import {routes, ServerApi} from "@lodestar/api";
import {
  CachedBeaconStateAllForks,
  computeStartSlotAtEpoch,
  proposerShufflingDecisionRoot,
  attesterShufflingDecisionRoot,
  getBlockRootAtSlot,
  computeEpochAtSlot,
  getCurrentSlot,
} from "@lodestar/state-transition";
import {GENESIS_SLOT, SLOTS_PER_EPOCH, SLOTS_PER_HISTORICAL_ROOT, SYNC_COMMITTEE_SUBNET_SIZE} from "@lodestar/params";
import {Root, Slot, ValidatorIndex, ssz, Epoch} from "@lodestar/types";
import {ExecutionStatus} from "@lodestar/fork-choice";
import {toHex} from "@lodestar/utils";
import {fromHexString} from "@chainsafe/ssz";
import {AttestationError, AttestationErrorCode, GossipAction, SyncCommitteeError} from "../../../chain/errors/index.js";
import {validateGossipAggregateAndProof} from "../../../chain/validation/index.js";
import {ZERO_HASH} from "../../../constants/index.js";
import {SyncState} from "../../../sync/index.js";
import {isOptimisticBlock} from "../../../util/forkChoice.js";
import {toGraffitiBuffer} from "../../../util/graffiti.js";
import {ApiError, NodeIsSyncing} from "../errors.js";
import {validateSyncCommitteeGossipContributionAndProof} from "../../../chain/validation/syncCommitteeContributionAndProof.js";
import {CommitteeSubscription} from "../../../network/subnets/index.js";
import {ApiModules} from "../types.js";
import {RegenCaller} from "../../../chain/regen/index.js";
import {getValidatorStatus} from "../beacon/state/utils.js";
import {computeSubnetForCommitteesAtSlot, getPubkeysForIndices} from "./utils.js";

/**
 * If the node is within this many epochs from the head, we declare it to be synced regardless of
 * the network sync state.
 *
 * This helps prevent attacks where nodes can convince us that we're syncing some non-existent
 * finalized head.
 *
 * TODO: Lighthouse uses 8 for the attack described above. However, 8 kills Lodestar since validators
 * can trigger regen to fast-forward head state 8 epochs to be immediately invalidated as sync sets
 * a new head. Then the checkpoint state cache grows unbounded with very different states (because
 * they are 8 epochs apart) and causes an OOM. Research a proper solution once regen and the state
 * caches are better.
 */
const SYNC_TOLERANCE_EPOCHS = 1;

/**
 * Server implementation for handling validator duties.
 * See `@lodestar/validator/src/api` for the client implementation).
 */
export function getValidatorApi({
  chain,
  config,
  logger,
  metrics,
  network,
  sync,
}: ApiModules): ServerApi<routes.validator.Api> {
  let genesisBlockRoot: Root | null = null;

  /**
   * Validator clock may be advanced from beacon's clock. If the validator requests a resource in a
   * future slot, wait some time instead of rejecting the request because it's in the future.
   * For very fast networks, reduce clock disparity to half a slot.
   */
  const MAX_API_CLOCK_DISPARITY_SEC = Math.min(1, config.SECONDS_PER_SLOT / 2);
  const MAX_API_CLOCK_DISPARITY_MS = MAX_API_CLOCK_DISPARITY_SEC * 1000;

  /** Compute and cache the genesis block root */
  async function getGenesisBlockRoot(state: CachedBeaconStateAllForks): Promise<Root> {
    if (!genesisBlockRoot) {
      // Close to genesis the genesis block may not be available in the DB
      if (state.slot < SLOTS_PER_HISTORICAL_ROOT) {
        genesisBlockRoot = state.blockRoots.get(0);
      }

      const genesisBlock = await chain.getCanonicalBlockAtSlot(GENESIS_SLOT);
      if (genesisBlock) {
        genesisBlockRoot = config.getForkTypes(genesisBlock.message.slot).SignedBeaconBlock.hashTreeRoot(genesisBlock);
      }
    }

