/*
 * 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.jclouds.json.internal;

import java.io.IOException;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;

import com.google.common.base.Optional;
import com.google.gson.Gson;
import com.google.gson.TypeAdapter;
import com.google.gson.TypeAdapterFactory;
import com.google.gson.reflect.TypeToken;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonToken;
import com.google.gson.stream.JsonWriter;

/**
 * Writes and reads Optional values as JSON
 */
public class OptionalTypeAdapterFactory implements TypeAdapterFactory {

   @SuppressWarnings("unchecked")
   @Override
   public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
      Type type = typeToken.getType();
      if (typeToken.getRawType() != Optional.class || !(type instanceof ParameterizedType)) {
         return null;
      }

      Type elementType = ((ParameterizedType) type).getActualTypeArguments()[0];
      TypeAdapter<?> elementAdapter = gson.getAdapter(TypeToken.get(elementType));
      return (TypeAdapter<T>) newOptionalAdapter(elementAdapter);
   }

   protected <E> TypeAdapter<Optional<E>> newOptionalAdapter(
         final TypeAdapter<E> elementAdapter) {
      return new TypeAdapter<Optional<E>>() {
         public void write(JsonWriter out, Optional<E> value) throws IOException {
            if (!value.isPresent()) {
               out.nullValue();
               return;
            }
            elementAdapter.write(out, value.get());
         }

         public Optional<E> read(JsonReader in) throws IOException {
            Optional<E> result = Optional.absent();
            if (in.peek() == JsonToken.NULL) {
               in.nextNull();
            } else {
               E element = elementAdapter.read(in);
               if (element != null) {
                  result = Optional.of(element);
               }
            }
            return result;
         }
      };
   }
}