/*
    * Copyright (C) 2007 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;
  
  import static com.google.common.base.Preconditions.checkState;
  
  import com.google.common.annotations.GwtCompatible;
  
  import java.util.NoSuchElementException;
  
  /**
   * This class provides a skeletal implementation of the {@code Iterator}
   * interface, to make this interface easier to implement for certain types of
   * data sources.
   *
   * <p>{@code Iterator} requires its implementations to support querying the
   * end-of-data status without changing the iterator's state, using the {@link
   * #hasNext} method. But many data sources, such as {@link
   * java.io.Reader#read()}, do not expose this information; the only way to
   * discover whether there is any data left is by trying to retrieve it. These
   * types of data sources are ordinarily difficult to write iterators for. But
   * using this class, one must implement only the {@link #computeNext} method,
   * and invoke the {@link #endOfData} method when appropriate.
   *
   * <p>Another example is an iterator that skips over null elements in a backing
   * iterator. This could be implemented as: <pre>   {@code
   *
   *   public static Iterator<String> skipNulls(final Iterator<String> in) {
   *     return new AbstractIterator<String>() {
   *       protected String computeNext() {
   *         while (in.hasNext()) {
   *           String s = in.next();
   *           if (s != null) {
   *             return s;
   *           }
   *         }
   *         return endOfData();
   *       }
   *     };
   *   }}</pre>
   *
   * <p>This class supports iterators that include null elements.
   *
   * @author Kevin Bourrillion
   * @since 2.0 (imported from Google Collections Library)
   */
  // When making changes to this class, please also update the copy at
  // com.google.common.base.AbstractIterator
  @GwtCompatible
  public abstract class AbstractIterator<T> extends UnmodifiableIterator<T> {
    private State state = State.NOT_READY;
  
    /** Constructor for use by subclasses. */
    protected AbstractIterator() {}
  
    private enum State {
      /** We have computed the next element and haven't returned it yet. */
      READY,
  
      /** We haven't yet computed or have already returned the element. */
      NOT_READY,
  
      /** We have reached the end of the data and are finished. */
      DONE,
  
      /** We've suffered an exception and are kaput. */
      FAILED,
    }
  
    private T next;
  
    /**
     * Returns the next element. <b>Note:</b> the implementation must call {@link
     * #endOfData()} when there are no elements left in the iteration. Failure to
     * do so could result in an infinite loop.
     *
     * <p>The initial invocation of {@link #hasNext()} or {@link #next()} calls
     * this method, as does the first invocation of {@code hasNext} or {@code
     * next} following each successful call to {@code next}. Once the
     * implementation either invokes {@code endOfData} or throws an exception,
     * {@code computeNext} is guaranteed to never be called again.
     *
     * <p>If this method throws an exception, it will propagate outward to the
     * {@code hasNext} or {@code next} invocation that invoked this method. Any
     * further attempts to use the iterator will result in an {@link
    * IllegalStateException}.
    *
    * <p>The implementation of this method may not invoke the {@code hasNext},
    * {@code next}, or {@link #peek()} methods on this instance; if it does, an
    * {@code IllegalStateException} will result.
    *
    * @return the next element if there was one. If {@code endOfData} was called
    *     during execution, the return value will be ignored.
    * @throws RuntimeException if any unrecoverable error happens. This exception
    *     will propagate outward to the {@code hasNext()}, {@code next()}, or
    *     {@code peek()} invocation that invoked this method. Any further
    *     attempts to use the iterator will result in an
    *     {@link IllegalStateException}.
    */
   protected abstract T computeNext();
 
   /**
    * Implementations of {@link #computeNext} <b>must</b> invoke this method when
    * there are no elements left in the iteration.
    *
    * @return {@code null}; a convenience so your {@code computeNext}
    *     implementation can use the simple statement {@code return endOfData();}
    */
   protected final T endOfData() {
     state = State.DONE;
     return null;
   }
 
   @Override
   public final boolean hasNext() {
     checkState(state != State.FAILED);
     switch (state) {
       case DONE:
         return false;
       case READY:
         return true;
       default:
     }
     return tryToComputeNext();
   }
 
   private boolean tryToComputeNext() {
     state = State.FAILED; // temporary pessimism
     next = computeNext();
     if (state != State.DONE) {
       state = State.READY;
       return true;
     }
     return false;
   }
 
   @Override
   public final T next() {
     if (!hasNext()) {
       throw new NoSuchElementException();
     }
     state = State.NOT_READY;
     T result = next;
     next = null;
     return result;
   }
 
   /**
    * Returns the next element in the iteration without advancing the iteration,
    * according to the contract of {@link PeekingIterator#peek()}.
    *
    * <p>Implementations of {@code AbstractIterator} that wish to expose this
    * functionality should implement {@code PeekingIterator}.
    */
   public final T peek() {
     if (!hasNext()) {
       throw new NoSuchElementException();
     }
     return next;
   }
 }