ciga-dapp/contracts/ShowOwnership.sol

pragma solidity ^0.4.23;

import "./ERC721.sol";
import "./SafeMath.sol";
import "./ShowHelper.sol";

contract ShowOwnership is ERC721, ShowHelper {
    event WantToExchange(uint mine, uint yours);
    event RefuseToExchange(uint indexed yours);

    using SafeMath for uint256;

    mapping (uint=>address) showApprovals;
    mapping (uint=>uint) public showExchange; // 0: not for exchange, n: exchange for the n_th show

    function balanceOf(address _owner) public view returns (uint256 _balance) {
        return ownerShowCount[_owner];
    }

    function ownerOf(uint256 _tokenId) public view returns (address _owner) {
        return showToOwner[_tokenId];
    }

    function transfer(address _to, uint256 _tokenId) public onlyOwnerOf(_tokenId) {
        _transfer(msg.sender, _to, _tokenId);
    }

    function approve(address _to, uint256 _tokenId) public onlyOwnerOf(_tokenId) {
        showApprovals[_tokenId] = _to;
        emit Approval(msg.sender, _to, _tokenId);
    }

    function takeOwnership(uint256 _tokenId) public {
        require(msg.sender == showApprovals[_tokenId] && showExchange[_tokenId] == 0);
        address owner = ownerOf(_tokenId);
        _transfer(owner, msg.sender, _tokenId);
    }

    function wantToExchange(uint mine, uint yours) public onlyOwnerOf(mine) {
        if (msg.sender == showApprovals[yours] && showExchange[yours] == mine.add(1)) {
            showExchange[yours] = 0;
            transfer(ownerOf(yours), mine);
            takeOwnership(yours);
        } else {
            showExchange[mine] = yours.add(1);
            showApprovals[mine] = ownerOf(yours);
            emit WantToExchange(mine, yours);
        }
    }

    function refuseToExchange(uint yours) public {
        require(msg.sender == showApprovals[yours]);
        require(showExchange[yours] != 0);
        require(msg.sender == showToOwner[showExchange[yours].sub(1)]);
        showApprovals[yours] = address(0);
        showExchange[yours] = 0;
    }

    function _transfer(address _from, address _to, uint _tokenId) internal {
        ownerShowCount[_from] = ownerShowCount[_from].sub(1);
        ownerShowCount[_to] = ownerShowCount[_to].add(1);
        showToOwner[_tokenId] = _to;
        emit Transfer(_from, _to, _tokenId);
    }
}