lib.rs

	///
	/// NOTE: returns actual amount of transferred value in `Ok` case.
	fn do_transfer_reserved(
		slashed: &T::AccountId,
		beneficiary: &T::AccountId,
		value: T::Balance,
		best_effort: bool,
		status: Status,
	) -> Result<T::Balance, DispatchError> {
		if value.is_zero() {
			return Ok(Zero::zero())
		}

		if slashed == beneficiary {
			return match status {
				Status::Free => Ok(value.saturating_sub(Self::unreserve(slashed, value))),
				Status::Reserved => Ok(value.saturating_sub(Self::reserved_balance(slashed))),
			}
		}

		let ((actual, _maybe_one_dust), _maybe_other_dust) = Self::try_mutate_account_with_dust(
			beneficiary,
			|to_account, is_new| -> Result<(T::Balance, DustCleaner<T, I>), DispatchError> {
				ensure!(!is_new, Error::<T, I>::DeadAccount);
				Self::try_mutate_account_with_dust(
					slashed,
					|from_account, _| -> Result<T::Balance, DispatchError> {
						let actual = cmp::min(from_account.reserved, value);
						ensure!(best_effort || actual == value, Error::<T, I>::InsufficientBalance);
						match status {
							Status::Free =>
								to_account.free = to_account
									.free
									.checked_add(&actual)
									.ok_or(ArithmeticError::Overflow)?,
							Status::Reserved =>
								to_account.reserved = to_account
									.reserved
									.checked_add(&actual)
									.ok_or(ArithmeticError::Overflow)?,
						}
						from_account.reserved -= actual;
						Ok(actual)
					},
				)
			},
		)?;

		Self::deposit_event(Event::ReserveRepatriated {
			from: slashed.clone(),
			to: beneficiary.clone(),
			amount: actual,
			destination_status: status,
		});
		Ok(actual)
	}
}

impl<T: Config<I>, I: 'static> fungible::Inspect<T::AccountId> for Pallet<T, I> {
	type Balance = T::Balance;

	fn total_issuance() -> Self::Balance {
		TotalIssuance::<T, I>::get()
	}
	fn active_issuance() -> Self::Balance {
		TotalIssuance::<T, I>::get().saturating_sub(InactiveIssuance::<T, I>::get())
	}
	fn minimum_balance() -> Self::Balance {
		T::ExistentialDeposit::get()
	}
	fn balance(who: &T::AccountId) -> Self::Balance {
		Self::account(who).total()
	}
	fn reducible_balance(who: &T::AccountId, keep_alive: bool) -> Self::Balance {
		let a = Self::account(who);
		// Liquid balance is what is neither reserved nor locked/frozen.
		let liquid = a.free.saturating_sub(a.fee_frozen.max(a.misc_frozen));
		if frame_system::Pallet::<T>::can_dec_provider(who) && !keep_alive {
			liquid
		} else {
			// `must_remain_to_exist` is the part of liquid balance which must remain to keep total
			// over ED.
			let must_remain_to_exist =
				T::ExistentialDeposit::get().saturating_sub(a.total() - liquid);
			liquid.saturating_sub(must_remain_to_exist)
		}
	}
	fn can_deposit(who: &T::AccountId, amount: Self::Balance, mint: bool) -> DepositConsequence {
		Self::deposit_consequence(who, amount, &Self::account(who), mint)
	}
	fn can_withdraw(
		who: &T::AccountId,
		amount: Self::Balance,
	) -> WithdrawConsequence<Self::Balance> {
		Self::withdraw_consequence(who, amount, &Self::account(who))
	}
}

impl<T: Config<I>, I: 'static> fungible::Mutate<T::AccountId> for Pallet<T, I> {
	fn mint_into(who: &T::AccountId, amount: Self::Balance) -> DispatchResult {
		if amount.is_zero() {
			return Ok(())
		}
		Self::try_mutate_account(who, |account, _is_new| -> DispatchResult {
			Self::deposit_consequence(who, amount, account, true).into_result()?;
			account.free += amount;
			Ok(())
		})?;
		TotalIssuance::<T, I>::mutate(|t| *t += amount);
		Self::deposit_event(Event::Deposit { who: who.clone(), amount });
		Ok(())
	}

