pragma solidity ^0.6.0;

import {Context} from '@openzeppelin/contracts/GSN/Context.sol';
import {IERC20} from '@openzeppelin/contracts/token/ERC20/IERC20.sol';
import {SafeMath} from '@openzeppelin/contracts/math/SafeMath.sol';
import {Address} from '@openzeppelin/contracts/utils/Address.sol';
import {DebtTokenBase} from './base/DebtTokenBase.sol';
import {MathUtils} from '../libraries/MathUtils.sol';
import {WadRayMath} from '../libraries/WadRayMath.sol';
import {IStableDebtToken} from './interfaces/IStableDebtToken.sol';

/**
* @title contract StableDebtToken
*
* @notice defines the interface for the stable debt token
*
* @dev it does not inherit from IERC20 to save in code size
*
* @author Aave
*
**/
contract StableDebtToken is IStableDebtToken, DebtTokenBase {
  using SafeMath for uint256;
  using WadRayMath for uint256;
  using Address for address;

  struct UserData {
    uint256 currentRate;
    uint40 lastUpdateTimestamp;
  }

  uint256 private avgStableRate;

  mapping(address => UserData) usersData;

  /**
  * @dev emitted when new stable debt is minted
  * @param _user the address of the user
  * @param _amount the amount minted
  * @param _previousBalance the previous balance of the user
  * @param _currentBalance the current balance of the user
  * @param _balanceIncrease the debt increase since the last update
  * @param _newRate the rate of the debt after the minting
  **/
  event mintDebt(
    address _user,
    uint256 _amount,
    uint256 _previousBalance,
    uint256 _currentBalance,
    uint256 _balanceIncrease,
    uint256 _newRate
  );

  /**
  * @dev emitted when new stable debt is burned
  * @param _user the address of the user
  * @param _amount the amount minted
  * @param _previousBalance the previous balance of the user
  * @param _currentBalance the current balance of the user
  * @param _balanceIncrease the debt increase since the last update
  **/
  event burnDebt(
    address _user,
    uint256 _amount,
    uint256 _previousBalance,
    uint256 _currentBalance,
    uint256 _balanceIncrease
  );

 /**
  * @dev returns the average stable rate across all the stable rate debt
  * @return the average stable rate
  **/
  function getAverageStableRate() external virtual override view returns (uint256) {
    return avgStableRate;
  }

 /**
  * @dev returns the timestamp of the last user action
  * @return the last update timestamp
  **/
  function getUserLastUpdated(address _user) external virtual override view returns (uint40) {
    return usersData[_user].lastUpdateTimestamp;
  }

 /**
  * @dev returns the stable rate of the user
  * @param _user the address of the user
  * @return the stable rate of _user
  **/
  function getUserStableRate(address _user) external virtual override view returns (uint256) {
    return usersData[_user].currentRate;
  }


  /**
  * @dev calculates the current user debt balance
  * @return the accumulated debt of the user
  **/
  function balanceOf(address account) public virtual override view returns (uint256) {
    if(balances[account] == 0) {
        return 0;
    }

    UserData storage userData = usersData[account];

    uint256 cumulatedInterest = MathUtils.calculateCompoundedInterest(
      userData.currentRate,
      userData.lastUpdateTimestamp
    );
    return balances[account].wadToRay().rayMul(cumulatedInterest).rayToWad();
  }


  struct MintLocalVars {
    uint256 newSupply;
    uint256 amountInRay;
    uint256 newStableRate;
  }

  /**
  * @dev mints debt token to the target user. The resulting rate is the weighted average
  * between the rate of the new debt and the rate of the previous debt
  * @param _user the address of the user
  * @param _amount the amount of debt tokens to mint
  * @param _rate the rate of the debt being minted.
  **/
  function mint(
    address _user,
    uint256 _amount,
    uint256 _rate
  ) public override onlyLendingPool {

    MintLocalVars memory vars;

    //cumulates the user debt
    (
      uint256 previousBalance,
      uint256 currentBalance,
      uint256 balanceIncrease
    ) = _cumulateBalance(_user);

    vars.newSupply = totalSupply.add(_amount);

    vars.amountInRay = _amount.wadToRay();

    //calculates the new stable rate for the user
    vars.newStableRate = usersData[_user]
      .currentRate
      .rayMul(currentBalance.wadToRay())
      .add(vars.amountInRay.rayMul(_rate))
      .rayDiv(currentBalance.add(_amount).wadToRay());

    usersData[_user].currentRate = vars.newStableRate;

    //solium-disable-next-line
    usersData[_user].lastUpdateTimestamp = uint40(block.timestamp);

    //calculates the updated average stable rate
    avgStableRate = avgStableRate
      .rayMul(totalSupply.wadToRay())
      .add(_rate.rayMul(vars.amountInRay))
      .rayDiv(vars.newSupply.wadToRay());

    _mint(_user, _amount);

    emit mintDebt(
      _user,
      _amount,
      previousBalance,
      currentBalance,
      balanceIncrease,
      vars.newStableRate
    );
  }

  /**
  * @dev burns debt of the target user.
  * @param _user the address of the user
  * @param _amount the amount of debt tokens to mint
  **/
  function burn(address _user, uint256 _amount) public override onlyLendingPool {
    (
      uint256 previousBalance,
      uint256 currentBalance,
      uint256 balanceIncrease
    ) = _cumulateBalance(_user);

    uint256 newSupply = totalSupply.sub(_amount);

    uint256 amountInRay = _amount.wadToRay();

    if (newSupply == 0) {
      avgStableRate = 0;
    } else {
      avgStableRate = avgStableRate
        .rayMul(totalSupply.wadToRay())
        .sub(usersData[_user].currentRate.rayMul(amountInRay))
        .rayDiv(newSupply.wadToRay());
    }

    if(_amount == currentBalance){
      usersData[_user].currentRate = 0;
      usersData[_user].lastUpdateTimestamp = 0;
    }

    _burn(_user, _amount);

    emit burnDebt(_user, _amount, previousBalance, currentBalance, balanceIncrease);
  }

  /**
   * @dev accumulates the accrued interest of the user to the principal balance
   * @param _user the address of the user for which the interest is being accumulated
   * @return the previous principal balance, the new principal balance, the balance increase
   **/
  function _cumulateBalance(address _user)
    internal
    returns (
      uint256,
      uint256,
      uint256
    )
  {
    uint256 previousPrincipalBalance = balances[_user];

    if(previousPrincipalBalance == 0){
      return (0,0,0);
    }

    //calculate the accrued interest since the last accumulation
    uint256 balanceIncrease = balanceOf(_user).sub(previousPrincipalBalance);
    //mints an amount of tokens equivalent to the amount accumulated
    _mint(_user, balanceIncrease);

    return (
      previousPrincipalBalance,
      previousPrincipalBalance.add(balanceIncrease),
      balanceIncrease
    );
  }
}