    // If for some reason the genesisBlockRoot is not able don't prevent validators from
    // proposing or attesting. If the genesisBlockRoot is wrong, at worst it may trigger a re-fetch of the duties
    return genesisBlockRoot || ZERO_HASH;
  }

  /**
   * If advancing the local clock `MAX_API_CLOCK_DISPARITY_MS` ticks to the requested slot, wait for its start
   * Prevents the validator from getting errors from the API if the clock is a bit advanced
   */
  async function waitForSlot(slot: Slot): Promise<void> {
    if (slot <= 0) {
      return;
    }

    const slotStartSec = chain.genesisTime + slot * config.SECONDS_PER_SLOT;
    const msToSlot = slotStartSec * 1000 - Date.now();

    if (msToSlot > MAX_API_CLOCK_DISPARITY_MS) {
      throw Error(`Requested slot ${slot} is in the future`);
    } else if (msToSlot > 0) {
      await chain.clock.waitForSlot(slot);
    }

    // else, clock already in slot or slot is in the past
  }

  /**
   * If advancing the local clock `MAX_API_CLOCK_DISPARITY_MS` ticks to the next epoch, wait for slot 0 of the next epoch.
   * Prevents a validator from not being able to get the attestater duties correctly if the beacon and validator clocks are off
   */
  async function waitForNextClosestEpoch(): Promise<void> {
    const nextEpoch = chain.clock.currentEpoch + 1;
    const secPerEpoch = SLOTS_PER_EPOCH * config.SECONDS_PER_SLOT;
    const nextEpochStartSec = chain.genesisTime + nextEpoch * secPerEpoch;
    const msToNextEpoch = nextEpochStartSec * 1000 - Date.now();
    if (msToNextEpoch > 0 && msToNextEpoch < MAX_API_CLOCK_DISPARITY_MS) {
      await chain.clock.waitForSlot(computeStartSlotAtEpoch(nextEpoch));
    }
  }

  function currentEpochWithDisparity(): Epoch {
    return computeEpochAtSlot(getCurrentSlot(config, chain.genesisTime - MAX_API_CLOCK_DISPARITY_SEC));
  }

  /**
   * Reject any request while the node is syncing
   */
  function notWhileSyncing(): void {
    // Consider node synced before or close to genesis
    if (chain.clock.currentSlot < SLOTS_PER_EPOCH) {
      return;
    }

    const syncState = sync.state;
    switch (syncState) {
      case SyncState.SyncingFinalized:
      case SyncState.SyncingHead: {
        const currentSlot = chain.clock.currentSlot;
        const headSlot = chain.forkChoice.getHead().slot;
        if (currentSlot - headSlot > SYNC_TOLERANCE_EPOCHS * SLOTS_PER_EPOCH) {
          throw new NodeIsSyncing(`headSlot ${headSlot} currentSlot ${currentSlot}`);
        } else {
          return;
        }
      }

      case SyncState.Synced:
        return;

      case SyncState.Stalled:
        throw new NodeIsSyncing("waiting for peers");
    }
  }

  /**
   * Post merge, the CL and EL could be out of step in the sync, and could result in
   * Syncing status of the chain head. To be precise:
   * 1. CL could be ahead of the EL, with the validity of head payload not yet verified
   * 2. CL could be on an invalid chain of execution blocks with a non-existent
   *    or non-available parent that never syncs up
   *
   * Both the above scenarios could be problematic and hence validator shouldn't participate
   * or weigh its vote on a head till it resolves to a Valid execution status.
   * Following activities should be skipped on an Optimistic head (with Syncing status):
   * 1. Attestation if targetRoot is optimistic
   * 2. SyncCommitteeContribution if if the root for which to produce contribution is Optimistic.
   * 3. ProduceBlock if the parentRoot (chain's current head is optimistic). However this doesn't
   *    need to be checked/aborted here as assembleBody would call EL's api for the latest
   *    executionStatus of the parentRoot. If still not validated, produceBlock will throw error.
   *
   * TODO/PENDING: SyncCommitteeSignatures should also be aborted, the best way to address this
   *   is still in flux and will be updated as and when other CL's figure this out.
   */

