lighthouse/consensus/fork_choice/src/fork_choice.rs

use std::marker::PhantomData;

use proto_array::{Block as ProtoBlock, ProtoArrayForkChoice};
use ssz_derive::{Decode, Encode};
use types::{
    BeaconBlock, BeaconState, BeaconStateError, Checkpoint, Epoch, EthSpec, Hash256,
    IndexedAttestation, RelativeEpoch, ShufflingId, Slot,
};

use crate::ForkChoiceStore;
use std::cmp::Ordering;

/// Defined here:
///
/// https://github.com/ethereum/eth2.0-specs/blob/v0.12.1/specs/phase0/fork-choice.md#configuration
pub const SAFE_SLOTS_TO_UPDATE_JUSTIFIED: u64 = 8;

#[derive(Debug)]
pub enum Error<T> {
    InvalidAttestation(InvalidAttestation),
    InvalidBlock(InvalidBlock),
    ProtoArrayError(String),
    InvalidProtoArrayBytes(String),
    MissingProtoArrayBlock(Hash256),
    UnknownAncestor {
        ancestor_slot: Slot,
        descendant_root: Hash256,
    },
    InconsistentOnTick {
        previous_slot: Slot,
        time: Slot,
    },
    BeaconStateError(BeaconStateError),
    AttemptToRevertJustification {
        store: Slot,
        state: Slot,
    },
    ForkChoiceStoreError(T),
    UnableToSetJustifiedCheckpoint(T),
    AfterBlockFailed(T),
}

impl<T> From<InvalidAttestation> for Error<T> {
    fn from(e: InvalidAttestation) -> Self {
        Error::InvalidAttestation(e)
    }
}

#[derive(Debug)]
pub enum InvalidBlock {
    UnknownParent(Hash256),
    FutureSlot {
        current_slot: Slot,
        block_slot: Slot,
    },
    FinalizedSlot {
        finalized_slot: Slot,
        block_slot: Slot,
    },
    NotFinalizedDescendant {
        finalized_root: Hash256,
        block_ancestor: Option<Hash256>,
    },
}

#[derive(Debug)]
pub enum InvalidAttestation {
    /// The attestations aggregation bits were empty when they shouldn't be.
    EmptyAggregationBitfield,
    /// The `attestation.data.beacon_block_root` block is unknown.
    UnknownHeadBlock { beacon_block_root: Hash256 },
    /// The `attestation.data.slot` is not from the same epoch as `data.target.epoch` and therefore
    /// the attestation is invalid.
    BadTargetEpoch { target: Epoch, slot: Slot },
    /// The target root of the attestation points to a block that we have not verified.
    UnknownTargetRoot(Hash256),
    /// The attestation is for an epoch in the future (with respect to the gossip clock disparity).
    FutureEpoch {
        attestation_epoch: Epoch,
        current_epoch: Epoch,
    },
    /// The attestation is for an epoch in the past (with respect to the gossip clock disparity).
    PastEpoch {
        attestation_epoch: Epoch,
        current_epoch: Epoch,
    },
    /// The attestation references a target root that does not match what is stored in our
    /// database.
    InvalidTarget {
        attestation: Hash256,
        local: Hash256,
    },
    /// The attestation is attesting to a state that is later than itself. (Viz., attesting to the
    /// future).
    AttestsToFutureBlock { block: Slot, attestation: Slot },
}

impl<T> From<String> for Error<T> {
    fn from(e: String) -> Self {
        Error::ProtoArrayError(e)
    }
}

/// Calculate how far `slot` lies from the start of its epoch.
///
/// ## Specification
///
/// Equivalent to:
///
/// https://github.com/ethereum/eth2.0-specs/blob/v0.12.1/specs/phase0/fork-choice.md#compute_slots_since_epoch_start
pub fn compute_slots_since_epoch_start<E: EthSpec>(slot: Slot) -> Slot {
    slot - slot
        .epoch(E::slots_per_epoch())
        .start_slot(E::slots_per_epoch())
}

