-- Returns HEX representation of num
-- minlen is optional 
function num2hex(num, minlen)
  minlen = minlen or 1
  local hexstr = '0123456789abcdef'
  local s = ''
  while num > 0 or #s < minlen do
    local mod = math.fmod(num, 16)
    s = string.sub(hexstr, mod+1, mod+1) .. s
    num = math.floor(num / 16)
  end
  return s
end