impls.rs

			Self::eras_start_session_index(current_era).unwrap_or(0);
		// Number of session in the current era or the maximum session per era if reached.
		let era_progress = current_session
			.saturating_sub(current_era_start_session_index)
			.min(T::SessionsPerEra::get());

		let until_this_session_end = T::NextNewSession::estimate_next_new_session(now)
			.0
			.unwrap_or_default()
			.saturating_sub(now);

		let session_length = T::NextNewSession::average_session_length();

		let sessions_left: T::BlockNumber = match ForceEra::<T>::get() {
			Forcing::ForceNone => Bounded::max_value(),
			Forcing::ForceNew | Forcing::ForceAlways => Zero::zero(),
			Forcing::NotForcing if era_progress >= T::SessionsPerEra::get() => Zero::zero(),
			Forcing::NotForcing => T::SessionsPerEra::get()
				.saturating_sub(era_progress)
				// One session is computed in this_session_end.
				.saturating_sub(1)
				.into(),
		};

		now.saturating_add(
			until_this_session_end.saturating_add(sessions_left.saturating_mul(session_length)),
		)
	}

	#[cfg(feature = "runtime-benchmarks")]
	fn add_voter(
		voter: T::AccountId,
		weight: VoteWeight,
		targets: BoundedVec<T::AccountId, Self::MaxVotesPerVoter>,
	) {
		let stake = <BalanceOf<T>>::try_from(weight).unwrap_or_else(|_| {
			panic!("cannot convert a VoteWeight into BalanceOf, benchmark needs reconfiguring.")
		});
		<Bonded<T>>::insert(voter.clone(), voter.clone());
		<Ledger<T>>::insert(
			voter.clone(),
			StakingLedger {
				stash: voter.clone(),
				active: stake,
				total: stake,
				unlocking: Default::default(),
				claimed_rewards: Default::default(),
			},
		);

		Self::do_add_nominator(&voter, Nominations { targets, submitted_in: 0, suppressed: false });
	}

	#[cfg(feature = "runtime-benchmarks")]
	fn add_target(target: T::AccountId) {
		let stake = MinValidatorBond::<T>::get() * 100u32.into();
		<Bonded<T>>::insert(target.clone(), target.clone());
		<Ledger<T>>::insert(
			target.clone(),
			StakingLedger {
				stash: target.clone(),
				active: stake,
				total: stake,
				unlocking: Default::default(),
				claimed_rewards: Default::default(),
			},
		);
		Self::do_add_validator(
			&target,
			ValidatorPrefs { commission: Perbill::zero(), blocked: false },
		);
	}

	#[cfg(feature = "runtime-benchmarks")]
	fn clear() {
		#[allow(deprecated)]
		<Bonded<T>>::remove_all(None);
		#[allow(deprecated)]
		<Ledger<T>>::remove_all(None);
		#[allow(deprecated)]
		<Validators<T>>::remove_all();
		#[allow(deprecated)]
		<Nominators<T>>::remove_all();

		T::VoterList::unsafe_clear();
	}

	#[cfg(feature = "runtime-benchmarks")]
	fn put_snapshot(
		voters: Vec<VoterOf<Self>>,
		targets: Vec<T::AccountId>,
		target_stake: Option<VoteWeight>,
	) {
		targets.into_iter().for_each(|v| {
			let stake: BalanceOf<T> = target_stake
				.and_then(|w| <BalanceOf<T>>::try_from(w).ok())
				.unwrap_or_else(|| MinNominatorBond::<T>::get() * 100u32.into());
			<Bonded<T>>::insert(v.clone(), v.clone());
			<Ledger<T>>::insert(
				v.clone(),
				StakingLedger {
					stash: v.clone(),
					active: stake,
					total: stake,
					unlocking: Default::default(),
					claimed_rewards: Default::default(),
				},
			);
			Self::do_add_validator(
				&v,
				ValidatorPrefs { commission: Perbill::zero(), blocked: false },
			);
		});

		voters.into_iter().for_each(|(v, s, t)| {
			let stake = <BalanceOf<T>>::try_from(s).unwrap_or_else(|_| {
				panic!("cannot convert a VoteWeight into BalanceOf, benchmark needs reconfiguring.")
			});
			<Bonded<T>>::insert(v.clone(), v.clone());
			<Ledger<T>>::insert(
				v.clone(),
				StakingLedger {
					stash: v.clone(),
					active: stake,
					total: stake,
					unlocking: Default::default(),
					claimed_rewards: Default::default(),
				},
			);
			Self::do_add_nominator(
				&v,
				Nominations { targets: t, submitted_in: 0, suppressed: false },
			);
		});
	}
}

