Feat(454): Exchange provider tests (#529)

### Description

This PR adds additional tests to to the exchange provider and includes
fuzz testing

### Related issues

- Fixes mento-protocol/mento-general#454
-

---------

Co-authored-by: bowd <dumbogdan@gmail.com>
Co-authored-by: philbow61 <philip.raetsch@gmail.com>
Co-authored-by: baroooo <baranseltekin@gmail.com>
Co-authored-by: philbow61 <80156619+philbow61@users.noreply.github.com>

contracts/goodDollar/GoodDollarExchangeProvider.sol

@@ -138,19 +138,21 @@ contract GoodDollarExchangeProvider is IGoodDollarExchangeProvider, BancorExchan
    */
   function mintFromExpansion(
     bytes32 exchangeId,
-    uint256 expansionScaler
+    uint256 reserveRatioScalar
   ) external onlyExpansionController whenNotPaused returns (uint256 amountToMint) {
-    require(expansionScaler > 0, "Expansion rate must be greater than 0");
+    require(reserveRatioScalar > 0, "Reserve ratio scalar must be greater than 0");
     PoolExchange memory exchange = getPoolExchange(exchangeId);
 
     UD60x18 scaledRatio = wrap(uint256(exchange.reserveRatio) * 1e10);
-    UD60x18 newRatio = scaledRatio.mul(wrap(expansionScaler));
+    UD60x18 newRatio = scaledRatio.mul(wrap(reserveRatioScalar));
+
+    uint32 newRatioUint = uint32(unwrap(newRatio) / 1e10);
+    require(newRatioUint > 0, "New ratio must be greater than 0");
 
     UD60x18 numerator = wrap(exchange.tokenSupply).mul(scaledRatio);
     numerator = numerator.sub(wrap(exchange.tokenSupply).mul(newRatio));
 
     uint256 scaledAmountToMint = unwrap(numerator.div(newRatio));
-    uint32 newRatioUint = uint32(unwrap(newRatio) / 1e10);
 
     exchanges[exchangeId].reserveRatio = newRatioUint;
     exchanges[exchangeId].tokenSupply += scaledAmountToMint;
@@ -193,10 +195,6 @@ contract GoodDollarExchangeProvider is IGoodDollarExchangeProvider, BancorExchan
   function updateRatioForReward(bytes32 exchangeId, uint256 reward) external onlyExpansionController whenNotPaused {
     PoolExchange memory exchange = getPoolExchange(exchangeId);
 
-    if (reward == 0) {
-      return;
-    }
-
     uint256 currentPriceScaled = currentPrice(exchangeId) * tokenPrecisionMultipliers[exchange.reserveAsset];
     uint256 rewardScaled = reward * tokenPrecisionMultipliers[exchange.tokenAddress];
 

contracts/goodDollar/GoodDollarExpansionController.sol

@@ -174,10 +174,10 @@ contract GoodDollarExpansionController is IGoodDollarExpansionController, Pausab
 
     bool shouldExpand = block.timestamp > config.lastExpansion + config.expansionFrequency;
     if (shouldExpand || config.lastExpansion == 0) {
-      uint256 expansionScaler = _getExpansionScaler(config);
+      uint256 reserveRatioScalar = _getReserveRatioScalar(config);
 
       exchangeExpansionConfigs[exchangeId].lastExpansion = uint32(block.timestamp);
-      amountMinted = goodDollarExchangeProvider.mintFromExpansion(exchangeId, expansionScaler);
+      amountMinted = goodDollarExchangeProvider.mintFromExpansion(exchangeId, reserveRatioScalar);
 
       IGoodDollar(exchange.tokenAddress).mint(address(distributionHelper), amountMinted);
       distributionHelper.onDistribution(amountMinted);
@@ -218,11 +218,11 @@ contract GoodDollarExpansionController is IGoodDollarExpansionController, Pausab
   }
 
   /**
-   * @notice Calculates the expansion scaler for the given expansion config.
+   * @notice Calculates the reserve ratio scalar for the given expansion config.
    * @param config The expansion config.
-   * @return expansionScaler The expansion scaler.
+   * @return reserveRatioScalar The reserve ratio scalar.
    */
-  function _getExpansionScaler(ExchangeExpansionConfig memory config) internal view returns (uint256) {
+  function _getReserveRatioScalar(ExchangeExpansionConfig memory config) internal view returns (uint256) {
     uint256 numberOfExpansions;
 
     // If there was no previous expansion, we expand once.
@@ -232,7 +232,7 @@ contract GoodDollarExpansionController is IGoodDollarExpansionController, Pausab
       numberOfExpansions = (block.timestamp - config.lastExpansion) / config.expansionFrequency;
     }
 
-    uint256 stepExpansionScaler = MAX_WEIGHT - config.expansionRate;
-    return unwrap(powu(wrap(stepExpansionScaler), numberOfExpansions));
+    uint256 stepReserveRatioScalar = MAX_WEIGHT - config.expansionRate;
+    return unwrap(powu(wrap(stepReserveRatioScalar), numberOfExpansions));
   }
 }

contracts/interfaces/IGoodDollarExchangeProvider.sol

@@ -55,10 +55,10 @@ interface IGoodDollarExchangeProvider {
   /**
    * @notice Calculates the amount of G$ tokens to be minted as a result of the expansion.
    * @param exchangeId The ID of the pool to calculate the expansion for.
-   * @param expansionScaler Scaler for calculating the new reserve ratio.
+   * @param reserveRatioScalar Scaler for calculating the new reserve ratio.
    * @return amountToMint Amount of G$ tokens to be minted as a result of the expansion.
    */
-  function mintFromExpansion(bytes32 exchangeId, uint256 expansionScaler) external returns (uint256 amountToMint);
+  function mintFromExpansion(bytes32 exchangeId, uint256 reserveRatioScalar) external returns (uint256 amountToMint);
 
   /**
    * @notice Calculates the amount of G$ tokens to be minted as a result of the collected reserve interest.