/*
    * 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.session.scope.internal;
  
  import java.util.Collection;
  import java.util.List;
  import java.util.Map;
  import java.util.concurrent.ConcurrentHashMap;
  import java.util.concurrent.CopyOnWriteArrayList;
  
  import com.google.inject.Key;
  import com.google.inject.OutOfScopeException;
  import com.google.inject.Provider;
  import com.google.inject.Scope;
  
  /**
   * SessionScope
   */
  public class SessionScope implements Scope {
  
      private static final Provider<Object> SEEDED_KEY_PROVIDER = () -> {
          throw new IllegalStateException();
      };
  
      /**
       * ScopeState
       */
      protected static final class ScopeState {
          private final Map<Key<?>, CachingProvider<?>> provided = new ConcurrentHashMap<>();
  
          public <T> void seed(Class<T> clazz, Provider<T> value) {
              provided.put(Key.get(clazz), new CachingProvider<>(value));
          }
  
          @SuppressWarnings("unchecked")
          public <T> Provider<T> scope(Key<T> key, Provider<T> unscoped) {
              Provider<?> provider = provided.computeIfAbsent(key, k -> new CachingProvider<>(unscoped));
              return (Provider<T>) provider;
          }
  
          public Collection<CachingProvider<?>> providers() {
              return provided.values();
          }
      }
  
      private final List<ScopeState> values = new CopyOnWriteArrayList<>();
  
      public void enter() {
          values.add(0, new ScopeState());
      }
  
      protected ScopeState getScopeState() {
          if (values.isEmpty()) {
              throw new OutOfScopeException("Cannot access session scope outside of a scoping block");
          }
          return values.get(0);
      }
  
      public void exit() {
          if (values.isEmpty()) {
              throw new IllegalStateException();
          }
          values.remove(0);
      }
  
      public <T> void seed(Class<T> clazz, Provider<T> value) {
          getScopeState().seed(clazz, value);
      }
  
      public <T> void seed(Class<T> clazz, final T value) {
          seed(clazz, (Provider<T>) () -> value);
      }
  
      public <T> Provider<T> scope(final Key<T> key, final Provider<T> unscoped) {
          // Lazy evaluating provider
          return () -> getScopeState().scope(key, unscoped).get();
      }
  
      /**
       * A provider wrapping an existing provider with a cache
       * @param <T> the provided type
       */
      protected static class CachingProvider<T> implements Provider<T> {
         private final Provider<T> provider;
         private volatile T value;
 
         CachingProvider(Provider<T> provider) {
             this.provider = provider;
         }
 
         public T value() {
             return value;
         }
 
         @Override
         public T get() {
             if (value == null) {
                 synchronized (this) {
                     if (value == null) {
                         value = provider.get();
                     }
                 }
             }
             return value;
         }
     }
 
     @SuppressWarnings({"unchecked"})
     public static <T> Provider<T> seededKeyProvider() {
         return (Provider<T>) SEEDED_KEY_PROVIDER;
     }
 }