//! Provides an `ObservedAggregates` struct which allows us to reject aggregated attestations or
//! sync committee contributions if we've already seen them.

use std::collections::HashSet;
use std::marker::PhantomData;
use tree_hash::TreeHash;
use types::consts::altair::{
    SYNC_COMMITTEE_SUBNET_COUNT, TARGET_AGGREGATORS_PER_SYNC_SUBCOMMITTEE,
};
use types::slot_data::SlotData;
use types::{Attestation, EthSpec, Hash256, Slot, SyncCommitteeContribution};

pub type ObservedSyncContributions<E> = ObservedAggregates<SyncCommitteeContribution<E>, E>;
pub type ObservedAggregateAttestations<E> = ObservedAggregates<Attestation<E>, E>;

/// A trait use to associate capacity constants with the type being stored in `ObservedAggregates`.
pub trait Consts {
    /// The default capacity of items stored per slot, in a single `SlotHashSet`.
    const DEFAULT_PER_SLOT_CAPACITY: usize;

    /// The maximum number of slots
    fn max_slot_capacity() -> usize;

    /// The maximum number of items stored per slot, in a single `SlotHashSet`.
    fn max_per_slot_capacity() -> usize;
}

impl<T: EthSpec> Consts for Attestation<T> {
    /// Use 128 as it's the target committee size for the mainnet spec. This is perhaps a little
    /// wasteful for the minimal spec, but considering it's approx. 128 * 32 bytes we're not wasting
    /// much.
    const DEFAULT_PER_SLOT_CAPACITY: usize = 128;

    /// We need to keep attestations for each slot of the current epoch.
    fn max_slot_capacity() -> usize {
        T::slots_per_epoch() as usize
    }

    /// As a DoS protection measure, the maximum number of distinct `Attestations` or
    /// `SyncCommitteeContributions` that will be recorded for each slot.
    ///
    /// Currently this is set to ~524k. If we say that each entry is 40 bytes (Hash256 (32 bytes) + an
    /// 8 byte hash) then this comes to about 20mb per slot. If we're storing 34 of these slots, then
    /// we're at 680mb. This is a lot of memory usage, but probably not a show-stopper for most
    /// reasonable hardware.
    ///
    /// Upstream conditions should strongly restrict the amount of attestations that can show up in
    /// this pool. The maximum size with respect to upstream restrictions is more likely on the order
    /// of the number of validators.
    fn max_per_slot_capacity() -> usize {
        1 << 19 // 524,288
    }
}

impl<T: EthSpec> Consts for SyncCommitteeContribution<T> {
    /// Set to `TARGET_AGGREGATORS_PER_SYNC_SUBCOMMITTEE * SYNC_COMMITTEE_SUBNET_COUNT`. This is the
    /// expected number of aggregators per slot across all subcommittees.
    const DEFAULT_PER_SLOT_CAPACITY: usize =
        (SYNC_COMMITTEE_SUBNET_COUNT * TARGET_AGGREGATORS_PER_SYNC_SUBCOMMITTEE) as usize;

    /// We only need to keep contributions related to the current slot.
    fn max_slot_capacity() -> usize {
        1
    }

    /// We should never receive more aggregates than there are sync committee participants.
    fn max_per_slot_capacity() -> usize {
        T::sync_committee_size()
    }
}

#[derive(Debug, PartialEq)]
pub enum ObserveOutcome {
    /// This item was already known.
    AlreadyKnown,
    /// This was the first time this item was observed.
    New,
}

#[derive(Debug, PartialEq)]
pub enum Error {
    SlotTooLow {
        slot: Slot,
        lowest_permissible_slot: Slot,
    },
    /// The function to obtain a set index failed, this is an internal error.
    InvalidSetIndex(usize),
    /// We have reached the maximum number of unique items that can be observed in a slot.
    /// This is a DoS protection function.
    ReachedMaxObservationsPerSlot(usize),
    IncorrectSlot {
        expected: Slot,
        attestation: Slot,
    },
}