/// In this implementation `new_session(session)` must be called before `end_session(session-1)`
/// i.e. the new session must be planned before the ending of the previous session.
///
/// Once the first new_session is planned, all session must start and then end in order, though
/// some session can lag in between the newest session planned and the latest session started.
impl<T: Config> pallet_session::SessionManager<T::AccountId> for Pallet<T> {
	fn new_session(new_index: SessionIndex) -> Option<Vec<T::AccountId>> {
		log!(trace, "planning new session {}", new_index);
		CurrentPlannedSession::<T>::put(new_index);
		Self::new_session(new_index, false).map(|v| v.into_inner())
	}
	fn new_session_genesis(new_index: SessionIndex) -> Option<Vec<T::AccountId>> {
		log!(trace, "planning new session {} at genesis", new_index);
		CurrentPlannedSession::<T>::put(new_index);
		Self::new_session(new_index, true).map(|v| v.into_inner())
	}
	fn start_session(start_index: SessionIndex) {
		log!(trace, "starting session {}", start_index);
		Self::start_session(start_index)
	}
	fn end_session(end_index: SessionIndex) {
		log!(trace, "ending session {}", end_index);
		Self::end_session(end_index)
	}
}

impl<T: Config> historical::SessionManager<T::AccountId, Exposure<T::AccountId, BalanceOf<T>>>
	for Pallet<T>
{
	fn new_session(
		new_index: SessionIndex,
	) -> Option<Vec<(T::AccountId, Exposure<T::AccountId, BalanceOf<T>>)>> {
		<Self as pallet_session::SessionManager<_>>::new_session(new_index).map(|validators| {
			let current_era = Self::current_era()
				// Must be some as a new era has been created.
				.unwrap_or(0);

			validators
				.into_iter()
				.map(|v| {
					let exposure = Self::eras_stakers(current_era, &v);
					(v, exposure)
				})
				.collect()
		})
	}
	fn new_session_genesis(
		new_index: SessionIndex,
	) -> Option<Vec<(T::AccountId, Exposure<T::AccountId, BalanceOf<T>>)>> {
		<Self as pallet_session::SessionManager<_>>::new_session_genesis(new_index).map(
			|validators| {
				let current_era = Self::current_era()
					// Must be some as a new era has been created.
					.unwrap_or(0);

				validators
					.into_iter()
					.map(|v| {
						let exposure = Self::eras_stakers(current_era, &v);
						(v, exposure)
					})
					.collect()
			},
		)
	}
	fn start_session(start_index: SessionIndex) {
		<Self as pallet_session::SessionManager<_>>::start_session(start_index)
	}
	fn end_session(end_index: SessionIndex) {
		<Self as pallet_session::SessionManager<_>>::end_session(end_index)
	}
}

/// Add reward points to block authors:
/// * 20 points to the block producer for producing a (non-uncle) block in the relay chain,
/// * 2 points to the block producer for each reference to a previously unreferenced uncle, and
/// * 1 point to the producer of each referenced uncle block.
impl<T> pallet_authorship::EventHandler<T::AccountId, T::BlockNumber> for Pallet<T>
where
	T: Config + pallet_authorship::Config + pallet_session::Config,
{
	fn note_author(author: T::AccountId) {
		Self::reward_by_ids(vec![(author, 20)])
	}
	fn note_uncle(uncle_author: T::AccountId, _age: T::BlockNumber) {
		// defensive-only: block author must exist.
		if let Some(block_author) = <pallet_authorship::Pallet<T>>::author() {
			Self::reward_by_ids(vec![(block_author, 2), (uncle_author, 1)])
		} else {
			crate::log!(warn, "block author not set, this should never happen");
		}
	}
}

