lib.rs

		/// DB Weight:
		/// - Reads: Stash Account, Bonded, Slashing Spans, Locks
		/// - Writes: Bonded, Slashing Spans (if S > 0), Ledger, Payee, Validators, Nominators, Stash Account, Locks
		/// - Writes Each: SpanSlash * S
		/// # </weight>
		#[weight = T::DbWeight::get().reads_writes(4, 7)
			.saturating_add(76 * WEIGHT_PER_MICROS)
			.saturating_add(
				WEIGHT_PER_MICROS.saturating_mul(2).saturating_mul(Weight::from(*num_slashing_spans))
			)
			.saturating_add(T::DbWeight::get().writes(Weight::from(*num_slashing_spans)))
			// if slashing spans is non-zero, add 1 more write
			.saturating_add(T::DbWeight::get().writes(Weight::from(*num_slashing_spans).min(1)))
		]
		fn reap_stash(_origin, stash: T::AccountId, num_slashing_spans: u32) {
			ensure!(T::Currency::total_balance(&stash).is_zero(), Error::<T>::FundedTarget);
			Self::kill_stash(&stash, num_slashing_spans)?;
			T::Currency::remove_lock(STAKING_ID, &stash);
		}

		/// Submit a phragmen result to the chain. If the solution:
		///
		/// 1. is valid.
		/// 2. has a better score than a potentially existing solution on chain.
		///
		/// then, it will be _put_ on chain.
		///
		/// A solution consists of two pieces of data:
		///
		/// 1. `winners`: a flat vector of all the winners of the round.
		/// 2. `assignments`: the compact version of an assignment vector that encodes the edge
		///    weights.
		///
		/// Both of which may be computed using [`phragmen`], or any other algorithm.
		///
		/// Additionally, the submitter must provide:
		///
		/// - The `score` that they claim their solution has.
		///
		/// Both validators and nominators will be represented by indices in the solution. The
		/// indices should respect the corresponding types ([`ValidatorIndex`] and
		/// [`NominatorIndex`]). Moreover, they should be valid when used to index into
		/// [`SnapshotValidators`] and [`SnapshotNominators`]. Any invalid index will cause the
		/// solution to be rejected. These two storage items are set during the election window and
		/// may be used to determine the indices.
		///
		/// A solution is valid if:
		///
		/// 0. It is submitted when [`EraElectionStatus`] is `Open`.
		/// 1. Its claimed score is equal to the score computed on-chain.
		/// 2. Presents the correct number of winners.
		/// 3. All indexes must be value according to the snapshot vectors. All edge values must
		///    also be correct and should not overflow the granularity of the ratio type (i.e. 256
		///    or billion).
		/// 4. For each edge, all targets are actually nominated by the voter.
		/// 5. Has correct self-votes.
		///
		/// A solutions score is consisted of 3 parameters:
		///
		/// 1. `min { support.total }` for each support of a winner. This value should be maximized.
		/// 2. `sum { support.total }` for each support of a winner. This value should be minimized.
		/// 3. `sum { support.total^2 }` for each support of a winner. This value should be
		///    minimized (to ensure less variance)
		///
		/// # <weight>
		/// E: number of edges. m: size of winner committee. n: number of nominators. d: edge degree
		/// (16 for now) v: number of on-chain validator candidates.
		///
		/// NOTE: given a solution which is reduced, we can enable a new check the ensure `|E| < n +
		/// m`. We don't do this _yet_, but our offchain worker code executes it nonetheless.
		///
		/// major steps (all done in `check_and_replace_solution`):
		///
		/// - Storage: O(1) read `ElectionStatus`.
		/// - Storage: O(1) read `PhragmenScore`.
		/// - Storage: O(1) read `ValidatorCount`.
		/// - Storage: O(1) length read from `SnapshotValidators`.
		///
		/// - Storage: O(v) reads of `AccountId` to fetch `snapshot_validators`.
		/// - Memory: O(m) iterations to map winner index to validator id.
		/// - Storage: O(n) reads `AccountId` to fetch `snapshot_nominators`.
		/// - Memory: O(n + m) reads to map index to `AccountId` for un-compact.
		///
		/// - Storage: O(e) accountid reads from `Nomination` to read correct nominations.
		/// - Storage: O(e) calls into `slashable_balance_of_vote_weight` to convert ratio to staked.
		///
		/// - Memory: build_support_map. O(e).
		/// - Memory: evaluate_support: O(E).
		///
		/// - Storage: O(e) writes to `QueuedElected`.
		/// - Storage: O(1) write to `QueuedScore`
		///
		/// The weight of this call is 1/10th of the blocks total weight.
		/// # </weight>
		#[weight = 100_000_000_000]
		pub fn submit_election_solution(
			origin,
			winners: Vec<ValidatorIndex>,
			compact_assignments: CompactAssignments,
			score: PhragmenScore,
			era: EraIndex,
		) {
			let _who = ensure_signed(origin)?;
			Self::check_and_replace_solution(
				winners,
				compact_assignments,
				ElectionCompute::Signed,
				score,
				era,
			)?
		}

