feat(rollup): seed snapshots

l1-contracts/src/core/RollupCore.sol

@@ -120,7 +120,6 @@ contract RollupCore is
     onlyOwner
   {
     CheatLib.cheat__InitialiseValidatorSet(_args);
-    setupEpoch();
   }
 
   function setEpochVerifier(address _verifier) external override(ITestRollup) onlyOwner {

l1-contracts/src/core/interfaces/IValidatorSelection.sol

@@ -3,25 +3,24 @@
 pragma solidity >=0.8.27;
 
 import {Timestamp, Slot, Epoch} from "@aztec/core/libraries/TimeLib.sol";
-
+import {Checkpoints} from "@oz/utils/structs/Checkpoints.sol";
 /**
  * @notice  The data structure for an epoch
  * @param committee - The attesters for the epoch
  * @param sampleSeed - The seed used to sample the attesters of the epoch
  * @param nextSeed - The seed used to influence the NEXT epoch
  */
+
 struct EpochData {
   // TODO: remove in favor of commitment to comittee
   address[] committee;
-  uint256 sampleSeed;
-  uint256 nextSeed;
 }
 
 struct ValidatorSelectionStorage {
   // A mapping to snapshots of the validator set
   mapping(Epoch => EpochData) epochs;
-  // The last stored randao value, same value as `seed` in the last inserted epoch
-  uint256 lastSeed;
+  // Checkpointed map of epoch -> sample seed
+  Checkpoints.Trace224 seeds;
   uint256 targetCommitteeSize;
 }
 

l1-contracts/src/core/libraries/rollup/ExtRollupLib.sol

@@ -3,15 +3,18 @@
 pragma solidity >=0.8.27;
 
 import {SubmitEpochRootProofArgs, PublicInputArgs} from "@aztec/core/interfaces/IRollup.sol";
+import {Epoch, Timestamp, TimeLib} from "@aztec/core/libraries/TimeLib.sol";
 import {StakingLib} from "./../staking/StakingLib.sol";
 import {ValidatorSelectionLib} from "./../validator-selection/ValidatorSelectionLib.sol";
 import {BlobLib} from "./BlobLib.sol";
 import {EpochProofLib} from "./EpochProofLib.sol";
 import {ProposeLib, ProposeArgs, Signature} from "./ProposeLib.sol";
-
 // We are using this library such that we can more easily "link" just a larger external library
 // instead of a few smaller ones.
+
 library ExtRollupLib {
+  using TimeLib for Timestamp;
+
   function submitEpochRootProof(SubmitEpochRootProofArgs calldata _args) external {
     EpochProofLib.submitEpochRootProof(_args);
   }
@@ -30,7 +33,8 @@ library ExtRollupLib {
   }
 
   function setupEpoch() external {
-    ValidatorSelectionLib.setupEpoch(StakingLib.getStorage());
+    Epoch currentEpoch = Timestamp.wrap(block.timestamp).epochFromTimestamp();
+    ValidatorSelectionLib.setupEpoch(StakingLib.getStorage(), currentEpoch);
   }
 
   function getEpochProofPublicInputs(

l1-contracts/src/core/libraries/rollup/ProposeLib.sol

@@ -99,10 +99,10 @@ library ProposeLib {
       BlobLib.validateBlobs(_blobInput, _checkBlob);
 
     Header memory header = HeaderLib.decode(_args.header);
-
     v.headerHash = HeaderLib.hash(_args.header);
 
-    ValidatorSelectionLib.setupEpoch(StakingLib.getStorage());
+    Epoch currentEpoch = Timestamp.wrap(block.timestamp).epochFromTimestamp();
+    ValidatorSelectionLib.setupEpoch(StakingLib.getStorage(), currentEpoch);
 
     ManaBaseFeeComponents memory components =
       getManaBaseFeeComponentsAt(Timestamp.wrap(block.timestamp), true);

l1-contracts/src/core/libraries/validator-selection/ValidatorSelectionLib.sol

@@ -16,6 +16,8 @@ import {
 } from "@aztec/core/libraries/staking/AddressSnapshotLib.sol";
 import {Timestamp, Slot, Epoch, TimeLib} from "@aztec/core/libraries/TimeLib.sol";
 import {MessageHashUtils} from "@oz/utils/cryptography/MessageHashUtils.sol";
+import {SafeCast} from "@oz/utils/math/SafeCast.sol";
+import {Checkpoints} from "@oz/utils/structs/Checkpoints.sol";
 import {EnumerableSet} from "@oz/utils/structs/EnumerableSet.sol";
 
 library ValidatorSelectionLib {
@@ -24,6 +26,8 @@ library ValidatorSelectionLib {
   using SignatureLib for Signature;
   using TimeLib for Timestamp;
   using AddressSnapshotLib for SnapshottedAddressSet;
+  using Checkpoints for Checkpoints.Trace224;
+  using SafeCast for *;
 
   bytes32 private constant VALIDATOR_SELECTION_STORAGE_POSITION =
     keccak256("aztec.validator_selection.storage");
@@ -36,23 +40,30 @@ library ValidatorSelectionLib {
   /**
    * @notice  Performs a setup of an epoch if needed. The setup will
    *          - Sample the validator set for the epoch
-   *          - Set the seed for the epoch
-   *          - Update the last seed
-   *
-   * @dev     Since this is a reference optimising for simplicity, we store the actual validator set in the epoch structure.
-   *          This is very heavy on gas, so start crying because the gas here will melt the poles
-   *          https://i.giphy.com/U1aN4HTfJ2SmgB2BBK.webp
+   *          - Set the seed for the next epoch
    */
-  function setupEpoch(StakingStorage storage _stakingStore) internal {
-    Epoch epochNumber = Timestamp.wrap(block.timestamp).epochFromTimestamp();
+  function setupEpoch(StakingStorage storage _stakingStore, Epoch _epochNumber) internal {
     ValidatorSelectionStorage storage store = getStorage();
-    EpochData storage epoch = store.epochs[epochNumber];
 
