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// Burton Computer Corporation
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package org.javimmutable.collections.cursors;

import org.javimmutable.collections.Cursor;
import org.javimmutable.collections.Holder;
import org.javimmutable.collections.Holders;

import javax.annotation.concurrent.Immutable;
import java.util.Iterator;

/**
 * Provides objects implementing the Cursor interface that can be used to traverse
 * an Iterable.  The Cursors are not actually immutable but are designed to behave
 * as though they are.  However since they depend on a mutable object tied to a
 * mutable collection for their state they are not thread safe.  They should only
 * be used when no other threads could be modifying the underlying collection.
 * Also their next() methods can throw exceptions due to concurrent modification
 * of the collection.
 */
@Immutable
public abstract class IterableCursor
{
    /**
     * Creates a mutable Cursor implementation that traverses the specified iterable.  Note that because
     * the Iterator and the collection it points to are not immutable outside factors can interfere with
     * the operation of this class.  In particular this class is not thread safe so the caller must take
     * precautions to synchronize access to the underlying collection.
     * 
     * The Cursor retains information obtained from the Iterator so that multiple passes can be made over
     * the data, intermediate Cursor values can be saved and resumed for look-ahead etc.
     *
     * @param iterable
     * @param 
     * @return
     */
    public static  Cursor of(Iterable iterable)
    {
        return ValueFunctionCursor.of(new Factory(iterable));
    }

    private static class Factory
            implements ValueFunctionFactory>
    {
        private final Iterable iterable;

        private Factory(Iterable iterable)
        {
            this.iterable = iterable;
        }

        @Override
        public Function createFunction()
        {
            return new Function(iterable.iterator());
        }
    }

    private static class Function
            implements ValueFunction
    {
        private final Iterator iterator;

        private Function(Iterator iterator)
        {
            this.iterator = iterator;
        }

        @Override
        public Holder nextValue()
        {
            return iterator.hasNext() ? Holders.of(iterator.next()) : Holders.of();
        }
    }
}