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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Updated: (DERBY-4752) CheapDateFormatter returns incorrect and invalid date strings
Date Thu, 22 Jul 2010 15:36:50 GMT

     [ https://issues.apache.org/jira/browse/DERBY-4752?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Knut Anders Hatlen updated DERBY-4752:
--------------------------------------

    Attachment: CheapFormatterPerfTest.java

Thanks for providing the historical background for the CheapDateFormatter class, Kathey.

I wrote a small performance test (see the attached CheapFormatterPerfTest.java) to see the
impact of the fix. On my laptop, running Java 6u21 with default options, the old implementation
managed to format about 280000 timestamps per second. The new implementation only manages
about 190000 timestamps per second, which is slower, but I cannot imagine that it would be
noticed in any realistic scenario.

> CheapDateFormatter returns incorrect and invalid date strings
> -------------------------------------------------------------
>
>                 Key: DERBY-4752
>                 URL: https://issues.apache.org/jira/browse/DERBY-4752
>             Project: Derby
>          Issue Type: Bug
>          Components: Services
>    Affects Versions: 10.7.0.0
>            Reporter: Knut Anders Hatlen
>            Assignee: Knut Anders Hatlen
>            Priority: Minor
>         Attachments: CheapFormatterPerfTest.java, derby-4752-1a.diff, derby-4752-1b.diff,
derby-4752-1c.diff
>
>
> CheapDateFormatter has multiple problems. These are the ones I'm aware of:
> 1) On the boundary between non-leap years and leap years it will return first day of
thirteenth month in previous year (for instance, 2011-13-01 instead of 2012-01-01)
> 2) It treats all years divisible by four as leap years. Those divisible by 100 and not
by 400 are not leap years. It attempts to adjust for that (see the snippet below) but it always
ends up setting leapYear=true if (year%4)==0.
> 		// It's a leap year if divisible by 4, unless divisible by 100,
> 		// unless divisible by 400.
> 		if ((year % 4L) == 0) {
> 			if ((year % 100L) == 0) {
> 				if ((year % 400L) == 0) {
> 					leapYear = true;
> 				}
> 			}
> 			leapYear = true;
> 		}
> 3) More leap year trouble. To find out which year it is, it calculates the number of
four year periods that have elapsed since 1970-01-01. A four year period is considered 365*3+366
days. Although most four year periods are of that length, some are shorter, so we'll get one
day off starting from year 2100, two days off from year 2200, and so on.

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