Re: Fun with metatables: Infix bitwise operators

[Dirk Laurie <dpl@...>]

On Sat, Sep 10, 2011 at 05:47:07AM +0200, HyperHacker wrote:
> Recently I was porting to Lua some horribly messy decompiled code like:
> x = (((((Param[1] & 0xF) << 0x12) | (((Param[1] & 0xF0) >> 4) << 0xE))
> | ((Param[0] & 0xF) << 8)) | (((Param[0] & 0xF0) >> 4) << 4))
> 
> I thought, trying to convert such a mess to prefix (bit.band(a,b)
> rather than a & b) would be a nightmare, and it sure would be nice if
> I had infix bitwise operators instead. 
...
> debug.setmetatable(0, {
...
> As an example, that messy fragment above (after stripping some
> redundant shifts) looks like:
> x = (((h) '&' (0x0F)) '<<' (0x12))
>   '|' (((h) '&' (0xF0)) '<<' (0xA))
>   '|' (((w) '&' (0x0F)) '<<' (8))
>   '|' ((w) '&' (0xF0))

If you're not married to C operator notation, another way is to override
existing metamethods for +, *, indexing etc.  Then your messy fragment 
becomes (with the aid of the file below):

    bits = require "bit32ops"
    Param = {0x86,0xE7}
    w, h = bits(Param[0]), bits(Param[1])   
    n0, n1 = 0xF, 0xF0
    x = (h*n0)^-18 + (h*n1)^-10 + (w*n0)^-8 + w*n1
    print(x)   --> 0x1F8680

Dirk

~~~~ file bit32ops.lua
--[[
bits.lua  (c) Dirk Laurie 2011
Released under the LHF public-domain license:
<http://www.tecgraf.puc-rio.br/~lhf/ftp/lua/install.html#license>

Infix operations on 32-bit values

Method: the 32-bit value is the entry 'value' of a table.

The following metamethods are available for use in expressions, with 
precedences of the corresponding operators from top (highest) to bottom:

index call
pow
len unm
mul div mod
add sub
concat
eq ne lt le gt ge

The following bit32 operations are available, which can be mapped as 
follows:

bnot                <==>    unm
band                <==>    mul
btest               <==>    mod
bor                 <==>    add
bxor                <==>    sub
rshift              <==>    pow
extract, replace    <==>    call
rrotate	            <==>    index
arshift	            <==>    concat

That leaves len, to convert back from `bits` to `number`.

lshift and lrotate can of course be done by rshift and rrotate.
The choice that right is positive, rather than left, is dictated
by the fact that arithmetic shift leaves no choice. A further emphasis 
of the uniqueness of arshift is that its precedence is anomalous.

If the second argument of a function is a 32-bit value, it may 
given as either be a table having a key 'value' or a number.  

Restrictions: 
1. many-argument use of band, btest, bor, bxor is not supported
2. 'replace' must have three arguments, value, field, width, 
   no default allowed.
   However, 'extract' may have only one argument, field: width=1 is supplied.

--]]

local val = function(b32)
    if type(b32)=='table' then return b32.value
    else return b32
    end end

local bits

bits = {
-- return number
__len = function (b32) return b32.value end;
-- return bits
__unm = function (b32) return bits(bit32.bnot(b32.value)) end;
__mul = function (a32,b32) return bits(bit32.band(a32.value,val(b32))) end;
__add = function (a32,b32) return bits(bit32.bor(a32.value,val(b32))) end;
__sub = function (a32,b32) return bits(bit32.bxor(a32.value,val(b32))) end;
__pow = function (a32,b) return bits(bit32.rshift(a32.value,b)) end;
__index = function (a32,b) return bits(bit32.rrotate(a32.value,b)) end;
__concat = function (a32,b) return bits(bit32.arshift(a32.value,b)) end; 
__call = function (b32,x,y,z) 
    if z==nil then return bits(bit32.extract(b32.value,x,y))
    else return bits(bit32.replace(b32.value,val(x),y,z))
        end end;
-- return string
__tostring = function(b32) return string.format('0x%X',b32.value) end;
-- return boolean
__mod = function (a32,b32) return bit32.btest(a32.value,val(b32)) end;
__eq = function (a32,b32) return a32.value==val(b32) end;
__ne = function (a32,b32) return a32.value~=val(b32) end;
__le = function (a32,b32) return a32.value<=val(b32) end;
__lt = function (a32,b32) return a32.value<val(b32) end;
__ge = function (a32,b32) return a32.value>=val(b32) end;
__gt = function (a32,b32) return a32.value>val(b32) end;
}

setmetatable(bits,{
__call = function(self,v) 
    local b32 = {value=v} 
    setmetatable (b32,self)
    return b32
    end
} )

return bits

--[[ EXAMPLES 
B=require"bit32ops"
x=B(639); print(x,#x,-x,#-x)      -->   0x27F  639  0xFFFFFD80  4294966656
y=B(159); print(y,x*y,x%y,x*159)  -->   0x9F  0x1F  true  0x1F
print(x+y,x-y)                    -->   0x2FF  0x2E0
print(x^-3,x^1,x[1],-x..1)        -->   0x13F8  0x13F  0x8000013F  0xFFFFFEC0 
print(x(0,8))                     -->   0x7F
print(x(0xf,24,4))                -->   0xF00027F
print(-x,(-x)..6,(-x)..6>-x)      -->   0xFFFFFD80  0xFFFFFFF6  true 
--]]
~~~~ end of file