		/// Unsigned version of `submit_election_solution`.
		///
		/// Note that this must pass the [`ValidateUnsigned`] check which only allows transactions
		/// from the local node to be included. In other words, only the block author can include a
		/// transaction in the block.
		#[weight = 100_000_000_000]
		pub fn submit_election_solution_unsigned(
			origin,
			winners: Vec<ValidatorIndex>,
			compact_assignments: CompactAssignments,
			score: PhragmenScore,
			era: EraIndex,
		) {
			ensure_none(origin)?;
			Self::check_and_replace_solution(
				winners,
				compact_assignments,
				ElectionCompute::Unsigned,
				score,
				era,
			)?
			// TODO: instead of returning an error, panic. This makes the entire produced block
			// invalid.
			// This ensures that block authors will not ever try and submit a solution which is not
			// an improvement, since they will lose their authoring points/rewards.
		}
	}
}

impl<T: Trait> Module<T> {
	/// The total balance that can be slashed from a stash account as of right now.
	pub fn slashable_balance_of(stash: &T::AccountId) -> BalanceOf<T> {
		Self::bonded(stash).and_then(Self::ledger).map(|l| l.active).unwrap_or_default()
	}

	/// internal impl of [`slashable_balance_of`] that returns [`VoteWeight`].
	fn slashable_balance_of_vote_weight(stash: &T::AccountId) -> VoteWeight {
		<T::CurrencyToVote as Convert<BalanceOf<T>, VoteWeight>>::convert(
			Self::slashable_balance_of(stash)
		)
	}

	/// Dump the list of validators and nominators into vectors and keep them on-chain.
	///
	/// This data is used to efficiently evaluate election results. returns `true` if the operation
	/// is successful.
	fn create_stakers_snapshot() -> (bool, Weight) {
		let mut consumed_weight = 0;
		let mut add_db_reads_writes = |reads, writes| {
			consumed_weight += T::DbWeight::get().reads_writes(reads, writes);
		};
		let validators = <Validators<T>>::iter().map(|(v, _)| v).collect::<Vec<_>>();
		let mut nominators = <Nominators<T>>::iter().map(|(n, _)| n).collect::<Vec<_>>();

		let num_validators = validators.len();
		let num_nominators = nominators.len();
		add_db_reads_writes((num_validators + num_nominators) as Weight, 0);

		if
			num_validators > MAX_VALIDATORS ||
			num_nominators.saturating_add(num_validators) > MAX_NOMINATORS
		{
			log!(
				warn,
				"💸 Snapshot size too big [{} <> {}][{} <> {}].",
				num_validators,
				MAX_VALIDATORS,
				num_nominators,
				MAX_NOMINATORS,
			);
			(false, consumed_weight)
		} else {
			// all validators nominate themselves;
			nominators.extend(validators.clone());

			<SnapshotValidators<T>>::put(validators);
			<SnapshotNominators<T>>::put(nominators);
			add_db_reads_writes(0, 2);
			(true, consumed_weight)
		}
	}

	/// Clears both snapshots of stakers.
	fn kill_stakers_snapshot() {
		<SnapshotValidators<T>>::kill();
		<SnapshotNominators<T>>::kill();
	}

	fn do_payout_nominator(ctrl: T::AccountId, era: EraIndex, validators: Vec<(T::AccountId, u32)>)
		-> DispatchResult
	{
		// validators len must not exceed `MAX_NOMINATIONS` to avoid querying more validator
		// exposure than necessary.
		if validators.len() > MAX_NOMINATIONS {
			return Err(Error::<T>::InvalidNumberOfNominations.into());
		}
		// If migrate_era is not populated, then you should use `payout_stakers`
		let migrate_era = MigrateEra::get().ok_or(Error::<T>::InvalidEraToReward)?;
		// This payout mechanism will only work for eras before the migration.
		// Subsequent payouts should use `payout_stakers`.
		ensure!(era < migrate_era, Error::<T>::InvalidEraToReward);
		let current_era = CurrentEra::get().ok_or(Error::<T>::InvalidEraToReward)?;
		ensure!(era <= current_era, Error::<T>::InvalidEraToReward);
		let history_depth = Self::history_depth();
		ensure!(era >= current_era.saturating_sub(history_depth), Error::<T>::InvalidEraToReward);

		// Note: if era has no reward to be claimed, era may be future. better not to update
		// `nominator_ledger.last_reward` in this case.
		let era_payout = <ErasValidatorReward<T>>::get(&era)
			.ok_or_else(|| Error::<T>::InvalidEraToReward)?;

		let mut nominator_ledger = <Ledger<T>>::get(&ctrl).ok_or_else(|| Error::<T>::NotController)?;

		ensure!(
			Self::era_election_status().is_closed() || Self::payee(&nominator_ledger.stash) != RewardDestination::Staked,
			Error::<T>::CallNotAllowed,
		);

		nominator_ledger.claimed_rewards.retain(|&x| x >= current_era.saturating_sub(history_depth));
		match nominator_ledger.claimed_rewards.binary_search(&era) {
			Ok(_) => Err(Error::<T>::AlreadyClaimed)?,
			Err(pos) => nominator_ledger.claimed_rewards.insert(pos, era),
		}

