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

[zsxwing]

Hi, this PR implemented the Operator: Min and MinBy #63 and Operator: Max and MaxBy #61. Every operator has 2 variants, one for Comparable, another for Comparator. Please take a look. Thanks!

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RxJava-pull-requests #404 SUCCESS
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[samuelgruetter]

OperationMin and OperationMax are exactly the same, except that < and > are swapped. Can't you implement one in terms of the other? Or create a general one which takes +1 or -1 to know whether we want < or >?

[zsxwing]

Is the following solution OK?

    public static <T extends Comparable<T>> Observable<T> min(
            Observable<T> source) {
        return minMax(source, -1);
    }

    public static <T extends Comparable<T>> Observable<T> max(
            Observable<T> source) {
        return minMax(source, 1);
    }

    public 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;
            }
        });
    }

flag must be long as Integer.MIN_VALUE == -1 * Integer.MIN_VALUE. But we will have one multiplication for every compareTo.

Another solution is:

    public static <T extends Comparable<T>> Observable<T> min(
            Observable<T> source) {
        return minMax(source, true);
    }

    public static <T extends Comparable<T>> Observable<T> max(
            Observable<T> source) {
        return minMax(source, false);
    }

    public static <T extends Comparable<T>> Observable<T> minMax(
            Observable<T> source, final boolean isMin) {
        return source.reduce(new Func2<T, T, T>() {
            @Override
            public T call(T acc, T value) {
                if (isMin) {
                    if (acc.compareTo(value) < 0) {
                        return acc;
                    }
                } else {
                    if (acc.compareTo(value) > 0) {
                        return acc;
                    }
                }
                return value;
            }
        });
    }

@samuelgruetter , do you have other better solution?

[samuelgruetter]

I like both of these two solutions, with a slight preference for the first one ;-)
Nice catch with Integer.MIN_VALUE :-)

A third solution would be to implement max using min and wrapping the given comparator such that it inverts the ordering, but I think the first solution is the best.

Another issue: What if there are several minimal elements? Does min return the first of them, the last of them, or is it unspecified? This should be documented.

[zsxwing]

@samuelgruetter , Thanks for your review. I used the '+1/-1' way to implement it and also updated the document.

[cloudbees-pull-request-builder]

RxJava-pull-requests #407 SUCCESS
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[benjchristensen]

Looks good @zsxwing and thanks @samuelgruetter for the review.

Some nice use of generics in those signatures!