package com.atlassian.plugins.rest.common.expand.resolver;
   
   import com.atlassian.plugins.rest.common.expand.EntityCrawler;
   import com.atlassian.plugins.rest.common.expand.EntityExpander;
   import com.atlassian.plugins.rest.common.expand.ExpandContext;
   import com.atlassian.plugins.rest.common.expand.entity.ListWrapper;
   import com.google.common.base.Preconditions;
   import com.google.common.collect.Sets;
   
  import java.lang.reflect.Field;
  import java.util.Collection;
  import java.util.Set;
  
  import static com.atlassian.plugins.rest.common.util.ReflectionUtils.setFieldValue;
  
  /**
   * An {@link EntityExpanderResolver entity expander resolver} that can create a {@link EntityExpander entity expander} for {@link ListWrapper list wrappers}
   */
  public class ListWrapperEntityExpanderResolver implements EntityExpanderResolver {
      static final ListWrapperEntityExpander EXPANDER = new ListWrapperEntityExpander();
  
      public boolean hasExpander(Class<?> type) {
          return ListWrapper.class.isAssignableFrom(Preconditions.checkNotNull(type));
      }
  
      public <T> EntityExpander<T> getExpander(Class<? extends T> type) {
          return ListWrapper.class.isAssignableFrom(type) ? (EntityExpander<T>) EXPANDER : null;
      }
  
      static class ListWrapperEntityExpander<T> implements EntityExpander<ListWrapper<T>> {
          public ListWrapper<T> expand(ExpandContext<ListWrapper<T>> context, EntityExpanderResolver expanderResolver, EntityCrawler entityCrawler) {
              final ListWrapper<T> entity = context.getEntity();
  
              final Set<Field> collectionFields = Sets.newHashSet();
              for (Class cls = entity.getClass(); cls != null; cls = cls.getSuperclass()) {
                  final Field[] fields = cls.getDeclaredFields();
                  for (Field field : fields) {
                      if (Collection.class.isAssignableFrom(field.getType())) {
                          collectionFields.add(field);
                      }
                  }
              }
              if (collectionFields.isEmpty()) {
                  throw new RuntimeException("Entity " + entity.getClass() + " has no collection field, cannot expand.");
              }
              if (collectionFields.size() > 1) {
                  throw new RuntimeException("Entity " + entity.getClass() + " has more than one collection field, cannot determine which collection to expand.");
              }
  
              setFieldValue(collectionFields.iterator().next(), entity, entity.getCallback().getItems(context.getEntityExpandParameter().getIndexes(context.getExpandable())));
  
              entityCrawler.crawl(entity, context.getEntityExpandParameter().getExpandParameter(context.getExpandable()), expanderResolver);
              return entity;
          }
  
      }
  }