syndicate-java/src/main/java/org/syndicate_lang/actors/Promise.java

package org.syndicate_lang.actors;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.*;
import java.util.function.Consumer;
import java.util.function.Function;

public class Promise<T> implements Future<T> {

    public static final TimeoutException TIMEOUT_WAITING_FOR_PROMISE_RESOLUTION = new TimeoutException("Waiting for promise resolution");

    public enum State {
        PENDING,
        FULFILLED,
        REJECTED
    }

    private volatile State _state = State.PENDING;
    private T _value = null;
    private Throwable _reason = null;
    private List<Consumer<T>> _resolvers = null;
    private List<Consumer<Throwable>> _rejecters = null;

    public Promise() {}

    public static<T> Promise<T> resolved() {
        return resolved(null);
    }

    public static<T> Promise<T> resolved(T v) {
        var p = new Promise<T>();
        p.resolveWith(v);
        return p;
    }

    public static<T> Promise<T> rejected(Throwable e) {
        var p = new Promise<T>();
        p.rejectWith(e);
        return p;
    }

    public State getState() {
        return _state;
    }

    public boolean isPending() {
        return _state == State.PENDING;
    }

    public boolean isFulfilled() {
        return _state == State.FULFILLED;
    }

    public boolean isRejected() {
        return _state == State.REJECTED;
    }

    public T getValue() {
        return _value;
    }

    public Throwable getReason() {
        return _reason;
    }

    public void resolve() {
        this.resolveWith(null);
    }

    public void resolveWith(T t) {
        List<Consumer<T>> worklist = null;
        synchronized (this) {
            if (this.isPending()) {
                this._value = t;
                this._state = State.FULFILLED;
                worklist = _resolvers;
                _resolvers = null;
                _rejecters = null;
            }
        }
        if (worklist != null) {
            for (var callback : worklist) {
                callback.accept(t);
            }
        }
    }

    public void chain(Promise<T> t) {
        if (this == t) {
            throw new IllegalArgumentException("cannot chain promise immediately to itself");
        }
        t.whenFulfilled(this::resolveWith);
        t.whenRejected(this::rejectWith);
    }

    public void rejectWith(Throwable e) {
        List<Consumer<Throwable>> worklist = null;
        synchronized (this) {
            if (this.isPending()) {
                this._reason = e;
                this._state = State.REJECTED;
                worklist = _rejecters;
                _resolvers = null;
                _rejecters = null;
            }
        }
        if (worklist != null) {
            for (var callback : worklist) {
                callback.accept(e);
            }
        }
    }

    public<R> Promise<R> andThen(Actor a, Function<T, R> ok) {
        return this.andThen(a, ok, null);
    }

    public void resolveCalling(ThrowingSupplier<T> f) {
        try {
            this.resolveWith(f.get());
        } catch (Throwable e) {
            this.rejectWith(e);
        }
    }

    public synchronized<R> Promise<R> andThen(Actor a0, final Function<T, R> ok, final Function<Throwable, R> fail) {
        final Actor a = a0 != null ? a0 : new Actor();
        Promise<R> p = new Promise<>();
        this.whenFulfilled((t) -> a.execute(
                () -> p.resolveCalling(() -> ok.apply(t)),
                () -> p.rejectWith(new ActorTerminated(a))));
        this.whenRejected((e) -> a.execute(
                () -> {
                    if (fail == null) {
                        p.rejectWith(e);
                    } else {
                        p.resolveCalling(() -> fail.apply(e));
                    }
                },
                () -> p.rejectWith(new ActorTerminated(a))));
        return p;
    }

    private static<T> T unexpectedTimeout(TimeoutException e) {
        throw new InternalError("await() without delay signalled TimeoutException", e);
    }

    public T await() {
        try {
            return this.await(-1);
        } catch (TimeoutException e) {
            return unexpectedTimeout(e);
        }
    }

    public T await(long delayMilliseconds) throws TimeoutException {
        try {
            return _await(delayMilliseconds, TimeUnit.MILLISECONDS);
        } catch (InterruptedException e) {
            this.rejectWith(e);
            throw new BrokenPromise(this);
        }
    }

    public T _await(long delay, TimeUnit unit) throws TimeoutException, InterruptedException {
        this.whenFulfilled((_t) -> { synchronized (this) { this.notifyAll(); } });
        this.whenRejected((_e) -> { synchronized (this) { this.notifyAll(); } });
        synchronized (this) {
            if (delay == -1) {
                while (this.isPending()) this.wait();
            } else {
                long targetTime = System.currentTimeMillis() + unit.toMillis(delay);
                while (this.isPending()) {
                    long now = System.currentTimeMillis();
                    if (now >= targetTime) throw TIMEOUT_WAITING_FOR_PROMISE_RESOLUTION;
                    this.wait(targetTime - now);
                }
            }
        }
        if (this.isFulfilled()) {
            return this._value;
        } else {
            throw new BrokenPromise(this);
        }
    }

    private synchronized void whenFulfilled(Consumer<T> callback) {
        switch (this._state) {
            case PENDING:
                if (_resolvers == null) _resolvers = new ArrayList<>();
                _resolvers.add(callback);
                break;
            case FULFILLED:
                callback.accept(this._value);
                break;
            case REJECTED:
                break;
        }
    }

    private synchronized void whenRejected(Consumer<Throwable> callback) {
        switch (this._state) {
            case PENDING:
                if (_rejecters == null) _rejecters = new ArrayList<>();
                _rejecters.add(callback);
                break;
            case FULFILLED:
                break;
            case REJECTED:
                callback.accept(this._reason);
                break;
        }
    }

    // Implementation of Future<T>

    @Override
    public boolean cancel(boolean b) {
        rejectWith(new CancellationException());
        return isRejected();
    }

    @Override
    public boolean isCancelled() {
        return isRejected();
    }

    @Override
    public boolean isDone() {
        return !isPending();
    }

    @Override
    public T get() throws InterruptedException, ExecutionException {
        try {
            return _await(-1, TimeUnit.MILLISECONDS);
        } catch (TimeoutException e) {
            return unexpectedTimeout(e);
        } catch (BrokenPromise e) {
            throw new ExecutionException(e.getReason());
        }
    }

    @Override
    public T get(long l, TimeUnit timeUnit) throws InterruptedException, ExecutionException, TimeoutException {
        try {
            return _await(l, timeUnit);
        } catch (BrokenPromise e) {
            throw new ExecutionException(e.getReason());
        }
    }
}