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/*
 * Copyright (c) 2021 dzikoysk
 *
 * 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.
 */

package panda.std.reactive;

import java.util.ArrayList;
import java.util.List;
import java.util.Objects;
import java.util.concurrent.CompletableFuture;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Supplier;

/**
 * Observable container for value that could be computed in the future.
 * This container is similar to {@link java.util.concurrent.CompletableFuture}, but without the concurrency layer.
 *
 * @param  type of value to store
 */
public class Completable implements Publisher, VALUE> {

    private VALUE value;
    private boolean ready;
    private List> subscribers = new ArrayList<>(3);

    public Completable() {
        this.value = null;
    }

    public static  Completable completed(VALUE value) {
        return new Completable().complete(value);
    }

    public static  Completable create() {
        return new Completable<>();
    }

    public boolean isReady() {
        return ready;
    }

    public boolean isUnprepared() {
        return !isReady();
    }

    public VALUE get() {
        if (isReady()) {
            return value;
        }

        throw new IllegalStateException("Option has not been completed");
    }

    public  VALUE orThrow(Supplier exception) throws ERROR {
        if (isReady()) {
            return value;
        }

        throw exception.get();
    }

    @Override
    public Completable subscribe(Subscriber subscriber) {
        if (isReady()) {
            subscriber.onComplete(get());
        }
        else {
            subscribers.add(subscriber);
        }

        return this;
    }

    public Completable complete(VALUE value) {
        if (isReady()) {
            return this;
        }

        this.ready = true;
        this.value = Objects.requireNonNull(value);

        for (Subscriber subscriber : subscribers) {
            subscriber.onComplete(value);
        }

        subscribers = null;
        return this;
    }

    public Completable then(Consumer consumer) {
        subscribe(consumer::accept);
        return this;
    }

    public  Completable thenApply(Function map) {
        Completable mappedOption = new Completable<>();
        subscribe(completedValue -> mappedOption.complete(map.apply(completedValue)));
        return mappedOption;
    }

    public  Completable thenCompose(Function> map) {
        Completable mappedOption = new Completable<>();
        subscribe(completedValue -> map.apply(completedValue).then(mappedOption::complete));
        return mappedOption;
    }

    public CompletableFuture toFuture() {
        CompletableFuture future = new CompletableFuture<>();
        then(future::complete);
        return future;
    }

}