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/**
 * Copyright 2014 Netflix, Inc.
 *
 * 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 rx.internal.operators;

import java.util.Queue;
import java.util.concurrent.atomic.AtomicLong;

import rx.*;
import rx.Observable.Operator;
import rx.exceptions.Exceptions;
import rx.functions.*;
import rx.internal.util.atomic.SpscLinkedAtomicQueue;
import rx.internal.util.unsafe.*;

/**
 * Returns an Observable that applies a function to the first item emitted by a source Observable, then feeds
 * the result of that function along with the second item emitted by an Observable into the same function, and
 * so on until all items have been emitted by the source Observable, emitting the result of each of these
 * iterations.
 * 
 * 
 * 

 * This sort of function is sometimes called an accumulator.
 * 

 * Note that when you pass a seed to {@code scan} the resulting Observable will emit that seed as its
 * first emitted item.
 *
 * @param  the aggregate and output type
 * @param  the input value type
 */
public final class OperatorScan implements Operator {

    private final Func0 initialValueFactory;
    final Func2 accumulator;
    // sentinel if we don't receive an initial value
    private static final Object NO_INITIAL_VALUE = new Object();

    /**
     * Applies an accumulator function over an observable sequence and returns each intermediate result with the
     * specified source and accumulator.
     *
     * @param initialValue
     *            the initial (seed) accumulator value
     * @param accumulator
     *            an accumulator function to be invoked on each element from the sequence
     * @see Observable.Scan(TSource, TAccumulate) Method (IObservable(TSource), TAccumulate, Func(TAccumulate, TSource,
     *      TAccumulate))
     */
    public OperatorScan(final R initialValue, Func2 accumulator) {
        this(new Func0() {

            @Override
            public R call() {
                return initialValue;
            }

        }, accumulator);
    }

    public OperatorScan(Func0 initialValueFactory, Func2 accumulator) {
        this.initialValueFactory = initialValueFactory;
        this.accumulator = accumulator;
    }

    /**
     * Applies an accumulator function over an observable sequence and returns each intermediate result with the
     * specified source and accumulator.
     *
     * @param accumulator
     *            an accumulator function to be invoked on each element from the sequence
     * @see Observable.Scan(TSource) Method (IObservable(TSource), Func(TSource, TSource, TSource))
     */
    @SuppressWarnings("unchecked")
    public OperatorScan(final Func2 accumulator) {
        this((R) NO_INITIAL_VALUE, accumulator);
    }

    @Override
    public Subscriber call(final Subscriber child) {
        final R initialValue = initialValueFactory.call();

        if (initialValue == NO_INITIAL_VALUE) {
            return new Subscriber(child) {
                boolean once;
                R value;
                @SuppressWarnings("unchecked")
                @Override
                public void onNext(T t) {
                    R v;
                    if (!once) {
                        once = true;
                        v = (R)t;
                    } else {
                        v = value;
                        try {
                            v = accumulator.call(v, t);
                        } catch (Throwable e) {
                            Exceptions.throwOrReport(e, child, t);
                            return;
                        }
                    }
                    value = v;
                    child.onNext(v);
                }
                @Override
                public void onError(Throwable e) {
                    child.onError(e);
                }
                @Override
                public void onCompleted() {
                    child.onCompleted();
                }
            };
        }

        final InitialProducer ip = new InitialProducer(initialValue, child);

        Subscriber parent = new Subscriber() {
            private R value = initialValue;

            @Override
            public void onNext(T currentValue) {
                R v = value;
                try {
                    v = accumulator.call(v, currentValue);
                } catch (Throwable e) {
                    Exceptions.throwOrReport(e, this, currentValue);
                    return;
                }
                value = v;
                ip.onNext(v);
            }

            @Override
            public void onError(Throwable e) {
                ip.onError(e);
            }

            @Override
            public void onCompleted() {
                ip.onCompleted();
            }

            @Override
            public void setProducer(final Producer producer) {
                ip.setProducer(producer);
            }
        };

        child.add(parent);
        child.setProducer(ip);
        return parent;
    }

    static final class InitialProducer implements Producer, Observer {
        final Subscriber child;
        final Queue