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Datastructures and utility classes for non-negative integers

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/*
 * Copyright (C) 2017 Tobias Meggendorfer
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see .
 */

package de.tum.in.naturals;

import it.unimi.dsi.fastutil.ints.IntIterator;
import java.util.NoSuchElementException;
import java.util.PrimitiveIterator;
import java.util.function.IntUnaryOperator;
import javax.annotation.Nonnegative;

/**
 * Transforms a given stream of natural numbers by a specified function, ignoring negative results.
 * Semantically, this transformation is equivalent to
 * {@code intStream.map(transformer).filter(i -> i >= 0)}.
 */
public final class NaturalsTransformer implements IntIterator {
    private final PrimitiveIterator.OfInt original;
    private final IntUnaryOperator transformer;
    // A negative value indicates that there are no more elements.
    private int next;

    public NaturalsTransformer(PrimitiveIterator.OfInt original, IntUnaryOperator transformer) {
        this.original = original;
        this.transformer = transformer;
        next = getNext();
    }

    private int getNext() {
        while (original.hasNext()) {
            int nextCandidate = transformer.applyAsInt(original.nextInt());

            // If the transformer returns a negative value, the element is skipped.
            if (nextCandidate >= 0) {
                return nextCandidate;
            }
        }

        return -1;
    }

    @Override
    public boolean hasNext() {
        return next >= 0;
    }

    @Override
    @Nonnegative
    public int nextInt() {
        if (next < 0) {
            throw new NoSuchElementException();
        }

        int value = next;
        next = getNext();
        return value;
    }
}