1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
// This file is part of Substrate.

// Copyright (C) Parity Technologies (UK) Ltd.
// SPDX-License-Identifier: Apache-2.0

// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// 	http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

//! `Inspect` and `Mutate` traits for working with regular balances.

use sp_std::marker::PhantomData;

use crate::{
	ensure,
	traits::{
		tokens::{
			misc::{
				Balance, DepositConsequence,
				Fortitude::{self, Force, Polite},
				Precision::{self, BestEffort, Exact},
				Preservation::{self, Expendable},
				Provenance::{self, Extant},
				WithdrawConsequence,
			},
			AssetId,
		},
		SameOrOther, TryDrop,
	},
};
use sp_arithmetic::traits::{CheckedAdd, CheckedSub, One};
use sp_runtime::{traits::Saturating, ArithmeticError, DispatchError, TokenError};

use super::{Credit, Debt, HandleImbalanceDrop, Imbalance};

/// Trait for providing balance-inspection access to a set of named fungible assets.
pub trait Inspect<AccountId>: Sized {
	/// Means of identifying one asset class from another.
	type AssetId: AssetId;

	/// Scalar type for representing balance of an account.
	type Balance: Balance;

	/// The total amount of issuance in the system.
	fn total_issuance(asset: Self::AssetId) -> Self::Balance;

	/// The total amount of issuance in the system excluding those which are controlled by the
	/// system.
	fn active_issuance(asset: Self::AssetId) -> Self::Balance {
		Self::total_issuance(asset)
	}

	/// The minimum balance any single account may have.
	fn minimum_balance(asset: Self::AssetId) -> Self::Balance;

	/// Get the total amount of funds whose ultimate beneficial ownership can be determined as
	/// `who`.
	///
	/// This may include funds which are wholly inaccessible to `who`, either temporarily or even
	/// indefinitely.
	///
	/// For the amount of the balance which is currently free to be removed from the account without
	/// error, use `reducible_balance`.
	///
	/// For the amount of the balance which may eventually be free to be removed from the account,
	/// use `balance()`.
	fn total_balance(asset: Self::AssetId, who: &AccountId) -> Self::Balance;

	/// Get the balance of `who` which does not include funds which are exclusively allocated to
	/// subsystems of the chain ("on hold" or "reserved").
	///
	/// In general this isn't especially useful outside of tests, and for practical purposes, you'll
	/// want to use `reducible_balance()`.
	fn balance(asset: Self::AssetId, who: &AccountId) -> Self::Balance;

	/// Get the maximum amount that `who` can withdraw/transfer successfully based on whether the
	/// account should be kept alive (`preservation`) or whether we are willing to force the
	/// transfer and potentially go below user-level restrictions on the minimum amount of the
	/// account.
	///
	/// Always less than `free_balance()`.
	fn reducible_balance(
		asset: Self::AssetId,
		who: &AccountId,
		preservation: Preservation,
		force: Fortitude,
	) -> Self::Balance;

	/// Returns `true` if the `asset` balance of `who` may be increased by `amount`.
	///
	/// - `asset`: The asset that should be deposited.
	/// - `who`: The account of which the balance should be increased by `amount`.
	/// - `amount`: How much should the balance be increased?
	/// - `mint`: Will `amount` be minted to deposit it into `account`?
	fn can_deposit(
		asset: Self::AssetId,
		who: &AccountId,
		amount: Self::Balance,
		provenance: Provenance,
	) -> DepositConsequence;

	/// Returns `Failed` if the `asset` balance of `who` may not be decreased by `amount`, otherwise
	/// the consequence.
	fn can_withdraw(
		asset: Self::AssetId,
		who: &AccountId,
		amount: Self::Balance,
	) -> WithdrawConsequence<Self::Balance>;

	/// Returns `true` if an `asset` exists.
	fn asset_exists(asset: Self::AssetId) -> bool;
}

/// Special dust type which can be type-safely converted into a `Credit`.
#[must_use]
pub struct Dust<A, T: Unbalanced<A>>(pub T::AssetId, pub T::Balance);

impl<A, T: Balanced<A>> Dust<A, T> {
	/// Convert `Dust` into an instance of `Credit`.
	pub fn into_credit(self) -> Credit<A, T> {
		Credit::<A, T>::new(self.0, self.1)
	}
}

