linux-kernel - Re: [PATCH] rtc: Modify leap year test for more simpler way

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Date:	Thu, 07 Feb 2013 10:15:57 +0900
From:	jonghwa3.lee@...sung.com
To:	Stephen Warren <swarren@...dotorg.org>
Cc:	Haojian Zhuang <haojian.zhuang@...il.com>,
	Venu Byravarasu <vbyravarasu@...dia.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"a.zummo@...ertech.it" <a.zummo@...ertech.it>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"rtc-linux@...glegroups.com" <rtc-linux@...glegroups.com>
Subject: Re: [PATCH] rtc: Modify leap year test for more simpler way

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@...sung.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
> 
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