Re: Fun with metatables: Infix bitwise operators
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- Subject: Re: Fun with metatables: Infix bitwise operators
- From: David Manura <dm.lua@...>
- Date: 2011年9月10日 01:07:40 -0400
On Fri, Sep 9, 2011 at 11:47 PM, HyperHacker <hyperhacker@gmail.com> wrote:
> so rather than "1 << n", you have to write "(1) '<<' (n)" [...]
> x = (((h) '&' (0x0F)) '<<' (0x12))
> '|' (((h) '&' (0xF0)) '<<' (0xA))
> '|' (((w) '&' (0x0F)) '<<' (8))
> '|' ((w) '&' (0xF0))
I don't seriously consider doing that type of thing [1] in real code,
though I wonder if LuaJit can work its magic on these approaches. I
would rather do the ordinary:
local O, A, L = bit.bor, bit.band, bit.lshift
x = O( L(A(h, 0x0F), 0x12),
L(A(h, 0xF0), 0xA),
L(A(w, 0x0F), 8),
A(w, 0xF0) )
You can also do this:
debug.setmetatable(0, {__index = require'bit32'})
x = (0):bor(
h:band(0x0F):lshift(0x12),
h:band(0xF0):lshift(0xA),
w:band(0x0F):lshift(8),
w:band(0xF0)
)
Under one of the syntax proposals, we could refine that to
x = (0):['|'](
h:['&'](0x0F):['<<'](0x12),
h:['&'](0xF0):['<<'](0xA),
w:['&'](0x0F):['<<'](8),
w:['&'](0xF0)
)
However, rather than abuse Lua syntax, it's more general to pass a
list of "tokens" to an evaluator:
x = eval(
'(', h, '&', 0x0F, '<<', 0x12, ')', '|',
'(', h, '&', 0xF0, '<<', 0xA, ')', '|',
'(', w, '&', 0x0F, '<<', 8, ')', '|',
'(', w, '&', 0xF0, ')'
)
A code generation technique like in [2] could make that quite efficient:
x = eval[[
((P1 & 0x0F) << 0x12) |
((P1 & 0xF0) << 0xA) |
((P2 & 0x0F) << 8) |
((P2 & 0xF0 )
]](h, w)
[1] http://lua-users.org/wiki/CustomOperators
[2] http://lua-users.org/wiki/ListComprehensions