/// A fungible token class where the balance can be set arbitrarily.
///
/// **WARNING**
/// Do not use this directly unless you want trouble, since it allows you to alter account balances
/// without keeping the issuance up to date. It has no safeguards against accidentally creating
/// token imbalances in your system leading to accidental inflation or deflation. It's really just
/// for the underlying datatype to implement so the user gets the much safer `Balanced` trait to
/// use.
pub trait Unbalanced<AccountId>: Inspect<AccountId> {
	/// Create some dust and handle it with `Self::handle_dust`. This is an unbalanced operation
	/// and it must only be used when an account is modified in a raw fashion, outside of the entire
	/// fungibles API. The `amount` is capped at `Self::minimum_balance() - 1`.
	///
	/// This should not be reimplemented.
	fn handle_raw_dust(asset: Self::AssetId, amount: Self::Balance) {
		Self::handle_dust(Dust(
			asset.clone(),
			amount.min(Self::minimum_balance(asset).saturating_sub(One::one())),
		))
	}

	/// Do something with the dust which has been destroyed from the system. `Dust` can be converted
	/// into a `Credit` with the `Balanced` trait impl.
	fn handle_dust(dust: Dust<AccountId, Self>);

	/// Forcefully set the balance of `who` to `amount`.
	///
	/// If this call executes successfully, you can `assert_eq!(Self::balance(), amount);`.
	///
	/// For implementations which include one or more balances on hold, then these are *not*
	/// included in the `amount`.
	///
	/// This function does its best to force the balance change through, but will not break system
	/// invariants such as any Existential Deposits needed or overflows/underflows.
	/// If this cannot be done for some reason (e.g. because the account cannot be created, deleted
	/// or would overflow) then an `Err` is returned.
	fn write_balance(
		asset: Self::AssetId,
		who: &AccountId,
		amount: Self::Balance,
	) -> Result<Option<Self::Balance>, DispatchError>;

	/// Set the total issuance to `amount`.
	fn set_total_issuance(asset: Self::AssetId, amount: Self::Balance);

	/// Reduce the balance of `who` by `amount`.
	///
	/// If `precision` is `Exact` and it cannot be reduced by that amount for
	/// some reason, return `Err` and don't reduce it at all. If `precision` is `BestEffort`, then
	/// reduce the balance of `who` by the most that is possible, up to `amount`.
	///
	/// In either case, if `Ok` is returned then the inner is the amount by which is was reduced.
	/// Minimum balance will be respected and thus the returned amount may be up to
	/// `Self::minimum_balance() - 1` greater than `amount` in the case that the reduction caused
	/// the account to be deleted.
	fn decrease_balance(
		asset: Self::AssetId,
		who: &AccountId,
		mut amount: Self::Balance,
		precision: Precision,
		preservation: Preservation,
		force: Fortitude,
	) -> Result<Self::Balance, DispatchError> {
		let old_balance = Self::balance(asset.clone(), who);
		let reducible = Self::reducible_balance(asset.clone(), who, preservation, force);
		match precision {
			BestEffort => amount = amount.min(reducible),
			Exact => ensure!(reducible >= amount, TokenError::FundsUnavailable),
		}
		let new_balance = old_balance.checked_sub(&amount).ok_or(TokenError::FundsUnavailable)?;
		if let Some(dust) = Self::write_balance(asset.clone(), who, new_balance)? {
			Self::handle_dust(Dust(asset, dust));
		}
		Ok(old_balance.saturating_sub(new_balance))
	}

	/// Increase the balance of `who` by `amount`.
	///
	/// If it cannot be increased by that amount for some reason, return `Err` and don't increase
	/// it at all. If Ok, return the imbalance.
	/// Minimum balance will be respected and an error will be returned if
	/// `amount < Self::minimum_balance()` when the account of `who` is zero.
	fn increase_balance(
		asset: Self::AssetId,
		who: &AccountId,
		amount: Self::Balance,
		precision: Precision,
	) -> Result<Self::Balance, DispatchError> {
		let old_balance = Self::balance(asset.clone(), who);
		let new_balance = if let BestEffort = precision {
			old_balance.saturating_add(amount)
		} else {
			old_balance.checked_add(&amount).ok_or(ArithmeticError::Overflow)?
		};
		if new_balance < Self::minimum_balance(asset.clone()) {
			// Attempt to increase from 0 to below minimum -> stays at zero.
			if let BestEffort = precision {
				Ok(Self::Balance::default())
			} else {
				Err(TokenError::BelowMinimum.into())
			}
		} else {
			if new_balance == old_balance {
				Ok(Self::Balance::default())
			} else {
				if let Some(dust) = Self::write_balance(asset.clone(), who, new_balance)? {
					Self::handle_dust(Dust(asset, dust));
				}
				Ok(new_balance.saturating_sub(old_balance))
			}
		}
	}

	/// Reduce the active issuance by some amount.
	fn deactivate(_asset: Self::AssetId, _: Self::Balance) {}

	/// Increase the active issuance by some amount, up to the outstanding amount reduced.
	fn reactivate(_asset: Self::AssetId, _: Self::Balance) {}
}

