// Copyright 2015 The Vanadium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package io.v.v23;

import com.google.common.util.concurrent.AsyncFunction;
import com.google.common.util.concurrent.Futures;
import com.google.common.util.concurrent.ListenableFuture;

import java.util.concurrent.CancellationException;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executor;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;

import io.v.v23.context.VContext;
import io.v.v23.verror.CanceledException;
import io.v.v23.verror.VException;

/**
 * Contains static utility methods that operate on or return objects of type {@link Future}.
 */
public class VFutures {
    /**
     * Waits for the computation embodied by the given {@code future} to complete and then
     * retrieves its result.
     * <p>
     * This method requires that {@code future}'s computation may only throw a {@link VException} -
     * any other exception type will result in a {@link RuntimeException}.
     * <p>
     * If the {@code future}'s execution raises an {@link CancellationException}, this method
     * will throw a {@link CanceledException} (which is a {@link VException}).
     * <p>
     * If the {@code future}'s execution raises an {@link InterruptedException}, this
     * method will raise a {@link RuntimeException}.
     *
     * @param future the future being executed
     * @return the computed result of the future
     * @throws VException if the computation embodied by the future threw a {@link VException}
     * @throws CanceledException if the future's execution raised an {@link CancellationException}
     * @throws RuntimeException if the computation embodied by the future threw an exception
     *                          other than {@link VException}, or if the future's executions raised
     *                          an {@link InterruptedException}
     */
    public static <T> T sync(Future<T> future) throws VException {
        try {
            return future.get();
        } catch (ExecutionException e) {
            if (e.getCause() instanceof VException) {
                throw (VException) e.getCause();
            } else if (e.getCause() instanceof CancellationException) {
                throw new CanceledException(null);
            } else if (e.getCause() instanceof RuntimeException) {
                throw (RuntimeException) e.getCause();
            }
            throw new RuntimeException(
                    "Vanadium futures may only raise a VException or a RuntimeException.",
                    e.getCause());
        } catch (InterruptedException e) {
            throw new RuntimeException("Vanadium future may not raise an InterruptedException.", e);
        } catch (CancellationException e) {
            throw new CanceledException(null);
        }
    }

    /**
     * Waits if necessary for at most the given time for the computation embodied by the given
     * {@code future} to complete and then retrieves its result, if available.
     * <p>
     * This method requires that {@code future}'s computation may only throw a {@link VException} -
     * any other exception type will result in a {@link RuntimeException}.
     * <p>
     * If the {@code future}'s execution raises an {@link CancellationException}, this method
     * will throw a {@link CanceledException} (which is a {@link VException}).
     * <p>
     * If the {@code future}'s execution results in an {@link InterruptedException}, this
     * method will raise a {@link RuntimeException}.
     *
     * @param future the future being executed
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @return the computed result of the future
     * @throws VException if the computation embodied by the future threw a {@link VException}
     * @throws CanceledException if the future's execution raised an {@link CancellationException}
     * @throws RuntimeException if the computation embodied by the future threw an exception
     *                          other than {@link VException}, or if the future's executions raised
     *                          an {@link InterruptedException}
     * @throws TimeoutException if the future didn't complete in the allotted time
     */
    public static <T> T sync(Future<T> future, long timeout, TimeUnit unit)
            throws VException, TimeoutException {
        try {
            return future.get(timeout, unit);
        } catch (ExecutionException e) {
            if (e.getCause() instanceof VException) {
                throw (VException) e.getCause();
            } else if (e.getCause() instanceof CancellationException) {
                throw new CanceledException(null);
            } else if (e.getCause() instanceof RuntimeException) {
                throw (RuntimeException) e.getCause();
            }
            throw new RuntimeException(
                    "Vanadium futures may only raise a VException or a RuntimeException.",
                    e.getCause());
        } catch (InterruptedException e) {
            throw new RuntimeException("Vanadium future may not raise an InterruptedException.", e);
        } catch (CancellationException e) {
            throw new CanceledException(null);
        }
    }

    /**
     * Returns a new {@link ListenableFuture} whose result is the same as the result of the
     * given {@code future} and is executed on an {@link Executor} specified in the given
     * {@code context}.
     * <p>
     * If no executor is specified in the context, the future may be executed on an arbitrary
     * thread.
     */
    public static <T> ListenableFuture<T> onExecutor(
            final VContext context, final ListenableFuture<T> future) {
        Executor executor = V.getExecutor(context);
        if (executor == null) {
            return future;
        }
        return Futures.transform(future, new AsyncFunction<T, T>() {
            @Override
            public ListenableFuture<T> apply(T input) throws Exception {
                return Futures.immediateFuture(input);
            }
        }, executor);
    }

    /**
     * Returns a new {@link ListenableFuture} that runs on an {@link Executor} specified in the
     * given {@code context}. If the future completes but the given {@code context} has been
     * canceled, the returned future is canceled as well.
     */
    public static <T> ListenableFuture<T> withUserLandChecks(
            final VContext context, final ListenableFuture<T> future) {
        Executor executor = V.getExecutor(context);
        if (executor == null) {
            throw new RuntimeException("NULL executor in context: did you derive this context " +
                    "from the context returned by V.init()?");
        }
        return Futures.transform(future, new AsyncFunction<T, T>() {
            @Override
            public ListenableFuture<T> apply(T input) throws Exception {
                if (context.isCanceled()) {
                    return Futures.immediateCancelledFuture();
                }
                return Futures.immediateFuture(input);
            }
        }, executor);
    }
}