Implemented the "Operator: Min and MinBy" and "Operator: Max and MaxBy"

rxjava-core/src/main/java/rx/Observable.java

@@ -19,6 +19,7 @@
 
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Comparator;
 import java.util.List;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.Future;
@@ -54,6 +55,7 @@
 import rx.operators.OperationMaterialize;
 import rx.operators.OperationMerge;
 import rx.operators.OperationMergeDelayError;
+import rx.operators.OperationMinMax;
 import rx.operators.OperationMulticast;
 import rx.operators.OperationObserveOn;
 import rx.operators.OperationOnErrorResumeNextViaFunction;
@@ -3631,6 +3633,126 @@ public static Observable<Double> averageDoubles(Observable<Double> source) {
         return OperationAverage.averageDoubles(source);
     }
 
+    /**
+     * Returns the minimum element in an observable sequence.
+     * If there are more than one minimum elements, returns the last one.
+     * For an empty source, it causes an {@link IllegalArgumentException}.
+     *
+     * @param source
+     *            an observable sequence to determine the minimum element of.
+     * @return an observable emitting the minimum element.
+     * @throws IllegalArgumentException
+     *            if the source is empty
+     * @see <a href="http://msdn.microsoft.com/en-us/library/hh229715(v=vs.103).aspx">MSDN: Observable.Min</a>
+     */
+    public static <T extends Comparable<T>> Observable<T> min(Observable<T> source) {
+        return OperationMinMax.min(source);
+    }
+
+    /**
+     * Returns the minimum element in an observable sequence according to the specified comparator.
+     * If there are more than one minimum elements, returns the last one.
+     * For an empty source, it causes an {@link IllegalArgumentException}.
+     *
+     * @param comparator
+     *            the comparer used to compare elements.
+     * @return an observable emitting the minimum value according to the specified comparator.
+     * @throws IllegalArgumentException
+     *            if the source is empty
+     * @see <a href="http://msdn.microsoft.com/en-us/library/hh229095(v=vs.103).aspx">MSDN: Observable.Min</a>
+     */
+    public Observable<T> min(Comparator<T> comparator) {
+        return OperationMinMax.min(this, comparator);
+    }
+
+    /**
+     * Returns the elements in an observable sequence with the minimum key value.
+     * For an empty source, it returns an observable emitting an empty List.
+     *
+     * @param selector
+     *            the key selector function.
+     * @return an observable emitting a List of the elements with the minimum key value.
+     * @see <a href="http://msdn.microsoft.com/en-us/library/hh228970(v=vs.103).aspx">MSDN: Observable.MinBy</a>
+     */
+    public <R extends Comparable<R>> Observable<List<T>> minBy(Func1<T, R> selector) {
+        return OperationMinMax.minBy(this, selector);
+    }
+
+    /**
+     * Returns the elements in an observable sequence with the minimum key value  according to the specified comparator.
+     * For an empty source, it returns an observable emitting an empty List.
+     *
+     * @param selector
+     *            the key selector function.
+     * @param comparator
+     *            the comparator used to compare key values.
+     * @return an observable emitting a List of the elements with the minimum key value  according to the specified comparator.
+     * @see <a href="http://msdn.microsoft.com/en-us/library/hh228970(v=vs.103).aspx">MSDN: Observable.MinBy</a>
+     */
+    public <R> Observable<List<T>> minBy(Func1<T, R> selector, Comparator<R> comparator) {
+        return OperationMinMax.minBy(this, selector, comparator);
+    }
+
+    /**
+     * Returns the maximum element in an observable sequence.
+     * If there are more than one maximum elements, returns the last one.
+     * For an empty source, it causes an {@link IllegalArgumentException}.
+     *
+     * @param source
+     *            an observable sequence to determine the maximum element of.
+     * @return an observable emitting the maximum element.
+     * @throws IllegalArgumentException
+     *            if the source is empty.
+     * @see <a href="http://msdn.microsoft.com/en-us/library/hh211837(v=vs.103).aspx">MSDN: Observable.Max</a>
+     */
+    public static <T extends Comparable<T>> Observable<T> max(Observable<T> source) {
+        return OperationMinMax.max(source);
+    }
+
+    /**
+     * Returns the maximum element in an observable sequence according to the specified comparator.
+     * If there are more than one maximum elements, returns the last one.
+     * For an empty source, it causes an {@link IllegalArgumentException}.
+     *
+     * @param comparator
+     *            the comparer used to compare elements.
+     * @return an observable emitting the maximum value according to the specified comparator.
+     * @throws IllegalArgumentException
+     *            if the source is empty.
+     * @see <a href="http://msdn.microsoft.com/en-us/library/hh211635(v=vs.103).aspx">MSDN: Observable.Max</a>
+     */
+    public Observable<T> max(Comparator<T> comparator) {
+        return OperationMinMax.max(this, comparator);
+    }
+
+    /**
+     * Returns the elements in an observable sequence with the maximum key value.
+     * For an empty source, it returns an observable emitting an empty List.
+     *
+     * @param selector
+     *            the key selector function.
+     * @return an observable emitting a List of the elements with the maximum key value.
+     * @see <a href="http://msdn.microsoft.com/en-us/library/hh229058(v=vs.103).aspx">MSDN: Observable.MaxBy</a>
+     */
+    public <R extends Comparable<R>> Observable<List<T>> maxBy(Func1<T, R> selector) {
+        return OperationMinMax.maxBy(this, selector);
+    }
+
+    /**
+     * Returns the elements in an observable sequence with the maximum key value  according to the specified comparator.
+     * For an empty source, it returns an observable emitting an empty List.
+     *
+     * @param selector
+     *            the key selector function.
+     * @param comparator
+     *            the comparator used to compare key values.
+     * @return an observable emitting a List of the elements with the maximum key value  according to the specified comparator.
+     * @see <a href="http://msdn.microsoft.com/en-us/library/hh244330(v=vs.103).aspx">MSDN: Observable.MaxBy</a>
+     */
+    public <R> Observable<List<T>> maxBy(Func1<T, R> selector, Comparator<R> comparator) {
+        return OperationMinMax.maxBy(this, selector, comparator);
+    }
+
     /**
      * Returns a {@link ConnectableObservable} that shares a single subscription to the underlying
      * Observable that will replay all of its items and notifications to any future {@link Observer}.

