lib.rs

		};
		imbalance.maybe_subsume(Self::make_payout(stash, validator_cut + off_the_table));
		T::Reward::on_unbalanced(imbalance);
	}

	/// Session has just ended. Provide the validator set for the next session if it's an era-end.
	fn new_session(session_index: SessionIndex) -> Option<Vec<T::AccountId>> {
		// accumulate good session reward
		let reward = Self::current_session_reward();
		<CurrentEraReward<T>>::mutate(|r| *r += reward);

		if ForceNewEra::take() || session_index % T::SessionsPerEra::get() == 0 {
			Self::new_era()
		} else {
			None
		}
	}

	/// The era has changed - enact new staking set.
	///
	/// NOTE: This always happens immediately before a session change to ensure that new validators
	/// get a chance to set their session keys.
	fn new_era() -> Option<Vec<T::AccountId>> {
		// Payout
		let reward = <CurrentEraReward<T>>::take();
		if !reward.is_zero() {
			let validators = Self::current_elected();
			for v in validators.iter() {
				Self::reward_validator(v, reward);
			}
			Self::deposit_event(RawEvent::Reward(reward));
			let len = validators.len() as u32; // validators length can never overflow u64
			let len: BalanceOf<T> = len.into();
			let total_minted = reward * len;
			let total_rewarded_stake = Self::slot_stake() * len;
			T::OnRewardMinted::on_dilution(total_minted, total_rewarded_stake);
		}

		// Increment current era.
		CurrentEra::mutate(|s| *s += 1);

		// Reassign all Stakers.
		let (slot_stake, maybe_new_validators) = Self::select_validators();

		// Update the balances for rewarding according to the stakes.
		<CurrentSessionReward<T>>::put(Self::session_reward() * slot_stake);

		maybe_new_validators
	}

	fn slashable_balance_of(stash: &T::AccountId) -> BalanceOf<T> {
		Self::bonded(stash).and_then(Self::ledger).map(|l| l.total).unwrap_or_default()
	}

	/// Select a new validator set from the assembled stakers and their role preferences.
	///
	/// Returns the new `SlotStake` value.
	fn select_validators() -> (BalanceOf<T>, Option<Vec<T::AccountId>>) {
		let maybe_elected_set = elect::<T, _, _, _>(
			Self::validator_count() as usize,
			Self::minimum_validator_count().max(1) as usize,
			<Validators<T>>::enumerate(),
			<Nominators<T>>::enumerate(),
			Self::slashable_balance_of,
		);

		if let Some(elected_set) = maybe_elected_set {
			let elected_stashes = elected_set.0;
			let assignments = elected_set.1;

			// helper closure.
			let to_balance = |b: ExtendedBalance|
				<T::CurrencyToVote as Convert<ExtendedBalance, BalanceOf<T>>>::convert(b);
			let to_votes = |b: BalanceOf<T>|
				<T::CurrencyToVote as Convert<BalanceOf<T>, u64>>::convert(b) as ExtendedBalance;

			// The return value of this is safe to be converted to u64.
			// The original balance, `b` is within the scope of u64. It is just extended to u128
			// to be properly multiplied by a ratio, which will lead to another value
			// less than u64 for sure. The result can then be safely passed to `to_balance`.
			// For now the backward convert is used. A simple `TryFrom<u64>` is also safe.
			let ratio_of = |b, p| (p as ExtendedBalance).saturating_mul(to_votes(b)) / ACCURACY;

			// Compute the actual stake from nominator's ratio.
			let assignments_with_stakes = assignments.iter().map(|(n, a)|(
				n.clone(),
				Self::slashable_balance_of(n),
				a.iter().map(|(acc, r)| (
					acc.clone(),
					*r,
					to_balance(ratio_of(Self::slashable_balance_of(n), *r)),
				))
				.collect::<Vec<Assignment<T>>>()
			)).collect::<Vec<(T::AccountId, BalanceOf<T>, Vec<Assignment<T>>)>>();

			// update elected candidate exposures.
			let mut exposures = <ExpoMap<T>>::new();
			elected_stashes
				.iter()
				.map(|e| (e, Self::slashable_balance_of(e)))
				.for_each(|(e, s)| {
					let item = Exposure { own: s, total: s, ..Default::default() };
					exposures.insert(e.clone(), item);
				});

			for (n, _, assignment) in &assignments_with_stakes {
				for (c, _, s) in assignment {
					if let Some(expo) = exposures.get_mut(c) {
						// NOTE: simple example where this saturates:
						// candidate with max_value stake. 1 nominator with max_value stake.
						// Nuked. Sadly there is not much that we can do about this.
						// See this test: phragmen_should_not_overflow_xxx()
						expo.total = expo.total.saturating_add(*s);
						expo.others.push( IndividualExposure { who: n.clone(), value: *s } );
					}
				}
			}

			if cfg!(feature = "equalize") {
				let tolerance = 0_u128;
				let iterations = 2_usize;
				let mut assignments_with_votes = assignments_with_stakes.iter()
					.map(|a| (
						a.0.clone(), a.1,
						a.2.iter()
							.map(|e| (e.0.clone(), e.1, to_votes(e.2)))
							.collect::<Vec<(T::AccountId, ExtendedBalance, ExtendedBalance)>>()
					))
					.collect::<Vec<(
						T::AccountId,
						BalanceOf<T>,
						Vec<(T::AccountId, ExtendedBalance, ExtendedBalance)>
					)>>();
				equalize::<T>(&mut assignments_with_votes, &mut exposures, tolerance, iterations);
			}