	fn burn_from(
		who: &T::AccountId,
		amount: Self::Balance,
	) -> Result<Self::Balance, DispatchError> {
		if amount.is_zero() {
			return Ok(Self::Balance::zero())
		}
		let actual = Self::try_mutate_account(
			who,
			|account, _is_new| -> Result<T::Balance, DispatchError> {
				let extra = Self::withdraw_consequence(who, amount, account).into_result()?;
				let actual = amount + extra;
				account.free -= actual;
				Ok(actual)
			},
		)?;
		TotalIssuance::<T, I>::mutate(|t| *t -= actual);
		Self::deposit_event(Event::Withdraw { who: who.clone(), amount });
		Ok(actual)
	}
}

impl<T: Config<I>, I: 'static> fungible::Transfer<T::AccountId> for Pallet<T, I> {
	fn transfer(
		source: &T::AccountId,
		dest: &T::AccountId,
		amount: T::Balance,
		keep_alive: bool,
	) -> Result<T::Balance, DispatchError> {
		let er = if keep_alive { KeepAlive } else { AllowDeath };
		<Self as Currency<T::AccountId>>::transfer(source, dest, amount, er).map(|_| amount)
	}

	fn deactivate(amount: Self::Balance) {
		InactiveIssuance::<T, I>::mutate(|b| b.saturating_accrue(amount));
	}

	fn reactivate(amount: Self::Balance) {
		InactiveIssuance::<T, I>::mutate(|b| b.saturating_reduce(amount));
	}
}

impl<T: Config<I>, I: 'static> fungible::Unbalanced<T::AccountId> for Pallet<T, I> {
	fn set_balance(who: &T::AccountId, amount: Self::Balance) -> DispatchResult {
		Self::mutate_account(who, |account| -> DispatchResult {
			// fungibles::Unbalanced::decrease_balance didn't check account.reserved
			// free = new_balance - reserved
			account.free =
				amount.checked_sub(&account.reserved).ok_or(ArithmeticError::Underflow)?;
			Self::deposit_event(Event::BalanceSet {
				who: who.clone(),
				free: account.free,
				reserved: account.reserved,
			});

			Ok(())
		})?
	}

	fn set_total_issuance(amount: Self::Balance) {
		TotalIssuance::<T, I>::mutate(|t| *t = amount);
	}
}

impl<T: Config<I>, I: 'static> fungible::InspectHold<T::AccountId> for Pallet<T, I> {
	fn balance_on_hold(who: &T::AccountId) -> T::Balance {
		Self::account(who).reserved
	}
	fn can_hold(who: &T::AccountId, amount: T::Balance) -> bool {
		let a = Self::account(who);
		let min_balance = T::ExistentialDeposit::get().max(a.frozen(Reasons::All));
		if a.reserved.checked_add(&amount).is_none() {
			return false
		}
		// We require it to be min_balance + amount to ensure that the full reserved funds may be
		// slashed without compromising locked funds or destroying the account.
		let required_free = match min_balance.checked_add(&amount) {
			Some(x) => x,
			None => return false,
		};
		a.free >= required_free
	}
}
impl<T: Config<I>, I: 'static> fungible::MutateHold<T::AccountId> for Pallet<T, I> {
	fn hold(who: &T::AccountId, amount: Self::Balance) -> DispatchResult {
		if amount.is_zero() {
			return Ok(())
		}
		ensure!(Self::can_reserve(who, amount), Error::<T, I>::InsufficientBalance);
		Self::mutate_account(who, |a| {
			a.free -= amount;
			a.reserved += amount;
		})?;
		Ok(())
	}
	fn release(
		who: &T::AccountId,
		amount: Self::Balance,
		best_effort: bool,
	) -> Result<T::Balance, DispatchError> {
		if amount.is_zero() {
			return Ok(amount)
		}
		// Done on a best-effort basis.
		Self::try_mutate_account(who, |a, _| {
			let new_free = a.free.saturating_add(amount.min(a.reserved));
			let actual = new_free - a.free;
			ensure!(best_effort || actual == amount, Error::<T, I>::InsufficientBalance);
			// ^^^ Guaranteed to be <= amount and <= a.reserved
			a.free = new_free;
			a.reserved = a.reserved.saturating_sub(actual);
			Ok(actual)
		})
	}
	fn transfer_held(
		source: &T::AccountId,
		dest: &T::AccountId,
		amount: Self::Balance,
		best_effort: bool,
		on_hold: bool,
	) -> Result<Self::Balance, DispatchError> {
		let status = if on_hold { Status::Reserved } else { Status::Free };
		Self::do_transfer_reserved(source, dest, amount, best_effort, status)
	}
}