/// This is intended to be used with `FilterHistoricalOffences`.
impl<T: Config>
	OnOffenceHandler<T::AccountId, pallet_session::historical::IdentificationTuple<T>, Weight>
	for Pallet<T>
where
	T: pallet_session::Config<ValidatorId = <T as frame_system::Config>::AccountId>,
	T: pallet_session::historical::Config<
		FullIdentification = Exposure<<T as frame_system::Config>::AccountId, BalanceOf<T>>,
		FullIdentificationOf = ExposureOf<T>,
	>,
	T::SessionHandler: pallet_session::SessionHandler<<T as frame_system::Config>::AccountId>,
	T::SessionManager: pallet_session::SessionManager<<T as frame_system::Config>::AccountId>,
	T::ValidatorIdOf: Convert<
		<T as frame_system::Config>::AccountId,
		Option<<T as frame_system::Config>::AccountId>,
	>,
{
	fn on_offence(
		offenders: &[OffenceDetails<
			T::AccountId,
			pallet_session::historical::IdentificationTuple<T>,
		>],
		slash_fraction: &[Perbill],
		slash_session: SessionIndex,
		disable_strategy: DisableStrategy,
	) -> Weight {
		let reward_proportion = SlashRewardFraction::<T>::get();
		let mut consumed_weight = Weight::from_ref_time(0);
		let mut add_db_reads_writes = |reads, writes| {
			consumed_weight += T::DbWeight::get().reads_writes(reads, writes);
		};

		let active_era = {
			let active_era = Self::active_era();
			add_db_reads_writes(1, 0);
			if active_era.is_none() {
				// This offence need not be re-submitted.
				return consumed_weight
			}
			active_era.expect("value checked not to be `None`; qed").index
		};
		let active_era_start_session_index = Self::eras_start_session_index(active_era)
			.unwrap_or_else(|| {
				frame_support::print("Error: start_session_index must be set for current_era");
				0
			});
		add_db_reads_writes(1, 0);

		let window_start = active_era.saturating_sub(T::BondingDuration::get());

		// Fast path for active-era report - most likely.
		// `slash_session` cannot be in a future active era. It must be in `active_era` or before.
		let slash_era = if slash_session >= active_era_start_session_index {
			active_era
		} else {
			let eras = BondedEras::<T>::get();
			add_db_reads_writes(1, 0);

			// Reverse because it's more likely to find reports from recent eras.
			match eras.iter().rev().find(|&&(_, ref sesh)| sesh <= &slash_session) {
				Some(&(ref slash_era, _)) => *slash_era,
				// Before bonding period. defensive - should be filtered out.
				None => return consumed_weight,
			}
		};

		add_db_reads_writes(1, 1);

		let slash_defer_duration = T::SlashDeferDuration::get();

		let invulnerables = Self::invulnerables();
		add_db_reads_writes(1, 0);

		for (details, slash_fraction) in offenders.iter().zip(slash_fraction) {
			let (stash, exposure) = &details.offender;

			// Skip if the validator is invulnerable.
			if invulnerables.contains(stash) {
				continue
			}

			let unapplied = slashing::compute_slash::<T>(slashing::SlashParams {
				stash,
				slash: *slash_fraction,
				exposure,
				slash_era,
				window_start,
				now: active_era,
				reward_proportion,
				disable_strategy,
			});

			Self::deposit_event(Event::<T>::SlashReported {
				validator: stash.clone(),
				fraction: *slash_fraction,
				slash_era,
			});

			if let Some(mut unapplied) = unapplied {
				let nominators_len = unapplied.others.len() as u64;
				let reporters_len = details.reporters.len() as u64;

