futur4j/futur-api/src/main/java/dev/tommyjs/futur/joiner/PromiseJoiner.java

package dev.tommyjs.futur.joiner;

import dev.tommyjs.futur.promise.CompletablePromise;
import dev.tommyjs.futur.promise.Promise;
import dev.tommyjs.futur.promise.PromiseCompletion;
import dev.tommyjs.futur.promise.PromiseFactory;
import dev.tommyjs.futur.util.PromiseUtil;
import org.jetbrains.annotations.NotNull;

import java.util.Iterator;
import java.util.concurrent.atomic.AtomicInteger;

public abstract class PromiseJoiner<T, Key, Value, Result> {

    private final CompletablePromise<Result> joined;

    protected PromiseJoiner(@NotNull PromiseFactory factory) {
        this.joined = factory.unresolved();
    }

    protected abstract Key getChildKey(T value);

    protected abstract @NotNull Promise<Value> getChildPromise(T value);

    protected abstract void onChildComplete(int index, Key key, @NotNull PromiseCompletion<Value> completion);

    protected abstract Result getResult();

    protected void join(@NotNull Iterator<T> promises) {
        AtomicInteger count = new AtomicInteger();

        int i = 0;
        do {
            if (joined.isCompleted()) {
                promises.forEachRemaining(v -> getChildPromise(v).cancel());
                return;
            }

            T value = promises.next();
            Promise<Value> p = getChildPromise(value);
            if (!p.isCompleted()) {
                PromiseUtil.cancelOnComplete(joined, p);
            }

            count.incrementAndGet();
            Key key = getChildKey(value);
            int index = i++;

            p.addAsyncListener(res -> {
                onChildComplete(index, key, res);
                if (res.isError()) {
                    assert res.getException() != null;
                    joined.completeExceptionally(res.getException());
                } else if (count.decrementAndGet() == -1) {
                    joined.complete(getResult());
                }
            });
        } while (promises.hasNext());

        if (count.decrementAndGet() == -1) {
            joined.complete(getResult());
        }
    }

    public @NotNull Promise<Result> joined() {
        return joined;
    }

}