aave-protocol-v2/contracts/protocol/lendingpool/DefaultReserveInterestRateStrategy.sol

// SPDX-License-Identifier: agpl-3.0
pragma solidity 0.6.12;

import {SafeMath} from '../../dependencies/openzeppelin/contracts/SafeMath.sol';
import {IReserveInterestRateStrategy} from '../../interfaces/IReserveInterestRateStrategy.sol';
import {WadRayMath} from '../libraries/math/WadRayMath.sol';
import {PercentageMath} from '../libraries/math/PercentageMath.sol';
import {ILendingPoolAddressesProvider} from '../../interfaces/ILendingPoolAddressesProvider.sol';
import {ILendingRateOracle} from '../../interfaces/ILendingRateOracle.sol';

/**
 * @title DefaultReserveInterestRateStrategy contract
 * @notice Implements the calculation of the interest rates depending on the reserve state
 * @dev The model of interest rate is based on 2 slopes, one before the `OPTIMAL_UTILIZATION_RATE`
 * point of utilization and another from that one to 100%
 * - An instance of this same contract, can't be used across different Aave markets, due to the caching
 *   of the LendingPoolAddressesProvider
 * @author Aave
 **/
contract DefaultReserveInterestRateStrategy is IReserveInterestRateStrategy {
  using WadRayMath for uint256;
  using SafeMath for uint256;
  using PercentageMath for uint256;

  /**
   * @dev this constant represents the utilization rate at which the pool aims to obtain most competitive borrow rates.
   * Expressed in ray
   **/
  uint256 public immutable OPTIMAL_UTILIZATION_RATE;

  /**
   * @dev This constant represents the excess utilization rate above the optimal. It's always equal to
   * 1-optimal utilization rate. Added as a constant here for gas optimizations.
   * Expressed in ray
   **/

  uint256 public immutable EXCESS_UTILIZATION_RATE;

  ILendingPoolAddressesProvider public immutable addressesProvider;

  // Base variable borrow rate when Utilization rate = 0. Expressed in ray
  uint256 internal immutable _baseVariableBorrowRate;

  // Slope of the variable interest curve when utilization rate > 0 and <= OPTIMAL_UTILIZATION_RATE. Expressed in ray
  uint256 internal immutable _variableRateSlope1;

  // Slope of the variable interest curve when utilization rate > OPTIMAL_UTILIZATION_RATE. Expressed in ray
  uint256 internal immutable _variableRateSlope2;

  // Slope of the stable interest curve when utilization rate > 0 and <= OPTIMAL_UTILIZATION_RATE. Expressed in ray
  uint256 internal immutable _stableRateSlope1;

  // Slope of the stable interest curve when utilization rate > OPTIMAL_UTILIZATION_RATE. Expressed in ray
  uint256 internal immutable _stableRateSlope2;

  constructor(
    ILendingPoolAddressesProvider provider,
    uint256 optimalUtilizationRate,
    uint256 baseVariableBorrowRate,
    uint256 variableRateSlope1,
    uint256 variableRateSlope2,
    uint256 stableRateSlope1,
    uint256 stableRateSlope2
  ) public {
    OPTIMAL_UTILIZATION_RATE = optimalUtilizationRate;
    EXCESS_UTILIZATION_RATE = WadRayMath.ray().sub(optimalUtilizationRate);
    addressesProvider = provider;
    _baseVariableBorrowRate = baseVariableBorrowRate;
    _variableRateSlope1 = variableRateSlope1;
    _variableRateSlope2 = variableRateSlope2;
    _stableRateSlope1 = stableRateSlope1;
    _stableRateSlope2 = stableRateSlope2;
  }

  function variableRateSlope1() external view returns (uint256) {
    return _variableRateSlope1;
  }

  function variableRateSlope2() external view returns (uint256) {
    return _variableRateSlope2;
  }

  function stableRateSlope1() external view returns (uint256) {
    return _stableRateSlope1;
  }

  function stableRateSlope2() external view returns (uint256) {
    return _stableRateSlope2;
  }

  function baseVariableBorrowRate() external view override returns (uint256) {
    return _baseVariableBorrowRate;
  }

  function getMaxVariableBorrowRate() external view override returns (uint256) {
    return _baseVariableBorrowRate.add(_variableRateSlope1).add(_variableRateSlope2);
  }

  struct CalcInterestRatesLocalVars {
    uint256 totalDebt;
    uint256 currentVariableBorrowRate;
    uint256 currentStableBorrowRate;
    uint256 currentLiquidityRate;
    uint256 utilizationRate;
  }