  function notOnOptimisticBlockRoot(beaconBlockRoot: Root): void {
    const protoBeaconBlock = chain.forkChoice.getBlock(beaconBlockRoot);
    if (!protoBeaconBlock) {
      throw new ApiError(400, "Block not in forkChoice");
    }

    if (protoBeaconBlock.executionStatus === ExecutionStatus.Syncing)
      throw new NodeIsSyncing(
        `Block's execution payload not yet validated, executionPayloadBlockHash=${protoBeaconBlock.executionPayloadBlockHash}`
      );
  }

  const produceBlindedBlock: ServerApi<routes.validator.Api>["produceBlindedBlock"] = async function produceBlindedBlock(
    slot,
    randaoReveal,
    graffiti
  ) {
    let timer;
    metrics?.blockProductionRequests.inc();
    try {
      notWhileSyncing();
      await waitForSlot(slot); // Must never request for a future slot > currentSlot

      // Error early for builder if builder flow not active
      if (!chain.executionBuilder) {
        throw Error("Execution builder not set");
      }
      if (!chain.executionBuilder.status) {
        throw Error("Execution builder disabled");
      }

      // Process the queued attestations in the forkchoice for correct head estimation
      // forkChoice.updateTime() might have already been called by the onSlot clock
      // handler, in which case this should just return.
      chain.forkChoice.updateTime(slot);
      chain.forkChoice.updateHead();

      timer = metrics?.blockProductionTime.startTimer();
      const {block, blockValue} = await chain.produceBlindedBlock({
        slot,
        randaoReveal,
        graffiti: toGraffitiBuffer(graffiti || ""),
      });
      metrics?.blockProductionSuccess.inc();
      return {data: block, version: config.getForkName(block.slot), blockValue};
    } finally {
      if (timer) timer();
    }
  };

  const produceBlock: ServerApi<routes.validator.Api>["produceBlockV2"] = async function produceBlock(
    slot,
    randaoReveal,
    graffiti
  ) {
    let timer;
    metrics?.blockProductionRequests.inc();
    try {
      notWhileSyncing();
      await waitForSlot(slot); // Must never request for a future slot > currentSlot

      // Process the queued attestations in the forkchoice for correct head estimation
      // forkChoice.updateTime() might have already been called by the onSlot clock
      // handler, in which case this should just return.
      chain.forkChoice.updateTime(slot);
      chain.recomputeForkChoiceHead();

      timer = metrics?.blockProductionTime.startTimer();
      const {block, blockValue} = await chain.produceBlock({
        slot,
        randaoReveal,
        graffiti: toGraffitiBuffer(graffiti || ""),
      });
      metrics?.blockProductionSuccess.inc();
      metrics?.blockProductionNumAggregated.observe(block.body.attestations.length);
      return {data: block, version: config.getForkName(block.slot), blockValue};
    } finally {
      if (timer) timer();
    }
  };

  return {
    produceBlock: produceBlock,
    produceBlockV2: produceBlock,
    produceBlindedBlock,

    async produceAttestationData(committeeIndex, slot) {
      notWhileSyncing();

      await waitForSlot(slot); // Must never request for a future slot > currentSlot

      // This needs a state in the same epoch as `slot` such that state.currentJustifiedCheckpoint is correct.
      // Note: This may trigger an epoch transition if there skipped slots at the beginning of the epoch.
      const headState = chain.getHeadState();
      const headSlot = headState.slot;
      const attEpoch = computeEpochAtSlot(slot);
      const headEpoch = computeEpochAtSlot(headSlot);
      const headBlockRootHex = chain.forkChoice.getHead().blockRoot;
      const headBlockRoot = fromHexString(headBlockRootHex);

      const beaconBlockRoot =
        slot >= headSlot
          ? // When attesting to the head slot or later, always use the head of the chain.
            headBlockRoot
          : // Permit attesting to slots *prior* to the current head. This is desirable when
            // the VC and BN are out-of-sync due to time issues or overloading.
            getBlockRootAtSlot(headState, slot);

      const targetSlot = computeStartSlotAtEpoch(attEpoch);
      const targetRoot =
        targetSlot >= headSlot
          ? // If the state is earlier than the target slot then the target *must* be the head block root.
            headBlockRoot
          : getBlockRootAtSlot(headState, targetSlot);

