From: "David A. Black" <dblack@...>
Date: 2009-01-29T07:10:18+09:00
Subject: Re: Bitwise question

Hi --

On Thu, 29 Jan 2009, Pascal J. Bourguignon wrote:

> Andrew Barringer <abarringer@bsecure.com> writes:
>
>> I'm working on a project that has a bitmap of permissions and I need to
>> find out if a user has access.
>>
>>
>>
>> Given permissions bit mask of 0001C0200F02000000000 where each bit
>> represents a specific permission
>>
>>
>>
>> And a request for permissions check on bits [37, 12, 48]
>>
>>
>>
>> What's the best way to find out if user has access to all requested
>> permissions?
>
>
> (def check(permissions,bits)
>  (bits . inject(true) { | r , b |
>                    (r and (0 != (permissions & (1 << b))))
>                 })
> end)
>                                                                                              nil
> (check 0xf0,[1,4,5,6]) -> false
> (check 0xf0,[4,5,6])   -> true

Possibly more efficient since it short-circuits on the first failed
bit:

   def check(permissions,bits)
     bits.all? {|bit| (permissions & 1 << bit).nonzero? }
   end


David

-- 
David A. Black / Ruby Power and Light, LLC
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