-    if (epoch.sampleSeed == 0) {
-      epoch.sampleSeed = getSampleSeed(epochNumber);
-      epoch.nextSeed = store.lastSeed = computeNextSeed(epochNumber);
-      epoch.committee = sampleValidators(_stakingStore, epochNumber, epoch.sampleSeed);
+    // Get the sample seed for this current epoch.
+    uint224 sampleSeed = getSampleSeed(_epochNumber);
+
+    // If no sample seed is set, we are in a genesis state, we set the sample seed to max and push it into the store
+    if (sampleSeed == 0) {
+      sampleSeed = type(uint224).max;
+      store.seeds.push(Epoch.unwrap(_epochNumber).toUint32(), sampleSeed);
+    }
+
+    // If the committee is not set for this epoch, we need to sample it
+    EpochData storage epoch = store.epochs[_epochNumber];
+    uint256 committeeLength = epoch.committee.length;
+    if (committeeLength == 0) {
+      epoch.committee = sampleValidators(_stakingStore, _epochNumber, sampleSeed);
     }
+
+    // Set the sample seed for the next epoch if required
+    // function handles the case where it is already set
+    setSampleSeedForEpoch(_epochNumber + Epoch.wrap(1));
   }
 
   /**
@@ -159,7 +170,7 @@ library ValidatorSelectionLib {
    *
    * @return The validators for the given epoch
    */
-  function sampleValidators(StakingStorage storage _stakingStore, Epoch _epoch, uint256 _seed)
+  function sampleValidators(StakingStorage storage _stakingStore, Epoch _epoch, uint224 _seed)
     internal
     returns (address[] memory)
   {
@@ -194,23 +205,35 @@ library ValidatorSelectionLib {
     ValidatorSelectionStorage storage store = getStorage();
     EpochData storage epoch = store.epochs[_epochNumber];
 
-    if (epoch.sampleSeed != 0) {
-      uint256 committeeSize = epoch.committee.length;
-      if (committeeSize == 0) {
-        return new address[](0);
-      }
-      return epoch.committee;
+    // If no committee has been stored, then we need to setup the epoch
+    uint256 committeeSize = epoch.committee.length;
+    if (committeeSize == 0) {
+      // This will set epoch.committee and the next sample seed in the store, meaning epoch.commitee on the line below will be set (storage reference)
+      setupEpoch(_stakingStore, _epochNumber);
     }
+    return epoch.committee;
+  }
 
-    // Allow anyone if there is no validator set
-    if (_stakingStore.attesters.length() == 0) {
-      return new address[](0);
-    }
+  function setSampleSeedForEpoch(Epoch _epoch) internal {
+    ValidatorSelectionStorage storage store = getStorage();
+    uint32 epoch = Epoch.unwrap(_epoch).toUint32();
 
-    // Emulate a sampling of the validators
-    uint256 sampleSeed = getSampleSeed(_epochNumber);
+    // Check if the latest checkpoint is for the next epoch
+    (bool exists, uint32 key,) = store.seeds.latestCheckpoint();
 
-    return sampleValidators(_stakingStore, _epochNumber, sampleSeed);
+    // If the sample seed for the next epoch is already set, we can skip the computation
+    // It should be impossible that zero epoch snapshots exist, as in the genesis state we push the first sample seed into the store
+    uint32 mostRecentSeedEpoch = exists ? key : 0;
+    if (mostRecentSeedEpoch == epoch) {
+      return;
+    }
+
+    // If the most recently stored seed is less than the epoch we are querying, then we need to compute it's seed for later use
+    if (mostRecentSeedEpoch < epoch) {
+      // Compute the sample seed for the next epoch
+      uint224 nextSeed = computeNextSeed(_epoch);
+      store.seeds.push(epoch, nextSeed);
+    }
   }
 
   /**
@@ -227,22 +250,13 @@ library ValidatorSelectionLib {
    *
    * @return The sample seed for the epoch
    */
-  function getSampleSeed(Epoch _epoch) internal view returns (uint256) {
-    if (Epoch.unwrap(_epoch) == 0) {
-      return type(uint256).max;
-    }
+  function getSampleSeed(Epoch _epoch) internal view returns (uint224) {
     ValidatorSelectionStorage storage store = getStorage();
-    uint256 sampleSeed = store.epochs[_epoch].sampleSeed;
-    if (sampleSeed != 0) {
-      return sampleSeed;
-    }
-
-    sampleSeed = store.epochs[_epoch - Epoch.wrap(1)].nextSeed;
-    if (sampleSeed != 0) {
-      return sampleSeed;
+    uint224 sampleSeed = store.seeds.upperLookup(Epoch.unwrap(_epoch).toUint32());
+    if (sampleSeed == 0) {
+      sampleSeed = type(uint224).max;
     }
-
-    return store.lastSeed;
+    return sampleSeed;
   }
 
   function getStorage() internal pure returns (ValidatorSelectionStorage storage storageStruct) {
@@ -262,8 +276,9 @@ library ValidatorSelectionLib {
    *
    * @return The computed seed
    */
-  function computeNextSeed(Epoch _epoch) private view returns (uint256) {
-    return uint256(keccak256(abi.encode(_epoch, block.prevrandao)));
+  function computeNextSeed(Epoch _epoch) private view returns (uint224) {
+    // Allow for unsafe (lossy) downcast as we do not care if we loose bits
+    return uint224(uint256(keccak256(abi.encode(_epoch, block.prevrandao))));
   }
 
   /**