/*
    * Copyright (C) 2009 The Guava Authors
    *
    * Licensed 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 com.google.common.collect.testing;
  
  import com.google.common.annotations.GwtCompatible;
  
  import java.util.Arrays;
  import java.util.Collection;
  import java.util.Iterator;
  
  /**
   * An implementation of {@code Iterable} which throws an exception on all
   * invocations of the {@link #iterator()} method after the first, and whose
   * iterator is always unmodifiable.
   *
   * <p>The {@code Iterable} specification does not make it absolutely clear what
   * should happen on a second invocation, so implementors have made various
   * choices, including:
   *
   * <ul>
   * <li>returning the same iterator again
   * <li>throwing an exception of some kind
   * <li>or the usual, <i>robust</i> behavior, which all known {@link Collection}
   *     implementations have, of returning a new, independent iterator
   * </ul>
   *
   * <p>Because of this situation, any public method accepting an iterable should
   * invoke the {@code iterator} method only once, and should be tested using this
   * class. Exceptions to this rule should be clearly documented.
   *
   * <p>Note that although your APIs should be liberal in what they accept, your
   * methods which <i>return</i> iterables should make every attempt to return
   * ones of the robust variety.
   *
   * <p>This testing utility is not thread-safe.
   *
   * @author Kevin Bourrillion
   */
  @GwtCompatible
  public final class MinimalIterable<E> implements Iterable<E> {
    /**
     * Returns an iterable whose iterator returns the given elements in order.
     */
    public static <E> MinimalIterable<E> of(E... elements) {
      // Make sure to get an unmodifiable iterator
      return new MinimalIterable<E>(Arrays.asList(elements).iterator());
    }
  
    /**
     * Returns an iterable whose iterator returns the given elements in order.
     * The elements are copied out of the source collection at the time this
     * method is called.
     */
    @SuppressWarnings("unchecked") // Es come in, Es go out
    public static <E> MinimalIterable<E> from(final Collection<E> elements) {
      return (MinimalIterable) of(elements.toArray());
    }
  
    private Iterator<E> iterator;
  
    private MinimalIterable(Iterator<E> iterator) {
      this.iterator = iterator;
    }
  
    @Override
    public Iterator<E> iterator() {
      if (iterator == null) {
        // TODO: throw something else? Do we worry that people's code and tests
        // might be relying on this particular type of exception?
        throw new IllegalStateException();
      }
      try {
        return iterator;
      } finally {
        iterator = null;
      }
    }
  }