Re: An operator syntax for bitfields using metatables

[David Burgess <dburgess@...>]

FWIW. I like the syntax.

On Sat, Jul 13, 2013 at 1:24 AM, John Hind <john.hind@zen.co.uk> 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[3]        -- Extract bit 3 as a boolean.
> n[23] = true    -- Set bit 23.
> n[6] = not n[6] -- Toggle bit 6.
>
> If you really need to bitwise and two numbers:
>
> for i=0, 31 do nr[i] = n1[i] and n2[i] end
>
> Nowhere near as efficient as:
>
> nr = bit32.band(n1,n2)
>
> But more expressive and intuitive especially when you need to manipulate a
> few bits in relatively complex ways.
>
> 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
> metamethod:
>
> 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.
>
>
>
>



-- 
David Burgess