/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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 org.apache.beam.runners.dataflow.worker.graph;

import static org.apache.beam.vendor.guava.v26_0_jre.com.google.common.base.Preconditions.checkArgument;
import static org.apache.beam.vendor.guava.v26_0_jre.com.google.common.base.Preconditions.checkNotNull;

import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.IdentityHashMap;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Queue;
import java.util.Set;
import java.util.function.Function;
import org.apache.beam.runners.dataflow.worker.graph.Nodes.Node;
import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.collect.ImmutableList;
import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.collect.ImmutableSet;
import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.collect.Maps;
import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.graph.EndpointPair;
import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.graph.MutableNetwork;
import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.graph.Network;

/** Static utility methods for {@link Network} instances that are directed. */
@SuppressWarnings({
  "nullness" // TODO(https://github.com/apache/beam/issues/20497)
})
public class Networks {
  /**
   * An abstract class that can be extended to apply a function in a type safe manner.
   *
   * <p>Applies {@link #typedApply} to all instances of {@code type}. Otherwise returns the existing
   * {@link Node} unmodified.
   */
  public abstract static class TypeSafeNodeFunction<T extends Node>
      implements Function<Node, Node> {
    private final Class<T> type;

    public TypeSafeNodeFunction(Class<T> type) {
      checkNotNull(type);
      this.type = type;
    }

    @SuppressWarnings("unchecked")
    @Override
    public final Node apply(Node input) {
      if (type.isInstance(input)) {
        return typedApply((T) input);
      }
      return input;
    }

    public abstract Node typedApply(T input);
  }

  /**
   * Applies the {@code function} to all nodes within the {@code network}. Replaces any node which
   * is not {@link #equals(Object)} to the original node, maintaining all existing edges between
   * nodes.
   */
  public static <N, E> void replaceDirectedNetworkNodes(
      MutableNetwork<N, E> network, Function<N, N> function) {
    checkArgument(network.isDirected(), "Only directed networks are supported, given %s", network);
    checkArgument(
        !network.allowsSelfLoops(),
        "Only networks without self loops are supported, given %s",
        network);

    // A map from the existing node to the replacement node
    Map<N, N> oldNodesToNewNodes = new HashMap<>(network.nodes().size());
    for (N currentNode : network.nodes()) {
      N newNode = function.apply(currentNode);
      // Skip updating the network if the old node is equivalent to the new node
      if (!currentNode.equals(newNode)) {
        oldNodesToNewNodes.put(currentNode, newNode);
      }
    }

    // For each replacement, connect up the existing predecessors and successors to the new node
    // and then remove the old node.
    for (Map.Entry<N, N> entry : oldNodesToNewNodes.entrySet()) {
      N oldNode = entry.getKey();
      N newNode = entry.getValue();
      network.addNode(newNode);
      for (N predecessor : ImmutableSet.copyOf(network.predecessors(oldNode))) {
        for (E edge : ImmutableSet.copyOf(network.edgesConnecting(predecessor, oldNode))) {
          network.removeEdge(edge);
          network.addEdge(predecessor, newNode, edge);
        }
      }
      for (N successor : ImmutableSet.copyOf(network.successors(oldNode))) {
        for (E edge : ImmutableSet.copyOf(network.edgesConnecting(oldNode, successor))) {
          network.removeEdge(edge);
          network.addEdge(newNode, successor, edge);
        }
      }
      network.removeNode(oldNode);
    }
  }

  /**
   * Returns the set of nodes that are reachable from {@code startNodes} up to and including {@code
   * endNodes}. Node B is defined as reachable from node A if there exists a path (a sequence of
   * adjacent outgoing edges) starting at node A and ending at node B which does not pass through
   * any node in {@code endNodes}. Note that a node is always reachable from itself via a
   * zero-length path.
   *
   * <p>This is a "snapshot" based on the current topology of the {@code network}, rather than a
   * live view of the set of nodes reachable from {@code node}. In other words, the returned {@link
   * Set} will not be updated after modifications to the {@code network}.
   */
  public static <N, E> Set<N> reachableNodes(
      Network<N, E> network, Set<N> startNodes, Set<N> endNodes) {
    Set<N> visitedNodes = new HashSet<>();
    Queue<N> queuedNodes = new ArrayDeque<>();
    queuedNodes.addAll(startNodes);
    // Perform a breadth-first traversal rooted at the input node.
    while (!queuedNodes.isEmpty()) {
      N currentNode = queuedNodes.remove();
      // If we have already visited this node or it is a terminal node than do not add any
      // successors.
      if (!visitedNodes.add(currentNode) || endNodes.contains(currentNode)) {
        continue;
      }
      queuedNodes.addAll(network.successors(currentNode));
    }
    return visitedNodes;
  }

