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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
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// Copyright (c) 2018, Burton Computer Corporation
// All rights reserved.
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package org.javimmutable.collections.iterators;

import org.javimmutable.collections.Indexed;
import org.javimmutable.collections.SplitIterator;
import org.javimmutable.collections.SplitableIterable;
import org.javimmutable.collections.SplitableIterator;
import org.javimmutable.collections.indexed.IndexedHelper;

import javax.annotation.Nonnull;
import java.util.NoSuchElementException;

public final class IndexedIterator
{
    private IndexedIterator()
    {
    }

    public static  SplitableIterator forward(@Nonnull Indexed values)
    {
        return new Forward<>(values, -1, values.size() - 1);
    }

    public static  SplitableIterable fwd(@Nonnull Indexed values)
    {
        return () -> forward(values);
    }

    public static  SplitableIterator iterator(@Nonnull Indexed values)
    {
        return forward(values);
    }

    public static  SplitableIterator reverse(@Nonnull Indexed values)
    {
        return new Reverse<>(values, values.size(), 0);
    }

    public static  SplitableIterable rev(@Nonnull Indexed values)
    {
        return () -> reverse(values);
    }

    public static SplitableIterator forRange(int low,
                                                      int high)
    {
        return iterator(IndexedHelper.range(low, high));
    }

    private static class Forward
        extends AbstractSplitableIterator
    {
        @Nonnull
        private final Indexed values;
        private final int limit;
        private int index;

        public Forward(@Nonnull Indexed values,
                       int index,
                       int limit)
        {
            this.values = values;
            this.limit = limit;
            this.index = index;
        }

        @Override
        public boolean hasNext()
        {
            return index < limit;
        }

        @Override
        public T next()
        {
            if (index >= limit) {
                throw new NoSuchElementException();
            }
            index += 1;
            return values.get(index);
        }

        @Override
        public boolean isSplitAllowed()
        {
            return (limit - index) > 1;
        }

        @Nonnull
        @Override
        public SplitIterator splitIterator()
        {
            final int splitIndex = index + (limit - index) / 2;
            if (splitIndex == index) {
                throw new UnsupportedOperationException();
            }
            return new SplitIterator<>(new Forward<>(values, index, splitIndex),
                                       new Forward<>(values, splitIndex, limit));
        }
    }

    private static class Reverse
        extends AbstractSplitableIterator
    {
        @Nonnull
        private final Indexed values;
        private final int limit;
        private int index;

        public Reverse(@Nonnull Indexed values,
                       int index,
                       int limit)
        {
            this.values = values;
            this.limit = limit;
            this.index = index;
        }

        @Override
        public boolean hasNext()
        {
            return index > limit;
        }

        @Override
        public T next()
        {
            if (index <= limit) {
                throw new NoSuchElementException();
            }
            index -= 1;
            return values.get(index);
        }

        @Override
        public boolean isSplitAllowed()
        {
            return (index - limit) > 1;
        }

        @Nonnull
        @Override
        public SplitIterator splitIterator()
        {
            final int splitIndex = index - (index - limit) / 2;
            if (splitIndex == index) {
                throw new UnsupportedOperationException();
            }
            return new SplitIterator<>(new Reverse<>(values, index, splitIndex),
                                       new Reverse<>(values, splitIndex, limit));
        }
    }
}