// wrapping these imbalances in a private module is necessary to ensure absolute privacy
// of the inner member.
mod imbalances {
	use super::{result, Config, Imbalance, RuntimeDebug, Saturating, TryDrop, Zero};
	use frame_support::traits::SameOrOther;
	use sp_std::mem;

	/// Opaque, move-only struct with private fields that serves as a token denoting that
	/// funds have been created without any equal and opposite accounting.
	#[must_use]
	#[derive(RuntimeDebug, PartialEq, Eq)]
	pub struct PositiveImbalance<T: Config<I>, I: 'static = ()>(T::Balance);

	impl<T: Config<I>, I: 'static> PositiveImbalance<T, I> {
		/// Create a new positive imbalance from a balance.
		pub fn new(amount: T::Balance) -> Self {
			PositiveImbalance(amount)
		}
	}

	/// Opaque, move-only struct with private fields that serves as a token denoting that
	/// funds have been destroyed without any equal and opposite accounting.
	#[must_use]
	#[derive(RuntimeDebug, PartialEq, Eq)]
	pub struct NegativeImbalance<T: Config<I>, I: 'static = ()>(T::Balance);

	impl<T: Config<I>, I: 'static> NegativeImbalance<T, I> {
		/// Create a new negative imbalance from a balance.
		pub fn new(amount: T::Balance) -> Self {
			NegativeImbalance(amount)
		}
	}

	impl<T: Config<I>, I: 'static> TryDrop for PositiveImbalance<T, I> {
		fn try_drop(self) -> result::Result<(), Self> {
			self.drop_zero()
		}
	}

	impl<T: Config<I>, I: 'static> Default for PositiveImbalance<T, I> {
		fn default() -> Self {
			Self::zero()
		}
	}

	impl<T: Config<I>, I: 'static> Imbalance<T::Balance> for PositiveImbalance<T, I> {
		type Opposite = NegativeImbalance<T, I>;

		fn zero() -> Self {
			Self(Zero::zero())
		}
		fn drop_zero(self) -> result::Result<(), Self> {
			if self.0.is_zero() {
				Ok(())
			} else {
				Err(self)
			}
		}
		fn split(self, amount: T::Balance) -> (Self, Self) {
			let first = self.0.min(amount);
			let second = self.0 - first;

			mem::forget(self);
			(Self(first), Self(second))
		}
		fn merge(mut self, other: Self) -> Self {
			self.0 = self.0.saturating_add(other.0);
			mem::forget(other);

			self
		}
		fn subsume(&mut self, other: Self) {
			self.0 = self.0.saturating_add(other.0);
			mem::forget(other);
		}
		fn offset(self, other: Self::Opposite) -> SameOrOther<Self, Self::Opposite> {
			let (a, b) = (self.0, other.0);
			mem::forget((self, other));

			if a > b {
				SameOrOther::Same(Self(a - b))
			} else if b > a {
				SameOrOther::Other(NegativeImbalance::new(b - a))
			} else {
				SameOrOther::None
			}
		}
		fn peek(&self) -> T::Balance {
			self.0
		}
	}

	impl<T: Config<I>, I: 'static> TryDrop for NegativeImbalance<T, I> {
		fn try_drop(self) -> result::Result<(), Self> {
			self.drop_zero()
		}
	}

	impl<T: Config<I>, I: 'static> Default for NegativeImbalance<T, I> {
		fn default() -> Self {
			Self::zero()
		}
	}

	impl<T: Config<I>, I: 'static> Imbalance<T::Balance> for NegativeImbalance<T, I> {
		type Opposite = PositiveImbalance<T, I>;

		fn zero() -> Self {
			Self(Zero::zero())
		}
		fn drop_zero(self) -> result::Result<(), Self> {
			if self.0.is_zero() {
				Ok(())
			} else {
				Err(self)
			}
		}
		fn split(self, amount: T::Balance) -> (Self, Self) {
			let first = self.0.min(amount);
			let second = self.0 - first;

			mem::forget(self);
			(Self(first), Self(second))
		}
		fn merge(mut self, other: Self) -> Self {
			self.0 = self.0.saturating_add(other.0);
			mem::forget(other);