		<Ledger<T>>::insert(&ctrl, &nominator_ledger);

		let mut reward = Perbill::zero();
		let era_reward_points = <ErasRewardPoints<T>>::get(&era);

		for (validator, nominator_index) in validators.into_iter() {
			let commission = Self::eras_validator_prefs(&era, &validator).commission;
			let validator_exposure = <ErasStakersClipped<T>>::get(&era, &validator);

			if let Some(nominator_exposure) = validator_exposure.others
				.get(nominator_index as usize)
			{
				if nominator_exposure.who != nominator_ledger.stash {
					continue;
				}

				let nominator_exposure_part = Perbill::from_rational_approximation(
					nominator_exposure.value,
					validator_exposure.total,
				);
				let validator_point = era_reward_points.individual.get(&validator)
					.map(|points| *points)
					.unwrap_or_else(|| Zero::zero());
				let validator_point_part = Perbill::from_rational_approximation(
					validator_point,
					era_reward_points.total,
				);
				reward = reward.saturating_add(
					validator_point_part
						.saturating_mul(Perbill::one().saturating_sub(commission))
						.saturating_mul(nominator_exposure_part)
				);
			}
		}

		if let Some(imbalance) = Self::make_payout(&nominator_ledger.stash, reward * era_payout) {
			Self::deposit_event(RawEvent::Reward(ctrl, imbalance.peek()));
		}

		Ok(())
	}

	fn do_payout_validator(ctrl: T::AccountId, era: EraIndex) -> DispatchResult {
		// If migrate_era is not populated, then you should use `payout_stakers`
		let migrate_era = MigrateEra::get().ok_or(Error::<T>::InvalidEraToReward)?;
		// This payout mechanism will only work for eras before the migration.
		// Subsequent payouts should use `payout_stakers`.
		ensure!(era < migrate_era, Error::<T>::InvalidEraToReward);
		let current_era = CurrentEra::get().ok_or(Error::<T>::InvalidEraToReward)?;
		ensure!(era <= current_era, Error::<T>::InvalidEraToReward);
		let history_depth = Self::history_depth();
		ensure!(era >= current_era.saturating_sub(history_depth), Error::<T>::InvalidEraToReward);

		// Note: if era has no reward to be claimed, era may be future. better not to update
		// `ledger.last_reward` in this case.
		let era_payout = <ErasValidatorReward<T>>::get(&era)
			.ok_or_else(|| Error::<T>::InvalidEraToReward)?;

		let mut ledger = <Ledger<T>>::get(&ctrl).ok_or_else(|| Error::<T>::NotController)?;

		ensure!(
			Self::era_election_status().is_closed() || Self::payee(&ledger.stash) != RewardDestination::Staked,
			Error::<T>::CallNotAllowed,
		);

		ledger.claimed_rewards.retain(|&x| x >= current_era.saturating_sub(history_depth));
		match ledger.claimed_rewards.binary_search(&era) {
			Ok(_) => Err(Error::<T>::AlreadyClaimed)?,
			Err(pos) => ledger.claimed_rewards.insert(pos, era),
		}

		<Ledger<T>>::insert(&ctrl, &ledger);

		let era_reward_points = <ErasRewardPoints<T>>::get(&era);
		let commission = Self::eras_validator_prefs(&era, &ledger.stash).commission;
		let exposure = <ErasStakersClipped<T>>::get(&era, &ledger.stash);

		let exposure_part = Perbill::from_rational_approximation(
			exposure.own,
			exposure.total,
		);
		let validator_point = era_reward_points.individual.get(&ledger.stash)
			.map(|points| *points)
			.unwrap_or_else(|| Zero::zero());
		let validator_point_part = Perbill::from_rational_approximation(
			validator_point,
			era_reward_points.total,
		);
		let reward = validator_point_part.saturating_mul(
			commission.saturating_add(
				Perbill::one().saturating_sub(commission).saturating_mul(exposure_part)
			)
		);

		if let Some(imbalance) = Self::make_payout(&ledger.stash, reward * era_payout) {
			Self::deposit_event(RawEvent::Reward(ctrl, imbalance.peek()));
		}

		Ok(())
	}

	fn do_payout_stakers(
		validator_stash: T::AccountId,
		era: EraIndex,
	) -> DispatchResult {
		// Validate input data
		let current_era = CurrentEra::get().ok_or(Error::<T>::InvalidEraToReward)?;
		ensure!(era <= current_era, Error::<T>::InvalidEraToReward);
		let history_depth = Self::history_depth();
		ensure!(era >= current_era.saturating_sub(history_depth), Error::<T>::InvalidEraToReward);

