/*
    * 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.base;
  
  import static com.google.common.base.Preconditions.checkState;
  
  import com.google.common.annotations.GwtCompatible;
  
  import java.util.Iterator;
  import java.util.NoSuchElementException;
  
  /**
   * Note this class is a copy of
   * {@link com.google.common.collect.AbstractIterator} (for dependency reasons).
   */
  @GwtCompatible
  abstract class AbstractIterator<T> implements Iterator<T> {
    private State state = State.NOT_READY;
  
    protected AbstractIterator() {}
  
    private enum State {
      READY, NOT_READY, DONE, FAILED,
    }
  
    private T next;
  
    protected abstract T computeNext();
  
    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;
    }
  
    @Override public final void remove() {
      throw new UnsupportedOperationException();
    }
  }