/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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.jena.atlas.iterator;

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

import org.apache.jena.atlas.lib.Lib ;

/** An Iterator with a one slot lookahead. */  
public abstract class IteratorSlotted<T> implements Iterator<T>
{
    private boolean finished = false ;
    private boolean slotIsSet = false ;
    private T slot = null ; 

    protected IteratorSlotted() { }

    // -------- The contract with the subclasses 
    
    /** Implement this, not next() or nextBinding() */
    protected abstract T moveToNext() ;
    
    /** Can return true here then null from moveToNext() to indicate end. */ 
    protected abstract boolean hasMore() ;
    // alter add a flag to say if null is a legal value.
    
    /** Close the iterator. */
    protected void closeIterator() { }
   
    // -------- The contract with the subclasses 

    protected boolean isFinished() { return finished ; }

    @Override
    public final boolean hasNext()
    {
        if ( finished )
            return false ;
        if ( slotIsSet )
            return true ;

        boolean r = hasMore() ;
        if ( ! r )
        {
            close() ;
            return false ;
        }
        
        slot = moveToNext() ;
        if ( slot == null ) {
            close() ;
            return false ;
        }
            
        slotIsSet = true ;
        return true ;
    }
    
    /** final - autoclose and registration relies on it - implement moveToNextBinding() */
    @Override
    public final T next()
    {
        if ( ! hasNext() ) throw new NoSuchElementException(Lib.className(this)) ;
        
        T obj = slot ;
        slot = null ;
        slotIsSet = false ;
        return obj ;
    }

    /** Look at the next element - returns null when there is no element */
    public final T peek()
    {
        return peek(null) ;
    }
    
    /** Look at the next element - returns dft when there is no element */
    public final T peek(T dft)
    {
        hasNext() ;
        if ( ! slotIsSet ) return dft ;
        return slot ;
    }
    
    @Override
    public final void remove()
    {
        throw new UnsupportedOperationException(Lib.className(this)+".remove") ;
    }
    
    public final void close()
    {
        if ( finished )
            return ;
        closeIterator() ;
        slotIsSet = false ;
        slot = null ;
        finished = true ;
    }
}