			self
		}
		fn subsume(&mut self, other: Self) {
			self.0 = self.0.saturating_add(other.0);
			mem::forget(other);
		}
		fn offset(self, other: Self::Opposite) -> SameOrOther<Self, Self::Opposite> {
			let (a, b) = (self.0, other.0);
			mem::forget((self, other));

			if a > b {
				SameOrOther::Same(Self(a - b))
			} else if b > a {
				SameOrOther::Other(PositiveImbalance::new(b - a))
			} else {
				SameOrOther::None
			}
		}
		fn peek(&self) -> T::Balance {
			self.0
		}
	}

	impl<T: Config<I>, I: 'static> Drop for PositiveImbalance<T, I> {
		/// Basic drop handler will just square up the total issuance.
		fn drop(&mut self) {
			<super::TotalIssuance<T, I>>::mutate(|v| *v = v.saturating_add(self.0));
		}
	}

	impl<T: Config<I>, I: 'static> Drop for NegativeImbalance<T, I> {
		/// Basic drop handler will just square up the total issuance.
		fn drop(&mut self) {
			<super::TotalIssuance<T, I>>::mutate(|v| *v = v.saturating_sub(self.0));
		}
	}
}

impl<T: Config<I>, I: 'static> Currency<T::AccountId> for Pallet<T, I>
where
	T::Balance: MaybeSerializeDeserialize + Debug,
{
	type Balance = T::Balance;
	type PositiveImbalance = PositiveImbalance<T, I>;
	type NegativeImbalance = NegativeImbalance<T, I>;

	fn total_balance(who: &T::AccountId) -> Self::Balance {
		Self::account(who).total()
	}

	// Check if `value` amount of free balance can be slashed from `who`.
	fn can_slash(who: &T::AccountId, value: Self::Balance) -> bool {
		if value.is_zero() {
			return true
		}
		Self::free_balance(who) >= value
	}

	fn total_issuance() -> Self::Balance {
		TotalIssuance::<T, I>::get()
	}

	fn active_issuance() -> Self::Balance {
		<Self as fungible::Inspect<T::AccountId>>::active_issuance()
	}

	fn deactivate(amount: Self::Balance) {
		<Self as fungible::Transfer<T::AccountId>>::deactivate(amount);
	}

	fn reactivate(amount: Self::Balance) {
		<Self as fungible::Transfer<T::AccountId>>::reactivate(amount);
	}

	fn minimum_balance() -> Self::Balance {
		T::ExistentialDeposit::get()
	}

	// Burn funds from the total issuance, returning a positive imbalance for the amount burned.
	// Is a no-op if amount to be burned is zero.
	fn burn(mut amount: Self::Balance) -> Self::PositiveImbalance {
		if amount.is_zero() {
			return PositiveImbalance::zero()
		}
		<TotalIssuance<T, I>>::mutate(|issued| {
			*issued = issued.checked_sub(&amount).unwrap_or_else(|| {
				amount = *issued;
				Zero::zero()
			});
		});
		PositiveImbalance::new(amount)
	}

	// Create new funds into the total issuance, returning a negative imbalance
	// for the amount issued.
	// Is a no-op if amount to be issued it zero.
	fn issue(mut amount: Self::Balance) -> Self::NegativeImbalance {
		if amount.is_zero() {
			return NegativeImbalance::zero()
		}
		<TotalIssuance<T, I>>::mutate(|issued| {
			*issued = issued.checked_add(&amount).unwrap_or_else(|| {
				amount = Self::Balance::max_value() - *issued;
				Self::Balance::max_value()
			})
		});
		NegativeImbalance::new(amount)
	}

	fn free_balance(who: &T::AccountId) -> Self::Balance {
		Self::account(who).free
	}

	// Ensure that an account can withdraw from their free balance given any existing withdrawal
	// restrictions like locks and vesting balance.
	// Is a no-op if amount to be withdrawn is zero.
	//
	// # <weight>
	// Despite iterating over a list of locks, they are limited by the number of
	// lock IDs, which means the number of runtime pallets that intend to use and create locks.
	// # </weight>
	fn ensure_can_withdraw(
		who: &T::AccountId,
		amount: T::Balance,
		reasons: WithdrawReasons,
		new_balance: T::Balance,
	) -> DispatchResult {
		if amount.is_zero() {
			return Ok(())
		}
		let min_balance = Self::account(who).frozen(reasons.into());
		ensure!(new_balance >= min_balance, Error::<T, I>::LiquidityRestrictions);
		Ok(())
	}