/// Trait for providing a basic fungible asset.
pub trait Mutate<AccountId>: Inspect<AccountId> + Unbalanced<AccountId>
where
	AccountId: Eq,
{
	/// Increase the balance of `who` by exactly `amount`, minting new tokens. If that isn't
	/// possible then an `Err` is returned and nothing is changed.
	fn mint_into(
		asset: Self::AssetId,
		who: &AccountId,
		amount: Self::Balance,
	) -> Result<Self::Balance, DispatchError> {
		Self::total_issuance(asset.clone())
			.checked_add(&amount)
			.ok_or(ArithmeticError::Overflow)?;
		let actual = Self::increase_balance(asset.clone(), who, amount, Exact)?;
		Self::set_total_issuance(
			asset.clone(),
			Self::total_issuance(asset.clone()).saturating_add(actual),
		);
		Self::done_mint_into(asset, who, amount);
		Ok(actual)
	}

	/// Decrease the balance of `who` by at least `amount`, possibly slightly more in the case of
	/// minimum-balance requirements, burning the tokens. If that isn't possible then an `Err` is
	/// returned and nothing is changed. If successful, the amount of tokens reduced is returned.
	fn burn_from(
		asset: Self::AssetId,
		who: &AccountId,
		amount: Self::Balance,
		precision: Precision,
		force: Fortitude,
	) -> Result<Self::Balance, DispatchError> {
		let actual = Self::reducible_balance(asset.clone(), who, Expendable, force).min(amount);
		ensure!(actual == amount || precision == BestEffort, TokenError::FundsUnavailable);
		Self::total_issuance(asset.clone())
			.checked_sub(&actual)
			.ok_or(ArithmeticError::Overflow)?;
		let actual =
			Self::decrease_balance(asset.clone(), who, actual, BestEffort, Expendable, force)?;
		Self::set_total_issuance(
			asset.clone(),
			Self::total_issuance(asset.clone()).saturating_sub(actual),
		);
		Self::done_burn_from(asset, who, actual);
		Ok(actual)
	}

	/// Attempt to decrease the `asset` balance of `who` by `amount`.
	///
	/// Equivalent to `burn_from`, except with an expectation that within the bounds of some
	/// universal issuance, the total assets `suspend`ed and `resume`d will be equivalent. The
	/// implementation may be configured such that the total assets suspended may never be less than
	/// the total assets resumed (which is the invariant for an issuing system), or the reverse
	/// (which the invariant in a non-issuing system).
	///
	/// Because of this expectation, any metadata associated with the asset is expected to survive
	/// the suspect-resume cycle.
	fn shelve(
		asset: Self::AssetId,
		who: &AccountId,
		amount: Self::Balance,
	) -> Result<Self::Balance, DispatchError> {
		let actual = Self::reducible_balance(asset.clone(), who, Expendable, Polite).min(amount);
		ensure!(actual == amount, TokenError::FundsUnavailable);
		Self::total_issuance(asset.clone())
			.checked_sub(&actual)
			.ok_or(ArithmeticError::Overflow)?;
		let actual =
			Self::decrease_balance(asset.clone(), who, actual, BestEffort, Expendable, Polite)?;
		Self::set_total_issuance(
			asset.clone(),
			Self::total_issuance(asset.clone()).saturating_sub(actual),
		);
		Self::done_shelve(asset, who, actual);
		Ok(actual)
	}

	/// Attempt to increase the `asset` balance of `who` by `amount`.
	///
	/// Equivalent to `mint_into`, except with an expectation that within the bounds of some
	/// universal issuance, the total assets `suspend`ed and `resume`d will be equivalent. The
	/// implementation may be configured such that the total assets suspended may never be less than
	/// the total assets resumed (which is the invariant for an issuing system), or the reverse
	/// (which the invariant in a non-issuing system).
	///
	/// Because of this expectation, any metadata associated with the asset is expected to survive
	/// the suspect-resume cycle.
	fn restore(
		asset: Self::AssetId,
		who: &AccountId,
		amount: Self::Balance,
	) -> Result<Self::Balance, DispatchError> {
		Self::total_issuance(asset.clone())
			.checked_add(&amount)
			.ok_or(ArithmeticError::Overflow)?;
		let actual = Self::increase_balance(asset.clone(), who, amount, Exact)?;
		Self::set_total_issuance(
			asset.clone(),
			Self::total_issuance(asset.clone()).saturating_add(actual),
		);
		Self::done_restore(asset, who, amount);
		Ok(actual)
	}

