/*
 * 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.ArrayList ;
import java.util.Iterator ;
import java.util.List ;
import java.util.NoSuchElementException ;

/**
 * Iterator that delays output by N slots so you can react to the output before
 * it's yielded. See also {@link PeekIterator} (which predates this code). See also
 * {@link IteratorWithHistory} for an iterator that remembers what it has yielded.
 * 
 * @see PeekIterator
 * @see PushbackIterator
 * @see IteratorWithHistory
 */
public class IteratorWithBuffer<T> implements Iterator<T> {
    private List<T>     lookahead ;
    private Iterator<T> iter ;
    private int         capacity ;
    private boolean     innerHasEnded = false ;

    public IteratorWithBuffer(Iterator<T> iter, int N) {
        if ( N < 0 )
            throw new IllegalArgumentException("Buffering size < 0") ;
        this.iter = iter ;
        this.lookahead = new ArrayList<>(N) ;
        this.capacity = N ;
        // Fill the lookahead.
        for ( int i = 0 ; i < N ; i++ ) {
            if ( !iter.hasNext() ) {
                atEndInner() ;
                break ;
            }
            T nextItem = iter.next() ;
            // System.out.println("Fill: "+nextItem) ;
            lookahead.add(nextItem) ;
        }
    }

    @Override
    public boolean hasNext() {
        return lookahead.size() > 0 ;
    }

    @Override
    public T next() {
        if ( !hasNext() )
            throw new NoSuchElementException(this.getClass().getName()) ;

        if ( !iter.hasNext() )
            atEndInner() ;

        T item = lookahead.remove(0) ;
        // System.out.println("remove: "+item) ;
        if ( iter.hasNext() ) {
            // Should not throw NoSuchElementException.
            T nextItem = iter.next() ;
            // System.out.println("add   : "+nextItem) ;
            lookahead.add(nextItem) ;
        }
        return item ;
    }

    @Override
    public void remove() {
        throw new UnsupportedOperationException("remove") ;
    }

    /**
     * Look at elements that will be returned by a subsequnet call of .next().
     * The next element is index 0, then index 1 etc. This operation is valid
     * immediately after the constructor returns. Returns null for no such
     * element (underlying iterator didn't yeild enough elements). Throws
     * IndexOutOfBoundsException if an attempt i smade to go beyond the
     * buffering window.
     */
    public T peek(int idx) {
        if ( idx < 0 || idx >= capacity )
            throw new IndexOutOfBoundsException("Index: " + idx) ;
        if ( idx >= lookahead.size() )
            return null ;
        return lookahead.get(idx) ;
    }

    /**
     * Return the current size of the lookahead. This can be used to tell the
     * difference between an iterator returning null and an iterator that is
     * just short.
     */
    public int currentSize() {
        return lookahead.size() ;
    }

    /**
     * Set the element to be returned by a subsequent .next(). Use with care.
     * The original element to be returned at this position is lost.
     */
    public void set(int idx, T item) {
        lookahead.set(idx, item) ;
    }

    /** Called when the underlying iterator ends */
    private void atEndInner() {
        if ( !innerHasEnded ) {
            innerHasEnded = true ;
            endReachedInner() ;
        }
    }

    /** Called, once, at the end of the wrapped iterator. */
    protected void endReachedInner() {}

}