lib.rs

		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	// Is a no-op if amount to be withdrawn is zero.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(Error::<T, I>::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>::is_provider_required(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(transactor.clone(), dest.clone(), 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), StoredMapError> {
					// 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(r) => return r,
				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(Error::<T, I>::Overflow)?;
			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() }

		let r = 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()),
			};

			Ok(PositiveImbalance::new(value))
		}).unwrap_or_else(|_| Self::PositiveImbalance::zero());

		r
	}

	/// 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;

			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;
			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(Error::<T, I>::Overflow)?;
			Self::ensure_can_withdraw(&who, value.clone(), WithdrawReasons::RESERVE, account.free)
		})?;

		Self::deposit_event(Event::Reserved(who.clone(), 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.
	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.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.clone(), actual.clone()));
		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(r) => return r,
				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> {
		if value.is_zero() { return Ok(Zero::zero()) }

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

		Self::deposit_event(Event::ReserveRepatriated(slashed.clone(), beneficiary.clone(), actual, status));
		Ok(value - actual)
	}
}

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

	type MaxLocks = T::MaxLocks;

	// Set a lock on the balance of `who`.
	// Is a no-op if lock amount is zero or `reasons` `is_none()`.
	fn set_lock(
		id: LockIdentifier,
		who: &T::AccountId,
		amount: T::Balance,
		reasons: WithdrawReasons,
	) {
		if amount.is_zero() || reasons.is_empty() { return }
		let mut new_lock = Some(BalanceLock { id, amount, reasons: reasons.into() });
		let mut locks = Self::locks(who).into_iter()
			.filter_map(|l| if l.id == id { new_lock.take() } else { Some(l) })
			.collect::<Vec<_>>();
		if let Some(lock) = new_lock {
			locks.push(lock)
		}
		Self::update_locks(who, &locks[..]);
	}

	// Extend a lock on the balance of `who`.
	// Is a no-op if lock amount is zero or `reasons` `is_none()`.
	fn extend_lock(
		id: LockIdentifier,
		who: &T::AccountId,
		amount: T::Balance,
		reasons: WithdrawReasons,
	) {
		if amount.is_zero() || reasons.is_empty() { return }
		let mut new_lock = Some(BalanceLock { id, amount, reasons: reasons.into() });
		let mut locks = Self::locks(who).into_iter().filter_map(|l|
			if l.id == id {
				new_lock.take().map(|nl| {
					BalanceLock {
						id: l.id,
						amount: l.amount.max(nl.amount),
						reasons: l.reasons | nl.reasons,
					}
				})
			} else {
				Some(l)
			}).collect::<Vec<_>>();
		if let Some(lock) = new_lock {
			locks.push(lock)
		}
		Self::update_locks(who, &locks[..]);
	}

	fn remove_lock(
		id: LockIdentifier,
		who: &T::AccountId,
	) {
		let mut locks = Self::locks(who);
		locks.retain(|l| l.id != id);
		Self::update_locks(who, &locks[..]);
	}
}