				{
					let upper_bound = 1 /* Validator/NominatorSlashInEra */ + 2 /* fetch_spans */;
					let rw = upper_bound + nominators_len * upper_bound;
					add_db_reads_writes(rw, rw);
				}
				unapplied.reporters = details.reporters.clone();
				if slash_defer_duration == 0 {
					// Apply right away.
					slashing::apply_slash::<T>(unapplied, slash_era);
					{
						let slash_cost = (6, 5);
						let reward_cost = (2, 2);
						add_db_reads_writes(
							(1 + nominators_len) * slash_cost.0 + reward_cost.0 * reporters_len,
							(1 + nominators_len) * slash_cost.1 + reward_cost.1 * reporters_len,
						);
					}
				} else {
					// Defer to end of some `slash_defer_duration` from now.
					log!(
						debug,
						"deferring slash of {:?}% happened in {:?} (reported in {:?}) to {:?}",
						slash_fraction,
						slash_era,
						active_era,
						slash_era + slash_defer_duration + 1,
					);
					<Self as Store>::UnappliedSlashes::mutate(
						slash_era.saturating_add(slash_defer_duration).saturating_add(One::one()),
						move |for_later| for_later.push(unapplied),
					);
					add_db_reads_writes(1, 1);
				}
			} else {
				add_db_reads_writes(4 /* fetch_spans */, 5 /* kick_out_if_recent */)
			}
		}

		consumed_weight
	}
}

impl<T: Config> ScoreProvider<T::AccountId> for Pallet<T> {
	type Score = VoteWeight;

	fn score(who: &T::AccountId) -> Self::Score {
		Self::weight_of(who)
	}

	#[cfg(feature = "runtime-benchmarks")]
	fn set_score_of(who: &T::AccountId, weight: Self::Score) {
		// this will clearly results in an inconsistent state, but it should not matter for a
		// benchmark.
		let active: BalanceOf<T> = weight.try_into().map_err(|_| ()).unwrap();
		let mut ledger = match Self::ledger(who) {
			None => StakingLedger::default_from(who.clone()),
			Some(l) => l,
		};
		ledger.active = active;

		<Ledger<T>>::insert(who, ledger);
		<Bonded<T>>::insert(who, who);

		// also, we play a trick to make sure that a issuance based-`CurrencyToVote` behaves well:
		// This will make sure that total issuance is zero, thus the currency to vote will be a 1-1
		// conversion.
		let imbalance = T::Currency::burn(T::Currency::total_issuance());
		// kinda ugly, but gets the job done. The fact that this works here is a HUGE exception.
		// Don't try this pattern in other places.
		sp_std::mem::forget(imbalance);
	}
}

/// A simple sorted list implementation that does not require any additional pallets. Note, this
/// does not provide validators in sorted order. If you desire nominators in a sorted order take
/// a look at [`pallet-bags-list`].
pub struct UseValidatorsMap<T>(sp_std::marker::PhantomData<T>);
impl<T: Config> SortedListProvider<T::AccountId> for UseValidatorsMap<T> {
	type Score = BalanceOf<T>;
	type Error = ();

	/// Returns iterator over voter list, which can have `take` called on it.
	fn iter() -> Box<dyn Iterator<Item = T::AccountId>> {
		Box::new(Validators::<T>::iter().map(|(v, _)| v))
	}
	fn iter_from(
		start: &T::AccountId,
	) -> Result<Box<dyn Iterator<Item = T::AccountId>>, Self::Error> {
		if Validators::<T>::contains_key(start) {
			let start_key = Validators::<T>::hashed_key_for(start);
			Ok(Box::new(Validators::<T>::iter_from(start_key).map(|(n, _)| n)))
		} else {
			Err(())
		}
	}
	fn count() -> u32 {
		Validators::<T>::count()
	}
	fn contains(id: &T::AccountId) -> bool {
		Validators::<T>::contains_key(id)
	}
	fn on_insert(_: T::AccountId, _weight: Self::Score) -> Result<(), Self::Error> {
		// nothing to do on insert.
		Ok(())
	}
	fn get_score(id: &T::AccountId) -> Result<Self::Score, Self::Error> {
		Ok(Pallet::<T>::weight_of(id).into())
	}
	fn on_update(_: &T::AccountId, _weight: Self::Score) -> Result<(), Self::Error> {
		// nothing to do on update.
		Ok(())
	}
	fn on_remove(_: &T::AccountId) -> Result<(), Self::Error> {
		// nothing to do on remove.
		Ok(())
	}
	fn unsafe_regenerate(
		_: impl IntoIterator<Item = T::AccountId>,
		_: Box<dyn Fn(&T::AccountId) -> Self::Score>,
	) -> u32 {
		// nothing to do upon regenerate.
		0
	}
	fn try_state() -> Result<(), &'static str> {
		Ok(())
	}
	fn unsafe_clear() {
		#[allow(deprecated)]
		Validators::<T>::remove_all();
	}