      // Check the execution status as validator shouldn't vote on an optimistic head
      // Check on target is sufficient as a valid target would imply a valid source
      notOnOptimisticBlockRoot(targetRoot);

      // To get the correct source we must get a state in the same epoch as the attestation's epoch.
      // An epoch transition may change state.currentJustifiedCheckpoint
      const attEpochState =
        attEpoch <= headEpoch
          ? headState
          : // Will advance the state to the correct next epoch if necessary
            await chain.regen.getBlockSlotState(headBlockRootHex, slot, RegenCaller.produceAttestationData);

      return {
        data: {
          slot,
          index: committeeIndex,
          beaconBlockRoot,
          source: attEpochState.currentJustifiedCheckpoint,
          target: {epoch: attEpoch, root: targetRoot},
        },
      };
    },

    /**
     * GET `/eth/v1/validator/sync_committee_contribution`
     *
     * Requests that the beacon node produce a sync committee contribution.
     *
     * https://github.com/ethereum/beacon-APIs/pull/138
     *
     * @param slot The slot for which a sync committee contribution should be created.
     * @param subcommitteeIndex The subcommittee index for which to produce the contribution.
     * @param beaconBlockRoot The block root for which to produce the contribution.
     */
    async produceSyncCommitteeContribution(slot, subcommitteeIndex, beaconBlockRoot) {
      // Check the execution status as validator shouldn't contribute on an optimistic head
      notOnOptimisticBlockRoot(beaconBlockRoot);

      const contribution = chain.syncCommitteeMessagePool.getContribution(subcommitteeIndex, slot, beaconBlockRoot);
      if (!contribution) throw new ApiError(500, "No contribution available");
      return {data: contribution};
    },

    async getProposerDuties(epoch) {
      notWhileSyncing();

      // Early check that epoch is within [current_epoch, current_epoch + 1], or allow for pre-genesis
      const currentEpoch = currentEpochWithDisparity();
      if (currentEpoch >= 0 && epoch !== currentEpoch && epoch !== currentEpoch + 1) {
        throw Error(`Requested epoch ${epoch} must equal current ${currentEpoch} or next epoch ${currentEpoch + 1}`);
      }

      // May request for an epoch that's in the future, for getBeaconProposersNextEpoch()
      await waitForNextClosestEpoch();

      const head = chain.forkChoice.getHead();
      const state = await chain.getHeadStateAtCurrentEpoch();

      const stateEpoch = state.epochCtx.epoch;
      let indexes: ValidatorIndex[] = [];

      if (epoch === stateEpoch) {
        indexes = state.epochCtx.getBeaconProposers();
      } else if (epoch === stateEpoch + 1) {
        // Requesting duties for next epoch is allow since they can be predicted with high probabilities.
        // @see `epochCtx.getBeaconProposersNextEpoch` JSDocs for rationale.
        indexes = state.epochCtx.getBeaconProposersNextEpoch();
      } else {
        // Should never happen, epoch is checked to be in bounds above
        throw Error(`Proposer duties for epoch ${epoch} not supported, current epoch ${stateEpoch}`);
      }

      // NOTE: this is the fastest way of getting compressed pubkeys.
      //       See benchmark -> packages/lodestar/test/perf/api/impl/validator/attester.test.ts
      // After dropping the flat caches attached to the CachedBeaconState it's no longer available.
      // TODO: Add a flag to just send 0x00 as pubkeys since the Lodestar validator does not need them.
      const pubkeys = getPubkeysForIndices(state.validators, indexes);

      const startSlot = computeStartSlotAtEpoch(stateEpoch);
      const duties: routes.validator.ProposerDuty[] = [];
      for (let i = 0; i < SLOTS_PER_EPOCH; i++) {
        duties.push({slot: startSlot + i, validatorIndex: indexes[i], pubkey: pubkeys[i]});
      }

