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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 javax.annotation.Nonnull;
import javax.annotation.Nullable;
import java.util.NoSuchElementException;
import java.util.function.Function;

public class LazyMultiIterator
    extends AbstractSplitableIterator
{
    @Nonnull
    private final SplitableIterator> source;
    @Nullable
    private SplitableIterator iterator;
    private boolean advanced;
    private boolean hasNext;
    private T nextValue;

    private LazyMultiIterator(@Nonnull SplitableIterator> source,
                              @Nullable SplitableIterator iterator,
                              boolean advanced,
                              boolean hasNext,
                              T nextValue)
    {
        this.source = source;
        this.iterator = iterator;
        this.advanced = advanced;
        this.hasNext = hasNext;
        this.nextValue = nextValue;
    }

    @Nonnull
    public static  LazyMultiIterator iterator(@Nonnull Indexed> source)
    {
        return iterator(IndexedIterator.iterator(source));
    }

    @Nonnull
    public static  LazyMultiIterator iterator(@Nonnull SplitableIterator> source)
    {
        return new LazyMultiIterator<>(source, null, false, false, null);
    }

    /**
     * Constructs an iterator that visits all of the values reachable from all of the
     * SplitableIterables visited by source.  All values are transformed using the provided method.
     */
    public static  LazyMultiIterator transformed(SplitableIterator source,
                                                          Function> transforminator)
    {
        return iterator(TransformIterator.of(source, transforminator));
    }

    /**
     * Constructs an iterator that visits all of the values reachable from all of the
     * SplitableIterables visited by source.  All values are transformed using the provided method.
     */
    public static  LazyMultiIterator transformed(Indexed source,
                                                          Function> transforminator)
    {
        return iterator(TransformIterator.of(IndexedIterator.iterator(source), transforminator));
    }

    @Override
    public boolean hasNext()
    {
        advance();
        return hasNext;
    }

    @Override
    public T next()
    {
        advance();
        if (!hasNext) {
            throw new NoSuchElementException();
        }
        advanced = false;
        return nextValue;
    }

    @Override
    public boolean isSplitAllowed()
    {
        return source.isSplitAllowed();
    }

    @Nonnull
    @Override
    public SplitIterator splitIterator()
    {
        final SplitIterator> split = source.splitIterator();
        return new SplitIterator<>(new LazyMultiIterator<>(split.getLeft(), iterator, advanced, hasNext, nextValue),
                                   new LazyMultiIterator<>(split.getRight(), null, false, false, null));
    }

    private void advance()
    {
        if (!advanced) {
            advanceImpl();
            advanced = true;
        }
    }

    private void advanceImpl()
    {
        if (iterator != null) {
            if (iterator.hasNext()) {
                hasNext = true;
                nextValue = iterator.next();
                return;
            }
        }

        assert (iterator == null) || !iterator.hasNext();

        while (source.hasNext()) {
            final SplitableIterable nextIterable = source.next();
            final SplitableIterator nextIterator = nextIterable.iterator();
            if (nextIterator.hasNext()) {
                iterator = nextIterator;
                hasNext = true;
                nextValue = iterator.next();
                return;
            }
        }

        hasNext = false;
    }
}