Re: bitwise ops -- make them enums! (Re: what's wrong?)

[Asko Kauppi <askok@...>]

Mike Pall kirjoitti 4.9.2005 kello 16.32:

> Hi,
>
> Asko Kauppi wrote:
>
> > n.b. normally the only place where enums are defined (within LuaX)
> > are the C modules. This comes from the simple reason that any
> > bitfields normally are used only in contact with C code.  In Lua only
> > world, there are other ways.
>
> IMHO writing a C function for every bitfield I want to use
> is not the way to go. Most of the bitfields I have are really
> N bit integers and not enumerations. Of course bitfields are
> a low-level approach, but they _are_ more generic. :-)

Hmm.. perhaps my (re)use of the enum name is misleading here. :)

Let's see your examples the way I'd have them (all features should  
build on practical samples, that's GOOD..:)

> A simple example:
>
>   local function wputd(n)
>     if n < 0 then
>       n = n + 4294967296
>     end
>     local r = n % 256
>     n = (n - r) / 256
>     wputb(r)
>     r = n % 256
>     n = (n - r) / 256
>     wputb(r)
>     r = n % 256
>     n = (n - r) / 256
>     wputb(r, n)
>   end
>
> Why oh why can't I simply write:
>
>   local function wputd(n)
>     wputb(bitfield(n, 0,8, 8,8, 16,8, 24,8))
>   end

You're right; your use seems really to involve bitops on plain  
integers.  For C API usage cases, that is not the issue: there the  
bitfields have both value and "type".

Let's have a go still:

Considering 'n' would be an "enum" with masks set up appropriately,  
the above would become:

local function wputd( n )
    wputb( n(MASK_000000FF) )
    wputb( n(MASK_0000FF00) )
    wputb( n(MASK_00FF0000) )
    wputb( n(MASK_FF000000) )
end

Call is overriden to provide the binary and (scaled to bit 0) of the  
value itself, and the mask.  I consider this easy to read.

-ak

> I spare you the code for some of the more uglier examples
> like splitting the ModRM and SIB bytes of an x86 opcode. :-)
>
> And I really don't want to write things like this anymore:
>
>   local opcode = op == "movzx" and 4023 or 4031
>
> But rather:
>
>   local opcode = op == "movzx" and 0x0fb7 or 0x0fbf

local opcode = op == "movzx" and hex'0fb7' or hex'0fbf'

(as many others have pointed out... but you're right, it's sorta slow)

> Just so this doesn't sound like a rant about a specific
> application: dealing with some network protocols (like DNS
> or TLS) is really, really ugly without hex numbers and
> bitfields. Don't tell me I have to write this in C ...
>
> Bye,
>      Mike