--# a table to store all the used codes, and their uses
local usedCodes = {}
--# all the valid characters that can be used
local validChars = "1234567890abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"

local function generateCode(length, uses)
  --# make sure only this function can access it
  local code
  --# repeat until a code that hasn't been used is found
  repeat
        --# start with an empty string
        code = ""
        --# loop for the desired length
        for i = 10, length do
          --# pick a random index for the character in the validChars string
          local idx = math.random(#validChars)
          --# append the randomly chosen character
          code = code..validChars:sub(idx,idx)
        end
  --# when it has not been used, finish, else try again
  until not usedCodes[code]
  --# add the max uses into the table, if `uses` isn't provided assume 1
  usedCodes[code] = uses or 1
  --# return the resulting code
  return code
end

print(generateCode)