		// If there was no migration, then this function is always valid.
		if let Some(migrate_era) = MigrateEra::get() {
			// This payout mechanism will only work for eras on and after the migration.
			// Payouts before then should use `payout_nominator`/`payout_validator`.
			ensure!(migrate_era <= era, Error::<T>::InvalidEraToReward);
		}

		// Note: if era has no reward to be claimed, era may be future. better not to update
		// `ledger.claimed_rewards` in this case.
		let era_payout = <ErasValidatorReward<T>>::get(&era)
			.ok_or_else(|| Error::<T>::InvalidEraToReward)?;

		let controller = Self::bonded(&validator_stash).ok_or(Error::<T>::NotStash)?;
		let mut ledger = <Ledger<T>>::get(&controller).ok_or_else(|| Error::<T>::NotController)?;

		ledger.claimed_rewards.retain(|&x| x >= current_era.saturating_sub(history_depth));
		match ledger.claimed_rewards.binary_search(&era) {
			Ok(_) => Err(Error::<T>::AlreadyClaimed)?,
			Err(pos) => ledger.claimed_rewards.insert(pos, era),
		}

		let exposure = <ErasStakersClipped<T>>::get(&era, &ledger.stash);

		/* Input data seems good, no errors allowed after this point */

		<Ledger<T>>::insert(&controller, &ledger);

		// Get Era reward points. It has TOTAL and INDIVIDUAL
		// Find the fraction of the era reward that belongs to the validator
		// Take that fraction of the eras rewards to split to nominator and validator
		//
		// Then look at the validator, figure out the proportion of their reward
		// which goes to them and each of their nominators.

		let era_reward_points = <ErasRewardPoints<T>>::get(&era);
		let total_reward_points = era_reward_points.total;
		let validator_reward_points = era_reward_points.individual.get(&ledger.stash)
			.map(|points| *points)
			.unwrap_or_else(|| Zero::zero());

		// Nothing to do if they have no reward points.
		if validator_reward_points.is_zero() { return Ok(())}

		// This is the fraction of the total reward that the validator and the
		// nominators will get.
		let validator_total_reward_part = Perbill::from_rational_approximation(
			validator_reward_points,
			total_reward_points,
		);

		// This is how much validator + nominators are entitled to.
		let validator_total_payout = validator_total_reward_part * era_payout;

		let validator_prefs = Self::eras_validator_prefs(&era, &validator_stash);
		// Validator first gets a cut off the top.
		let validator_commission = validator_prefs.commission;
		let validator_commission_payout = validator_commission * validator_total_payout;

		let validator_leftover_payout = validator_total_payout - validator_commission_payout;
		// Now let's calculate how this is split to the validator.
		let validator_exposure_part = Perbill::from_rational_approximation(
			exposure.own,
			exposure.total,
		);
		let validator_staking_payout = validator_exposure_part * validator_leftover_payout;

		// We can now make total validator payout:
		if let Some(imbalance) = Self::make_payout(
			&ledger.stash,
			validator_staking_payout + validator_commission_payout
		) {
			Self::deposit_event(RawEvent::Reward(ledger.stash, imbalance.peek()));
		}

		// Lets now calculate how this is split to the nominators.
		// Sort nominators by highest to lowest exposure, but only keep `max_nominator_payouts` of them.
		for nominator in exposure.others.iter() {
			let nominator_exposure_part = Perbill::from_rational_approximation(
				nominator.value,
				exposure.total,
			);

			let nominator_reward: BalanceOf<T> = nominator_exposure_part * validator_leftover_payout;
			// We can now make nominator payout:
			if let Some(imbalance) = Self::make_payout(&nominator.who, nominator_reward) {
				Self::deposit_event(RawEvent::Reward(nominator.who.clone(), imbalance.peek()));
			}
		}

		Ok(())
	}

	/// Update the ledger for a controller. This will also update the stash lock. The lock will
	/// will lock the entire funds except paying for further transactions.
	fn update_ledger(
		controller: &T::AccountId,
		ledger: &StakingLedger<T::AccountId, BalanceOf<T>>
	) {
		T::Currency::set_lock(
			STAKING_ID,
			&ledger.stash,
			ledger.total,
			WithdrawReasons::all(),
		);
		<Ledger<T>>::insert(controller, ledger);
	}

	/// Chill a stash account.
	fn chill_stash(stash: &T::AccountId) {
		<Validators<T>>::remove(stash);
		<Nominators<T>>::remove(stash);
	}

	/// Actually make a payment to a staker. This uses the currency's reward function
	/// to pay the right payee for the given staker account.
	fn make_payout(stash: &T::AccountId, amount: BalanceOf<T>) -> Option<PositiveImbalanceOf<T>> {
		let dest = Self::payee(stash);
		match dest {
			RewardDestination::Controller => Self::bonded(stash)
				.and_then(|controller|
					T::Currency::deposit_into_existing(&controller, amount).ok()
				),
			RewardDestination::Stash =>
				T::Currency::deposit_into_existing(stash, amount).ok(),
			RewardDestination::Staked => Self::bonded(stash)
				.and_then(|c| Self::ledger(&c).map(|l| (c, l)))
				.and_then(|(controller, mut l)| {
					l.active += amount;
					l.total += amount;
					let r = T::Currency::deposit_into_existing(stash, amount).ok();
					Self::update_ledger(&controller, &l);
					r
				}),
		}
	}

