pragma solidity ^0.5.16;

// Inheritance
import "./Owned.sol";
import "./ExternStateToken.sol";
import "./MixinResolver.sol";
import "./interfaces/ISynth.sol";
import "./interfaces/IERC20.sol";

// Internal references
import "./interfaces/ISystemStatus.sol";
import "./interfaces/IFeePool.sol";
import "./interfaces/IExchanger.sol";
import "./interfaces/IIssuer.sol";


// https://docs.synthetix.io/contracts/source/contracts/synth
contract Synth is Owned, IERC20, ExternStateToken, MixinResolver, ISynth {
    /* ========== STATE VARIABLES ========== */

    // Currency key which identifies this Synth to the Synthetix system
    bytes32 public currencyKey;

    uint8 public constant DECIMALS = 18;

    // Where fees are pooled in sUSD
    address public constant FEE_ADDRESS = 0xfeEFEEfeefEeFeefEEFEEfEeFeefEEFeeFEEFEeF;

    /* ========== ADDRESS RESOLVER CONFIGURATION ========== */

    bytes32 private constant CONTRACT_SYSTEMSTATUS = "SystemStatus";
    bytes32 private constant CONTRACT_EXCHANGER = "Exchanger";
    bytes32 private constant CONTRACT_ISSUER = "Issuer";
    bytes32 private constant CONTRACT_FEEPOOL = "FeePool";

    /* ========== CONSTRUCTOR ========== */

    constructor(
        address payable _proxy,
        TokenState _tokenState,
        string memory _tokenName,
        string memory _tokenSymbol,
        address _owner,
        bytes32 _currencyKey,
        uint _totalSupply,
        address _resolver
    )
        public
        ExternStateToken(_proxy, _tokenState, _tokenName, _tokenSymbol, _totalSupply, DECIMALS, _owner)
        MixinResolver(_resolver)
    {
        require(_proxy != address(0), "_proxy cannot be 0");
        require(_owner != address(0), "_owner cannot be 0");

        currencyKey = _currencyKey;
    }

    /* ========== MUTATIVE FUNCTIONS ========== */

    function transfer(address to, uint value) public optionalProxy returns (bool) {
        _ensureCanTransfer(messageSender, value);

        // transfers to FEE_ADDRESS will be exchanged into sUSD and recorded as fee
        if (to == FEE_ADDRESS) {
            return _transferToFeeAddress(to, value);
        }

        // transfers to 0x address will be burned
        if (to == address(0)) {
            return _internalBurn(messageSender, value);
        }

        return super._internalTransfer(messageSender, to, value);
    }

    function transferAndSettle(address to, uint value) public optionalProxy returns (bool) {
        // Exchanger.settle ensures synth is active
        (, , uint numEntriesSettled) = exchanger().settle(messageSender, currencyKey);

        // Save gas instead of calling transferableSynths
        uint balanceAfter = value;

        if (numEntriesSettled > 0) {
            balanceAfter = tokenState.balanceOf(messageSender);
        }

        // Reduce the value to transfer if balance is insufficient after reclaimed
        value = value > balanceAfter ? balanceAfter : value;

        return super._internalTransfer(messageSender, to, value);
    }

    function transferFrom(
        address from,
        address to,
        uint value
    ) public optionalProxy returns (bool) {
        _ensureCanTransfer(from, value);

        return _internalTransferFrom(from, to, value);
    }

    function transferFromAndSettle(
        address from,
        address to,
        uint value
    ) public optionalProxy returns (bool) {
        // Exchanger.settle() ensures synth is active
        (, , uint numEntriesSettled) = exchanger().settle(from, currencyKey);

        // Save gas instead of calling transferableSynths
        uint balanceAfter = value;

        if (numEntriesSettled > 0) {
            balanceAfter = tokenState.balanceOf(from);
        }

        // Reduce the value to transfer if balance is insufficient after reclaimed
        value = value >= balanceAfter ? balanceAfter : value;

        return _internalTransferFrom(from, to, value);
    }

    /**
     * @notice _transferToFeeAddress function
     * non-sUSD synths are exchanged into sUSD via synthInitiatedExchange
     * notify feePool to record amount as fee paid to feePool */
    function _transferToFeeAddress(address to, uint value) internal returns (bool) {
        uint amountInUSD;

        // sUSD can be transferred to FEE_ADDRESS directly
        if (currencyKey == "sUSD") {
            amountInUSD = value;
            super._internalTransfer(messageSender, to, value);
        } else {
            // else exchange synth into sUSD and send to FEE_ADDRESS
            amountInUSD = exchanger().exchange(messageSender, currencyKey, value, "sUSD", FEE_ADDRESS);
        }