			// Clear Stakers and reduce their slash_count.
			for v in Self::current_elected().iter() {
				<Stakers<T>>::remove(v);
				let slash_count = <SlashCount<T>>::take(v);
				if slash_count > 1 {
					<SlashCount<T>>::insert(v, slash_count - 1);
				}
			}

			// Populate Stakers and figure out the minimum stake behind a slot.
			let mut slot_stake = BalanceOf::<T>::max_value();
			for (c, e) in exposures.iter() {
				if e.total < slot_stake {
					slot_stake = e.total;
				}
				<Stakers<T>>::insert(c.clone(), e.clone());
			}

			// Update slot stake.
			<SlotStake<T>>::put(&slot_stake);

			// Set the new validator set in sessions.
			<CurrentElected<T>>::put(&elected_stashes);
			let validators = elected_stashes.into_iter()
				.map(|s| Self::bonded(s).unwrap_or_default())
				.collect::<Vec<_>>();
			(slot_stake, Some(validators))
		} 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
			(Self::slot_stake(), None)
		}
	}

	fn apply_force_new_era() {
		ForceNewEra::put(true);
	}

	/// Call when a validator is determined to be offline. `count` is the
	/// number of offenses the validator has committed.
	///
	/// NOTE: This is called with the controller (not the stash) account id.
	pub fn on_offline_validator(controller: T::AccountId, count: usize) {
		if let Some(l) = Self::ledger(&controller) {
			let stash = l.stash;

			// Early exit if validator is invulnerable.
			if Self::invulnerables().contains(&stash) {
				return
			}

			let slash_count = Self::slash_count(&stash);
			let new_slash_count = slash_count + count as u32;
			<SlashCount<T>>::insert(&stash, new_slash_count);
			let grace = Self::offline_slash_grace();

			if RECENT_OFFLINE_COUNT > 0 {
				let item = (stash.clone(), <system::Module<T>>::block_number(), count as u32);
				<RecentlyOffline<T>>::mutate(|v| if v.len() >= RECENT_OFFLINE_COUNT {
					let index = v.iter()
						.enumerate()
						.min_by_key(|(_, (_, block, _))| block)
						.expect("v is non-empty; qed")
						.0;
					v[index] = item;
				} else {
					v.push(item);
				});
			}

			let prefs = Self::validators(&stash);
			let unstake_threshold = prefs.unstake_threshold.min(MAX_UNSTAKE_THRESHOLD);
			let max_slashes = grace + unstake_threshold;

			let event = if new_slash_count > max_slashes {
				let slash_exposure = Self::stakers(&stash).total;
				let offline_slash_base = Self::offline_slash() * slash_exposure;
				// They're bailing.
				let slash = offline_slash_base
					// Multiply slash_mantissa by 2^(unstake_threshold with upper bound)
					.checked_shl(unstake_threshold)
					.map(|x| x.min(slash_exposure))
					.unwrap_or(slash_exposure);
				let _ = Self::slash_validator(&stash, slash);
				let _ = <session::Module<T>>::disable(&controller);

				RawEvent::OfflineSlash(stash.clone(), slash)
			} else {
				RawEvent::OfflineWarning(stash.clone(), slash_count)
			};

			Self::deposit_event(event);
		}
	}
}

impl<T: Trait> OnSessionEnding<T::AccountId> for Module<T> {
	fn on_session_ending(i: SessionIndex) -> Option<Vec<T::AccountId>> {
		Self::new_session(i + 1)
	}
}

impl<T: Trait> OnFreeBalanceZero<T::AccountId> for Module<T> {
	fn on_free_balance_zero(stash: &T::AccountId) {
		if let Some(controller) = <Bonded<T>>::take(stash) {
			<Ledger<T>>::remove(&controller);
		}
		<Payee<T>>::remove(stash);
		<SlashCount<T>>::remove(stash);
		<Validators<T>>::remove(stash);
		<Nominators<T>>::remove(stash);
	}
}

impl<T: Trait> SelectInitialValidators<T> for Module<T> {
	fn select_initial_validators() -> Option<Vec<T::AccountId>> {
		<Module<T>>::select_validators().1
	}
}