package org.apache.maven.session.scope.internal;
   
   /*
    * 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.
   */
  
  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;
     }
 }