  /**
   * @dev Calculates the interest rates depending on the reserve's state and configurations
   * @param reserve The address of the reserve
   * @param availableLiquidity The liquidity available in the reserve
   * @param totalStableDebt The total borrowed from the reserve a stable rate
   * @param totalVariableDebt The total borrowed from the reserve at a variable rate
   * @param averageStableBorrowRate The weighted average of all the stable rate loans
   * @param reserveFactor The reserve portion of the interest that goes to the treasury of the market
   * @return The liquidity rate, the stable borrow rate and the variable borrow rate
   **/
  function calculateInterestRates(
    address reserve,
    uint256 availableLiquidity,
    uint256 totalStableDebt,
    uint256 totalVariableDebt,
    uint256 averageStableBorrowRate,
    uint256 reserveFactor
  )
    external
    view
    override
    returns (
      uint256,
      uint256,
      uint256
    )
  {
    CalcInterestRatesLocalVars memory vars;

    vars.totalDebt = totalStableDebt.add(totalVariableDebt);
    vars.currentVariableBorrowRate = 0;
    vars.currentStableBorrowRate = 0;
    vars.currentLiquidityRate = 0;

    uint256 utilizationRate =
      vars.totalDebt == 0 ? 0 : vars.totalDebt.rayDiv(availableLiquidity.add(vars.totalDebt));

    vars.currentStableBorrowRate = ILendingRateOracle(addressesProvider.getLendingRateOracle())
      .getMarketBorrowRate(reserve);

    if (utilizationRate > OPTIMAL_UTILIZATION_RATE) {
      uint256 excessUtilizationRateRatio =
        utilizationRate.sub(OPTIMAL_UTILIZATION_RATE).rayDiv(EXCESS_UTILIZATION_RATE);

      vars.currentStableBorrowRate = vars.currentStableBorrowRate.add(_stableRateSlope1).add(
        _stableRateSlope2.rayMul(excessUtilizationRateRatio)
      );

      vars.currentVariableBorrowRate = _baseVariableBorrowRate.add(_variableRateSlope1).add(
        _variableRateSlope2.rayMul(excessUtilizationRateRatio)
      );
    } else {
      vars.currentStableBorrowRate = vars.currentStableBorrowRate.add(
        _stableRateSlope1.rayMul(utilizationRate.rayDiv(OPTIMAL_UTILIZATION_RATE))
      );
      vars.currentVariableBorrowRate = _baseVariableBorrowRate.add(
        utilizationRate.rayMul(_variableRateSlope1).rayDiv(OPTIMAL_UTILIZATION_RATE)
      );
    }

    vars.currentLiquidityRate = _getOverallBorrowRate(
      totalStableDebt,
      totalVariableDebt,
      vars
        .currentVariableBorrowRate,
      averageStableBorrowRate
    )
      .rayMul(utilizationRate)
      .percentMul(PercentageMath.PERCENTAGE_FACTOR.sub(reserveFactor));

    return (
      vars.currentLiquidityRate,
      vars.currentStableBorrowRate,
      vars.currentVariableBorrowRate
    );
  }

  /**
   * @dev Calculates the overall borrow rate as the weighted average between the total variable debt and total stable debt
   * @param totalStableDebt The total borrowed from the reserve a stable rate
   * @param totalVariableDebt The total borrowed from the reserve at a variable rate
   * @param currentVariableBorrowRate The current variable borrow rate of the reserve
   * @param currentAverageStableBorrowRate The current weighted average of all the stable rate loans
   * @return The weighted averaged borrow rate
   **/
  function _getOverallBorrowRate(
    uint256 totalStableDebt,
    uint256 totalVariableDebt,
    uint256 currentVariableBorrowRate,
    uint256 currentAverageStableBorrowRate
  ) internal pure returns (uint256) {
    uint256 totalDebt = totalStableDebt.add(totalVariableDebt);

    if (totalDebt == 0) return 0;

    uint256 weightedVariableRate = totalVariableDebt.wadToRay().rayMul(currentVariableBorrowRate);

    uint256 weightedStableRate = totalStableDebt.wadToRay().rayMul(currentAverageStableBorrowRate);

    uint256 overallBorrowRate =
      weightedVariableRate.add(weightedStableRate).rayDiv(totalDebt.wadToRay());

    return overallBorrowRate;
  }
}