	// Transfer some free balance from `transactor` to `dest`, respecting existence requirements.
	// Is a no-op if value to be transferred is zero or the `transactor` is the same as `dest`.
	fn transfer(
		transactor: &T::AccountId,
		dest: &T::AccountId,
		value: Self::Balance,
		existence_requirement: ExistenceRequirement,
	) -> DispatchResult {
		if value.is_zero() || transactor == dest {
			return Ok(())
		}

		Self::try_mutate_account_with_dust(
			dest,
			|to_account, _| -> Result<DustCleaner<T, I>, DispatchError> {
				Self::try_mutate_account_with_dust(
					transactor,
					|from_account, _| -> DispatchResult {
						from_account.free = from_account
							.free
							.checked_sub(&value)
							.ok_or(Error::<T, I>::InsufficientBalance)?;

						// NOTE: total stake being stored in the same type means that this could
						// never overflow but better to be safe than sorry.
						to_account.free =
							to_account.free.checked_add(&value).ok_or(ArithmeticError::Overflow)?;

						let ed = T::ExistentialDeposit::get();
						ensure!(to_account.total() >= ed, Error::<T, I>::ExistentialDeposit);

						Self::ensure_can_withdraw(
							transactor,
							value,
							WithdrawReasons::TRANSFER,
							from_account.free,
						)
						.map_err(|_| Error::<T, I>::LiquidityRestrictions)?;

						// TODO: This is over-conservative. There may now be other providers, and
						// this pallet may not even be a provider.
						let allow_death = existence_requirement == ExistenceRequirement::AllowDeath;
						let allow_death =
							allow_death && system::Pallet::<T>::can_dec_provider(transactor);
						ensure!(
							allow_death || from_account.total() >= ed,
							Error::<T, I>::KeepAlive
						);

						Ok(())
					},
				)
				.map(|(_, maybe_dust_cleaner)| maybe_dust_cleaner)
			},
		)?;

		// Emit transfer event.
		Self::deposit_event(Event::Transfer {
			from: transactor.clone(),
			to: dest.clone(),
			amount: value,
		});

		Ok(())
	}

	/// Slash a target account `who`, returning the negative imbalance created and any left over
	/// amount that could not be slashed.
	///
	/// Is a no-op if `value` to be slashed is zero or the account does not exist.
	///
	/// NOTE: `slash()` prefers free balance, but assumes that reserve balance can be drawn
	/// from in extreme circumstances. `can_slash()` should be used prior to `slash()` to avoid
	/// having to draw from reserved funds, however we err on the side of punishment if things are
	/// inconsistent or `can_slash` wasn't used appropriately.
	fn slash(who: &T::AccountId, value: Self::Balance) -> (Self::NegativeImbalance, Self::Balance) {
		if value.is_zero() {
			return (NegativeImbalance::zero(), Zero::zero())
		}
		if Self::total_balance(who).is_zero() {
			return (NegativeImbalance::zero(), value)
		}

		for attempt in 0..2 {
			match Self::try_mutate_account(
				who,
				|account,
				 _is_new|
				 -> Result<(Self::NegativeImbalance, Self::Balance), DispatchError> {
					// Best value is the most amount we can slash following liveness rules.
					let best_value = match attempt {
						// First attempt we try to slash the full amount, and see if liveness issues
						// happen.
						0 => value,
						// If acting as a critical provider (i.e. first attempt failed), then slash
						// as much as possible while leaving at least at ED.
						_ => value.min(
							(account.free + account.reserved)
								.saturating_sub(T::ExistentialDeposit::get()),
						),
					};

					let free_slash = cmp::min(account.free, best_value);
					account.free -= free_slash; // Safe because of above check
					let remaining_slash = best_value - free_slash; // Safe because of above check

					if !remaining_slash.is_zero() {
						// If we have remaining slash, take it from reserved balance.
						let reserved_slash = cmp::min(account.reserved, remaining_slash);
						account.reserved -= reserved_slash; // Safe because of above check
						Ok((
							NegativeImbalance::new(free_slash + reserved_slash),
							value - free_slash - reserved_slash, /* Safe because value is gt or
							                                      * eq total slashed */
						))
					} else {
						// Else we are done!
						Ok((
							NegativeImbalance::new(free_slash),
							value - free_slash, // Safe because value is gt or eq to total slashed
						))
					}
				},
			) {
				Ok((imbalance, not_slashed)) => {
					Self::deposit_event(Event::Slashed {
						who: who.clone(),
						amount: value.saturating_sub(not_slashed),
					});
					return (imbalance, not_slashed)
				},
				Err(_) => (),
			}
		}