rxjava-core/src/main/java/rx/operators/OperationMinMax.java

@@ -0,0 +1,147 @@
+/**
+ * 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.ArrayList;
+import java.util.Comparator;
+import java.util.List;
+
+import rx.Observable;
+import rx.util.functions.Func1;
+import rx.util.functions.Func2;
+
+/**
+ * Returns the minimum element in an observable sequence.
+ */
+public class OperationMinMax {
+
+    public static <T extends Comparable<T>> Observable<T> min(
+            Observable<T> source) {
+        return minMax(source, -1L);
+    }
+
+    public static <T> Observable<T> min(Observable<T> source,
+            final Comparator<T> comparator) {
+        return minMax(source, comparator, -1L);
+    }
+
+    public static <T, R extends Comparable<R>> Observable<List<T>> minBy(
+            Observable<T> source, final Func1<T, R> selector) {
+        return minMaxBy(source, selector, -1L);
+    }
+
+    public static <T, R> Observable<List<T>> minBy(Observable<T> source,
+            final Func1<T, R> selector, final Comparator<R> comparator) {
+        return minMaxBy(source, selector, comparator, -1L);
+    }
+
+    public static <T extends Comparable<T>> Observable<T> max(
+            Observable<T> source) {
+        return minMax(source, 1L);
+    }
+
+    public static <T> Observable<T> max(Observable<T> source,
+            final Comparator<T> comparator) {
+        return minMax(source, comparator, 1L);
+    }
+
+    public static <T, R extends Comparable<R>> Observable<List<T>> maxBy(
+            Observable<T> source, final Func1<T, R> selector) {
+        return minMaxBy(source, selector, 1L);
+    }
+
+    public static <T, R> Observable<List<T>> maxBy(Observable<T> source,
+            final Func1<T, R> selector, final Comparator<R> comparator) {
+        return minMaxBy(source, selector, comparator, 1L);
+    }
+
+    private static <T extends Comparable<T>> Observable<T> minMax(
+            Observable<T> source, final long flag) {
+        return source.reduce(new Func2<T, T, T>() {
+            @Override
+            public T call(T acc, T value) {
+                if (flag * acc.compareTo(value) > 0) {
+                    return acc;
+                }
+                return value;
+            }
+        });
+    }
+
+    private static <T> Observable<T> minMax(Observable<T> source,
+            final Comparator<T> comparator, final long flag) {
+        return source.reduce(new Func2<T, T, T>() {
+            @Override
+            public T call(T acc, T value) {
+                if (flag * comparator.compare(acc, value) > 0) {
+                    return acc;
+                }
+                return value;
+            }
+        });
+    }
+
+    private static <T, R extends Comparable<R>> Observable<List<T>> minMaxBy(
+            Observable<T> source, final Func1<T, R> selector, final long flag) {
+        return source.reduce(new ArrayList<T>(),
+                new Func2<List<T>, T, List<T>>() {
+
+                    @Override
+                    public List<T> call(List<T> acc, T value) {
+                        if (acc.isEmpty()) {
+                            acc.add(value);
+                        } else {
+                            int compareResult = selector.call(acc.get(0))
+                                    .compareTo(selector.call(value));
+                            if (compareResult == 0) {
+                                acc.add(value);
+                            } else if (flag * compareResult < 0) {
+                                acc.clear();
+                                acc.add(value);
+                            }
+                        }
+                        return acc;
+                    }
+                });
+    }
+
+    private static <T, R> Observable<List<T>> minMaxBy(Observable<T> source,
+            final Func1<T, R> selector, final Comparator<R> comparator,
+            final long flag) {
+        return source.reduce(new ArrayList<T>(),
+                new Func2<List<T>, T, List<T>>() {
+
+                    @Override
+                    public List<T> call(List<T> acc, T value) {
+                        if (acc.isEmpty()) {
+                            acc.add(value);
+                        } else {
+                            int compareResult = comparator.compare(
+                                    selector.call(acc.get(0)),
+                                    selector.call(value));
+                            if (compareResult == 0) {
+                                acc.add(value);
+                            } else if (flag * compareResult < 0) {
+                                acc.clear();
+                                acc.add(value);
+                            }
+                        }
+                        return acc;
+                    }
+                });
+    }
+
+}