Add average uptime calculation

contracts/interfaces/ISFC.sol

@@ -111,6 +111,8 @@ interface ISFC {
 
     function getEpochAccumulatedUptime(uint256 epoch, uint256 validatorID) external view returns (uint256);
 
+    function getEpochAverageUptime(uint256 epoch, uint256 validatorID) external view returns (uint32);
+
     function getEpochAccumulatedOriginatedTxsFee(uint256 epoch, uint256 validatorID) external view returns (uint256);
 
     function getEpochOfflineTime(uint256 epoch, uint256 validatorID) external view returns (uint256);

contracts/sfc/ConstantsManager.sol

@@ -29,6 +29,14 @@ contract ConstantsManager is Ownable {
     uint256 public targetGasPowerPerSecond;
     uint256 public gasPriceBalancingCounterweight;
 
+    // The number of epochs to calculate the average uptime ratio from, acceptable bound [10, 87600].
+    // Is also the minimum number of epochs necessary for deactivation of offline validators.
+    uint32 public averageUptimeEpochWindow;
+
+    // Minimum average uptime ratio in fixed-point format; acceptable bounds [0,0.9].
+    // Zero to disable validators deactivation by this metric.
+    uint64 public minAverageUptime;
+
     /**
      * @dev Given value is too small
      */
@@ -153,4 +161,22 @@ contract ConstantsManager is Ownable {
         }
         gasPriceBalancingCounterweight = v;
     }
+
+    function updateAverageUptimeEpochWindow(uint32 v) external virtual onlyOwner {
+        if (v < 10) {
+            // needs to be long enough to allow permissible downtime for validators maintenance
+            revert ValueTooSmall();
+        }
+        if (v > 87600) {
+            revert ValueTooLarge();
+        }
+        averageUptimeEpochWindow = v;
+    }
+
+    function updateMinAverageUptime(uint64 v) external virtual onlyOwner {
+        if (v > ((Decimal.unit() * 9) / 10)) {
+            revert ValueTooLarge();
+        }
+        minAverageUptime = v;
+    }
 }

contracts/sfc/NetworkInitializer.sol

@@ -37,6 +37,8 @@ contract NetworkInitializer {
         consts.updateOfflinePenaltyThresholdBlocksNum(1000);
         consts.updateTargetGasPowerPerSecond(2000000);
         consts.updateGasPriceBalancingCounterweight(3600);
+        consts.updateAverageUptimeEpochWindow(100);
+        consts.updateMinAverageUptime(0); // check disabled by default
         consts.transferOwnership(_owner);
 
         ISFC(_sfc).initialize(sealedEpoch, totalSupply, _auth, address(consts), _owner);

contracts/sfc/SFC.sol

@@ -17,6 +17,7 @@ contract SFC is Initializable, Ownable, Version {
     uint256 internal constant OK_STATUS = 0;
     uint256 internal constant WITHDRAWN_BIT = 1;
     uint256 internal constant OFFLINE_BIT = 1 << 3;
+    uint256 internal constant OFFLINE_AVG_BIT = 1 << 4;
     uint256 internal constant DOUBLESIGN_BIT = 1 << 7;
     uint256 internal constant CHEATER_MASK = DOUBLESIGN_BIT;
 
@@ -68,13 +69,25 @@ contract SFC is Initializable, Ownable, Version {
     // delegator => validator ID => current stake
     mapping(address delegator => mapping(uint256 validatorID => uint256 stake)) public getStake;
 
+    // data structure to compute average uptime for each active validator
+    struct AverageUptime {
+        // average uptime ratio as a value between 0 and 1e18
+        uint64 averageUptime;
+        // remainder from the division in the average calculation
+        uint32 remainder;
+        // number of epochs in the average (at most averageUptimeEpochsWindow)
+        uint32 epochs;
+    }
+
     struct EpochSnapshot {
         // validator ID => validator weight in the epoch
         mapping(uint256 => uint256) receivedStake;
         // validator ID => accumulated ( delegatorsReward * 1e18 / receivedStake )
         mapping(uint256 => uint256) accumulatedRewardPerToken;
         // validator ID => accumulated online time
         mapping(uint256 => uint256) accumulatedUptime;
+        // validator ID => average uptime as a percentage
+        mapping(uint256 => AverageUptime) averageUptime;
         // validator ID => gas fees from txs originated by the validator
         mapping(uint256 => uint256) accumulatedOriginatedTxsFee;
         mapping(uint256 => uint256) offlineTime;
@@ -288,6 +301,7 @@ contract SFC is Initializable, Ownable, Version {
                 epochDuration = _now() - prevSnapshot.endTime;
             }
             _sealEpochRewards(epochDuration, snapshot, prevSnapshot, validatorIDs, uptimes, originatedTxsFee);
+            _sealEpochAverageUptime(epochDuration, snapshot, prevSnapshot, validatorIDs, uptimes);
         }
 
