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- Subject: Re: An operator syntax for bitfields using metatables
- From: Dirk Laurie <dirk.laurie@...>
- Date: Sat, 13 Jul 2013 10:40:05 +0200
2013/7/13 David Burgess <firstname.lastname@example.org>:
> FWIW. I like the syntax.
> On Sat, Jul 13, 2013 at 1:24 AM, John Hind <email@example.com> wrote:
>> I've been following the discussion on the bit32 library and share the desire
>> for an operator rather than a functional syntax. However I realised that 99%
>> of my use of this library is for cracking and assembling bitfields for
>> communications protocols and register mapping in embedded systems. It is
>> straightforward to add a syntax for this in a C library by providing a
>> metatable for the number type with 'index' and 'newindex' metamethods. Then:
>> b = n -- Extract bit 3 as a boolean.
>> n = true -- Set bit 23.
>> n = not n -- Toggle bit 6.
Up to here I like it too.
>> This concept can be extended to extract and replace numeric fields within a
>> larger field (like the bit32 functions with these names). This requires that
>> the index encode two five bit integers which could be done using bitfields
>> within a single number or by using a string index which gets parsed in the
>> bitrange = function(s,e) return ((e * 0xFF) + 1) + s end -- Range testing
>> omitted for clarity
>> nr = n[bitrange(10,15)] -- Mask out bits 10 through 15 and shift right 10.
>> nr = n[bitrange(15,10)] -- Mask out bits 10 through 15, shift and reverse
>> the bit order.
>> nr = n[bitrange(12,12)] -- nr = 1 if bit 12 is set, else 0.
>> b = n[bitrange(12)] -- b = true only if bit 12 is set (as before).
>> n[bitrange(4,6)] = 2 -- replace bits 4 through 6 with 010.
Here I would prefer string-valued indexing.
nr = n["10:15"]
n["4:6"] = 2
String-valued assignment can be handled too.
n["4:6"] = "010"
The available metamethods for numbers have not been exhausted yet.
__len -- number of 1's in n
__call -- iterator over the positions of the 1's in n