/// Calculate the first slot in `epoch`.
///
/// ## Specification
///
/// Equivalent to:
///
/// https://github.com/ethereum/eth2.0-specs/blob/v0.12.1/specs/phase0/beacon-chain.md#compute_start_slot_at_epoch
fn compute_start_slot_at_epoch<E: EthSpec>(epoch: Epoch) -> Slot {
    epoch.start_slot(E::slots_per_epoch())
}

/// Called whenever the current time increases.
///
/// ## Specification
///
/// Equivalent to:
///
/// https://github.com/ethereum/eth2.0-specs/blob/v0.12.1/specs/phase0/fork-choice.md#on_tick
fn on_tick<T, E>(store: &mut T, time: Slot) -> Result<(), Error<T::Error>>
where
    T: ForkChoiceStore<E>,
    E: EthSpec,
{
    let previous_slot = store.get_current_slot();

    if time > previous_slot + 1 {
        return Err(Error::InconsistentOnTick {
            previous_slot,
            time,
        });
    }

    // Update store time.
    store.set_current_slot(time);

    let current_slot = store.get_current_slot();
    if !(current_slot > previous_slot && compute_slots_since_epoch_start::<E>(current_slot) == 0) {
        return Ok(());
    }

    if store.best_justified_checkpoint().epoch > store.justified_checkpoint().epoch {
        store
            .set_justified_checkpoint(*store.best_justified_checkpoint())
            .map_err(Error::ForkChoiceStoreError)?;
    }

    Ok(())
}

/// Used for queuing attestations from the current slot. Only contains the minimum necessary
/// information about the attestation.
#[derive(Clone, PartialEq, Encode, Decode)]
pub struct QueuedAttestation {
    slot: Slot,
    attesting_indices: Vec<u64>,
    block_root: Hash256,
    target_epoch: Epoch,
}

impl<E: EthSpec> From<&IndexedAttestation<E>> for QueuedAttestation {
    fn from(a: &IndexedAttestation<E>) -> Self {
        Self {
            slot: a.data.slot,
            attesting_indices: a.attesting_indices[..].to_vec(),
            block_root: a.data.beacon_block_root,
            target_epoch: a.data.target.epoch,
        }
    }
}

/// Returns all values in `self.queued_attestations` that have a slot that is earlier than the
/// current slot. Also removes those values from `self.queued_attestations`.
fn dequeue_attestations(
    current_slot: Slot,
    queued_attestations: &mut Vec<QueuedAttestation>,
) -> Vec<QueuedAttestation> {
    let remaining = queued_attestations.split_off(
        queued_attestations
            .iter()
            .position(|a| a.slot >= current_slot)
            .unwrap_or_else(|| queued_attestations.len()),
    );

    std::mem::replace(queued_attestations, remaining)
}

/// Provides an implementation of "Ethereum 2.0 Phase 0 -- Beacon Chain Fork Choice":
///
/// https://github.com/ethereum/eth2.0-specs/blob/v0.12.1/specs/phase0/fork-choice.md#ethereum-20-phase-0----beacon-chain-fork-choice
///
/// ## Detail
///
/// This struct wraps `ProtoArrayForkChoice` and provides:
///
/// - Management of the justified state and caching of balances.
/// - Queuing of attestations from the current slot.
pub struct ForkChoice<T, E> {
    /// Storage for `ForkChoice`, modelled off the spec `Store` object.
    fc_store: T,
    /// The underlying representation of the block DAG.
    proto_array: ProtoArrayForkChoice,
    /// Attestations that arrived at the current slot and must be queued for later processing.
    queued_attestations: Vec<QueuedAttestation>,
    _phantom: PhantomData<E>,
}

impl<T, E> PartialEq for ForkChoice<T, E>
where
    T: ForkChoiceStore<E> + PartialEq,
    E: EthSpec,
{
    fn eq(&self, other: &Self) -> bool {
        self.fc_store == other.fc_store
            && self.proto_array == other.proto_array
            && self.queued_attestations == other.queued_attestations
    }
}

