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Library providing immutable/persistent collection classes for Java. While collections are immutable they provide methods for adding and removing values by creating new modified copies of themselves. Each copy shares almost all of its structure with other copies to minimize memory consumption.

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///###////////////////////////////////////////////////////////////////////////
//
// Burton Computer Corporation
// http://www.burton-computer.com
//
// Copyright (c) 2018, Burton Computer Corporation
// All rights reserved.
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package org.javimmutable.collections.cursors;

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

import javax.annotation.Nonnull;

/**
 * Provides standard Cursor objects for iterating over the values of arbitrary functions.
 * The cursors are actually created using a factory object that can produce the functions.
 * The factories ensure that a Cursor is restartable and permits lazy evaluation of
 * the beginning of iteration.  The function will only be created if the starting
 * Cursor's start() or next() method is invoked.
 * 
 * The Cursors are not immutable but are designed to act as though they are.
 * Cursors remember their next value between invocations so that standard look ahead
 * behavior is preserved.
 * 
 * The Cursors are not thread safe.
 */
public abstract class ValueFunctionCursor
{
    /**
     * Produces a non-thread safe Cursor to traverse the values returned by a function.  The function
     * will be created using the factory when the traversal is initiated by calling the next() method.
     * Multiple invocations of the initial Cursor's next() method will produce multiple functions
     * using the factory.
     *
     * @param factory
     * @param 
     * @return
     */
    public static , A extends ValueFunctionFactory> Cursor of(A factory)
    {
        return new Start(factory);
    }

    protected static class Start, A extends ValueFunctionFactory>
            extends AbstractStartCursor
    {
        private final A factory;

        protected Start(A factory)
        {
            this.factory = factory;
        }

        @Nonnull
        @Override
        public Cursor next()
        {
            final F function = createFunction();
            final Holder firstValue = function.nextValue();
            return firstValue.isFilled() ? new Started(function, firstValue.getValue()) : super.next();
        }

        protected F createFunction()
        {
            return factory.createFunction();
        }
    }

    private static class Started>
            extends AbstractStartedCursor
    {
        private final F function;
        private Cursor next;
        private final T value;

        private Started(F function,
                        T value)
        {
            this.function = function;
            this.value = value;
        }

        @Override
        public T getValue()
        {
            return value;
        }

        @Nonnull
        @Override
        public Cursor next()
        {
            if (next == null) {
                final Holder nextValue = function.nextValue();
                next = nextValue.isFilled() ? new Started(function, nextValue.getValue()) : super.next();
            }
            return next;
        }
    }
}