LUA FFI byte union struct

test.lua

local ffi = require 'ffi'

local ct = [[
  union {
    struct {
      uint8_t nlow:4;
      uint8_t nhigh:4;
    };
    struct {
      uint8_t b0:1;
      uint8_t b1:1;
      uint8_t b2:1;
      uint8_t b3:1;
      uint8_t b4:1;
      uint8_t b5:1;
      uint8_t b6:1;
      uint8_t b7:1;
    };
    uint8_t value;
  }
]]

local _byte = ffi.metatype(ct, {
  __call = function(obj, p1, p2)
    local lut = {
      [0] = 'b0',
      [1] = 'b1',
      [2] = 'b2',
      [3] = 'b3',
      [4] = 'b4',
      [5] = 'b5',
      [6] = 'b6',
      [7] = 'b7'
    }
    if p2 == nil then
      return obj[lut[p1]]
    else
      local b = 0
      local s = p1 - p2
      for i=p1,p2,-1 do
        b = bit.bor(b, bit.lshift(obj[lut[i]], s))
        s = s - 1
      end
      return b
    end
  end;
})

test = ffi.new(_byte)
test.value = 0xab
assert(test(3, 0) == test.nlow, 'invalid')
assert(test(7, 4) == test.nhigh, 'invalid')