fix(contracts): `RateLimit` minor changes (#4575)

### Description

- Added a check for invalid capacity and event for token level change

### Drive-by changes

None

### Related issues

- fixes https://github.com/chainlight-io/2024-08-hyperlane/issues/14

### Backward compatibility

Yes

### Testing

Unit tests

solidity/contracts/libs/RateLimited.sol

@@ -1,5 +1,19 @@
 // SPDX-License-Identifier: MIT OR Apache-2.0
 pragma solidity >=0.8.0;
+
+/*@@@@@@@       @@@@@@@@@
+ @@@@@@@@@       @@@@@@@@@
+  @@@@@@@@@       @@@@@@@@@
+   @@@@@@@@@       @@@@@@@@@
+    @@@@@@@@@@@@@@@@@@@@@@@@@
+     @@@@@  HYPERLANE  @@@@@@@
+    @@@@@@@@@@@@@@@@@@@@@@@@@
+   @@@@@@@@@       @@@@@@@@@
+  @@@@@@@@@       @@@@@@@@@
+ @@@@@@@@@       @@@@@@@@@
+@@@@@@@@@       @@@@@@@@*/
+
+// ============ External Imports ============
 import {OwnableUpgradeable} from "@openzeppelin/contracts-upgradeable/access/OwnableUpgradeable.sol";
 
 /**
@@ -7,16 +21,26 @@ import {OwnableUpgradeable} from "@openzeppelin/contracts-upgradeable/access/Own
  * @notice A contract used to keep track of an address sender's token amount limits.
  * @dev Implements a modified token bucket algorithm where the bucket is full in the beginning and gradually refills
  * See: https://dev.to/satrobit/rate-limiting-using-the-token-bucket-algorithm-3cjh
- **/
+ *
+ */
 contract RateLimited is OwnableUpgradeable {
     uint256 public constant DURATION = 1 days; // 86400
-    uint256 public filledLevel; /// @notice Current filled level
-    uint256 public refillRate; /// @notice Tokens per second refill rate
-    uint256 public lastUpdated; /// @notice Timestamp of the last time an action has been taken TODO prob can be uint40
+    /// @notice Current filled level
+    uint256 public filledLevel;
+    /// @notice Tokens per second refill rate
+    uint256 public refillRate;
+    /// @notice Timestamp of the last time an action has been taken
+    uint256 public lastUpdated;
 
     event RateLimitSet(uint256 _oldCapacity, uint256 _newCapacity);
 
+    event ConsumedFilledLevel(uint256 filledLevel, uint256 lastUpdated);
+
     constructor(uint256 _capacity) {
+        require(
+            _capacity >= DURATION,
+            "Capacity must be greater than DURATION"
+        );
         _transferOwnership(msg.sender);
         setRefillRate(_capacity);
         filledLevel = _capacity;
@@ -88,20 +112,22 @@ contract RateLimited is OwnableUpgradeable {
 
     /**
      * Validate an amount and decreases the currentCapacity
-     * @param _newAmount The amount to consume the fill level
+     * @param _consumedAmount The amount to consume the fill level
      * @return The new filled level
      */
     function validateAndConsumeFilledLevel(
-        uint256 _newAmount
+        uint256 _consumedAmount
     ) public returns (uint256) {
         uint256 adjustedFilledLevel = calculateCurrentLevel();
-        require(_newAmount <= adjustedFilledLevel, "RateLimitExceeded");
+        require(_consumedAmount <= adjustedFilledLevel, "RateLimitExceeded");
 
         // Reduce the filledLevel and update lastUpdated
-        uint256 _filledLevel = adjustedFilledLevel - _newAmount;
+        uint256 _filledLevel = adjustedFilledLevel - _consumedAmount;
         filledLevel = _filledLevel;
         lastUpdated = block.timestamp;
 
+        emit ConsumedFilledLevel(filledLevel, lastUpdated);
+
         return _filledLevel;
     }
 }

solidity/test/lib/RateLimited.t.sol

@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: MIT or Apache-2.0
 pragma solidity ^0.8.13;
+
 import {Test} from "forge-std/Test.sol";
 import {RateLimited} from "../../contracts/libs/RateLimited.sol";
 
@@ -13,8 +14,13 @@ contract RateLimitLibTest is Test {
         rateLimited = new RateLimited(MAX_CAPACITY);
     }
 
+    function testConstructor_revertsWhen_lowCapacity() public {
+        vm.expectRevert("Capacity must be greater than DURATION");
+        new RateLimited(1 days - 1);
+    }
+
     function testRateLimited_setsNewLimit() external {
-        rateLimited.setRefillRate(2 ether);
+        assert(rateLimited.setRefillRate(2 ether) > 0);
         assertApproxEqRel(rateLimited.maxCapacity(), 2 ether, ONE_PERCENT);
         assertEq(rateLimited.refillRate(), uint256(2 ether) / 1 days); // 2 ether / 1 day
     }
@@ -45,6 +51,25 @@ contract RateLimitLibTest is Test {
         rateLimited.setRefillRate(1 ether);
     }
 
+    function testConsumedFilledLevelEvent() public {
+        uint256 consumeAmount = 0.5 ether;
+
+        vm.expectEmit(true, true, false, true);
+        emit RateLimited.ConsumedFilledLevel(
+            499999999999993600,
+            block.timestamp
+        ); // precision loss
+        rateLimited.validateAndConsumeFilledLevel(consumeAmount);
+
+        assertApproxEqRelDecimal(
+            rateLimited.filledLevel(),
+            MAX_CAPACITY - consumeAmount,
+            1e14,
+            0
+        );
+        assertEq(rateLimited.lastUpdated(), block.timestamp);
+    }
+
     function testRateLimited_neverReturnsGtMaxLimit(
         uint256 _newAmount,
         uint40 _newTime
@@ -104,4 +129,24 @@ contract RateLimitLibTest is Test {
         currentTargetLimit = rateLimited.calculateCurrentLevel();
         assertApproxEqRel(currentTargetLimit, MAX_CAPACITY, ONE_PERCENT);
     }
+
+    function testCalculateCurrentLevel_revertsWhenCapacityIsZero() public {
+        rateLimited.setRefillRate(0);
+
+        vm.expectRevert("RateLimitNotSet");
+        rateLimited.calculateCurrentLevel();
+    }
+
+    function testValidateAndConsumeFilledLevel_revertsWhenExceedingLimit()
+        public
+    {
+        vm.warp(1 days);
+        uint256 initialLevel = rateLimited.calculateCurrentLevel();
+
+        uint256 excessAmount = initialLevel + 1 ether;
+
+        vm.expectRevert("RateLimitExceeded");
+        rateLimited.validateAndConsumeFilledLevel(excessAmount);
+        assertEq(rateLimited.calculateCurrentLevel(), initialLevel);
+    }
 }