impl<T, E> ForkChoice<T, E>
where
    T: ForkChoiceStore<E>,
    E: EthSpec,
{
    /// Instantiates `Self` from the genesis parameters.
    pub fn from_genesis(
        fc_store: T,
        genesis_block_root: Hash256,
        genesis_block: &BeaconBlock<E>,
        genesis_state: &BeaconState<E>,
    ) -> Result<Self, Error<T::Error>> {
        let finalized_block_slot = genesis_block.slot;
        let finalized_block_state_root = genesis_block.state_root;
        let current_epoch_shuffling_id =
            ShufflingId::new(genesis_block_root, genesis_state, RelativeEpoch::Current)
                .map_err(Error::BeaconStateError)?;
        let next_epoch_shuffling_id =
            ShufflingId::new(genesis_block_root, genesis_state, RelativeEpoch::Next)
                .map_err(Error::BeaconStateError)?;

        let proto_array = ProtoArrayForkChoice::new(
            finalized_block_slot,
            finalized_block_state_root,
            fc_store.justified_checkpoint().epoch,
            fc_store.finalized_checkpoint().epoch,
            fc_store.finalized_checkpoint().root,
            current_epoch_shuffling_id,
            next_epoch_shuffling_id,
        )?;

        Ok(Self {
            fc_store,
            proto_array,
            queued_attestations: vec![],
            _phantom: PhantomData,
        })
    }

    /// Instantiates `Self` from some existing components.
    ///
    /// This is useful if the existing components have been loaded from disk after a process
    /// restart.
    pub fn from_components(
        fc_store: T,
        proto_array: ProtoArrayForkChoice,
        queued_attestations: Vec<QueuedAttestation>,
    ) -> Self {
        Self {
            fc_store,
            proto_array,
            queued_attestations,
            _phantom: PhantomData,
        }
    }

    /// Returns the block root of an ancestor of `block_root` at the given `slot`. (Note: `slot` refers
    /// to the block that is *returned*, not the one that is supplied.)
    ///
    /// The result may be `Ok(None)` if the block does not descend from the finalized block. This
    /// is an artifact of proto-array, sometimes it contains descendants of blocks that have been
    /// pruned.
    ///
    /// ## Specification
    ///
    /// Equivalent to:
    ///
    /// https://github.com/ethereum/eth2.0-specs/blob/v0.12.1/specs/phase0/fork-choice.md#get_ancestor
    fn get_ancestor(
        &self,
        block_root: Hash256,
        ancestor_slot: Slot,
    ) -> Result<Option<Hash256>, Error<T::Error>>
    where
        T: ForkChoiceStore<E>,
        E: EthSpec,
    {
        let block = self
            .proto_array
            .get_block(&block_root)
            .ok_or_else(|| Error::MissingProtoArrayBlock(block_root))?;

        match block.slot.cmp(&ancestor_slot) {
            Ordering::Greater => Ok(self
                .proto_array
                .core_proto_array()
                .iter_block_roots(&block_root)
                // Search for a slot that is **less than or equal to** the target slot. We check
                // for lower slots to account for skip slots.
                .find(|(_, slot)| *slot <= ancestor_slot)
                .map(|(root, _)| root)),
            Ordering::Less => Ok(Some(block_root)),
            Ordering::Equal =>
            // Root is older than queried slot, thus a skip slot. Return most recent root prior
            // to slot.
            {
                Ok(Some(block_root))
            }
        }
    }

    /// Run the fork choice rule to determine the head.
    ///
    /// ## Specification
    ///
    /// Is equivalent to:
    ///
    /// https://github.com/ethereum/eth2.0-specs/blob/v0.12.1/specs/phase0/fork-choice.md#get_head
    pub fn get_head(&mut self, current_slot: Slot) -> Result<Hash256, Error<T::Error>> {
        self.update_time(current_slot)?;

        let store = &mut self.fc_store;

        self.proto_array
            .find_head(
                store.justified_checkpoint().epoch,
                store.justified_checkpoint().root,
                store.finalized_checkpoint().epoch,
                store.justified_balances(),
            )
            .map_err(Into::into)
    }

