Strpad function in LUA

strpad.lua

-- strpad(input, pad_length, [pad_string], [pad_type])
-- (php-style) implemented in LUA (inspired from https://gist.github.com/2625581)
-- @FGRibreau - Francois-Guillaume Ribreau
-- @Redsmin - A full-feature client for Redis http://redsmin.com
local function strpad(input, pad_length, pad_string, pad_type)
  local output = input

  if not pad_string then pad_string = ' ' end
  if not pad_type   then pad_type   = 'STR_PAD_RIGHT' end

  if pad_type == 'STR_PAD_BOTH' then
    local j = 0
    while string.len(output) < pad_length do
      output = j % 2 == 0 and output .. pad_string or pad_string .. output
      j = j + 1
    end
  else
    while string.len(output) < pad_length do
      output = pad_type == 'STR_PAD_LEFT' and pad_string .. output or output .. pad_string
    end
  end

  return output
end