	/// Plan a new session potentially trigger a new era.
	fn new_session(session_index: SessionIndex) -> Option<Vec<T::AccountId>> {
		if let Some(current_era) = Self::current_era() {
			// Initial era has been set.

			let current_era_start_session_index = Self::eras_start_session_index(current_era)
				.unwrap_or_else(|| {
					frame_support::print("Error: start_session_index must be set for current_era");
					0
				});

			let era_length = session_index.checked_sub(current_era_start_session_index)
				.unwrap_or(0); // Must never happen.

			match ForceEra::get() {
				Forcing::ForceNew => ForceEra::kill(),
				Forcing::ForceAlways => (),
				Forcing::NotForcing if era_length >= T::SessionsPerEra::get() => (),
				_ => {
					// not forcing, not a new era either. If final, set the flag.
					if era_length + 1 >= T::SessionsPerEra::get() {
						IsCurrentSessionFinal::put(true);
					}
					return None
				},
			}

			// new era.
			IsCurrentSessionFinal::put(false);
			Self::new_era(session_index)
		} else {
			// Set initial era
			Self::new_era(session_index)
		}
	}

	/// Basic and cheap checks that we perform in validate unsigned, and in the execution.
	pub fn pre_dispatch_checks(score: PhragmenScore, era: EraIndex) -> Result<(), Error<T>> {
		// discard solutions that are not in-time
		// check window open
		ensure!(
			Self::era_election_status().is_open(),
			Error::<T>::PhragmenEarlySubmission,
		);

		// check current era.
		if let Some(current_era) = Self::current_era() {
			ensure!(
				current_era == era,
				Error::<T>::PhragmenEarlySubmission,
			)
		}

		// assume the given score is valid. Is it better than what we have on-chain, if we have any?
		if let Some(queued_score) = Self::queued_score() {
			ensure!(
				is_score_better(queued_score, score),
				Error::<T>::PhragmenWeakSubmission,
			)
		}

		Ok(())
	}

	/// Checks a given solution and if correct and improved, writes it on chain as the queued result
	/// of the next round. This may be called by both a signed and an unsigned transaction.
	pub fn check_and_replace_solution(
		winners: Vec<ValidatorIndex>,
		compact_assignments: CompactAssignments,
		compute: ElectionCompute,
		claimed_score: PhragmenScore,
		era: EraIndex,
	) -> Result<(), Error<T>> {
		// Do the basic checks. era, claimed score and window open.
		Self::pre_dispatch_checks(claimed_score, era)?;

		// Check that the number of presented winners is sane. Most often we have more candidates
		// that we need. Then it should be Self::validator_count(). Else it should be all the
		// candidates.
		let snapshot_length = <SnapshotValidators<T>>::decode_len()
			.ok_or_else(|| Error::<T>::SnapshotUnavailable)?;

		// check the winner length only here and when we know the length of the snapshot validators
		// length.
		let desired_winners = Self::validator_count().min(snapshot_length as u32);
		ensure!(winners.len() as u32 == desired_winners, Error::<T>::PhragmenBogusWinnerCount);

		// decode snapshot validators.
		let snapshot_validators = Self::snapshot_validators()
			.ok_or(Error::<T>::SnapshotUnavailable)?;

		// check if all winners were legit; this is rather cheap. Replace with accountId.
		let winners = winners.into_iter().map(|widx| {
			// NOTE: at the moment, since staking is explicitly blocking any offence until election
			// is closed, we don't check here if the account id at `snapshot_validators[widx]` is
			// actually a validator. If this ever changes, this loop needs to also check this.
			snapshot_validators.get(widx as usize).cloned().ok_or(Error::<T>::PhragmenBogusWinner)
		}).collect::<Result<Vec<T::AccountId>, Error<T>>>()?;

		// decode the rest of the snapshot.
		let snapshot_nominators = <Module<T>>::snapshot_nominators()
			.ok_or(Error::<T>::SnapshotUnavailable)?;

		// helpers
		let nominator_at = |i: NominatorIndex| -> Option<T::AccountId> {
			snapshot_nominators.get(i as usize).cloned()
		};
		let validator_at = |i: ValidatorIndex| -> Option<T::AccountId> {
			snapshot_validators.get(i as usize).cloned()
		};

		// un-compact.
		let assignments = compact_assignments.into_assignment(
			nominator_at,
			validator_at,
		).map_err(|e| {
			// log the error since it is not propagated into the runtime error.
			log!(warn, "💸 un-compacting solution failed due to {:?}", e);
			Error::<T>::PhragmenBogusCompact
		})?;

