The Boston Diaries

The ongoing saga of a programmer who doesn't live in Boston, nor does he even like Boston, but yet named his weblog/journal “The Boston Diaries.”

Go figure.

Thursday, August 29, 2019

Okay, so I wasn't really done with the profiling

Last week I was surprised to find this bit of Lua code as the hot spot in “Project: Sippy-Cup:”

local h16 = Cmt(HEXDIG^1,function(_,position,capture)
  local n = tonumber(capture,16)
  if n < 65536 then
    return position
  end
end)

Last night I realized that this code was stupid in this context. The code was originally based upon the original IP address parsing module which converted the text into a binary representation of the code. So in that context, converting the text into a number made sense. When I made the text-only version, I did the minimum amount of work required and thus, left in some sub-optimal code.

But here? I'm just looking at text, expecting up to four hex digits. A string of four hex digits will, by definition, always be less than 65,536. And LPeg has a way of specifying “up to four, but no more” of a pattern. It's just:

local h16 = HEXDIG^-4

I made that change to the code, and reprofiled “Project: Sippy-Cup.” It didn't change the results at the C level all that much (the LPEG C function merge() is still third, as I do quite a bit of parsing so that's expected), but the results in Lua are vastly different—it's now the code that verifies the incoming phone numbers that's the hot spot. It doesn't surprise me very much as we do that twice per request (one for the caller, and one for the recipient), and it's not nearly as bad a hot spot as the above code was.

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