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

[pjc50.livejournal.com]

http://portal.acm.org/citation.cfm?id=806321&dl= , possibly?

I remember his approach to fast multiplication (FFT, convolve, FFT); I'd guess that he had some fast method for computing 1/x, which would make a/b as fast as a*(1/b).

[happydisciple.livejournal.com]

[...] his approach to fast multiplication (FFT, convolve, FFT); [...]

Wuh?

Whenever I need to do a convolution or, more likely, a correlation/autocorrelation, I tend to go FFT, multiply, FFT. In what case/by what algorithm is a direct convolution faster than multiplication?