src/main/java/com/google/devtools/build/lib/actions/BipartiteVisitor.java

// Copyright 2014 The Bazel Authors. All rights reserved.
//
// 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.devtools.build.lib.actions;

import java.util.HashMap;
import java.util.Map;

/**
 * A visitor helper class for bipartite graphs.  The alternate kinds of nodes
 * are arbitrarily designated "black" or "white".
 *
 * <p> Subclasses implement the black() and white() hook functions which are
 * called as nodes are visited.  The class holds a mapping from each node to a
 * small integer; this is available to subclasses if they wish.
 */
public abstract class BipartiteVisitor<BLACK, WHITE> {

  protected BipartiteVisitor() {}

  private int nextNodeId = 0;

  // Maps each visited black node to a small integer.
  protected final Map<BLACK, Integer> visitedBlackNodes = new HashMap<>();

  // Maps each visited white node to a small integer.
  protected final Map<WHITE, Integer> visitedWhiteNodes = new HashMap<>();

  /**
   *  Visit the specified black node.  If this node has not already been
   *  visited, the black() hook is called and true is returned; otherwise,
   *  false is returned.
   */
  public final boolean visitBlackNode(BLACK blackNode) {
    if (blackNode == null) { throw new NullPointerException(); }
    if (!visitedBlackNodes.containsKey(blackNode)) {
      visitedBlackNodes.put(blackNode, nextNodeId++);
      black(blackNode);
      return true;
    }
    return false;
  }

  /**
   * Visit all specified black nodes.
   */
  public final void visitBlackNodes(Iterable<BLACK> blackNodes) {
    for (BLACK blackNode : blackNodes) {
      visitBlackNode(blackNode);
    }
  }

  /**
   *  Visit the specified white node.  If this node has not already been
   *  visited, the white() hook is called and true is returned; otherwise,
   *  false is returned.
   */
  public final boolean visitWhiteNode(WHITE whiteNode) {
    if (whiteNode == null) {
      throw new NullPointerException();
    }
    if (!visitedWhiteNodes.containsKey(whiteNode)) {
      visitedWhiteNodes.put(whiteNode, nextNodeId++);
      white(whiteNode);
      return true;
    }
    return false;
  }

  /**
   * Visit all specified white nodes.
   */
  public final void visitWhiteNodes(Iterable<WHITE> whiteNodes) {
    for (WHITE whiteNode : whiteNodes) {
      visitWhiteNode(whiteNode);
    }
  }

  /**
   * Called whenever a white node is visited.  Hook for subclasses.
   */
  protected abstract void white(WHITE whiteNode);

  /**
   * Called whenever a black node is visited.  Hook for subclasses.
   */
  protected abstract void black(BLACK blackNode);
}