The contract is based from ERC721, but in order to saving gas fee, we choose to inherit ERC721A.
Basically, contract is close to original ERC721, but we did some changes to fit requirements,
the following are some explanation for the function or user scenario.

• Period Procedure
  We want to divide total NFT supply into many rounds of sale, and each round is called as Period.
  We don't record all numbers of supplement for each period in contract, but only the number of current period supply (_periodTokenSupply) and the sum of numbers of supplement from the first period to the current period (_sumOfPeriodTokenSupply). In this way, we may save some contract storage.
  While starting next period (functionsetPeriodTokenSupply), admin needs to set up the number of next period maximum NFT supply.
  Also, if admin wants to start next period, he needs to make sure the NFT supply of previous period are sold out. Otherwise, he can not start next period.
  Last but not least, _sumOfPeriodTokenSupply should not be greater than total NFT supply (_sumOfLevelTokenSupply).

• Users Buy NFT Procedure
  Only buyers on whitelisting can buy NFT, the detailed method is listed the below function checkTokenAndMint.
  The following will explain about the connection of _sumOfPeriodTokenSupply and selling NFT.
  If the buyer is able to mint NFT, the contract will make sure the _currentIndex plus quantity which will be the id of the NFT to be minted is not greater than _sumOfPeriodTokenSupply.
  If the above situation is invalid, it means the number of minted NFT is greater than _sumOfPeriodTokenSupply. The buyer needs to wait the admin starting the next period. Otherwise, he cannot mint NFT.
  If the situation is valid, he would be able to mint NFT with token(ERC20).

• checkTokenAndMint
  Only address on whitelisting can mint NFT, and this part we implement through EIP712.
  Every address need to call checkTokenAndMint function with arguments: uuid, userAddress, deadline, uri, and signature. The signature will be signed by the address assigned by the contract admin, and that's ERC721AStorageCustom.layout()._signerAddress.
  In addition, we use uuid but not nonce,
  because if there are two addresses try to request token for mint NFT at the same time,
  one of the address must fail because they got the same nonce,
  and the failed executor may be confused about why would the signature be wrong.
  The signature is provided by _signerAddress and should be accurate.
  In order to optimize user experience, we choose to use uuid though it may cost much more contract storage.

• setLevel onlyWorker
  To set up price and maxAgentAmount of each level's NFTs.

• setTokenSupply onlyWorker
  To set up the token supply of specific level, and the new supply must be greater than the old supply.

• switchMintable onlyWorker
  To switch the specific level's status of mintable.

• setSignerAddress onlyWorker
  Admin can change the signerAddress who can sign the signature for the raiser who is able to mint NFT.

• batchMint onlyAdmin
  Admin can batch mint NFT, but it's also needed to be in Period.

• startPeriod onlyWorker
  Worker can start next period, and this function will call _setPeriodTokenSupply to set up the token supply of next peiord. Also, Only while the current period is sold out, Worker can start a new period.

• endPeriod onlyWorker
  Worker can end current period.

• withdraw onlyAdmin
  When user mint address, the contract will get token.
  The admin can withdraw the token back or transfer to whoever he want.

• transferAdmin onlyAdmin
  Admin can transfer his role to another address, but this one only record the transfer information. The another address needs to call updateAdmin to complete transfer.

• updateAdmin
  If Admin transferred its role to msgSender, msgSender needs to call this function to get the Admin role, and also revoke the old Admin role from old Admin to complete the transfer.

• cancelTransferAdmin onlyAdmin
  If the Admin regrets about transferring its role, he can delete the transfer information if the transfer hasn't completed.