	#[cfg(feature = "runtime-benchmarks")]
	fn score_update_worst_case(_who: &T::AccountId, _is_increase: bool) -> Self::Score {
		unimplemented!()
	}
}

/// A simple voter list implementation that does not require any additional pallets. Note, this
/// does not provided nominators in sorted ordered. If you desire nominators in a sorted order take
/// a look at [`pallet-bags-list].
pub struct UseNominatorsAndValidatorsMap<T>(sp_std::marker::PhantomData<T>);
impl<T: Config> SortedListProvider<T::AccountId> for UseNominatorsAndValidatorsMap<T> {
	type Error = ();
	type Score = VoteWeight;

	fn iter() -> Box<dyn Iterator<Item = T::AccountId>> {
		Box::new(
			Validators::<T>::iter()
				.map(|(v, _)| v)
				.chain(Nominators::<T>::iter().map(|(n, _)| n)),
		)
	}
	fn iter_from(
		start: &T::AccountId,
	) -> Result<Box<dyn Iterator<Item = T::AccountId>>, Self::Error> {
		if Validators::<T>::contains_key(start) {
			let start_key = Validators::<T>::hashed_key_for(start);
			Ok(Box::new(
				Validators::<T>::iter_from(start_key)
					.map(|(n, _)| n)
					.chain(Nominators::<T>::iter().map(|(x, _)| x)),
			))
		} else if Nominators::<T>::contains_key(start) {
			let start_key = Nominators::<T>::hashed_key_for(start);
			Ok(Box::new(Nominators::<T>::iter_from(start_key).map(|(n, _)| n)))
		} else {
			Err(())
		}
	}
	fn count() -> u32 {
		Nominators::<T>::count().saturating_add(Validators::<T>::count())
	}
	fn contains(id: &T::AccountId) -> bool {
		Nominators::<T>::contains_key(id) || Validators::<T>::contains_key(id)
	}
	fn on_insert(_: T::AccountId, _weight: Self::Score) -> Result<(), Self::Error> {
		// nothing to do on insert.
		Ok(())
	}
	fn get_score(id: &T::AccountId) -> Result<Self::Score, Self::Error> {
		Ok(Pallet::<T>::weight_of(id))
	}
	fn on_update(_: &T::AccountId, _weight: Self::Score) -> Result<(), Self::Error> {
		// nothing to do on update.
		Ok(())
	}
	fn on_remove(_: &T::AccountId) -> Result<(), Self::Error> {
		// nothing to do on remove.
		Ok(())
	}
	fn unsafe_regenerate(
		_: impl IntoIterator<Item = T::AccountId>,
		_: Box<dyn Fn(&T::AccountId) -> Self::Score>,
	) -> u32 {
		// nothing to do upon regenerate.
		0
	}
	fn try_state() -> Result<(), &'static str> {
		Ok(())
	}

	fn unsafe_clear() {
		// NOTE: Caller must ensure this doesn't lead to too many storage accesses. This is a
		// condition of SortedListProvider::unsafe_clear.
		#[allow(deprecated)]
		Nominators::<T>::remove_all();
		#[allow(deprecated)]
		Validators::<T>::remove_all();
	}

	#[cfg(feature = "runtime-benchmarks")]
	fn score_update_worst_case(_who: &T::AccountId, _is_increase: bool) -> Self::Score {
		unimplemented!()
	}
}