    /// Returns `true` if the given `store` should be updated to set
    /// `state.current_justified_checkpoint` its `justified_checkpoint`.
    ///
    /// ## Specification
    ///
    /// Is equivalent to:
    ///
    /// https://github.com/ethereum/eth2.0-specs/blob/v0.12.1/specs/phase0/fork-choice.md#should_update_justified_checkpoint
    fn should_update_justified_checkpoint(
        &mut self,
        current_slot: Slot,
        state: &BeaconState<E>,
    ) -> Result<bool, Error<T::Error>> {
        self.update_time(current_slot)?;

        let new_justified_checkpoint = &state.current_justified_checkpoint;

        if compute_slots_since_epoch_start::<E>(self.fc_store.get_current_slot())
            < SAFE_SLOTS_TO_UPDATE_JUSTIFIED
        {
            return Ok(true);
        }

        let justified_slot =
            compute_start_slot_at_epoch::<E>(self.fc_store.justified_checkpoint().epoch);

        // This sanity check is not in the spec, but the invariant is implied.
        if justified_slot >= state.slot {
            return Err(Error::AttemptToRevertJustification {
                store: justified_slot,
                state: state.slot,
            });
        }

        // We know that the slot for `new_justified_checkpoint.root` is not greater than
        // `state.slot`, since a state cannot justify its own slot.
        //
        // We know that `new_justified_checkpoint.root` is an ancestor of `state`, since a `state`
        // only ever justifies ancestors.
        //
        // A prior `if` statement protects against a justified_slot that is greater than
        // `state.slot`
        let justified_ancestor =
            self.get_ancestor(new_justified_checkpoint.root, justified_slot)?;
        if justified_ancestor != Some(self.fc_store.justified_checkpoint().root) {
            return Ok(false);
        }

        Ok(true)
    }

    /// Add `block` to the fork choice DAG.
    ///
    /// - `block_root` is the root of `block.
    /// - The root of `state` matches `block.state_root`.
    ///
    /// ## Specification
    ///
    /// Approximates:
    ///
    /// https://github.com/ethereum/eth2.0-specs/blob/v0.12.1/specs/phase0/fork-choice.md#on_block
    ///
    /// It only approximates the specification since it does not run the `state_transition` check.
    /// That should have already been called upstream and it's too expensive to call again.
    ///
    /// ## Notes:
    ///
    /// The supplied block **must** pass the `state_transition` function as it will not be run
    /// here.
    pub fn on_block(
        &mut self,
        current_slot: Slot,
        block: &BeaconBlock<E>,
        block_root: Hash256,
        state: &BeaconState<E>,
    ) -> Result<(), Error<T::Error>> {
        let current_slot = self.update_time(current_slot)?;

        // Parent block must be known.
        if !self.proto_array.contains_block(&block.parent_root) {
            return Err(Error::InvalidBlock(InvalidBlock::UnknownParent(
                block.parent_root,
            )));
        }

        // Blocks cannot be in the future. If they are, their consideration must be delayed until
        // the are in the past.
        //
        // Note: presently, we do not delay consideration. We just drop the block.
        if block.slot > current_slot {
            return Err(Error::InvalidBlock(InvalidBlock::FutureSlot {
                current_slot,
                block_slot: block.slot,
            }));
        }

        // Check that block is later than the finalized epoch slot (optimization to reduce calls to
        // get_ancestor).
        let finalized_slot =
            compute_start_slot_at_epoch::<E>(self.fc_store.finalized_checkpoint().epoch);
        if block.slot <= finalized_slot {
            return Err(Error::InvalidBlock(InvalidBlock::FinalizedSlot {
                finalized_slot,
                block_slot: block.slot,
            }));
        }

        // Check block is a descendant of the finalized block at the checkpoint finalized slot.
        //
        // Note: the specification uses `hash_tree_root(block)` instead of `block.parent_root` for
        // the start of this search. I claim that since `block.slot > finalized_slot` it is
        // equivalent to use the parent root for this search. Doing so reduces a single lookup
        // (trivial), but more importantly, it means we don't need to have added `block` to
        // `self.proto_array` to do this search. See:
        //
        // https://github.com/ethereum/eth2.0-specs/pull/1884
        let block_ancestor = self.get_ancestor(block.parent_root, finalized_slot)?;
        let finalized_root = self.fc_store.finalized_checkpoint().root;
        if block_ancestor != Some(finalized_root) {
            return Err(Error::InvalidBlock(InvalidBlock::NotFinalizedDescendant {
                finalized_root,
                block_ancestor,
            }));
        }

