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

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Message-ID: <CAN1soZwtchSj_idRCu=6Q5uxY8LfZ+HTOQU2Y72Cspbd49xeuw@mail.gmail.com>
Date:	Wed, 6 Feb 2013 21:00:38 +0800
From:	Haojian Zhuang <haojian.zhuang@...il.com>
To:	jonghwa3.lee@...sung.com
Cc:	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 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?

Regards
Haojian
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