/// A `HashSet` that contains entries related to some `Slot`.
struct SlotHashSet {
    set: HashSet<Hash256>,
    slot: Slot,
    max_capacity: usize,
}

impl SlotHashSet {
    pub fn new(slot: Slot, initial_capacity: usize, max_capacity: usize) -> Self {
        Self {
            slot,
            set: HashSet::with_capacity(initial_capacity),
            max_capacity,
        }
    }

    /// Store the items in self so future observations recognise its existence.
    pub fn observe_item<T: SlotData>(
        &mut self,
        item: &T,
        root: Hash256,
    ) -> Result<ObserveOutcome, Error> {
        if item.get_slot() != self.slot {
            return Err(Error::IncorrectSlot {
                expected: self.slot,
                attestation: item.get_slot(),
            });
        }

        if self.set.contains(&root) {
            Ok(ObserveOutcome::AlreadyKnown)
        } else {
            // Here we check to see if this slot has reached the maximum observation count.
            //
            // The resulting behaviour is that we are no longer able to successfully observe new
            // items, however we will continue to return `is_known` values. We could also
            // disable `is_known`, however then we would stop forwarding items across the
            // gossip network and I think that this is a worse case than sending some invalid ones.
            // The underlying libp2p network is responsible for removing duplicate messages, so
            // this doesn't risk a broadcast loop.
            if self.set.len() >= self.max_capacity {
                return Err(Error::ReachedMaxObservationsPerSlot(self.max_capacity));
            }

            self.set.insert(root);

            Ok(ObserveOutcome::New)
        }
    }

    /// Indicates if `item` has been observed before.
    pub fn is_known<T: SlotData>(&self, item: &T, root: Hash256) -> Result<bool, Error> {
        if item.get_slot() != self.slot {
            return Err(Error::IncorrectSlot {
                expected: self.slot,
                attestation: item.get_slot(),
            });
        }

        Ok(self.set.contains(&root))
    }

    /// The number of observed items in `self`.
    pub fn len(&self) -> usize {
        self.set.len()
    }
}

/// Stores the roots of objects for some number of `Slots`, so we can determine if
/// these have previously been seen on the network.
pub struct ObservedAggregates<T: TreeHash + SlotData + Consts, E: EthSpec> {
    lowest_permissible_slot: Slot,
    sets: Vec<SlotHashSet>,
    _phantom_spec: PhantomData<E>,
    _phantom_tree_hash: PhantomData<T>,
}

impl<T: TreeHash + SlotData + Consts, E: EthSpec> Default for ObservedAggregates<T, E> {
    fn default() -> Self {
        Self {
            lowest_permissible_slot: Slot::new(0),
            sets: vec![],
            _phantom_spec: PhantomData,
            _phantom_tree_hash: PhantomData,
        }
    }
}

impl<T: TreeHash + SlotData + Consts, E: EthSpec> ObservedAggregates<T, E> {
    /// Store the root of `item` in `self`.
    ///
    /// `root` must equal `item.tree_hash_root()`.
    pub fn observe_item(
        &mut self,
        item: &T,
        root_opt: Option<Hash256>,
    ) -> Result<ObserveOutcome, Error> {
        let index = self.get_set_index(item.get_slot())?;
        let root = root_opt.unwrap_or_else(|| item.tree_hash_root());

        self.sets
            .get_mut(index)
            .ok_or(Error::InvalidSetIndex(index))
            .and_then(|set| set.observe_item(item, root))
    }

    /// Check to see if the `root` of `item` is in self.
    ///
    /// `root` must equal `a.tree_hash_root()`.
    #[allow(clippy::wrong_self_convention)]
    pub fn is_known(&mut self, item: &T, root: Hash256) -> Result<bool, Error> {
        let index = self.get_set_index(item.get_slot())?;

        self.sets
            .get(index)
            .ok_or(Error::InvalidSetIndex(index))
            .and_then(|set| set.is_known(item, root))
    }

    /// The maximum number of slots that items are stored for.
    fn max_capacity(&self) -> u64 {
        // We add `2` in order to account for one slot either side of the range due to
        // `MAXIMUM_GOSSIP_CLOCK_DISPARITY`.
        (T::max_slot_capacity() + 2) as u64
    }