      // Returns `null` on the one-off scenario where the genesis block decides its own shuffling.
      // It should be set to the latest block applied to `self` or the genesis block root.
      const dependentRoot = proposerShufflingDecisionRoot(state) || (await getGenesisBlockRoot(state));

      return {
        data: duties,
        dependentRoot: toHex(dependentRoot),
        executionOptimistic: isOptimisticBlock(head),
      };
    },

    async getAttesterDuties(epoch, validatorIndices) {
      notWhileSyncing();

      if (validatorIndices.length === 0) {
        throw new ApiError(400, "No validator to get attester duties");
      }

      // May request for an epoch that's in the future
      await waitForNextClosestEpoch();

      // should not compare to headEpoch in order to handle skipped slots
      // Check if the epoch is in the future after waiting for requested slot
      if (epoch > chain.clock.currentEpoch + 1) {
        throw new ApiError(400, "Cannot get duties for epoch more than one ahead");
      }

      const head = chain.forkChoice.getHead();
      const state = await chain.getHeadStateAtCurrentEpoch();

      // TODO: Determine what the current epoch would be if we fast-forward our system clock by
      // `MAXIMUM_GOSSIP_CLOCK_DISPARITY`.
      //
      // Most of the time, `tolerantCurrentEpoch` will be equal to `currentEpoch`. However, during
      // the first `MAXIMUM_GOSSIP_CLOCK_DISPARITY` duration of the epoch `tolerantCurrentEpoch`
      // will equal `currentEpoch + 1`

      // Check that all validatorIndex belong to the state before calling getCommitteeAssignments()
      const pubkeys = getPubkeysForIndices(state.validators, validatorIndices);
      const committeeAssignments = state.epochCtx.getCommitteeAssignments(epoch, validatorIndices);
      const duties: routes.validator.AttesterDuty[] = [];
      for (let i = 0, len = validatorIndices.length; i < len; i++) {
        const validatorIndex = validatorIndices[i];
        const duty = committeeAssignments.get(validatorIndex) as routes.validator.AttesterDuty | undefined;
        if (duty) {
          // Mutate existing object instead of re-creating another new object with spread operator
          // Should be faster and require less memory
          duty.pubkey = pubkeys[i];
          duties.push(duty);
        }
      }

      const dependentRoot = attesterShufflingDecisionRoot(state, epoch) || (await getGenesisBlockRoot(state));

      return {
        data: duties,
        dependentRoot: toHex(dependentRoot),
        executionOptimistic: isOptimisticBlock(head),
      };
    },

    /**
     * `POST /eth/v1/validator/duties/sync/{epoch}`
     *
     * Requests the beacon node to provide a set of sync committee duties for a particular epoch.
     * - Although pubkey can be inferred from the index we return it to keep this call analogous with the one that
     *   fetches attester duties.
     * - `sync_committee_index` is the index of the validator in the sync committee. This can be used to infer the
     *   subnet to which the contribution should be broadcast. Note, there can be multiple per validator.
     *
     * https://github.com/ethereum/beacon-APIs/pull/134
     *
     * @param validatorIndices an array of the validator indices for which to obtain the duties.
     */
    async getSyncCommitteeDuties(epoch, validatorIndices) {
      notWhileSyncing();

      if (validatorIndices.length === 0) {
        throw new ApiError(400, "No validator to get attester duties");
      }

      // May request for an epoch that's in the future
      await waitForNextClosestEpoch();

      // sync committee duties have a lookahead of 1 day. Assuming the validator only requests duties for upcoming
      // epochs, the head state will very likely have the duties available for the requested epoch.
      // Note: does not support requesting past duties
      const head = chain.forkChoice.getHead();
      const state = chain.getHeadState();

