From: Rob Biedenharn <Rob@...>
Date: 2009-01-29T07:07:29+09:00
Subject: Re: Bitwise question


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

irb> 0x0001C0200F02000000000.as_bits
=> [68, 67, 66, 57, 47, 46, 45, 44, 37]
irb> 0x0001C0200F02000000000.check_permissions 37, 12, 48
=> false
irb> 0x0001C0200F02000000000.check_permissions 37, 47, 57
=> true

And written in normal Ruby rather than LispRuby ;-)

-Rob

Rob Biedenharn		http://agileconsultingllc.com
Rob@AgileConsultingLLC.com