Messages in this thread | | | Date | Thu, 07 Feb 2013 10:15:57 +0900 | From | jonghwa3.lee@samsung ... | Subject | Re: [PATCH] rtc: Modify leap year test for more simpler way |
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On 2013년 02월 07일 03:11, Stephen Warren wrote: > On 02/06/2013 06:00 AM, Haojian Zhuang wrote: >> On Wed, Feb 6, 2013 at 8:43 PM, <jonghwa3.lee@samsung.com> wrote: >>> On 2013년 02월 06일 20:42, Venu Byravarasu wrote: >>>> By definition, leap year is one, which is a divisible by 4 & 400, excluding multiples of 100s. >>>> Hence I feel this patch is not correct. >>> >>> No, I think you might misunderstood the it's meaning. The former code checks >>> whether if it is multiple of 4 or not. Formal mathematical way to verify multiple of 4 >>> is just checks the last two digits are multiple of 4. This '(!year % 4) && (year % 100)' >>> part does it. But with only that checking, it may miss the case of multiple of 400 which >>> is also multiple of 4. Then my modification checks in hexadecimal, whether if number >>> has any of 1st and 2nd bit with value 1. Because any number which has all bits above >>> the 3rd can be divided with 4(2^2). >>> (e.g. 44(0b101100) = 2^5+2^3+2^2 = 2^2(2^3 + 2 + 1)) >>> So It does same things with less instructions. >> >> I still can't understand your logic. >> >> Please check whether 200 year is leap year. >> >> 200(decimal) = 2b11001000 >> >> !(200 & 0x3) = 1 (Your condition said that 200 year is a leap year.) >> >> According to this logic in below. >> if year mod 4 = 0 and year mod 100 <> 0 or year mod 400 = 0, then >> it's a leap year. >> >> This tells us that 200 year isn't a leap year because 200 mod 100 == >> 0. So who is wrong? > > The rule is: it's a leap year if divisible by 4, unless it's divisible > by 100, but actually also including years divisible by 400. So, the > current code is correct, and the patch is wrong. >
Sorry, I was wrong. I didn't know the definition of leap year not including multiple of 4. Sorry for making noise.
Thanks, Jonghwa
> http://en.wikipedia.org/wiki/Leap_year#Algorithm > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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