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

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

/** Iterator of Iterators
 *  IteratorConcat is better when there are lots of iterators to be joined.
 *  IteratorCons is slightly better for two iterators. 
 */

public class IteratorConcat<T> implements Iterator<T>
{
    private List<Iterator<T>> iterators = DS.list(); 
    int idx = -1 ;
    private Iterator<T> current = null ;
    private Iterator<T> removeFrom = null ;
    boolean finished = false ;
    
    /** 
     * Usually, it is better to create an IteratorConcat explicitly and add iterator if there are going to be many.
     * @param iter1
     * @param iter2
     * @return Iterator
     * @see IteratorCons
     */
    public static <T> Iterator<T> concat(Iterator<T> iter1, Iterator<T> iter2)
    {
        if (iter2 == null) return iter1 ;
        if (iter1 == null) return iter2 ;
        IteratorConcat<T> c = new IteratorConcat<>() ;
        c.add(iter1) ;
        c.add(iter2) ;
        return c ;
    }
 
    public IteratorConcat() {}

    
    public void add(Iterator<T> iter) { iterators.add(iter) ; }
    
    @Override
    public boolean hasNext()
    {
        if ( finished )
            return false ;

        if ( current != null && current.hasNext() )
            return true ;
        
        while ( idx < iterators.size()-1 )
        {
            idx++ ;
            current = iterators.get(idx) ;
            if ( current.hasNext() )
                return true ;
            // Nothing here - move on.
            current = null ;
        }
        // idx has run off the end.
        return false ;
    }

    @Override
    public T next()
    {
        if ( ! hasNext() )
            throw new NoSuchElementException() ; 
        removeFrom = current ;
        return current.next();
    }

    @Override
    public void remove()
    { 
        if ( null == removeFrom )
            throw new IllegalStateException("no calls to next() since last call to remove()") ;
        
        removeFrom.remove() ;
        removeFrom = null ;

    }

}