/*
    * 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.maven.model.building;
  
  import java.util.Collection;
  import java.util.Collections;
  import java.util.HashMap;
  import java.util.HashSet;
  import java.util.LinkedHashMap;
  import java.util.LinkedList;
  import java.util.List;
  import java.util.Map;
  import java.util.Set;
  
  class Graph {
  
      final Map<String, Set<String>> graph = new LinkedHashMap<>();
  
      synchronized void addEdge(String from, String to) throws CycleDetectedException {
          if (graph.computeIfAbsent(from, l -> new HashSet<>()).add(to)) {
              List<String> cycle = visitCycle(graph, Collections.singleton(to), new HashMap<>(), new LinkedList<>());
              if (cycle != null) {
                  // remove edge which introduced cycle
                  throw new CycleDetectedException(
                          "Edge between '" + from + "' and '" + to + "' introduces to cycle in the graph", cycle);
              }
          }
      }
  
      private enum DfsState {
          VISITING,
          VISITED
      }
  
      private static List<String> visitCycle(
              Map<String, Set<String>> graph,
              Collection<String> children,
              Map<String, DfsState> stateMap,
              LinkedList<String> cycle) {
          if (children != null) {
              for (String v : children) {
                  DfsState state = stateMap.putIfAbsent(v, DfsState.VISITING);
                  if (state == null) {
                      cycle.addLast(v);
                      List<String> ret = visitCycle(graph, graph.get(v), stateMap, cycle);
                      if (ret != null) {
                          return ret;
                      }
                      cycle.removeLast();
                      stateMap.put(v, DfsState.VISITED);
                  } else if (state == DfsState.VISITING) {
                      // we are already visiting this vertex, this mean we have a cycle
                      int pos = cycle.lastIndexOf(v);
                      List<String> ret = cycle.subList(pos, cycle.size());
                      ret.add(v);
                      return ret;
                  }
              }
          }
          return null;
      }
  
      public static class CycleDetectedException extends Exception {
          private final List<String> cycle;
  
          CycleDetectedException(String message, List<String> cycle) {
              super(message);
              this.cycle = cycle;
          }
  
          public List<String> getCycle() {
              return cycle;
          }
  
          @Override
          public String getMessage() {
              return super.getMessage() + " " + String.join(" --> ", cycle);
          }
      }
  }