HMCL/HMCLAPI/src/main/java/rx/operators/OperatorGroupBy.java

/**
 * Copyright 2013 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.operators;

import java.util.concurrent.ConcurrentHashMap;

import rx.Observable;
import rx.Observer;
import rx.Subscription;
import rx.observables.GroupedObservable;
import rx.util.functions.Func1;
import rx.util.functions.Functions;

public final class OperatorGroupBy {

    public static <K, T, R> Func1<Observer<GroupedObservable<K, R>>, Subscription> groupBy(Observable<T> source, final Func1<T, K> keySelector, final Func1<T, R> elementSelector) {

        final Observable<KeyValue<K, R>> keyval = source.map(new Func1<T, KeyValue<K, R>>() {
            @Override
            public KeyValue<K, R> call(T t) {
                K key = keySelector.call(t);
                R value = elementSelector.call(t);

                return new KeyValue<K, R>(key, value);
            }
        });

        return new GroupBy<K, R>(keyval);
    }

    public static <K, T> Func1<Observer<GroupedObservable<K, T>>, Subscription> groupBy(Observable<T> source, final Func1<T, K> keySelector) {
        return groupBy(source, keySelector, Functions.<T> identity());
    }

    private static class GroupBy<K, V> implements Func1<Observer<GroupedObservable<K, V>>, Subscription> {
        private final Observable<KeyValue<K, V>> source;
        private final ConcurrentHashMap<K, Boolean> keys = new ConcurrentHashMap<K, Boolean>();

        private GroupBy(Observable<KeyValue<K, V>> source) {
            this.source = source;
        }

        @Override
        public Subscription call(final Observer<GroupedObservable<K, V>> observer) {

            return source.subscribe(new Observer<KeyValue<K, V>>() {

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

                @Override
                public void onError(Exception e) {
                    observer.onError(e);
                }

                @Override
                public void onNext(final KeyValue<K, V> args) {
                    K key = args.key;
                    boolean newGroup = keys.putIfAbsent(key, true) == null;
                    if (newGroup) {
                        observer.onNext(buildObservableFor(source, key));
                    }
                }

            });
        }
    }

    private static <K, R> GroupedObservable<K, R> buildObservableFor(Observable<KeyValue<K, R>> source, final K key) {
        final Observable<R> observable = source.filter(new Func1<KeyValue<K, R>, Boolean>() {
            @Override
            public Boolean call(KeyValue<K, R> pair) {
                return key.equals(pair.key);
            }
        }).map(new Func1<KeyValue<K, R>, R>() {
            @Override
            public R call(KeyValue<K, R> pair) {
                return pair.value;
            }
        });
        return new GroupedObservable<K, R>(key, new Func1<Observer<R>, Subscription>() {

            @Override
            public Subscription call(Observer<R> observer) {
                return observable.subscribe(observer);
            }

        });
    }

    private static class KeyValue<K, V> {
        private final K key;
        private final V value;

        private KeyValue(K key, V value) {
            this.key = key;
            this.value = value;
        }
    }

}