// NOTE: in this entire impl block, the assumption is that `who` is a stash account.
impl<T: Config> StakingInterface for Pallet<T> {
	type AccountId = T::AccountId;
	type Balance = BalanceOf<T>;

	fn minimum_nominator_bond() -> Self::Balance {
		MinNominatorBond::<T>::get()
	}

	fn minimum_validator_bond() -> Self::Balance {
		MinValidatorBond::<T>::get()
	}

	fn desired_validator_count() -> u32 {
		ValidatorCount::<T>::get()
	}

	fn election_ongoing() -> bool {
		T::ElectionProvider::ongoing()
	}

	fn force_unstake(who: Self::AccountId) -> sp_runtime::DispatchResult {
		let num_slashing_spans = Self::slashing_spans(&who).iter().count() as u32;
		Self::force_unstake(RawOrigin::Root.into(), who.clone(), num_slashing_spans)
	}

	fn stash_by_ctrl(controller: &Self::AccountId) -> Result<Self::AccountId, DispatchError> {
		Self::ledger(controller)
			.map(|l| l.stash)
			.ok_or(Error::<T>::NotController.into())
	}

	fn is_exposed_in_era(who: &Self::AccountId, era: &EraIndex) -> bool {
		ErasStakers::<T>::iter_prefix(era).any(|(validator, exposures)| {
			validator == *who || exposures.others.iter().any(|i| i.who == *who)
		})
	}

	fn bonding_duration() -> EraIndex {
		T::BondingDuration::get()
	}

	fn current_era() -> EraIndex {
		Self::current_era().unwrap_or(Zero::zero())
	}

	fn stake(who: &Self::AccountId) -> Result<Stake<Self>, DispatchError> {
		Self::bonded(who)
			.and_then(|c| Self::ledger(c))
			.map(|l| Stake { stash: l.stash, total: l.total, active: l.active })
			.ok_or(Error::<T>::NotStash.into())
	}

	fn bond_extra(who: &Self::AccountId, extra: Self::Balance) -> DispatchResult {
		Self::bond_extra(RawOrigin::Signed(who.clone()).into(), extra)
	}

	fn unbond(who: &Self::AccountId, value: Self::Balance) -> DispatchResult {
		let ctrl = Self::bonded(who).ok_or(Error::<T>::NotStash)?;
		Self::unbond(RawOrigin::Signed(ctrl).into(), value)
			.map_err(|with_post| with_post.error)
			.map(|_| ())
	}

	fn chill(who: &Self::AccountId) -> DispatchResult {
		// defensive-only: any account bonded via this interface has the stash set as the
		// controller, but we have to be sure. Same comment anywhere else that we read this.
		let ctrl = Self::bonded(who).ok_or(Error::<T>::NotStash)?;
		Self::chill(RawOrigin::Signed(ctrl).into())
	}

	fn withdraw_unbonded(
		who: Self::AccountId,
		num_slashing_spans: u32,
	) -> Result<bool, DispatchError> {
		let ctrl = Self::bonded(who).ok_or(Error::<T>::NotStash)?;
		Self::withdraw_unbonded(RawOrigin::Signed(ctrl.clone()).into(), num_slashing_spans)
			.map(|_| !Ledger::<T>::contains_key(&ctrl))
			.map_err(|with_post| with_post.error)
	}

	fn bond(
		who: &Self::AccountId,
		value: Self::Balance,
		payee: &Self::AccountId,
	) -> DispatchResult {
		Self::bond(
			RawOrigin::Signed(who.clone()).into(),
			T::Lookup::unlookup(who.clone()),
			value,
			RewardDestination::Account(payee.clone()),
		)
	}

	fn nominate(who: &Self::AccountId, targets: Vec<Self::AccountId>) -> DispatchResult {
		let ctrl = Self::bonded(who).ok_or(Error::<T>::NotStash)?;
		let targets = targets.into_iter().map(T::Lookup::unlookup).collect::<Vec<_>>();
		Self::nominate(RawOrigin::Signed(ctrl).into(), targets)
	}

	sp_staking::runtime_benchmarks_enabled! {
		fn nominations(who: Self::AccountId) -> Option<Vec<T::AccountId>> {
			Nominators::<T>::get(who).map(|n| n.targets.into_inner())
		}

		fn add_era_stakers(
			current_era: &EraIndex,
			stash: &T::AccountId,
			exposures: Vec<(Self::AccountId, Self::Balance)>,
		) {
			let others = exposures
				.iter()
				.map(|(who, value)| IndividualExposure { who: who.clone(), value: value.clone() })
				.collect::<Vec<_>>();
			let exposure = Exposure { total: Default::default(), own: Default::default(), others };
			<ErasStakers<T>>::insert(&current_era, &stash, &exposure);
		}