		// check all nominators actually including the claimed vote. Also check correct self votes.
		// Note that we assume all validators and nominators in `assignments` are properly bonded,
		// because they are coming from the snapshot via a given index.
		for Assignment { who, distribution } in assignments.iter() {
			let is_validator = <Validators<T>>::contains_key(&who);
			let maybe_nomination = Self::nominators(&who);

			if !(maybe_nomination.is_some() ^ is_validator) {
				// all of the indices must map to either a validator or a nominator. If this is ever
				// not the case, then the locking system of staking is most likely faulty, or we
				// have bigger problems.
				log!(error, "💸 detected an error in the staking locking and snapshot.");
				// abort.
				return Err(Error::<T>::PhragmenBogusNominator);
			}

			if !is_validator {
				// a normal vote
				let nomination = maybe_nomination.expect(
					"exactly one of `maybe_validator` and `maybe_nomination.is_some` is true. \
					is_validator is false; maybe_nomination is some; qed"
				);

				// NOTE: we don't really have to check here if the sum of all edges are the
				// nominator correct. Un-compacting assures this by definition.

				for (t, _) in distribution {
					// each target in the provided distribution must be actually nominated by the
					// nominator after the last non-zero slash.
					if nomination.targets.iter().find(|&tt| tt == t).is_none() {
						return Err(Error::<T>::PhragmenBogusNomination);
					}

					if <Self as Store>::SlashingSpans::get(&t).map_or(
						false,
						|spans| nomination.submitted_in < spans.last_nonzero_slash(),
					) {
						return Err(Error::<T>::PhragmenSlashedNomination);
					}
				}
			} else {
				// a self vote
				ensure!(distribution.len() == 1, Error::<T>::PhragmenBogusSelfVote);
				ensure!(distribution[0].0 == *who, Error::<T>::PhragmenBogusSelfVote);
				// defensive only. A compact assignment of length one does NOT encode the weight and
				// it is always created to be 100%.
				ensure!(
					distribution[0].1 == OffchainAccuracy::one(),
					Error::<T>::PhragmenBogusSelfVote,
				);
			}
		}

		// convert into staked assignments.
		let staked_assignments = sp_phragmen::assignment_ratio_to_staked(
			assignments,
			Self::slashable_balance_of_vote_weight,
		);

		// build the support map thereof in order to evaluate.
		// OPTIMIZATION: loop to create the staked assignments but it would bloat the code. Okay for
		// now as it does not add to the complexity order.
		let (supports, num_error) = build_support_map::<T::AccountId>(
			&winners,
			&staked_assignments,
		);
		// This technically checks that all targets in all nominators were among the winners.
		ensure!(num_error == 0, Error::<T>::PhragmenBogusEdge);

		// Check if the score is the same as the claimed one.
		let submitted_score = evaluate_support(&supports);
		ensure!(submitted_score == claimed_score, Error::<T>::PhragmenBogusScore);

		// At last, alles Ok. Exposures and store the result.
		let exposures = Self::collect_exposure(supports);
		log!(
			info,
			"💸 A better solution (with compute {:?}) has been validated and stored on chain.",
			compute,
		);

		// write new results.
		<QueuedElected<T>>::put(ElectionResult {
			elected_stashes: winners,
			compute,
			exposures,
		});
		QueuedScore::put(submitted_score);

		Ok(())

	}

	/// Start a session potentially starting an era.
	fn start_session(start_session: SessionIndex) {
		let next_active_era = Self::active_era().map(|e| e.index + 1).unwrap_or(0);
		if let Some(next_active_era_start_session_index) =
			Self::eras_start_session_index(next_active_era)
		{
			if next_active_era_start_session_index == start_session {
				Self::start_era(start_session);
			} else if next_active_era_start_session_index < start_session {
				// This arm should never happen, but better handle it than to stall the
				// staking pallet.
				frame_support::print("Warning: A session appears to have been skipped.");
				Self::start_era(start_session);
			}
		}
	}

	/// End a session potentially ending an era.
	fn end_session(session_index: SessionIndex) {
		if let Some(active_era) = Self::active_era() {
			if let Some(next_active_era_start_session_index) =
				Self::eras_start_session_index(active_era.index + 1)
			{
				if next_active_era_start_session_index == session_index + 1 {
					Self::end_era(active_era, session_index);
				}
			}
		}
	}

	/// * Increment `active_era.index`,
	/// * reset `active_era.start`,
	/// * update `BondedEras` and apply slashes.
	fn start_era(start_session: SessionIndex) {
		let active_era = ActiveEra::mutate(|active_era| {
			let new_index = active_era.as_ref().map(|info| info.index + 1).unwrap_or(0);
			*active_era = Some(ActiveEraInfo {
				index: new_index,
				// Set new active era start in next `on_finalize`. To guarantee usage of `Time`
				start: None,
			});
			new_index
		});

		let bonding_duration = T::BondingDuration::get();

