linux-kernel - Re: [PATCH] Add "is_power_of

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Date:	Wed, 31 Jan 2007 11:13:40 +0100 (CET)
From:	"Vegard Nossum" <vegard@...tkore.net>
To:	"Jan Engelhardt" <jengelh@...ux01.gwdg.de>
Cc:	"Robert P. J. Day" <rpjday@...dspring.com>,
	"Nick Piggin" <nickpiggin@...oo.com.au>,
	"Linux kernel mailing list" <linux-kernel@...r.kernel.org>,
	"Andrew Morton" <akpm@...l.org>,
	"Paul Mackerras" <paulus@...ba.org>, dhowells@...hat.com,
	galak@...nel.crashing.org
Subject: Re: [PATCH] Add "is_power_of_2" checking to log2.h.

On Tue, January 30, 2007 3:12 pm, Jan Engelhardt wrote:
>
>>> Why the qualifier? Zero *is* not a power of 2, is it?
>
> No, it is not:
>
> 	In[1]:= Solve[2^n == 0, n]
>
> 	Out[1]= {}
>
> So says Mathematica5.

As a side note, I would just like to point out that Mathematica does not
deal with modular arithmetic by default (which programmers very much do).
In fact, in modular arithmetic, zero IS a power of two.

2^n = 0 (mod 2^n)

To see if it holds for bytes, substitute n = 8, and you get 2^8 = 0 (mod
256). In other words: Zero is the eighth power of two modulo 256.

Modular arithmetic is, however, very often a source of errors in
programming (unchecked-for overflows and underflows), and it is
questionable whether the programmer would really want 0 reported as a
power of two.

Vegard

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