contract ChainLog {

    event Rely(address usr);
    event Deny(address usr);
    event UpdateVersion(string version);
    event UpdateSha256sum(string sha256sum);
    event UpdateIPFS(string ipfs);
    event UpdateAddress(bytes32 key, address addr);
    event RemoveAddress(bytes32 key);

    // --- Auth ---
    mapping (address => uint) public wards;
    function rely(address usr) external auth { wards[usr] = 1; emit Rely(usr); }
    function deny(address usr) external auth { wards[usr] = 0; emit Deny(usr); }
    modifier auth {
        require(wards[msg.sender] == 1, "ChainLog/not-authorized");
        _;
    }

    struct Location {
        uint256  pos;
        address  addr;
    }
    mapping (bytes32 => Location) location;

    bytes32[] public keys;

    string public version;
    string public sha256sum;
    string public ipfs;

    constructor() public {
        wards[msg.sender] = 1;
        setVersion("0.0.0");
        setAddress("CHANGELOG", address(this));
    }

    /// @notice Set the "version" of the current changelog
    /// @param _version The version string (optional)
    function setVersion(string memory _version) public auth {
        version = _version;
        emit UpdateVersion(_version);
    }

    /// @notice Set the "sha256sum" of some current external changelog
    /// @dev designed to store sha256 of changelog.makerdao.com hosted log
    /// @param _sha256sum The sha256 sum (optional)
    function setSha256sum(string memory _sha256sum) public auth {
        sha256sum = _sha256sum;
        emit UpdateSha256sum(_sha256sum);
    }

    /// @notice Set the IPFS hash of a pinned changelog
    /// @dev designed to store IPFS pin hash that can retreive changelog json
    /// @param _ipfs The ipfs pin hash of an ipfs hosted log (optional)
    function setIPFS(string memory _ipfs) public auth {
        ipfs = _ipfs;
        emit UpdateIPFS(_ipfs);
    }

    /// @notice Set the key-value pair for a changelog item
    /// @param _key  the changelog key (ex. MCD_VAT)
    /// @param _addr the address to the contract
    function setAddress(bytes32 _key, address _addr) public auth {
        if (count() > 0 && _key == keys[location[_key].pos]) {
            location[_key].addr = _addr;   // Key exists in keys (update)
        } else {
            _addAddress(_key, _addr);      // Add key to keys array
        }
        emit UpdateAddress(_key, _addr);
    }

    /// @notice Removes the key from the keys list()
    /// @dev removes the item from the array but moves the last element to it's place
    //   WARNING: To save the expense of shifting an array on-chain,
    //     this will replace the key to be deleted with the last key
    //     in the array, and can therefore result in keys being out
    //     of order. Use this only if you intend to reorder the list(),
    //     otherwise consider using `setAddress("KEY", address(0));`
    /// @param _key the key to be removed
    function removeAddress(bytes32 _key) public auth {
        _removeAddress(_key);
        emit RemoveAddress(_key);
    }

    /// @notice Returns the number of keys being tracked in the keys array
    /// @return the number of keys as uint256
    function count() public view returns (uint256) {
        return keys.length;
    }

    /// @notice Returns the key and address of an item in the changelog array (for enumeration)
    /// @dev _index is 0-indexed to the underlying array
    /// @return a tuple containing the key and address associated with that key
    function get(uint256 _index) public view returns (bytes32, address) {
        return (keys[_index], location[keys[_index]].addr);
    }

    /// @notice Returns the list of keys being tracked by the changelog
    /// @dev May fail if keys is too large, if so, call count() and iterate with get()
    function list() public view returns (bytes32[] memory) {
        return keys;
    }

    /// @notice Returns the address for a particular key
    /// @param _key a bytes32 key (ex. MCD_VAT)
    /// @return addr the contract address associated with the key
    function getAddress(bytes32 _key) public view returns (address addr) {
        addr = location[_key].addr;
        require(addr != address(0), "dss-chain-log/invalid-key");
    }

    function _addAddress(bytes32 _key, address _addr) internal {
        keys.push(_key);
        location[keys[keys.length - 1]] = Location(
            keys.length - 1,
            _addr
        );
    }

    function _removeAddress(bytes32 _key) internal {
        uint256 index = location[_key].pos;       // Get pos in array
        require(keys[index] == _key, "dss-chain-log/invalid-key");
        bytes32 move  = keys[keys.length - 1];    // Get last key
        keys[index] = move;                       // Replace
        location[move].pos = index;               // Update array pos
        keys.pop();                               // Trim last key
        delete location[_key];                    // Delete struct data
    }
}
/// @title An on-chain governance-managed contract registry
/// @notice Publicly readable data; mutating functions must be called by an authorized user
function count() public view returns (uint256) {
    return keys.length;
}

/// @notice Returns the number of keys being tracked in the keys array
/// @return the number of keys as uint256