        // Update justified checkpoint.
        if state.current_justified_checkpoint.epoch > self.fc_store.justified_checkpoint().epoch {
            if state.current_justified_checkpoint.epoch
                > self.fc_store.best_justified_checkpoint().epoch
            {
                self.fc_store
                    .set_best_justified_checkpoint(state.current_justified_checkpoint);
            }
            if self.should_update_justified_checkpoint(current_slot, state)? {
                self.fc_store
                    .set_justified_checkpoint(state.current_justified_checkpoint)
                    .map_err(Error::UnableToSetJustifiedCheckpoint)?;
            }
        }

        // Update finalized checkpoint.
        if state.finalized_checkpoint.epoch > self.fc_store.finalized_checkpoint().epoch {
            self.fc_store
                .set_finalized_checkpoint(state.finalized_checkpoint);
            let finalized_slot =
                compute_start_slot_at_epoch::<E>(self.fc_store.finalized_checkpoint().epoch);

            // Note: the `if` statement here is not part of the specification, but I claim that it
            // is an optimization and equivalent to the specification. See this PR for more
            // information:
            //
            // https://github.com/ethereum/eth2.0-specs/pull/1880
            if *self.fc_store.justified_checkpoint() != state.current_justified_checkpoint
                && (state.current_justified_checkpoint.epoch
                    > self.fc_store.justified_checkpoint().epoch
                    || self
                        .get_ancestor(self.fc_store.justified_checkpoint().root, finalized_slot)?
                        != Some(self.fc_store.finalized_checkpoint().root))
            {
                self.fc_store
                    .set_justified_checkpoint(state.current_justified_checkpoint)
                    .map_err(Error::UnableToSetJustifiedCheckpoint)?;
            }
        }

        let target_slot = block
            .slot
            .epoch(E::slots_per_epoch())
            .start_slot(E::slots_per_epoch());
        let target_root = if block.slot == target_slot {
            block_root
        } else {
            *state
                .get_block_root(target_slot)
                .map_err(Error::BeaconStateError)?
        };

        self.fc_store
            .on_verified_block(block, block_root, state)
            .map_err(Error::AfterBlockFailed)?;

        // This does not apply a vote to the block, it just makes fork choice aware of the block so
        // it can still be identified as the head even if it doesn't have any votes.
        self.proto_array.process_block(ProtoBlock {
            slot: block.slot,
            root: block_root,
            parent_root: Some(block.parent_root),
            target_root,
            current_epoch_shuffling_id: ShufflingId::new(block_root, state, RelativeEpoch::Current)
                .map_err(Error::BeaconStateError)?,
            next_epoch_shuffling_id: ShufflingId::new(block_root, state, RelativeEpoch::Next)
                .map_err(Error::BeaconStateError)?,
            state_root: block.state_root,
            justified_epoch: state.current_justified_checkpoint.epoch,
            finalized_epoch: state.finalized_checkpoint.epoch,
        })?;

        Ok(())
    }

    /// Validates the `indexed_attestation` for application to fork choice.
    ///
    /// ## Specification
    ///
    /// Equivalent to:
    ///
    /// https://github.com/ethereum/eth2.0-specs/blob/v0.12.1/specs/phase0/fork-choice.md#validate_on_attestation
    fn validate_on_attestation(
        &self,
        indexed_attestation: &IndexedAttestation<E>,
    ) -> Result<(), InvalidAttestation> {
        // There is no point in processing an attestation with an empty bitfield. Reject
        // it immediately.
        //
        // This is not in the specification, however it should be transparent to other nodes. We
        // return early here to avoid wasting precious resources verifying the rest of it.
        if indexed_attestation.attesting_indices.is_empty() {
            return Err(InvalidAttestation::EmptyAggregationBitfield);
        }

        let slot_now = self.fc_store.get_current_slot();
        let epoch_now = slot_now.epoch(E::slots_per_epoch());
        let target = indexed_attestation.data.target;

