Trying to write the smaller possible UTF-8 encoder for Lua. The code relies on bit32 library available from Lua 5.2 and backported to Lua 5.1

utf8_encode.lua

-- Encode the input number value into a UTF-8 string in Lua 5.2
function utf8(c)
  local t,C={},string.char
  if c<128 then return C(c) end
  for s=1,3 do
    if c<2^(5*s+6) then
      for i=s*6,0,-6 do
        t[#t+1]=128+bit32.extract(c,i,6)
      end
      t[1]=t[1]+({64,96,112})[s]
      return C(table.unpack(t))
    end
  end
end