package com.carrotsearch.hppc;

import java.util.Iterator;
import java.util.NoSuchElementException;

/**
 * Simplifies the implementation of iterators a bit. Modeled loosely after
 * Google Guava's API.
 */
public abstract class AbstractIterator<E> implements Iterator<E> {
  private final static int NOT_CACHED = 0;
  private final static int CACHED = 1;
  private final static int AT_END = 2;

  /** Current iterator state. */
  private int state = NOT_CACHED;

  /**
   * The next element to be returned from {@link #next()} if fetched.
   */
  private E nextElement;

  /**
   * {@inheritDoc}
   */
  @Override
  public boolean hasNext() {
    if (state == NOT_CACHED) {
      state = CACHED;
      nextElement = fetch();
    }
    return state == CACHED;
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public E next() {
    if (!hasNext()) {
      throw new NoSuchElementException();
    }

    state = NOT_CACHED;
    return nextElement;
  }

  /**
   * Default implementation throws {@link UnsupportedOperationException}.
   */
  @Override
  public void remove() {
    throw new UnsupportedOperationException();
  }

  /**
   * Fetch next element. The implementation must return {@link #done()} when all
   * elements have been fetched.
   * 
   * @return Returns the next value for the iterator or chain-calls {@link #done()}. 
   */
  protected abstract E fetch();

  /**
   * Call when done.
   * 
   * @return Returns a unique sentinel value to indicate end-of-iteration.
   */
  protected final E done() {
    state = AT_END;
    return null;
  }
}