		fn set_current_era(era: EraIndex) {
			CurrentEra::<T>::put(era);
		}
	}
}

#[cfg(any(test, feature = "try-runtime"))]
impl<T: Config> Pallet<T> {
	pub(crate) fn do_try_state(_: BlockNumberFor<T>) -> Result<(), &'static str> {
		ensure!(
			T::VoterList::iter()
				.all(|x| <Nominators<T>>::contains_key(&x) || <Validators<T>>::contains_key(&x)),
			"VoterList contains non-staker"
		);

		Self::check_nominators()?;
		Self::check_exposures()?;
		Self::check_ledgers()?;
		Self::check_count()
	}

	fn check_count() -> Result<(), &'static str> {
		ensure!(
			<T as Config>::VoterList::count() ==
				Nominators::<T>::count() + Validators::<T>::count(),
			"wrong external count"
		);
		ensure!(
			<T as Config>::TargetList::count() == Validators::<T>::count(),
			"wrong external count"
		);
		ensure!(
			ValidatorCount::<T>::get() <=
				<T::ElectionProvider as frame_election_provider_support::ElectionProviderBase>::MaxWinners::get(),
			"validator count exceeded election max winners"
		);
		Ok(())
	}

	fn check_ledgers() -> Result<(), &'static str> {
		Bonded::<T>::iter()
			.map(|(_, ctrl)| Self::ensure_ledger_consistent(ctrl))
			.collect::<Result<_, _>>()
	}

	fn check_exposures() -> Result<(), &'static str> {
		// a check per validator to ensure the exposure struct is always sane.
		let era = Self::active_era().unwrap().index;
		ErasStakers::<T>::iter_prefix_values(era)
			.map(|expo| {
				ensure!(
					expo.total ==
						expo.own +
							expo.others
								.iter()
								.map(|e| e.value)
								.fold(Zero::zero(), |acc, x| acc + x),
					"wrong total exposure.",
				);
				Ok(())
			})
			.collect::<Result<_, _>>()
	}

	fn check_nominators() -> Result<(), &'static str> {
		// a check per nominator to ensure their entire stake is correctly distributed. Will only
		// kick-in if the nomination was submitted before the current era.
		let era = Self::active_era().unwrap().index;
		<Nominators<T>>::iter()
			.filter_map(
				|(nominator, nomination)| {
					if nomination.submitted_in < era {
						Some(nominator)
					} else {
						None
					}
				},
			)
			.map(|nominator| {
				// must be bonded.
				Self::ensure_is_stash(&nominator)?;
				let mut sum = BalanceOf::<T>::zero();
				T::SessionInterface::validators()
					.iter()
					.map(|v| Self::eras_stakers(era, v))
					.map(|e| {
						let individual =
							e.others.iter().filter(|e| e.who == nominator).collect::<Vec<_>>();
						let len = individual.len();
						match len {
							0 => { /* not supporting this validator at all. */ },
							1 => sum += individual[0].value,
							_ => return Err("nominator cannot back a validator more than once."),
						};
						Ok(())
					})
					.collect::<Result<_, _>>()
			})
			.collect::<Result<_, _>>()
	}

	fn ensure_is_stash(who: &T::AccountId) -> Result<(), &'static str> {
		ensure!(Self::bonded(who).is_some(), "Not a stash.");
		Ok(())
	}

	fn ensure_ledger_consistent(ctrl: T::AccountId) -> Result<(), &'static str> {
		// ensures ledger.total == ledger.active + sum(ledger.unlocking).
		let ledger = Self::ledger(ctrl.clone()).ok_or("Not a controller.")?;
		let real_total: BalanceOf<T> =
			ledger.unlocking.iter().fold(ledger.active, |a, c| a + c.value);
		ensure!(real_total == ledger.total, "ledger.total corrupt");

		if !(ledger.active >= T::Currency::minimum_balance() || ledger.active.is_zero()) {
			log!(warn, "ledger.active less than ED: {:?}, {:?}", ctrl, ledger)
		}

		Ok(())
	}
}