         currentSealedEpoch = currentEpoch();
@@ -520,6 +534,11 @@ contract SFC is Initializable, Ownable, Version {
         return getEpochSnapshot[epoch].accumulatedUptime[validatorID];
     }
 
+    /// Get average uptime for a validator in a given epoch.
+    function getEpochAverageUptime(uint256 epoch, uint256 validatorID) public view returns (uint64) {
+        return getEpochSnapshot[epoch].averageUptime[validatorID].averageUptime;
+    }
+
     /// Get accumulated originated txs fee for a validator in a given epoch.
     function getEpochAccumulatedOriginatedTxsFee(uint256 epoch, uint256 validatorID) public view returns (uint256) {
         return getEpochSnapshot[epoch].accumulatedOriginatedTxsFee[validatorID];
@@ -901,6 +920,64 @@ contract SFC is Initializable, Ownable, Version {
         }
     }
 
+    /// Seal epoch - recalculate average uptime time of validators
+    function _sealEpochAverageUptime(
+        uint256 epochDuration,
+        EpochSnapshot storage snapshot,
+        EpochSnapshot storage prevSnapshot,
+        uint256[] memory validatorIDs,
+        uint256[] memory uptimes
+    ) internal {
+        for (uint256 i = 0; i < validatorIDs.length; i++) {
+            uint256 validatorID = validatorIDs[i];
+            // compute normalised uptime as a percentage in the fixed-point format
+            uint256 normalisedUptime = (uptimes[i] * Decimal.unit()) / epochDuration;
+            if (normalisedUptime > Decimal.unit()) {
+                normalisedUptime = Decimal.unit();
+            }
+            AverageUptime memory previous = prevSnapshot.averageUptime[validatorID];
+            AverageUptime memory current = _addElementIntoAverageUptime(uint64(normalisedUptime), previous);
+            snapshot.averageUptime[validatorID] = current;
+
+            // remove validator if average uptime drops below min average uptime
+            // (by setting minAverageUptime to zero, this check is ignored)
+            if (current.averageUptime < c.minAverageUptime() && current.epochs >= c.averageUptimeEpochWindow()) {
+                _setValidatorDeactivated(validatorID, OFFLINE_AVG_BIT);
+                _syncValidator(validatorID, false);
+            }
+        }
+    }
+
+    function _addElementIntoAverageUptime(
+        uint64 newValue,
+        AverageUptime memory prev
+    ) private view returns (AverageUptime memory) {
+        AverageUptime memory cur;
+        if (prev.epochs == 0) {
+            cur.averageUptime = newValue; // the only element for the average
+            cur.epochs = 1;
+            return cur;
+        }
+
+        // the number of elements the average is calculated from
+        uint128 n = prev.epochs + 1;
+        // add new value into the average
+        uint128 tmp = (n - 1) * uint128(prev.averageUptime) + uint128(newValue) + prev.remainder;
+
+        cur.averageUptime = uint64(tmp / n);
+        cur.remainder = uint32(tmp % n);
+
+        if (cur.averageUptime > Decimal.unit()) {
+            cur.averageUptime = uint64(Decimal.unit());
+        }
+        if (prev.epochs < c.averageUptimeEpochWindow()) {
+            cur.epochs = prev.epochs + 1;
+        } else {
+            cur.epochs = prev.epochs;
+        }
+        return cur;
+    }
+
     /// Create a new validator.
     function _createValidator(address auth, bytes calldata pubkey) internal {
         uint256 validatorID = ++lastValidatorID;

contracts/test/UnitTestSFC.sol

@@ -75,6 +75,8 @@ contract UnitTestNetworkInitializer {
         consts.updateOfflinePenaltyThresholdBlocksNum(1000);
         consts.updateTargetGasPowerPerSecond(2000000);
         consts.updateGasPriceBalancingCounterweight(6 * 60 * 60);
+        consts.updateAverageUptimeEpochWindow(10);
+        consts.updateMinAverageUptime(0); // check disabled by default
         consts.transferOwnership(_owner);
 
