feedback on chunk

[David Rio Deiros <driodeiros@...>]

I was wondering if I could get some feedback on the following chunk of 
lua code. Since I did not get any replies in my previous email I thought
I'd try to be more specific:

-------------------------------------------------------------------------
local function slide_over_read(read, pl)
  local i      = 1
  local ps     = 7 
  local fs     = 3
  local n_hits = 0

  while ps + i <= #read+1 do -- while the window is within the size of the read
    sub_read = read:sub(i, ps+i-1)
    nt_value = sub_read:sub(fs+1, fs+1)
    sub_read = sub_read:sub(1, fs) .. "N" .. sub_read:sub(fs+2)
    print("Trying: " .. sub_read)
    if pl[sub_read] then -- We have a probe with that sequence
      if not pl[sub_read].hits then -- No previous hits
        pl[sub_read].hits = {A=0, C=0, G=0, T=0, N=0}
      end
      pl[sub_read].hits[nt_value] = pl[sub_read].hits[nt_value] + 1
      n_hits = n_hits + 1
    end
    i = i + 1
  end
  return n_hits
end

-- Main
probes = {}
probes["123N567"] = {}
read = "123A56789"
print("hits: " .. slide_over_read(read, probes))
print("--> " .. probes["123N567"].hits["A"])
print("---------------")

probes["345N789"] = {}
read = "12345T789"
print("hits: " .. slide_over_read(read, probes))
print("--> " .. probes["345N789"].hits["T"])
-------------------------------------------------------------------------

My next step is to implement slide_over_read in C, but prior to that I thought you guys
may have some ideas on how to improve it.

That chunk will be called million of times, that's why I want to squeeze as many cpu
cycles as possible.

-drd