    /// Removes any items with a slot lower than `current_slot` and bars any future
    /// item with a slot lower than `current_slot - SLOTS_RETAINED`.
    pub fn prune(&mut self, current_slot: Slot) {
        let lowest_permissible_slot = current_slot.saturating_sub(self.max_capacity() - 1);

        self.sets.retain(|set| set.slot >= lowest_permissible_slot);

        self.lowest_permissible_slot = lowest_permissible_slot;
    }

    /// Returns the index of `self.set` that matches `slot`.
    ///
    /// If there is no existing set for this slot one will be created. If `self.sets.len() >=
    /// Self::max_capacity()`, the set with the lowest slot will be replaced.
    fn get_set_index(&mut self, slot: Slot) -> Result<usize, Error> {
        let lowest_permissible_slot = self.lowest_permissible_slot;

        if slot < lowest_permissible_slot {
            return Err(Error::SlotTooLow {
                slot,
                lowest_permissible_slot,
            });
        }

        // Prune the pool if this item indicates that the current slot has advanced.
        if lowest_permissible_slot + self.max_capacity() < slot + 1 {
            self.prune(slot)
        }

        if let Some(index) = self.sets.iter().position(|set| set.slot == slot) {
            return Ok(index);
        }

        // To avoid re-allocations, try and determine a rough initial capacity for the new set
        // by obtaining the mean size of all items in earlier epoch.
        let (count, sum) = self
            .sets
            .iter()
            // Only include slots that are less than the given slot in the average. This should
            // generally avoid including recent slots that are still "filling up".
            .filter(|set| set.slot < slot)
            .map(|set| set.len())
            .fold((0, 0), |(count, sum), len| (count + 1, sum + len));
        // If we are unable to determine an average, just use the `self.default_per_slot_capacity`.
        let initial_capacity = sum
            .checked_div(count)
            .unwrap_or(T::DEFAULT_PER_SLOT_CAPACITY);

        if self.sets.len() < self.max_capacity() as usize || self.sets.is_empty() {
            let index = self.sets.len();
            self.sets.push(SlotHashSet::new(
                slot,
                initial_capacity,
                T::max_per_slot_capacity(),
            ));
            return Ok(index);
        }

        let index = self
            .sets
            .iter()
            .enumerate()
            .min_by_key(|(_i, set)| set.slot)
            .map(|(i, _set)| i)
            .expect("sets cannot be empty due to previous .is_empty() check");

        self.sets[index] = SlotHashSet::new(slot, initial_capacity, T::max_per_slot_capacity());

        Ok(index)
    }
}

#[cfg(test)]
#[cfg(not(debug_assertions))]
mod tests {
    use super::*;
    use tree_hash::TreeHash;
    use types::{test_utils::test_random_instance, Hash256};

    type E = types::MainnetEthSpec;

    fn get_attestation(slot: Slot, beacon_block_root: u64) -> Attestation<E> {
        let mut a: Attestation<E> = test_random_instance();
        a.data.slot = slot;
        a.data.beacon_block_root = Hash256::from_low_u64_be(beacon_block_root);
        a
    }

    fn get_sync_contribution(slot: Slot, beacon_block_root: u64) -> SyncCommitteeContribution<E> {
        let mut a: SyncCommitteeContribution<E> = test_random_instance();
        a.slot = slot;
        a.beacon_block_root = Hash256::from_low_u64_be(beacon_block_root);
        a
    }

    macro_rules! test_suite {
        ($mod_name: ident, $type: ident, $method_name: ident) => {
            #[cfg(test)]
            mod $mod_name {
                use super::*;

                const NUM_ELEMENTS: usize = 8;

                fn single_slot_test(store: &mut $type<E>, slot: Slot) {
                    let items = (0..NUM_ELEMENTS as u64)
                        .map(|i| $method_name(slot, i))
                        .collect::<Vec<_>>();

                    for a in &items {
                        assert_eq!(
                            store.is_known(a, a.tree_hash_root()),
                            Ok(false),
                            "should indicate an unknown attestation is unknown"
                        );
                        assert_eq!(
                            store.observe_item(a, None),
                            Ok(ObserveOutcome::New),
                            "should observe new attestation"
                        );
                    }

                    for a in &items {
                        assert_eq!(
                            store.is_known(a, a.tree_hash_root()),
                            Ok(true),
                            "should indicate a known attestation is known"
                        );
                        assert_eq!(
                            store.observe_item(a, Some(a.tree_hash_root())),
                            Ok(ObserveOutcome::AlreadyKnown),
                            "should acknowledge an existing attestation"
                        );
                    }
                }

