/*
 * Copyright 2012-present Facebook, Inc.
 *
 * 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.facebook.buck.graph;

import com.google.common.base.Preconditions;
import com.google.common.collect.Iterables;
import com.google.common.collect.Lists;
import com.google.common.collect.Maps;
import com.google.common.collect.Sets;

import java.util.Map;
import java.util.Queue;
import java.util.Set;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * Class that performs a "bottom-up" traversal of a DAG. For any given node, every node to which it
 * has an outgoing edge will be visited before the given node.
 */
public abstract class AbstractBottomUpTraversal<T, V> {

  private final TraversableGraph<T> graph;

  private final Set<T> visitedNodes;

  private final Queue<T> nodesToExplore;

  // AtomicInteger is used to decrement the integer value in-place.
  private final Map<T, AtomicInteger> effectiveOutDegreesOfExplorableNodes;

  public AbstractBottomUpTraversal(TraversableGraph<T> graph) {
    this.graph = graph;
    this.visitedNodes = Sets.newHashSet();
    this.nodesToExplore = Lists.newLinkedList();
    this.effectiveOutDegreesOfExplorableNodes = Maps.newHashMap();
  }

  public final void traverse() {
    Iterables.addAll(nodesToExplore, graph.getNodesWithNoOutgoingEdges());
    while (!nodesToExplore.isEmpty()) {
      T node = nodesToExplore.remove();
      if (visitedNodes.contains(node)) {
        Preconditions.checkState(false,
            "The queue of nodes to explore should not contain a node that has already been" +
            " visited.");
      }

      visit(node);
      visitedNodes.add(node);

      // Only add a node to the set of nodes to be explored if all the nodes it depends on have
      // been visited already. We achieve the same by keeping track of the out degrees of explorable
      // nodes. After visiting a node, decrement the out degree of each of its parent node. When the
      // out degree reaches zero, it is safe to add that node to the list of nodes to explore next.
      for (T exploreCandidate : graph.getIncomingNodesFor(node)) {
        if (!effectiveOutDegreesOfExplorableNodes.containsKey(exploreCandidate)) {
          effectiveOutDegreesOfExplorableNodes.put(exploreCandidate,
              new AtomicInteger(Iterables.size(graph.getOutgoingNodesFor(exploreCandidate))));
        }
        AtomicInteger outDegree = effectiveOutDegreesOfExplorableNodes.get(exploreCandidate);
        Preconditions.checkNotNull(outDegree);
        if (outDegree.decrementAndGet() == 0) {
          nodesToExplore.add(exploreCandidate);
        }
      }
    }
  }

  public abstract void visit(T node);

  public abstract V getResult();
}