		// Should never get here. But we'll be defensive anyway.
		(Self::NegativeImbalance::zero(), value)
	}

	/// Deposit some `value` into the free balance of an existing target account `who`.
	///
	/// Is a no-op if the `value` to be deposited is zero.
	fn deposit_into_existing(
		who: &T::AccountId,
		value: Self::Balance,
	) -> Result<Self::PositiveImbalance, DispatchError> {
		if value.is_zero() {
			return Ok(PositiveImbalance::zero())
		}

		Self::try_mutate_account(
			who,
			|account, is_new| -> Result<Self::PositiveImbalance, DispatchError> {
				ensure!(!is_new, Error::<T, I>::DeadAccount);
				account.free = account.free.checked_add(&value).ok_or(ArithmeticError::Overflow)?;
				Self::deposit_event(Event::Deposit { who: who.clone(), amount: value });
				Ok(PositiveImbalance::new(value))
			},
		)
	}

	/// Deposit some `value` into the free balance of `who`, possibly creating a new account.
	///
	/// This function is a no-op if:
	/// - the `value` to be deposited is zero; or
	/// - the `value` to be deposited is less than the required ED and the account does not yet
	///   exist; or
	/// - the deposit would necessitate the account to exist and there are no provider references;
	///   or
	/// - `value` is so large it would cause the balance of `who` to overflow.
	fn deposit_creating(who: &T::AccountId, value: Self::Balance) -> Self::PositiveImbalance {
		if value.is_zero() {
			return Self::PositiveImbalance::zero()
		}

		Self::try_mutate_account(
			who,
			|account, is_new| -> Result<Self::PositiveImbalance, DispatchError> {
				let ed = T::ExistentialDeposit::get();
				ensure!(value >= ed || !is_new, Error::<T, I>::ExistentialDeposit);

				// defensive only: overflow should never happen, however in case it does, then this
				// operation is a no-op.
				account.free = match account.free.checked_add(&value) {
					Some(x) => x,
					None => return Ok(Self::PositiveImbalance::zero()),
				};

				Self::deposit_event(Event::Deposit { who: who.clone(), amount: value });
				Ok(PositiveImbalance::new(value))
			},
		)
		.unwrap_or_else(|_| Self::PositiveImbalance::zero())
	}

	/// Withdraw some free balance from an account, respecting existence requirements.
	///
	/// Is a no-op if value to be withdrawn is zero.
	fn withdraw(
		who: &T::AccountId,
		value: Self::Balance,
		reasons: WithdrawReasons,
		liveness: ExistenceRequirement,
	) -> result::Result<Self::NegativeImbalance, DispatchError> {
		if value.is_zero() {
			return Ok(NegativeImbalance::zero())
		}

		Self::try_mutate_account(
			who,
			|account, _| -> Result<Self::NegativeImbalance, DispatchError> {
				let new_free_account =
					account.free.checked_sub(&value).ok_or(Error::<T, I>::InsufficientBalance)?;

				// bail if we need to keep the account alive and this would kill it.
				let ed = T::ExistentialDeposit::get();
				let would_be_dead = new_free_account + account.reserved < ed;
				let would_kill = would_be_dead && account.free + account.reserved >= ed;
				ensure!(liveness == AllowDeath || !would_kill, Error::<T, I>::KeepAlive);

				Self::ensure_can_withdraw(who, value, reasons, new_free_account)?;

				account.free = new_free_account;

				Self::deposit_event(Event::Withdraw { who: who.clone(), amount: value });
				Ok(NegativeImbalance::new(value))
			},
		)
	}