        // Attestation must be from the current or previous epoch.
        if target.epoch > epoch_now {
            return Err(InvalidAttestation::FutureEpoch {
                attestation_epoch: target.epoch,
                current_epoch: epoch_now,
            });
        } else if target.epoch + 1 < epoch_now {
            return Err(InvalidAttestation::PastEpoch {
                attestation_epoch: target.epoch,
                current_epoch: epoch_now,
            });
        }

        if target.epoch != indexed_attestation.data.slot.epoch(E::slots_per_epoch()) {
            return Err(InvalidAttestation::BadTargetEpoch {
                target: target.epoch,
                slot: indexed_attestation.data.slot,
            });
        }

        // Attestation target must be for a known block.
        //
        // We do not delay the block for later processing to reduce complexity and DoS attack
        // surface.
        if !self.proto_array.contains_block(&target.root) {
            return Err(InvalidAttestation::UnknownTargetRoot(target.root));
        }

        // Load the block for `attestation.data.beacon_block_root`.
        //
        // This indirectly checks to see if the `attestation.data.beacon_block_root` is in our fork
        // choice. Any known, non-finalized block should be in fork choice, so this check
        // immediately filters out attestations that attest to a block that has not been processed.
        //
        // Attestations must be for a known block. If the block is unknown, we simply drop the
        // attestation and do not delay consideration for later.
        let block = self
            .proto_array
            .get_block(&indexed_attestation.data.beacon_block_root)
            .ok_or_else(|| InvalidAttestation::UnknownHeadBlock {
                beacon_block_root: indexed_attestation.data.beacon_block_root,
            })?;

        // If an attestation points to a block that is from an earlier slot than the attestation,
        // then all slots between the block and attestation must be skipped. Therefore if the block
        // is from a prior epoch to the attestation, then the target root must be equal to the root
        // of the block that is being attested to.
        let expected_target = if target.epoch > block.slot.epoch(E::slots_per_epoch()) {
            indexed_attestation.data.beacon_block_root
        } else {
            block.target_root
        };

        if expected_target != target.root {
            return Err(InvalidAttestation::InvalidTarget {
                attestation: target.root,
                local: expected_target,
            });
        }

        // Attestations must not be for blocks in the future. If this is the case, the attestation
        // should not be considered.
        if block.slot > indexed_attestation.data.slot {
            return Err(InvalidAttestation::AttestsToFutureBlock {
                block: block.slot,
                attestation: indexed_attestation.data.slot,
            });
        }

        Ok(())
    }

    /// Register `attestation` with the fork choice DAG so that it may influence future calls to
    /// `Self::get_head`.
    ///
    /// ## Specification
    ///
    /// Approximates:
    ///
    /// https://github.com/ethereum/eth2.0-specs/blob/v0.12.1/specs/phase0/fork-choice.md#on_attestation
    ///
    /// It only approximates the specification since it does not perform
    /// `is_valid_indexed_attestation` since that should already have been called upstream and it's
    /// too expensive to call again.
    ///
    /// ## Notes:
    ///
    /// The supplied `attestation` **must** pass the `in_valid_indexed_attestation` function as it
    /// will not be run here.
    pub fn on_attestation(
        &mut self,
        current_slot: Slot,
        attestation: &IndexedAttestation<E>,
    ) -> Result<(), Error<T::Error>> {
        // Ensure the store is up-to-date.
        self.update_time(current_slot)?;