        // Notify feePool to record sUSD to distribute as fees
        feePool().recordFeePaid(amountInUSD);

        return true;
    }

    function issue(address account, uint amount) external onlyInternalContracts {
        _internalIssue(account, amount);
    }

    function burn(address account, uint amount) external onlyInternalContracts {
        _internalBurn(account, amount);
    }

    function _internalIssue(address account, uint amount) internal {
        tokenState.setBalanceOf(account, tokenState.balanceOf(account).add(amount));
        totalSupply = totalSupply.add(amount);
        emitTransfer(address(0), account, amount);
        emitIssued(account, amount);
    }

    function _internalBurn(address account, uint amount) internal returns (bool) {
        tokenState.setBalanceOf(account, tokenState.balanceOf(account).sub(amount));
        totalSupply = totalSupply.sub(amount);
        emitTransfer(account, address(0), amount);
        emitBurned(account, amount);

        return true;
    }

    // Allow owner to set the total supply on import.
    function setTotalSupply(uint amount) external optionalProxy_onlyOwner {
        totalSupply = amount;
    }

    /* ========== VIEWS ========== */

    // Note: use public visibility so that it can be invoked in a subclass
    function resolverAddressesRequired() public view returns (bytes32[] memory addresses) {
        addresses = new bytes32[](4);
        addresses[0] = CONTRACT_SYSTEMSTATUS;
        addresses[1] = CONTRACT_EXCHANGER;
        addresses[2] = CONTRACT_ISSUER;
        addresses[3] = CONTRACT_FEEPOOL;
    }

    function systemStatus() internal view returns (ISystemStatus) {
        return ISystemStatus(requireAndGetAddress(CONTRACT_SYSTEMSTATUS));
    }

    function feePool() internal view returns (IFeePool) {
        return IFeePool(requireAndGetAddress(CONTRACT_FEEPOOL));
    }

    function exchanger() internal view returns (IExchanger) {
        return IExchanger(requireAndGetAddress(CONTRACT_EXCHANGER));
    }

    function issuer() internal view returns (IIssuer) {
        return IIssuer(requireAndGetAddress(CONTRACT_ISSUER));
    }

    function _ensureCanTransfer(address from, uint value) internal view {
        require(exchanger().maxSecsLeftInWaitingPeriod(from, currencyKey) == 0, "Cannot transfer during waiting period");
        require(transferableSynths(from) >= value, "Insufficient balance after any settlement owing");
        systemStatus().requireSynthActive(currencyKey);
    }

    function transferableSynths(address account) public view returns (uint) {
        (uint reclaimAmount, , ) = exchanger().settlementOwing(account, currencyKey);

        // Note: ignoring rebate amount here because a settle() is required in order to
        // allow the transfer to actually work

        uint balance = tokenState.balanceOf(account);

        if (reclaimAmount > balance) {
            return 0;
        } else {
            return balance.sub(reclaimAmount);
        }
    }

    /* ========== INTERNAL FUNCTIONS ========== */

    function _internalTransferFrom(
        address from,
        address to,
        uint value
    ) internal returns (bool) {
        // Skip allowance update in case of infinite allowance
        if (tokenState.allowance(from, messageSender) != uint(-1)) {
            // Reduce the allowance by the amount we're transferring.
            // The safeSub call will handle an insufficient allowance.
            tokenState.setAllowance(from, messageSender, tokenState.allowance(from, messageSender).sub(value));
        }

        return super._internalTransfer(from, to, value);
    }

    /* ========== MODIFIERS ========== */

    modifier onlyInternalContracts() {
        bool isFeePool = msg.sender == address(feePool());
        bool isExchanger = msg.sender == address(exchanger());
        bool isIssuer = msg.sender == address(issuer());

        require(isFeePool || isExchanger || isIssuer, "Only FeePool, Exchanger or Issuer contracts allowed");
        _;
    }

    /* ========== EVENTS ========== */
    event Issued(address indexed account, uint value);
    bytes32 private constant ISSUED_SIG = keccak256("Issued(address,uint256)");

    function emitIssued(address account, uint value) internal {
        proxy._emit(abi.encode(value), 2, ISSUED_SIG, addressToBytes32(account), 0, 0);
    }

    event Burned(address indexed account, uint value);
    bytes32 private constant BURNED_SIG = keccak256("Burned(address,uint256)");

    function emitBurned(address account, uint value) internal {
        proxy._emit(abi.encode(value), 2, BURNED_SIG, addressToBytes32(account), 0, 0);
    }
}