	/// Force the new free balance of a target account `who` to some new value `balance`.
	fn make_free_balance_be(
		who: &T::AccountId,
		value: Self::Balance,
	) -> SignedImbalance<Self::Balance, Self::PositiveImbalance> {
		Self::try_mutate_account(
			who,
			|account,
			 is_new|
			 -> Result<SignedImbalance<Self::Balance, Self::PositiveImbalance>, DispatchError> {
				let ed = T::ExistentialDeposit::get();
				let total = value.saturating_add(account.reserved);
				// If we're attempting to set an existing account to less than ED, then
				// bypass the entire operation. It's a no-op if you follow it through, but
				// since this is an instance where we might account for a negative imbalance
				// (in the dust cleaner of set_account) before we account for its actual
				// equal and opposite cause (returned as an Imbalance), then in the
				// instance that there's no other accounts on the system at all, we might
				// underflow the issuance and our arithmetic will be off.
				ensure!(total >= ed || !is_new, Error::<T, I>::ExistentialDeposit);

				let imbalance = if account.free <= value {
					SignedImbalance::Positive(PositiveImbalance::new(value - account.free))
				} else {
					SignedImbalance::Negative(NegativeImbalance::new(account.free - value))
				};
				account.free = value;
				Self::deposit_event(Event::BalanceSet {
					who: who.clone(),
					free: account.free,
					reserved: account.reserved,
				});
				Ok(imbalance)
			},
		)
		.unwrap_or_else(|_| SignedImbalance::Positive(Self::PositiveImbalance::zero()))
	}
}

impl<T: Config<I>, I: 'static> ReservableCurrency<T::AccountId> for Pallet<T, I>
where
	T::Balance: MaybeSerializeDeserialize + Debug,
{
	/// Check if `who` can reserve `value` from their free balance.
	///
	/// Always `true` if value to be reserved is zero.
	fn can_reserve(who: &T::AccountId, value: Self::Balance) -> bool {
		if value.is_zero() {
			return true
		}
		Self::account(who).free.checked_sub(&value).map_or(false, |new_balance| {
			Self::ensure_can_withdraw(who, value, WithdrawReasons::RESERVE, new_balance).is_ok()
		})
	}

	fn reserved_balance(who: &T::AccountId) -> Self::Balance {
		Self::account(who).reserved
	}

	/// Move `value` from the free balance from `who` to their reserved balance.
	///
	/// Is a no-op if value to be reserved is zero.
	fn reserve(who: &T::AccountId, value: Self::Balance) -> DispatchResult {
		if value.is_zero() {
			return Ok(())
		}

		Self::try_mutate_account(who, |account, _| -> DispatchResult {
			account.free =
				account.free.checked_sub(&value).ok_or(Error::<T, I>::InsufficientBalance)?;
			account.reserved =
				account.reserved.checked_add(&value).ok_or(ArithmeticError::Overflow)?;
			Self::ensure_can_withdraw(&who, value, WithdrawReasons::RESERVE, account.free)
		})?;

		Self::deposit_event(Event::Reserved { who: who.clone(), amount: value });
		Ok(())
	}

	/// Unreserve some funds, returning any amount that was unable to be unreserved.
	///
	/// Is a no-op if the value to be unreserved is zero or the account does not exist.
	///
	/// NOTE: returns amount value which wasn't successfully unreserved.
	fn unreserve(who: &T::AccountId, value: Self::Balance) -> Self::Balance {
		if value.is_zero() {
			return Zero::zero()
		}
		if Self::total_balance(who).is_zero() {
			return value
		}

		let actual = match Self::mutate_account(who, |account| {
			let actual = cmp::min(account.reserved, value);
			account.reserved -= actual;
			// defensive only: this can never fail since total issuance which is at least
			// free+reserved fits into the same data type.
			account.free = account.free.defensive_saturating_add(actual);
			actual
		}) {
			Ok(x) => x,
			Err(_) => {
				// This should never happen since we don't alter the total amount in the account.
				// If it ever does, then we should fail gracefully though, indicating that nothing
				// could be done.
				return value
			},
		};

		Self::deposit_event(Event::Unreserved { who: who.clone(), amount: actual });
		value - actual
	}

	/// Slash from reserved balance, returning the negative imbalance created,
	/// and any amount that was unable to be slashed.
	///
	/// Is a no-op if the value to be slashed is zero or the account does not exist.
	fn slash_reserved(
		who: &T::AccountId,
		value: Self::Balance,
	) -> (Self::NegativeImbalance, Self::Balance) {
		if value.is_zero() {
			return (NegativeImbalance::zero(), Zero::zero())
		}
		if Self::total_balance(who).is_zero() {
			return (NegativeImbalance::zero(), value)
		}

		// NOTE: `mutate_account` may fail if it attempts to reduce the balance to the point that an
		//   account is attempted to be illegally destroyed.

