From: "David A. Black" <dblack@...>
Date: 2009-01-29T11:16:21+09:00
Subject: Re: Bitwise question

Hi --

On Thu, 29 Jan 2009, Rob Biedenharn wrote:

>
> On Jan 28, 2009, at 4:32 PM, 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
>> 
>> -- 
>> __Pascal Bourguignon__
>
> Or a simpler version:
>
> class Integer
> def as_bits
>   ary = []
>   (self.size*8).times {|i| ary.unshift(i) if self[i].nonzero?}
>   ary
> end
>
> def check_permissions(*bits)
>   bits.all? {|bit| self[bit].nonzero? }
> end
> end

Thanks for the reminder about Integer#[], one of those cool Ruby
things that come back and re-surprise me with their coolness
occasionally :-)


David

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