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/*
* 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 org.apache.cassandra.index.sasi.utils;
import java.util.NoSuchElementException;
import com.google.common.collect.PeekingIterator;
import static com.google.common.base.Preconditions.checkState;
// This is fork of the Guava AbstractIterator, the only difference
// is that state & next variables are now protected, this was required
// for SkippableIterator.skipTo(..) to void all previous state.
public abstract class AbstractIterator implements PeekingIterator
{
protected State state = State.NOT_READY;
/** Constructor for use by subclasses. */
protected AbstractIterator() {}
protected 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,
}
protected T next;
/**
* Returns the next element. Note: 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.
*
* 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.
*
*
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}.
*
*
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} must 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;
}
public final boolean hasNext()
{
checkState(state != State.FAILED);
switch (state)
{
case DONE:
return false;
case READY:
return true;
default:
}
return tryToComputeNext();
}
protected boolean tryToComputeNext()
{
state = State.FAILED; // temporary pessimism
next = computeNext();
if (state != State.DONE)
{
state = State.READY;
return true;
}
return false;
}
public final T next()
{
if (!hasNext())
throw new NoSuchElementException();
state = State.NOT_READY;
return next;
}
public void remove()
{
throw new UnsupportedOperationException();
}
/**
* Returns the next element in the iteration without advancing the iteration,
* according to the contract of {@link PeekingIterator#peek()}.
*
*
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;
}
}