import java.util.Arrays;
import java.util.List;
import java.util.concurrent.atomic.AtomicReference;
import java.util.function.BiConsumer;
import java.util.stream.IntStream;

public class Pairs {
    public static void main(String[] args) {
        List<Double> list = Arrays.asList(-100.0, -90.0, -2.0, 0.0, 3.0, 50.0);
//        List<Double> list = Arrays.asList(-100.0, -90.0, -2.0, 0.0, 3.0, 50.0);
//        List<Double> list = Arrays.asList(-100.0, -90.0);
//        List<Double> list = Arrays.asList(3.0, 50.0);
        BiConsumer<Double, Double> action = (value, refValue) -> System.out.println(value+", "+refValue);

        {
            System.out.println("readability: good, performance 2xO(n) for array list, 2xO(n^2) for linked list");
            IntStream
                .range(0, list.size())
                .filter(idx -> list.get(idx) < 0)
                .forEach(idx -> action.accept(list.get(idx), getIf(list, idx+1)));
            IntStream
                .range(0, list.size())
                .filter(idx -> list.get(idx) > 0)
                .forEach(idx -> action.accept(list.get(idx), getIf(list, idx-1)));
        }

        {
            System.out.println("readability: fair, performance O(n) for array list, O(n^2) for linked list");
            IntStream
                .range(0, list.size())
                .filter(idx -> list.get(idx) != 0)
                .forEach(idx -> { double v = list.get(idx); action.accept(v, getIf(list, idx + (v < 0 ? 1 : -1)));});
        }
        {
            System.out.println("readability: bad, performance O(n) for all lists");
            AtomicReference<Double> prev = new AtomicReference<>();
            list.stream().forEach(n -> {
                Double p = prev.getAndSet(n);
                if (n > 0) action.accept(n, p);
                else if (p != null && p < 0) action.accept(p, n);
            });
            if (prev.get() != null && prev.get() < 0) action.accept(prev.get(), null);
        }
    }

    private static Double getIf(List<Double> list, int index) {
        return (index < 0 || index >= list.size()) ? null : list.get(index);
    }
}