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javascriptjavacomparisonchain
Nick Bull
Nick Bull

Reputation: 9866

Guava's ComparisonChain conversion to JavaScript

I'm converting some code to JavaScript from Java, and I've come across this code:

static final Comparator<Point> compareXCoord = new Comparator<Point>() {
    @Override
    public int compare(Point o1, Point o2) {
        return ComparisonChain.start().
                compare(o1.x, o2.x).
                compare(o1.y, o2.y).
                result();
    }
};

static final Comparator<Point> compareYCoord = new Comparator<Point>() {
    @Override
    public int compare(Point o1, Point o2) {
        return ComparisonChain.start().
                compare(o1.y, o2.y).
                compare(o1.x, o2.x).result();
    }
};

...

Arrays.binarySearch(ypoints, Point.make(Double.NEGATIVE_INFINITY, ymin),
            compareYCoord);

Seems fair enough, as it'll return the resultant object of the matching comparison chain.

But what I don't understand is that both methods seem to my naive eyes to do the exact same thing: return if either the x or y properties match. The only difference is the first checks the x property first, i.e., the order of the checks is different.

Also, if that's true, then the Arrays.binarySearch method will return ypoints elements with equal x properties. I don't feel this is what the function is intended to do.

So, my JavaScript translation would be:

function compareXCoord(p1, p2) {
  return (p1.x === p2.x) ? p1 : 
         (p1.y === p2.y) ? p1 : undefined;
}

function compareYCoord(p1, p2) {
  return (p1.y === p2.y) ? p1 : 
         (p1.x === p2.x) ? p1 : undefined;
}

But both of these could be simplified to return (p1.y === p2.y || p1.x === p2.x) ? p1 : undefined;.

I feel I'm certainly misunderstanding how ComparisonChain works. Is the order of the chain important? Bonus points for directions into how to translate this to JavaScript.

Upvotes: 0

Views: 101

Answers (1)

Oliver Charlesworth
Oliver Charlesworth

Reputation: 272647

You need to bear in mind that the Java Comparator return value is three-valued (i.e. higher, lower, or equal). Thus the order in which you compare the coordinates is important - sorting by x first is not the same as sorting by y first.

Upvotes: 1

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