Tony Finch – blog

So I needed some Lua functions to extract bit fields from an integral value, specifically struct stat.st_mode. Lua only has floating point numbers, and its standard library doesn't extend beyond ANSI C. So I wrote the following. (Note that a % b == a - floor(a/b)*b.)

   function mask(lobit, hibit, num)
      local toolo = num % 2^lobit
      return (num - toolo) % 2^hibit
   end

   function bit(bit, num)
      return num / 2^bit % 2 >= 1
   end