  /** Returns a set of nodes sorted in topological order. */
  public static <N, E> Set<N> topologicalOrder(Network<N, E> network) {
    // TODO: Upgrade Guava and remove this method if topological sorting becomes
    // supported externally or remove this comment if its not going to be supported externally.

    checkArgument(network.isDirected(), "Only directed networks are supported, given %s", network);
    checkArgument(
        !network.allowsSelfLoops(),
        "Only networks without self loops are supported, given %s",
        network);

    // Linked hashset will prevent duplicates from appearing and will maintain insertion order.
    LinkedHashSet<N> nodes = new LinkedHashSet<>(network.nodes().size());
    Queue<N> processingOrder = new ArrayDeque<>();
    // Add all the roots
    for (N node : network.nodes()) {
      if (network.inDegree(node) == 0) {
        processingOrder.add(node);
      }
    }

    while (!processingOrder.isEmpty()) {
      N current = processingOrder.remove();
      // If all predecessors have already been added, then we can add this node, otherwise
      // we need to add the node to the back of the processing queue.
      if (nodes.containsAll(network.predecessors(current))) {
        nodes.add(current);
        processingOrder.addAll(network.successors(current));
      } else {
        processingOrder.add(current);
      }
    }

    return nodes;
  }

  public static <N, E> String toDot(Network<N, E> network) {
    StringBuilder builder = new StringBuilder();
    builder.append("digraph network {\n");
    IdentityHashMap<N, String> nodeName = Maps.newIdentityHashMap();
    network.nodes().forEach(node -> nodeName.put(node, "n" + nodeName.size()));
    for (Entry<N, String> nodeEntry : nodeName.entrySet()) {
      builder.append(
          String.format(
              "  %s [fontname=\"Courier New\" label=\"%s\"];%n",
              nodeEntry.getValue(), escapeDot(nodeEntry.getKey().toString())));
    }
    for (E edge : network.edges()) {
      EndpointPair<N> endpoints = network.incidentNodes(edge);
      builder.append(
          String.format(
              "  %s -> %s [fontname=\"Courier New\" label=\"%s\"];%n",
              nodeName.get(endpoints.source()),
              nodeName.get(endpoints.target()),
              escapeDot(edge.toString())));
    }
    builder.append("}");
    return builder.toString();
  }

  private static String escapeDot(String s) {
    return s.replace("\\", "\\\\")
        .replace("\"", "\\\"")
        // http://www.graphviz.org/doc/info/attrs.html#k:escString
        // The escape sequences "\n", "\l" and "\r" divide the label into lines, centered,
        // left-justified, and right-justified, respectively.
        .replace("\n", "\\l");
  }

  /**
   * Returns a list of all distinct paths from roots of the network to leaves. The list can be in
   * arbitrary orders and can contain duplicate paths if there are multiple edges from two nodes.
   */
  public static <NodeT, EdgeT> List<List<NodeT>> allPathsFromRootsToLeaves(
      Network<NodeT, EdgeT> network) {
    ArrayDeque<List<NodeT>> paths = new ArrayDeque<>();
    // Populate the list with all roots
    for (NodeT node : network.nodes()) {
      if (network.inDegree(node) == 0) {
        paths.add(ImmutableList.of(node));
      }
    }

    List<List<NodeT>> distinctPathsFromRootsToLeaves = new ArrayList<>();
    while (!paths.isEmpty()) {
      List<NodeT> path = paths.removeFirst();
      NodeT lastNode = path.get(path.size() - 1);
      if (network.outDegree(lastNode) == 0) {
        distinctPathsFromRootsToLeaves.add(new ArrayList<>(path));
      } else {
        for (EdgeT edge : network.outEdges(lastNode)) {
          paths.addFirst(
              ImmutableList.<NodeT>builder()
                  .addAll(path)
                  .add(network.incidentNodes(edge).target())
                  .build());
        }
      }
    }
    return distinctPathsFromRootsToLeaves;
  }

  // Hide visibility to prevent instantiation
  private Networks() {}
}