        // Ignore any attestations to the zero hash.
        //
        // This is an edge case that results from the spec aliasing the zero hash to the genesis
        // block. Attesters may attest to the zero hash if they have never seen a block.
        //
        // We have two options here:
        //
        //  1. Apply all zero-hash attestations to the genesis block.
        //  2. Ignore all attestations to the zero hash.
        //
        // (1) becomes weird once we hit finality and fork choice drops the genesis block. (2) is
        // fine because votes to the genesis block are not useful; all validators implicitly attest
        // to genesis just by being present in the chain.
        if attestation.data.beacon_block_root == Hash256::zero() {
            return Ok(());
        }

        self.validate_on_attestation(attestation)?;

        if attestation.data.slot < self.fc_store.get_current_slot() {
            for validator_index in attestation.attesting_indices.iter() {
                self.proto_array.process_attestation(
                    *validator_index as usize,
                    attestation.data.beacon_block_root,
                    attestation.data.target.epoch,
                )?;
            }
        } else {
            // The spec declares:
            //
            // ```
            // Attestations can only affect the fork choice of subsequent slots.
            // Delay consideration in the fork choice until their slot is in the past.
            // ```
            self.queued_attestations
                .push(QueuedAttestation::from(attestation));
        }

        Ok(())
    }

    /// Call `on_tick` for all slots between `fc_store.get_current_slot()` and the provided
    /// `current_slot`. Returns the value of `self.fc_store.get_current_slot`.
    pub fn update_time(&mut self, current_slot: Slot) -> Result<Slot, Error<T::Error>> {
        while self.fc_store.get_current_slot() < current_slot {
            let previous_slot = self.fc_store.get_current_slot();
            // Note: we are relying upon `on_tick` to update `fc_store.time` to ensure we don't
            // get stuck in a loop.
            on_tick(&mut self.fc_store, previous_slot + 1)?
        }

        // Process any attestations that might now be eligible.
        self.process_attestation_queue()?;

        Ok(self.fc_store.get_current_slot())
    }

    /// Processes and removes from the queue any queued attestations which may now be eligible for
    /// processing due to the slot clock incrementing.
    fn process_attestation_queue(&mut self) -> Result<(), Error<T::Error>> {
        for attestation in dequeue_attestations(
            self.fc_store.get_current_slot(),
            &mut self.queued_attestations,
        ) {
            for validator_index in attestation.attesting_indices.iter() {
                self.proto_array.process_attestation(
                    *validator_index as usize,
                    attestation.block_root,
                    attestation.target_epoch,
                )?;
            }
        }

        Ok(())
    }

    /// Returns `true` if the block is known **and** a descendant of the finalized root.
    pub fn contains_block(&self, block_root: &Hash256) -> bool {
        self.proto_array.contains_block(block_root) && self.is_descendant_of_finalized(*block_root)
    }

    /// Returns a `ProtoBlock` if the block is known **and** a descendant of the finalized root.
    pub fn get_block(&self, block_root: &Hash256) -> Option<ProtoBlock> {
        if self.is_descendant_of_finalized(*block_root) {
            self.proto_array.get_block(block_root)
        } else {
            None
        }
    }

    /// Return `true` if `block_root` is equal to the finalized root, or a known descendant of it.
    pub fn is_descendant_of_finalized(&self, block_root: Hash256) -> bool {
        self.proto_array
            .is_descendant(self.fc_store.finalized_checkpoint().root, block_root)
    }

    /// Return the current finalized checkpoint.
    pub fn finalized_checkpoint(&self) -> Checkpoint {
        *self.fc_store.finalized_checkpoint()
    }

    /// Returns the latest message for a given validator, if any.
    ///
    /// Returns `(block_root, block_slot)`.
    ///
    /// ## Notes
    ///
    /// It may be prudent to call `Self::update_time` before calling this function,
    /// since some attestations might be queued and awaiting processing.
    pub fn latest_message(&self, validator_index: usize) -> Option<(Hash256, Epoch)> {
        self.proto_array.latest_message(validator_index)
    }

    /// Returns a reference to the underlying fork choice DAG.
    pub fn proto_array(&self) -> &ProtoArrayForkChoice {
        &self.proto_array
    }

    /// Returns a reference to the underlying `fc_store`.
    pub fn fc_store(&self) -> &T {
        &self.fc_store
    }

    /// Returns a reference to the currently queued attestations.
    pub fn queued_attestations(&self) -> &[QueuedAttestation] {
        &self.queued_attestations
    }