         ISFC(_sfc).initialize(sealedEpoch, totalSupply, _auth, address(consts), _owner);

test/SFC.ts

@@ -951,4 +951,82 @@ describe('SFC', () => {
       await expect(this.sfc._syncValidator(33, false)).to.be.revertedWithCustomError(this.sfc, 'ValidatorNotExists');
     });
   });
+
+  describe('Average uptime calculation', () => {
+    const validatorsFixture = async function (this: Context) {
+      const [validator] = await ethers.getSigners();
+      const pubkey =
+        '0xc000a2941866e485442aa6b17d67d77f8a6c4580bb556894cc1618473eff1e18203d8cce50b563cf4c75e408886079b8f067069442ed52e2ac9e556baa3f8fcc525f';
+      const blockchainNode = new BlockchainNode(this.sfc);
+
+      await this.sfc.rebaseTime();
+      await this.sfc.enableNonNodeCalls();
+
+      await blockchainNode.handleTx(
+        await this.sfc.connect(validator).createValidator(pubkey, { value: ethers.parseEther('10') }),
+      );
+
+      const validatorId = await this.sfc.getValidatorID(validator);
+
+      await blockchainNode.sealEpoch(0);
+
+      return {
+        validatorId,
+        blockchainNode,
+      };
+    };
+
+    beforeEach(async function () {
+      return Object.assign(this, await loadFixture(validatorsFixture.bind(this)));
+    });
+
+    it('Should calculate uptime correctly', async function () {
+      // validator online 100% of time in the first epoch => average 100%
+      await this.blockchainNode.sealEpoch(
+        100,
+        new Map<number, ValidatorMetrics>([[this.validatorId as number, new ValidatorMetrics(0, 0, 100, 0n)]]),
+      );
+      expect(await this.sfc.getEpochAverageUptime(await this.sfc.currentSealedEpoch(), this.validatorId)).to.equal(
+        1000000000000000000n,
+      );
+
+      // validator online 20% of time in the second epoch => average 60%
+      await this.blockchainNode.sealEpoch(
+        100,
+        new Map<number, ValidatorMetrics>([[this.validatorId as number, new ValidatorMetrics(0, 0, 20, 0n)]]),
+      );
+      expect(await this.sfc.getEpochAverageUptime(await this.sfc.currentSealedEpoch(), this.validatorId)).to.equal(
+        600000000000000000n,
+      );
+
+      // validator online 30% of time in the third epoch => average 50%
+      await this.blockchainNode.sealEpoch(
+        100,
+        new Map<number, ValidatorMetrics>([[this.validatorId as number, new ValidatorMetrics(0, 0, 30, 0n)]]),
+      );
+      expect(await this.sfc.getEpochAverageUptime(await this.sfc.currentSealedEpoch(), this.validatorId)).to.equal(
+        500000000000000000n,
+      );
+
+      // fill the averaging window
+      for (let i = 0; i < 10; i++) {
+        await this.blockchainNode.sealEpoch(
+          100,
+          new Map<number, ValidatorMetrics>([[this.validatorId as number, new ValidatorMetrics(0, 0, 50, 0n)]]),
+        );
+        expect(await this.sfc.getEpochAverageUptime(await this.sfc.currentSealedEpoch(), this.validatorId)).to.equal(
+          500000000000000000n,
+        );
+      }
+
+      // (50 * 10 + 28) / 11 = 48
+      await this.blockchainNode.sealEpoch(
+        100,
+        new Map<number, ValidatorMetrics>([[this.validatorId as number, new ValidatorMetrics(0, 0, 28, 0n)]]),
+      );
+      expect(await this.sfc.getEpochAverageUptime(await this.sfc.currentSealedEpoch(), this.validatorId)).to.equal(
+        480000000000000000n,
+      );
+    });
+  });
 });