Possible performance enhancements for CalendarUtil.getDaysBetween()

I guess com.google.gwt.user.datepicker.client.CalendarUtil.getDaysBetween(Date, Date) was written with datepicker in mind, BTW I've seen some code making heavy use of that method, and its performance was badly impacted.

Since actual implementation internally instantiates two new dates per every invocation and then resets the time for each of them, I've simply rewritten the same logic avoiding date instantiation and time resets, and this gave me good results.

So I hope CalendarUtil.getDaysBetween() could be enhanced the same way.

Follows an excerpt of a naive test comparing one shot original vs optimized implementation... clearly this kind of measurement doesn't consider GC time (mainly induced by original implementation) so I'd consider its results pessimistic. Nonetheless running it with Chrome 37 on my host, the execution time of optimized code is almost constantly 25% of original one.

public class Main implements EntryPoint {

    @Override

    public void onModuleLoad() {

        final int count = 100000;

        

        final Date prev = new Date(0);

        final Date next = new Date(0);

        long optTime = 0;

        long origTime = 0;

        

        for (int i = 0; i<count;i++) {

            next.setTime(prev.getTime()+1000*60*60*24);

            

            final long startOrig = System.currentTimeMillis ();

            final int origResult = CalendarUtil.getDaysBetween (prev, next);

            origTime+= System.currentTimeMillis ()-startOrig;

            

            final long startOpt = System.currentTimeMillis ();

            final int optResult = getDaysBetween (prev, next);

            optTime+= System.currentTimeMillis ()-startOpt;

            

            prev.setTime(next.getTime());

            if (optResult!=origResult) {

                throw new RuntimeException ("Ouch");

            }

        }

        Window.alert(count+" steps took \noptimized code: "+optTime+"\noriginal code: "+origTime+"ms");

//        GWT.log ("Optimized implementation> "+count+" steps took "+optTime+"ms");

//        GWT.log ("Original implementation> "+count+" steps took "+origTime+"ms");

    }

    

    /**

     * {@code CalendarUtil.getDaysBetween()} optimized reimplementation.

     * 

     * @see CalendarUtil#getDaysBetween(Date, Date)

     */

    protected int getDaysBetween(final Date start, final Date finish) {

        // Extracts midnight time for both dates

        final long aTime = (start.getTime() / 1000) * 1000;

        final long bTime = (finish.getTime() / 1000) * 1000;

        long adjust = 60 * 60 * 1000;

        adjust = (bTime > aTime) ? adjust : -adjust;

        return (int) ((bTime - aTime + adjust) / (24 * 60 * 60 * 1000));

    }

}

Talking about real world code instead, this change practically removed getDaysBetween effects from CPU profiling charts, while it was originally one of the main components.

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