    /// Prunes the underlying fork choice DAG.
    pub fn prune(&mut self) -> Result<(), Error<T::Error>> {
        let finalized_root = self.fc_store.finalized_checkpoint().root;

        self.proto_array
            .maybe_prune(finalized_root)
            .map_err(Into::into)
    }

    /// Instantiate `Self` from some `PersistedForkChoice` generated by a earlier call to
    /// `Self::to_persisted`.
    pub fn from_persisted(
        persisted: PersistedForkChoice,
        fc_store: T,
    ) -> Result<Self, Error<T::Error>> {
        let proto_array = ProtoArrayForkChoice::from_bytes(&persisted.proto_array_bytes)
            .map_err(Error::InvalidProtoArrayBytes)?;

        Ok(Self {
            fc_store,
            proto_array,
            queued_attestations: persisted.queued_attestations,
            _phantom: PhantomData,
        })
    }

    /// Takes a snapshot of `Self` and stores it in `PersistedForkChoice`, allowing this struct to
    /// be instantiated again later.
    pub fn to_persisted(&self) -> PersistedForkChoice {
        PersistedForkChoice {
            proto_array_bytes: self.proto_array().as_bytes(),
            queued_attestations: self.queued_attestations().to_vec(),
        }
    }
}

/// Helper struct that is used to encode/decode the state of the `ForkChoice` as SSZ bytes.
///
/// This is used when persisting the state of the fork choice to disk.
#[derive(Encode, Decode, Clone)]
pub struct PersistedForkChoice {
    proto_array_bytes: Vec<u8>,
    queued_attestations: Vec<QueuedAttestation>,
}

#[cfg(test)]
mod tests {
    use types::{EthSpec, MainnetEthSpec};

    use super::*;

    type E = MainnetEthSpec;

    #[test]
    fn slots_since_epoch_start() {
        for epoch in 0..3 {
            for slot in 0..E::slots_per_epoch() {
                let input = epoch * E::slots_per_epoch() + slot;
                assert_eq!(compute_slots_since_epoch_start::<E>(Slot::new(input)), slot)
            }
        }
    }

    #[test]
    fn start_slot_at_epoch() {
        for epoch in 0..3 {
            assert_eq!(
                compute_start_slot_at_epoch::<E>(Epoch::new(epoch)),
                epoch * E::slots_per_epoch()
            )
        }
    }

    fn get_queued_attestations() -> Vec<QueuedAttestation> {
        (1..4)
            .into_iter()
            .map(|i| QueuedAttestation {
                slot: Slot::new(i),
                attesting_indices: vec![],
                block_root: Hash256::zero(),
                target_epoch: Epoch::new(0),
            })
            .collect()
    }

    fn get_slots(queued_attestations: &[QueuedAttestation]) -> Vec<u64> {
        queued_attestations.iter().map(|a| a.slot.into()).collect()
    }

    fn test_queued_attestations(current_time: Slot) -> (Vec<u64>, Vec<u64>) {
        let mut queued = get_queued_attestations();
        let dequeued = dequeue_attestations(current_time, &mut queued);

        (get_slots(&queued), get_slots(&dequeued))
    }

    #[test]
    fn dequeing_attestations() {
        let (queued, dequeued) = test_queued_attestations(Slot::new(0));
        assert_eq!(queued, vec![1, 2, 3]);
        assert!(dequeued.is_empty());

        let (queued, dequeued) = test_queued_attestations(Slot::new(1));
        assert_eq!(queued, vec![1, 2, 3]);
        assert!(dequeued.is_empty());

        let (queued, dequeued) = test_queued_attestations(Slot::new(2));
        assert_eq!(queued, vec![2, 3]);
        assert_eq!(dequeued, vec![1]);

        let (queued, dequeued) = test_queued_attestations(Slot::new(3));
        assert_eq!(queued, vec![3]);
        assert_eq!(dequeued, vec![1, 2]);

        let (queued, dequeued) = test_queued_attestations(Slot::new(4));
        assert!(queued.is_empty());
        assert_eq!(dequeued, vec![1, 2, 3]);
    }
}