      // Check that all validatorIndex belong to the state before calling getCommitteeAssignments()
      const pubkeys = getPubkeysForIndices(state.validators, validatorIndices);
      // Ensures `epoch // EPOCHS_PER_SYNC_COMMITTEE_PERIOD <= current_epoch // EPOCHS_PER_SYNC_COMMITTEE_PERIOD + 1`
      const syncCommitteeCache = state.epochCtx.getIndexedSyncCommitteeAtEpoch(epoch);
      const syncCommitteeValidatorIndexMap = syncCommitteeCache.validatorIndexMap;

      const duties: routes.validator.SyncDuty[] = [];
      for (let i = 0, len = validatorIndices.length; i < len; i++) {
        const validatorIndex = validatorIndices[i];
        const validatorSyncCommitteeIndices = syncCommitteeValidatorIndexMap.get(validatorIndex);
        if (validatorSyncCommitteeIndices) {
          duties.push({
            pubkey: pubkeys[i],
            validatorIndex,
            validatorSyncCommitteeIndices,
          });
        }
      }

      return {
        data: duties,
        executionOptimistic: isOptimisticBlock(head),
      };
    },

    async getAggregatedAttestation(attestationDataRoot, slot) {
      notWhileSyncing();

      await waitForSlot(slot); // Must never request for a future slot > currentSlot

      return {
        data: chain.attestationPool.getAggregate(slot, attestationDataRoot),
      };
    },

    async publishAggregateAndProofs(signedAggregateAndProofs) {
      notWhileSyncing();

      const seenTimestampSec = Date.now() / 1000;
      const errors: Error[] = [];

      await Promise.all(
        signedAggregateAndProofs.map(async (signedAggregateAndProof, i) => {
          try {
            // TODO: Validate in batch
            const {indexedAttestation, committeeIndices} = await validateGossipAggregateAndProof(
              chain,
              signedAggregateAndProof,
              true // skip known attesters check
            );

            chain.aggregatedAttestationPool.add(
              signedAggregateAndProof.message.aggregate,
              indexedAttestation.attestingIndices.length,
              committeeIndices
            );
            const sentPeers = await network.gossip.publishBeaconAggregateAndProof(signedAggregateAndProof);
            metrics?.submitAggregatedAttestation(seenTimestampSec, indexedAttestation, sentPeers);
          } catch (e) {
            if (e instanceof AttestationError && e.type.code === AttestationErrorCode.AGGREGATOR_ALREADY_KNOWN) {
              logger.debug("Ignoring known signedAggregateAndProof");
              return; // Ok to submit the same aggregate twice
            }

            errors.push(e as Error);
            logger.error(
              `Error on publishAggregateAndProofs [${i}]`,
              {
                slot: signedAggregateAndProof.message.aggregate.data.slot,
                index: signedAggregateAndProof.message.aggregate.data.index,
              },
              e as Error
            );
            if (e instanceof AttestationError && e.action === GossipAction.REJECT) {
              chain.persistInvalidSszValue(ssz.phase0.SignedAggregateAndProof, signedAggregateAndProof, "api_reject");
            }
          }
        })
      );

      if (errors.length > 1) {
        throw Error("Multiple errors on publishAggregateAndProofs\n" + errors.map((e) => e.message).join("\n"));
      } else if (errors.length === 1) {
        throw errors[0];
      }
    },

    /**
     * POST `/eth/v1/validator/contribution_and_proofs`
     *
     * Publish multiple signed sync committee contribution and proofs
     *
     * https://github.com/ethereum/beacon-APIs/pull/137
     */
    async publishContributionAndProofs(contributionAndProofs) {
      notWhileSyncing();

      const errors: Error[] = [];

      await Promise.all(
        contributionAndProofs.map(async (contributionAndProof, i) => {
          try {
            // TODO: Validate in batch
            const {syncCommitteeParticipants} = await validateSyncCommitteeGossipContributionAndProof(
              chain,
              contributionAndProof,
              true // skip known participants check
            );
            chain.syncContributionAndProofPool.add(contributionAndProof.message, syncCommitteeParticipants);
            await network.gossip.publishContributionAndProof(contributionAndProof);
          } catch (e) {
            errors.push(e as Error);
            logger.error(
              `Error on publishContributionAndProofs [${i}]`,
              {
                slot: contributionAndProof.message.contribution.slot,
                subcommitteeIndex: contributionAndProof.message.contribution.subcommitteeIndex,
              },
              e as Error
            );
            if (e instanceof SyncCommitteeError && e.action === GossipAction.REJECT) {
              chain.persistInvalidSszValue(ssz.altair.SignedContributionAndProof, contributionAndProof, "api_reject");
            }
          }
        })
      );

