floaty fiddling

[fanf]

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

Comments

[bellinghman.livejournal.com]

On a high theoretical level, all additions are subtractions, and all divisions are multiplications. In the latter case, finding the multiplicand is an exercise for the reader.

(Though it is an easy optimisation if it's a compile time known value. Or loop-invariant.)