Creating streams of Fibonacci numbers using Gatherers

Fibonacci.java

/*
 * Fibonacci.java
 * Copyright 2025 Rob Spoor
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import java.util.stream.Gatherer;
import java.util.stream.IntStream;
import java.util.stream.Stream;

final class Fibonacci {

    private Fibonacci() {
    }

    public static Stream<Integer> stream() {
        return IntStream.generate(() -> 0)
                .boxed()
                .gather(Gatherer.ofSequential(() -> new int[2], (state, _, downstream) -> {
                    if (state[1] == 0) {
                        state[0] = 0;
                        state[1] = 1;
                        downstream.push(0);
                        downstream.push(1);
                    }
                    int value = state[0] + state[1];
                    state[0] = state[1];
                    state[1] = value;
                    return downstream.push(value);
                }));
    }
}