                #[test]
                fn single_slot() {
                    let mut store = $type::default();

                    single_slot_test(&mut store, Slot::new(0));

                    assert_eq!(store.sets.len(), 1, "should have a single set stored");
                    assert_eq!(
                        store.sets[0].len(),
                        NUM_ELEMENTS,
                        "set should have NUM_ELEMENTS elements"
                    );
                }

                #[test]
                fn mulitple_contiguous_slots() {
                    let mut store = $type::default();
                    let max_cap = store.max_capacity();

                    for i in 0..max_cap * 3 {
                        let slot = Slot::new(i);

                        single_slot_test(&mut store, slot);

                        /*
                         * Ensure that the number of sets is correct.
                         */

                        if i < max_cap {
                            assert_eq!(
                                store.sets.len(),
                                i as usize + 1,
                                "should have a {} sets stored",
                                i + 1
                            );
                        } else {
                            assert_eq!(
                                store.sets.len(),
                                max_cap as usize,
                                "should have max_capacity sets stored"
                            );
                        }

                        /*
                         * Ensure that each set contains the correct number of elements.
                         */

                        for set in &store.sets[..] {
                            assert_eq!(
                                set.len(),
                                NUM_ELEMENTS,
                                "each store should have NUM_ELEMENTS elements"
                            )
                        }

                        /*
                         *  Ensure that all the sets have the expected slots
                         */

                        let mut store_slots =
                            store.sets.iter().map(|set| set.slot).collect::<Vec<_>>();

                        assert!(
                            store_slots.len() <= store.max_capacity() as usize,
                            "store size should not exceed max"
                        );

                        store_slots.sort_unstable();

                        let expected_slots = (i.saturating_sub(max_cap - 1)..=i)
                            .map(Slot::new)
                            .collect::<Vec<_>>();

                        assert_eq!(expected_slots, store_slots, "should have expected slots");
                    }
                }

                #[test]
                fn mulitple_non_contiguous_slots() {
                    let mut store = $type::default();
                    let max_cap = store.max_capacity();

                    let to_skip = vec![1_u64, 2, 3, 5, 6, 29, 30, 31, 32, 64];
                    let slots = (0..max_cap * 3)
                        .into_iter()
                        .filter(|i| !to_skip.contains(i))
                        .collect::<Vec<_>>();

                    for &i in &slots {
                        if to_skip.contains(&i) {
                            continue;
                        }

                        let slot = Slot::from(i);

                        single_slot_test(&mut store, slot);

                        /*
                         * Ensure that each set contains the correct number of elements.
                         */

                        for set in &store.sets[..] {
                            assert_eq!(
                                set.len(),
                                NUM_ELEMENTS,
                                "each store should have NUM_ELEMENTS elements"
                            )
                        }

                        /*
                         *  Ensure that all the sets have the expected slots
                         */

                        let mut store_slots =
                            store.sets.iter().map(|set| set.slot).collect::<Vec<_>>();

                        store_slots.sort_unstable();

                        assert!(
                            store_slots.len() <= store.max_capacity() as usize,
                            "store size should not exceed max"
                        );

                        let lowest = store.lowest_permissible_slot.as_u64();
                        let highest = slot.as_u64();
                        let expected_slots = (lowest..=highest)
                            .filter(|i| !to_skip.contains(i))
                            .map(Slot::new)
                            .collect::<Vec<_>>();

                        assert_eq!(
                            expected_slots,
                            &store_slots[..],
                            "should have expected slots"
                        );
                    }
                }
            }
        };
    }
    test_suite!(
        observed_sync_aggregates,
        ObservedSyncContributions,
        get_sync_contribution
    );
    test_suite!(
        observed_aggregate_attestations,
        ObservedAggregateAttestations,
        get_attestation
    );
}