		for attempt in 0..2 {
			match Self::mutate_account(who, |account| {
				let best_value = match attempt {
					0 => value,
					// If acting as a critical provider (i.e. first attempt failed), then ensure
					// slash leaves at least the ED.
					_ => value.min(
						(account.free + account.reserved)
							.saturating_sub(T::ExistentialDeposit::get()),
					),
				};

				let actual = cmp::min(account.reserved, best_value);
				account.reserved -= actual;

				// underflow should never happen, but it if does, there's nothing to be done here.
				(NegativeImbalance::new(actual), value - actual)
			}) {
				Ok((imbalance, not_slashed)) => {
					Self::deposit_event(Event::Slashed {
						who: who.clone(),
						amount: value.saturating_sub(not_slashed),
					});
					return (imbalance, not_slashed)
				},
				Err(_) => (),
			}
		}
		// Should never get here as we ensure that ED is left in the second attempt.
		// In case we do, though, then we fail gracefully.
		(Self::NegativeImbalance::zero(), value)
	}

	/// Move the reserved balance of one account into the balance of another, according to `status`.
	///
	/// Is a no-op if:
	/// - the value to be moved is zero; or
	/// - the `slashed` id equal to `beneficiary` and the `status` is `Reserved`.
	fn repatriate_reserved(
		slashed: &T::AccountId,
		beneficiary: &T::AccountId,
		value: Self::Balance,
		status: Status,
	) -> Result<Self::Balance, DispatchError> {
		let actual = Self::do_transfer_reserved(slashed, beneficiary, value, true, status)?;
		Ok(value.saturating_sub(actual))
	}
}

impl<T: Config<I>, I: 'static> NamedReservableCurrency<T::AccountId> for Pallet<T, I>
where
	T::Balance: MaybeSerializeDeserialize + Debug,
{
	type ReserveIdentifier = T::ReserveIdentifier;

	fn reserved_balance_named(id: &Self::ReserveIdentifier, who: &T::AccountId) -> Self::Balance {
		let reserves = Self::reserves(who);
		reserves
			.binary_search_by_key(id, |data| data.id)
			.map(|index| reserves[index].amount)
			.unwrap_or_default()
	}

	/// Move `value` from the free balance from `who` to a named reserve balance.
	///
	/// Is a no-op if value to be reserved is zero.
	fn reserve_named(
		id: &Self::ReserveIdentifier,
		who: &T::AccountId,
		value: Self::Balance,
	) -> DispatchResult {
		if value.is_zero() {
			return Ok(())
		}

		Reserves::<T, I>::try_mutate(who, |reserves| -> DispatchResult {
			match reserves.binary_search_by_key(id, |data| data.id) {
				Ok(index) => {
					// this add can't overflow but just to be defensive.
					reserves[index].amount = reserves[index].amount.defensive_saturating_add(value);
				},
				Err(index) => {
					reserves
						.try_insert(index, ReserveData { id: *id, amount: value })
						.map_err(|_| Error::<T, I>::TooManyReserves)?;
				},
			};
			<Self as ReservableCurrency<_>>::reserve(who, value)?;
			Ok(())
		})
	}

	/// Unreserve some funds, returning any amount that was unable to be unreserved.
	///
	/// Is a no-op if the value to be unreserved is zero.
	fn unreserve_named(
		id: &Self::ReserveIdentifier,
		who: &T::AccountId,
		value: Self::Balance,
	) -> Self::Balance {
		if value.is_zero() {
			return Zero::zero()
		}

		Reserves::<T, I>::mutate_exists(who, |maybe_reserves| -> Self::Balance {
			if let Some(reserves) = maybe_reserves.as_mut() {
				match reserves.binary_search_by_key(id, |data| data.id) {
					Ok(index) => {
						let to_change = cmp::min(reserves[index].amount, value);

						let remain = <Self as ReservableCurrency<_>>::unreserve(who, to_change);

						// remain should always be zero but just to be defensive here.
						let actual = to_change.defensive_saturating_sub(remain);

						// `actual <= to_change` and `to_change <= amount`; qed;
						reserves[index].amount -= actual;

						if reserves[index].amount.is_zero() {
							if reserves.len() == 1 {
								// no more named reserves
								*maybe_reserves = None;
							} else {
								// remove this named reserve
								reserves.remove(index);
							}
						}

						value - actual
					},
					Err(_) => value,
				}
			} else {
				value
			}
		})
	}