Fixes #68

contracts/lendingpool/LendingPool.sol

@@ -106,13 +106,12 @@ contract LendingPool is VersionedInitializable, ILendingPool, LendingPoolStorage
     reserve.updateState();
     reserve.updateInterestRates(asset, aToken, amount, 0);
 
-    bool isFirstDeposit = IAToken(aToken).balanceOf(onBehalfOf) == 0;
+    bool isFirstDeposit = IAToken(aToken).mint(onBehalfOf, amount, reserve.liquidityIndex);
+
     if (isFirstDeposit) {
       _usersConfig[onBehalfOf].setUsingAsCollateral(reserve.id, true);
     }
 
-    IAToken(aToken).mint(onBehalfOf, amount, reserve.liquidityIndex);
-
     //transfer to the aToken contract
     IERC20(asset).safeTransferFrom(msg.sender, aToken, amount);
 

contracts/libraries/logic/ReserveLogic.sol

@@ -147,21 +147,22 @@ library ReserveLogic {
    * a formal specification.
    * @param reserve the reserve object
    **/
-  function updateState(ReserveData storage reserve) external {
-    address variableDebtToken = reserve.variableDebtTokenAddress;
+  function updateState(ReserveData storage reserve) internal {
+    uint256 scaledVariableDebt = IVariableDebtToken(reserve.variableDebtTokenAddress)
+      .scaledTotalSupply();
     uint256 previousVariableBorrowIndex = reserve.variableBorrowIndex;
     uint256 previousLiquidityIndex = reserve.liquidityIndex;
 
     (uint256 newLiquidityIndex, uint256 newVariableBorrowIndex) = _updateIndexes(
       reserve,
-      variableDebtToken,
+      scaledVariableDebt,
       previousLiquidityIndex,
       previousVariableBorrowIndex
     );
 
     _mintToTreasury(
       reserve,
-      variableDebtToken,
+      scaledVariableDebt,
       previousVariableBorrowIndex,
       newLiquidityIndex,
       newVariableBorrowIndex
@@ -179,7 +180,7 @@ library ReserveLogic {
     ReserveData storage reserve,
     uint256 totalLiquidity,
     uint256 amount
-  ) external {
+  ) internal {
     uint256 amountToLiquidityRatio = amount.wadToRay().rayDiv(totalLiquidity.wadToRay());
 
     uint256 result = amountToLiquidityRatio.add(WadRayMath.ray());
@@ -227,6 +228,7 @@ library ReserveLogic {
     uint256 newStableRate;
     uint256 newVariableRate;
     uint256 avgStableRate;
+    uint256 totalVariableDebt;
   }
 
   /**
@@ -242,7 +244,7 @@ library ReserveLogic {
     address aTokenAddress,
     uint256 liquidityAdded,
     uint256 liquidityTaken
-  ) external {
+  ) internal {
     UpdateInterestRatesLocalVars memory vars;
 
     vars.stableDebtTokenAddress = reserve.stableDebtTokenAddress;
@@ -250,6 +252,13 @@ library ReserveLogic {
     (vars.totalStableDebt, vars.avgStableRate) = IStableDebtToken(vars.stableDebtTokenAddress)
       .getTotalSupplyAndAvgRate();
 
+    //calculates the total variable debt locally using the scaled total supply instead
+    //of totalSupply(), as it's noticeably cheaper. Also, the index has been
+    //updated by the previous updateState() call
+    vars.totalVariableDebt = IVariableDebtToken(reserve.variableDebtTokenAddress)
+      .scaledTotalSupply()
+      .rayMul(reserve.variableBorrowIndex);
+
     vars.availableLiquidity = IERC20(reserveAddress).balanceOf(aTokenAddress);
 
     (
@@ -260,7 +269,7 @@ library ReserveLogic {
       reserveAddress,
       vars.availableLiquidity.add(liquidityAdded).sub(liquidityTaken),
       vars.totalStableDebt,
-      IERC20(reserve.variableDebtTokenAddress).totalSupply(),
+      vars.totalVariableDebt,
       vars.avgStableRate,
       reserve.configuration.getReserveFactor()
     );
@@ -287,7 +296,6 @@ library ReserveLogic {
     uint256 principalStableDebt;
     uint256 previousStableDebt;
     uint256 currentVariableDebt;
-    uint256 scaledVariableDebt;
     uint256 previousVariableDebt;
     uint256 avgStableRate;
     uint256 cumulatedStableInterest;
@@ -301,14 +309,14 @@ library ReserveLogic {
    * @dev mints part of the repaid interest to the reserve treasury, depending on the reserveFactor for the
    * specific asset.
    * @param reserve the reserve reserve to be updated
-   * @param variableDebtToken the debt token address
+   * @param scaledVariableDebt the current scaled total variable debt
    * @param previousVariableBorrowIndex the variable borrow index before the last accumulation of the interest
    * @param newLiquidityIndex the new liquidity index
    * @param newVariableBorrowIndex the variable borrow index after the last accumulation of the interest
    **/
   function _mintToTreasury(
     ReserveData storage reserve,
-    address variableDebtToken,
+    uint256 scaledVariableDebt,
     uint256 previousVariableBorrowIndex,
     uint256 newLiquidityIndex,
     uint256 newVariableBorrowIndex
@@ -321,9 +329,6 @@ library ReserveLogic {
       return;
     }
 
