/**
 * Licensed to jclouds, Inc. (jclouds) under one or more
 * contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  jclouds 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.suppliers;

import java.util.Map;

import javax.annotation.Resource;

import org.jclouds.logging.Logger;

import com.google.common.base.Supplier;
import com.google.common.base.Suppliers;
import com.google.common.collect.ImmutableBiMap;
import com.google.common.collect.Iterables;
import com.google.common.collect.Maps;

/**
 * Allows you to lazy discover a key by value. This is useful for example in service discovery,
 * where you need to see what the "current" service name is based on a map of service names to
 * endpoints. <h3>note</h3> take care to memoize this using {@link Suppliers#memoize(Supplier)}
 * 
 * @author Adrian Cole
 */
public class SupplyKeyMatchingValueOrNull<K, V> implements Supplier<K> {
   @Resource
   protected Logger logger = Logger.NULL;

   private final Supplier<Map<K, Supplier<V>>> supplier;
   private final Supplier<V> valueSupplier;

   public SupplyKeyMatchingValueOrNull(Supplier<Map<K, Supplier<V>>> supplier, Supplier<V> valueSupplier) {
      this.valueSupplier = valueSupplier;
      this.supplier = supplier;
   }

   @Override
   public K get() {
      V uri = valueSupplier.get();
      // eagerly get all the values, so we can see which is default
      Map<K, V> map = Maps.transformValues(supplier.get(), Suppliers.<V> supplierFunction());
      K region = ImmutableBiMap.copyOf(map).inverse().get(uri);
      if (region == null && map.size() > 0) {
         region = Iterables.get(map.keySet(), 0);
         logger.warn("failed to find key for value %s in %s; choosing first: %s", uri, map, region);
      }
      return region;
   }
}