		BondedEras::mutate(|bonded| {
			bonded.push((active_era, start_session));

			if active_era > bonding_duration {
				let first_kept = active_era - bonding_duration;

				// prune out everything that's from before the first-kept index.
				let n_to_prune = bonded.iter()
					.take_while(|&&(era_idx, _)| era_idx < first_kept)
					.count();

				// kill slashing metadata.
				for (pruned_era, _) in bonded.drain(..n_to_prune) {
					slashing::clear_era_metadata::<T>(pruned_era);
				}

				if let Some(&(_, first_session)) = bonded.first() {
					T::SessionInterface::prune_historical_up_to(first_session);
				}
			}
		});

		Self::apply_unapplied_slashes(active_era);
	}

	/// Compute payout for era.
	fn end_era(active_era: ActiveEraInfo, _session_index: SessionIndex) {
		// Note: active_era_start can be None if end era is called during genesis config.
		if let Some(active_era_start) = active_era.start {
			let now_as_millis_u64 = T::UnixTime::now().as_millis().saturated_into::<u64>();

			let era_duration = now_as_millis_u64 - active_era_start;
			let (validator_payout, max_payout) = inflation::compute_total_payout(
				&T::RewardCurve::get(),
				Self::eras_total_stake(&active_era.index),
				T::Currency::total_issuance(),
				// Duration of era; more than u64::MAX is rewarded as u64::MAX.
				era_duration.saturated_into::<u64>(),
			);
			let rest = max_payout.saturating_sub(validator_payout);

			Self::deposit_event(RawEvent::EraPayout(active_era.index, validator_payout, rest));

			// Set ending era reward.
			<ErasValidatorReward<T>>::insert(&active_era.index, validator_payout);
			T::RewardRemainder::on_unbalanced(T::Currency::issue(rest));
		}
	}

	/// Plan a new era. Return the potential new staking set.
	fn new_era(start_session_index: SessionIndex) -> Option<Vec<T::AccountId>> {
		// Increment or set current era.
		let current_era = CurrentEra::mutate(|s| {
			*s = Some(s.map(|s| s + 1).unwrap_or(0));
			s.unwrap()
		});
		ErasStartSessionIndex::insert(&current_era, &start_session_index);

		// Clean old era information.
		if let Some(old_era) = current_era.checked_sub(Self::history_depth() + 1) {
			Self::clear_era_information(old_era);
		}

		// Set staking information for new era.
		let maybe_new_validators = Self::select_and_update_validators(current_era);

		maybe_new_validators
	}

	/// Select the new validator set at the end of the era.
	///
	/// Runs [`try_do_phragmen`] and updates the following storage items:
	/// - [`EraElectionStatus`]: with `None`.
	/// - [`ErasStakers`]: with the new staker set.
	/// - [`ErasStakersClipped`].
	/// - [`ErasValidatorPrefs`].
	/// - [`ErasTotalStake`]: with the new total stake.
	/// - [`SnapshotValidators`] and [`SnapshotNominators`] are both removed.
	///
	/// Internally, [`QueuedElected`], snapshots and [`QueuedScore`] are also consumed.
	///
	/// If the election has been successful, It passes the new set upwards.
	///
	/// This should only be called at the end of an era.
	fn select_and_update_validators(current_era: EraIndex) -> Option<Vec<T::AccountId>> {
		if let Some(ElectionResult::<T::AccountId, BalanceOf<T>> {
			elected_stashes,
			exposures,
			compute,
		}) = Self::try_do_phragmen() {
			// We have chosen the new validator set. Submission is no longer allowed.
			<EraElectionStatus<T>>::put(ElectionStatus::Closed);

			// kill the snapshots.
			Self::kill_stakers_snapshot();

			// Populate Stakers and write slot stake.
			let mut total_stake: BalanceOf<T> = Zero::zero();
			exposures.into_iter().for_each(|(stash, exposure)| {
				total_stake = total_stake.saturating_add(exposure.total);
				<ErasStakers<T>>::insert(current_era, &stash, &exposure);

				let mut exposure_clipped = exposure;
				let clipped_max_len = T::MaxNominatorRewardedPerValidator::get() as usize;
				if exposure_clipped.others.len() > clipped_max_len {
					exposure_clipped.others.sort_unstable_by(|a, b| a.value.cmp(&b.value).reverse());
					exposure_clipped.others.truncate(clipped_max_len);
				}
				<ErasStakersClipped<T>>::insert(&current_era, &stash, exposure_clipped);
			});

			// Insert current era staking information
			<ErasTotalStake<T>>::insert(&current_era, total_stake);

			// collect the pref of all winners
			for stash in &elected_stashes {
				let pref = Self::validators(stash);
				<ErasValidatorPrefs<T>>::insert(&current_era, stash, pref);
			}

			// emit event
			Self::deposit_event(RawEvent::StakingElection(compute));

			log!(
				info,
				"💸 new validator set of size {:?} has been elected via {:?} for era {:?}",
				elected_stashes.len(),
				compute,
				current_era,
			);

