GAMMA V2

pragma solidity 0.5.17;
contract GAMMA { // Γ - mv - NFT - mkt - γ
    address payable public dao = 0x057e820D740D5AAaFfa3c6De08C5c98d990dB00d;
    uint256 public constant GAMMA_MAX = 5772156649015328606065120900824024310421;
    uint256 public totalSupply;
    string public name = "GAMMA";
    string public symbol = "GAMMA";
    mapping(address => uint256) public balanceOf;
    mapping(uint256 => address) public getApproved;
    mapping(uint256 => address) public ownerOf;
    mapping(uint256 => uint256) public tokenByIndex;
    mapping(uint256 => string) public tokenURI;
    mapping(uint256 => Sale) public sale;
    mapping(bytes4 => bool) public supportsInterface; // eip-165 
    mapping(address => mapping(address => bool)) public isApprovedForAll;
    mapping(address => mapping(uint256 => uint256)) public tokenOfOwnerByIndex;
    event Approval(address indexed approver, address indexed spender, uint256 indexed tokenId);
    event ApprovalForAll(address indexed owner, address indexed operator, bool approved);
    event Transfer(address indexed from, address indexed to, uint256 indexed tokenId);
    event UpdateSale(uint256 indexed ethPrice, uint256 indexed tokenId, bool forSale);
    struct Sale {
        uint256 ethPrice;
        bool forSale;
    }
    constructor () public {
        supportsInterface[0x80ac58cd] = true; // ERC721 
        supportsInterface[0x5b5e139f] = true; // METADATA
        supportsInterface[0x780e9d63] = true; // ENUMERABLE
    }
    function approve(address spender, uint256 tokenId) external {
        require(msg.sender == ownerOf[tokenId] || isApprovedForAll[ownerOf[tokenId]][msg.sender], "!owner/operator");
        getApproved[tokenId] = spender;
        emit Approval(msg.sender, spender, tokenId); 
    }
    function mint(uint256 ethPrice, string calldata _tokenURI, bool forSale) external { 
        totalSupply++;
        require(totalSupply <= GAMMA_MAX, "maxed");
        uint256 tokenId = totalSupply;
        balanceOf[msg.sender]++;
        ownerOf[tokenId] = msg.sender;
        tokenByIndex[tokenId - 1] = tokenId;
        tokenURI[tokenId] = _tokenURI;
        sale[tokenId].ethPrice = ethPrice;
        sale[tokenId].forSale = forSale;
        tokenOfOwnerByIndex[msg.sender][tokenId - 1] = tokenId;
        emit Transfer(address(0), msg.sender, tokenId); 
        emit UpdateSale(ethPrice, tokenId, forSale);
    }
    function purchase(uint256 tokenId) payable external {
        require(msg.value == sale[tokenId].ethPrice, "!ethPrice");
        require(sale[tokenId].forSale, "!forSale");
        address owner = ownerOf[tokenId];
        (bool success, ) = owner.call.value(msg.value)("");
        require(success, "!transfer");
        _transfer(owner, msg.sender, tokenId);
    }
    function setApprovalForAll(address operator, bool approved) external {
        isApprovedForAll[msg.sender][operator] = approved;
        emit ApprovalForAll(msg.sender, operator, approved);
    }
    function _transfer(address from, address to, uint256 tokenId) internal {
        balanceOf[from]--; 
        balanceOf[to]++; 
        getApproved[tokenId] = address(0);
        ownerOf[tokenId] = to;
        sale[tokenId].forSale = false;
        tokenOfOwnerByIndex[from][tokenId - 1] = 0;
        tokenOfOwnerByIndex[to][tokenId - 1] = tokenId;
        emit Transfer(from, to, tokenId); 
    }
    function transfer(address to, uint256 tokenId) external {
        require(msg.sender == ownerOf[tokenId], "!owner");
        _transfer(msg.sender, to, tokenId);
    }
    function transferBatch(address[] calldata to, uint256[] calldata tokenId) external {
        require(to.length == tokenId.length, "!to/tokenId");
        for (uint256 i = 0; i < to.length; i++) {
            require(msg.sender == ownerOf[tokenId[i]], "!owner");
            _transfer(msg.sender, to[i], tokenId[i]);
        }
    }
    function transferFrom(address from, address to, uint256 tokenId) external {
        require(msg.sender == ownerOf[tokenId] || getApproved[tokenId] == msg.sender || isApprovedForAll[ownerOf[tokenId]][msg.sender], "!owner/spender/operator");
        _transfer(from, to, tokenId);
    }
    function updateDao(address payable _dao) external {
        require(msg.sender == dao, "!dao");
        dao = _dao;
    }
    function updateSale(uint256 ethPrice, uint256 tokenId, bool forSale) payable external {
        require(msg.sender == ownerOf[tokenId], "!owner");
        sale[tokenId].ethPrice = ethPrice;
        sale[tokenId].forSale = forSale;
        (bool success, ) = dao.call.value(msg.value)("");
        require(success, "!transfer");
        emit UpdateSale(ethPrice, tokenId, forSale);
    }
}

{
  "compilationTarget": {
    "GAMMA.sol": "GAMMA"
  },
  "evmVersion": "istanbul",
  "libraries": {},
  "optimizer": {
    "enabled": true,
    "runs": 200
  },
  "remappings": []
}