
    return true;
  }

  @SuppressWarnings("unchecked")
  public Matchable<T> asMatchable() {
    checkState(isMatchable(), "not matchable");
    return (Matchable<T>) this;
  }

  /** Returns 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;
    }
  }
}