      if (errors.length > 1) {
        throw Error("Multiple errors on publishContributionAndProofs\n" + errors.map((e) => e.message).join("\n"));
      } else if (errors.length === 1) {
        throw errors[0];
      }
    },

    async prepareBeaconCommitteeSubnet(subscriptions) {
      notWhileSyncing();

      network.prepareBeaconCommitteeSubnet(
        subscriptions.map(({validatorIndex, slot, isAggregator, committeesAtSlot, committeeIndex}) => ({
          validatorIndex: validatorIndex,
          subnet: computeSubnetForCommitteesAtSlot(slot, committeesAtSlot, committeeIndex),
          slot: slot,
          isAggregator: isAggregator,
        }))
      );

      // TODO:
      // If the discovery mechanism isn't disabled, attempt to set up a peer discovery for the
      // required subnets.

      if (metrics) {
        for (const subscription of subscriptions) {
          metrics.registerLocalValidator(subscription.validatorIndex);
        }
      }
    },

    /**
     * POST `/eth/v1/validator/sync_committee_subscriptions`
     *
     * Subscribe to a number of sync committee subnets.
     * Sync committees are not present in phase0, but are required for Altair networks.
     * Subscribing to sync committee subnets is an action performed by VC to enable network participation in Altair networks,
     * and only required if the VC has an active validator in an active sync committee.
     *
     * https://github.com/ethereum/beacon-APIs/pull/136
     */
    async prepareSyncCommitteeSubnets(subscriptions) {
      notWhileSyncing();

      // A `validatorIndex` can be in multiple subnets, so compute the CommitteeSubscription with double for loop
      const subs: CommitteeSubscription[] = [];
      for (const sub of subscriptions) {
        for (const committeeIndex of sub.syncCommitteeIndices) {
          const subnet = Math.floor(committeeIndex / SYNC_COMMITTEE_SUBNET_SIZE);
          subs.push({
            validatorIndex: sub.validatorIndex,
            subnet: subnet,
            // Subscribe until the end of `untilEpoch`: https://github.com/ethereum/beacon-APIs/pull/136#issuecomment-840315097
            slot: computeStartSlotAtEpoch(sub.untilEpoch + 1),
            isAggregator: true,
          });
        }
      }

      network.prepareSyncCommitteeSubnets(subs);
    },

    async prepareBeaconProposer(proposers) {
      await chain.updateBeaconProposerData(chain.clock.currentEpoch, proposers);
    },

    async getLiveness(indices: ValidatorIndex[], epoch: Epoch) {
      if (indices.length === 0) {
        return {
          data: [],
        };
      }
      const currentEpoch = chain.clock.currentEpoch;
      if (epoch < currentEpoch - 1 || epoch > currentEpoch + 1) {
        throw new Error(
          `Request epoch ${epoch} is more than one epoch before or after the current epoch ${currentEpoch}`
        );
      }

      return {
        data: indices.map((index: ValidatorIndex) => ({
          index,
          epoch,
          isLive: chain.validatorSeenAtEpoch(index, epoch),
        })),
      };
    },

    async registerValidator(registrations) {
      // should only send active or pending validator to builder
      // Spec: https://ethereum.github.io/builder-specs/#/Builder/registerValidator
      const headState = chain.getHeadState();
      const currentEpoch = chain.clock.currentEpoch;

      const filteredRegistrations = registrations.filter((registration) => {
        const {pubkey} = registration.message;
        const validatorIndex = headState.epochCtx.pubkey2index.get(pubkey);
        if (validatorIndex === undefined) return false;

        const validator = headState.validators.getReadonly(validatorIndex);
        const status = getValidatorStatus(validator, currentEpoch);
        return (
          status === "active" ||
          status === "active_exiting" ||
          status === "active_ongoing" ||
          status === "active_slashed" ||
          status === "pending_initialized" ||
          status === "pending_queued"
        );
      });

      return chain.executionBuilder?.registerValidator(filteredRegistrations);
    },
  };
}