			Some(elected_stashes)
		} else {
			None
		}
	}

	/// Select a new validator set from the assembled stakers and their role preferences. It tries
	/// first to peek into [`QueuedElected`]. Otherwise, it runs a new phragmen.
	///
	/// If [`QueuedElected`] and [`QueuedScore`] exists, they are both removed. No further storage
	/// is updated.
	fn try_do_phragmen() -> Option<ElectionResult<T::AccountId, BalanceOf<T>>> {
		// a phragmen result from either a stored submission or locally executed one.
		let next_result = <QueuedElected<T>>::take().or_else(||
			Self::do_phragmen_with_post_processing::<ChainAccuracy>(ElectionCompute::OnChain)
		);

		// either way, kill this. We remove it here to make sure it always has the exact same
		// lifetime as `QueuedElected`.
		QueuedScore::kill();

		next_result
	}

	/// Execute phragmen and return the new results. The edge weights are processed into support
	/// values.
	///
	/// This is basically a wrapper around [`do_phragmen`] which translates `PhragmenResult` into
	/// `ElectionResult`.
	///
	/// No storage item is updated.
	fn do_phragmen_with_post_processing<Accuracy: PerThing>(compute: ElectionCompute)
		-> Option<ElectionResult<T::AccountId, BalanceOf<T>>>
	where
		Accuracy: sp_std::ops::Mul<ExtendedBalance, Output=ExtendedBalance>,
		ExtendedBalance: From<<Accuracy as PerThing>::Inner>,
	{
		if let Some(phragmen_result) = Self::do_phragmen::<Accuracy>() {
			let elected_stashes = phragmen_result.winners.iter()
				.map(|(s, _)| s.clone())
				.collect::<Vec<T::AccountId>>();
			let assignments = phragmen_result.assignments;

			let staked_assignments = sp_phragmen::assignment_ratio_to_staked(
				assignments,
				Self::slashable_balance_of_vote_weight,
			);

			let (supports, _) = build_support_map::<T::AccountId>(
				&elected_stashes,
				&staked_assignments,
			);

			// collect exposures
			let exposures = Self::collect_exposure(supports);

			// In order to keep the property required by `on_session_ending` that we must return the
			// new validator set even if it's the same as the old, as long as any underlying
			// economic conditions have changed, we don't attempt to do any optimization where we
			// compare against the prior set.
			Some(ElectionResult::<T::AccountId, BalanceOf<T>> {
				elected_stashes,
				exposures,
				compute,
			})
		} else {
			// There were not enough candidates for even our minimal level of functionality. This is
			// bad. We should probably disable all functionality except for block production and let
			// the chain keep producing blocks until we can decide on a sufficiently substantial
			// set. TODO: #2494
			None
		}
	}

	/// Execute phragmen and return the new results. No post-processing is applied and the raw edge
	/// weights are returned.
	///
	/// Self votes are added and nominations before the most recent slashing span are reaped.
	///
	/// No storage item is updated.
	fn do_phragmen<Accuracy: PerThing>() -> Option<PhragmenResult<T::AccountId, Accuracy>> {
		let mut all_nominators: Vec<(T::AccountId, VoteWeight, Vec<T::AccountId>)> = Vec::new();
		let mut all_validators = Vec::new();
		for (validator, _) in <Validators<T>>::iter() {
			// append self vote
			let self_vote = (validator.clone(), Self::slashable_balance_of_vote_weight(&validator), vec![validator.clone()]);
			all_nominators.push(self_vote);
			all_validators.push(validator);
		}

		let nominator_votes = <Nominators<T>>::iter().map(|(nominator, nominations)| {
			let Nominations { submitted_in, mut targets, suppressed: _ } = nominations;

			// Filter out nomination targets which were nominated before the most recent
			// slashing span.
			targets.retain(|stash| {
				<Self as Store>::SlashingSpans::get(&stash).map_or(
					true,
					|spans| submitted_in >= spans.last_nonzero_slash(),
				)
			});

			(nominator, targets)
		});
		all_nominators.extend(nominator_votes.map(|(n, ns)| {
			let s = Self::slashable_balance_of_vote_weight(&n);
			(n, s, ns)
		}));

		elect::<_, Accuracy>(
			Self::validator_count() as usize,
			Self::minimum_validator_count().max(1) as usize,
			all_validators,
			all_nominators,
		)
	}

	/// Consume a set of [`Supports`] from [`sp_phragmen`] and collect them into a [`Exposure`]
	fn collect_exposure(supports: SupportMap<T::AccountId>) -> Vec<(T::AccountId, Exposure<T::AccountId, BalanceOf<T>>)> {
		let to_balance = |e: ExtendedBalance|
			<T::CurrencyToVote as Convert<ExtendedBalance, BalanceOf<T>>>::convert(e);

		supports.into_iter().map(|(validator, support)| {
			// build `struct exposure` from `support`
			let mut others = Vec::new();
			let mut own: BalanceOf<T> = Zero::zero();