-    //fetching the last scaled total variable debt
-    vars.scaledVariableDebt = IVariableDebtToken(variableDebtToken).scaledTotalSupply();
-
     //fetching the principal, total stable debt and the avg stable rate
     (
       vars.principalStableDebt,
@@ -333,10 +338,10 @@ library ReserveLogic {
     ) = IStableDebtToken(reserve.stableDebtTokenAddress).getSupplyData();
 
     //calculate the last principal variable debt
-    vars.previousVariableDebt = vars.scaledVariableDebt.rayMul(previousVariableBorrowIndex);
+    vars.previousVariableDebt = scaledVariableDebt.rayMul(previousVariableBorrowIndex);
 
     //calculate the new total supply after accumulation of the index
-    vars.currentVariableDebt = vars.scaledVariableDebt.rayMul(newVariableBorrowIndex);
+    vars.currentVariableDebt = scaledVariableDebt.rayMul(newVariableBorrowIndex);
 
     //calculate the stable debt until the last timestamp update
     vars.cumulatedStableInterest = MathUtils.calculateCompoundedInterest(
@@ -361,13 +366,13 @@ library ReserveLogic {
   /**
    * @dev updates the reserve indexes and the timestamp of the update
    * @param reserve the reserve reserve to be updated
-   * @param variableDebtToken the debt token address
+   * @param scaledVariableDebt the scaled variable debt
    * @param liquidityIndex the last stored liquidity index
    * @param variableBorrowIndex the last stored variable borrow index
    **/
   function _updateIndexes(
     ReserveData storage reserve,
-    address variableDebtToken,
+    uint256 scaledVariableDebt,
     uint256 liquidityIndex,
     uint256 variableBorrowIndex
   ) internal returns (uint256, uint256) {
@@ -391,7 +396,7 @@ library ReserveLogic {
 
       //as the liquidity rate might come only from stable rate loans, we need to ensure
       //that there is actual variable debt before accumulating
-      if (IERC20(variableDebtToken).totalSupply() > 0) {
+      if (scaledVariableDebt != 0) {
         uint256 cumulatedVariableBorrowInterest = MathUtils.calculateCompoundedInterest(
           reserve.currentVariableBorrowRate,
           timestamp

contracts/libraries/logic/ValidationLogic.sol

@@ -34,7 +34,7 @@ library ValidationLogic {
    * @param reserve the reserve state on which the user is depositing
    * @param amount the amount to be deposited
    */
-  function validateDeposit(ReserveLogic.ReserveData storage reserve, uint256 amount) external view {
+  function validateDeposit(ReserveLogic.ReserveData storage reserve, uint256 amount) internal view {
     (bool isActive, bool isFreezed, , ) = reserve.configuration.getFlags();
 
     require(amount > 0, Errors.AMOUNT_NOT_GREATER_THAN_0);
@@ -57,7 +57,7 @@ library ValidationLogic {
     mapping(uint256 => address) storage reserves,
     uint256 reservesCount,
     address oracle
-  ) external view {
+  ) internal view {
     require(amount > 0, Errors.AMOUNT_NOT_GREATER_THAN_0);
 
     require(amount <= userBalance, Errors.NOT_ENOUGH_AVAILABLE_USER_BALANCE);

contracts/tokenization/AToken.sol

@@ -119,12 +119,15 @@ contract AToken is VersionedInitializable, IncentivizedERC20, IAToken {
    * @param user the address receiving the minted tokens
    * @param amount the amount of tokens to mint
    * @param index the the last index of the reserve
+   * @return true if the the previous balance of the user is 0
    */
   function mint(
     address user,
     uint256 amount,
     uint256 index
-  ) external override onlyLendingPool {
+  ) external override onlyLendingPool returns (bool) {
+    uint256 previousBalance = super.balanceOf(user);
+
     uint256 amountScaled = amount.rayDiv(index);
     require(amountScaled != 0, Errors.INVALID_MINT_AMOUNT);
     _mint(user, amountScaled);
@@ -132,6 +135,8 @@ contract AToken is VersionedInitializable, IncentivizedERC20, IAToken {
     //transfer event to track balances
     emit Transfer(address(0), user, amount);
     emit Mint(user, amount, index);
+
+    return previousBalance == 0;
   }
 
   /**

contracts/tokenization/VariableDebtToken.sol

@@ -54,12 +54,14 @@ contract VariableDebtToken is DebtTokenBase, IVariableDebtToken {
    * @param user the user receiving the debt
    * @param amount the amount of debt being minted
    * @param index the variable debt index of the reserve
+   * @return true if the the previous balance of the user is 0
    **/
   function mint(
     address user,
     uint256 amount,
     uint256 index
-  ) external override onlyLendingPool {
+  ) external override onlyLendingPool returns (bool) {
+    uint256 previousBalance = super.balanceOf(user);
     uint256 amountScaled = amount.rayDiv(index);
     require(amountScaled != 0, Errors.INVALID_MINT_AMOUNT);
 
@@ -67,6 +69,8 @@ contract VariableDebtToken is DebtTokenBase, IVariableDebtToken {
 
     emit Transfer(address(0), user, amount);
     emit Mint(user, amount, index);
+
+    return previousBalance == 0;
   }
 
   /**

contracts/tokenization/interfaces/IScaledBalanceToken.sol

@@ -21,7 +21,7 @@ interface IScaledBalanceToken {
     address user,
     uint256 amount,
     uint256 index
-  ) external;
+  ) external returns (bool);
 
   /**
    * @dev returns the principal balance of the user. The principal balance is the last