	/// Transfer funds from one account into another.
	///
	/// A transfer where the source and destination account are identical is treated as No-OP after
	/// checking the preconditions.
	fn transfer(
		asset: Self::AssetId,
		source: &AccountId,
		dest: &AccountId,
		amount: Self::Balance,
		preservation: Preservation,
	) -> Result<Self::Balance, DispatchError> {
		let _extra = Self::can_withdraw(asset.clone(), source, amount)
			.into_result(preservation != Expendable)?;
		Self::can_deposit(asset.clone(), dest, amount, Extant).into_result()?;
		if source == dest {
			return Ok(amount)
		}

		Self::decrease_balance(asset.clone(), source, amount, BestEffort, preservation, Polite)?;
		// This should never fail as we checked `can_deposit` earlier. But we do a best-effort
		// anyway.
		let _ = Self::increase_balance(asset.clone(), dest, amount, BestEffort);
		Self::done_transfer(asset, source, dest, amount);
		Ok(amount)
	}

	/// Simple infallible function to force an account to have a particular balance, good for use
	/// in tests and benchmarks but not recommended for production code owing to the lack of
	/// error reporting.
	///
	/// Returns the new balance.
	fn set_balance(asset: Self::AssetId, who: &AccountId, amount: Self::Balance) -> Self::Balance {
		let b = Self::balance(asset.clone(), who);
		if b > amount {
			Self::burn_from(asset, who, b - amount, BestEffort, Force).map(|d| b.saturating_sub(d))
		} else {
			Self::mint_into(asset, who, amount - b).map(|d| b.saturating_add(d))
		}
		.unwrap_or(b)
	}
	fn done_mint_into(_asset: Self::AssetId, _who: &AccountId, _amount: Self::Balance) {}
	fn done_burn_from(_asset: Self::AssetId, _who: &AccountId, _amount: Self::Balance) {}
	fn done_shelve(_asset: Self::AssetId, _who: &AccountId, _amount: Self::Balance) {}
	fn done_restore(_asset: Self::AssetId, _who: &AccountId, _amount: Self::Balance) {}
	fn done_transfer(
		_asset: Self::AssetId,
		_source: &AccountId,
		_dest: &AccountId,
		_amount: Self::Balance,
	) {
	}
}

/// Simple handler for an imbalance drop which increases the total issuance of the system by the
/// imbalance amount. Used for leftover debt.
pub struct IncreaseIssuance<AccountId, U>(PhantomData<(AccountId, U)>);
impl<AccountId, U: Unbalanced<AccountId>> HandleImbalanceDrop<U::AssetId, U::Balance>
	for IncreaseIssuance<AccountId, U>
{
	fn handle(asset: U::AssetId, amount: U::Balance) {
		U::set_total_issuance(asset.clone(), U::total_issuance(asset).saturating_add(amount))
	}
}

/// Simple handler for an imbalance drop which decreases the total issuance of the system by the
/// imbalance amount. Used for leftover credit.
pub struct DecreaseIssuance<AccountId, U>(PhantomData<(AccountId, U)>);
impl<AccountId, U: Unbalanced<AccountId>> HandleImbalanceDrop<U::AssetId, U::Balance>
	for DecreaseIssuance<AccountId, U>
{
	fn handle(asset: U::AssetId, amount: U::Balance) {
		U::set_total_issuance(asset.clone(), U::total_issuance(asset).saturating_sub(amount))
	}
}

/// A fungible token class where any creation and deletion of tokens is semi-explicit and where the
/// total supply is maintained automatically.
///
/// This is auto-implemented when a token class has `Unbalanced` implemented.
pub trait Balanced<AccountId>: Inspect<AccountId> + Unbalanced<AccountId> {
	/// The type for managing what happens when an instance of `Debt` is dropped without being used.
	type OnDropDebt: HandleImbalanceDrop<Self::AssetId, Self::Balance>;
	/// The type for managing what happens when an instance of `Credit` is dropped without being
	/// used.
	type OnDropCredit: HandleImbalanceDrop<Self::AssetId, Self::Balance>;

	/// Reduce the total issuance by `amount` and return the according imbalance. The imbalance will
	/// typically be used to reduce an account by the same amount with e.g. `settle`.
	///
	/// This is infallible, but doesn't guarantee that the entire `amount` is burnt, for example
	/// in the case of underflow.
	fn rescind(asset: Self::AssetId, amount: Self::Balance) -> Debt<AccountId, Self> {
		let old = Self::total_issuance(asset.clone());
		let new = old.saturating_sub(amount);
		Self::set_total_issuance(asset.clone(), new);
		let delta = old - new;
		Self::done_rescind(asset.clone(), delta);
		Imbalance::<Self::AssetId, Self::Balance, Self::OnDropDebt, Self::OnDropCredit>::new(
			asset, delta,
		)
	}

	/// Increase the total issuance by `amount` and return the according imbalance. The imbalance
	/// will typically be used to increase an account by the same amount with e.g.
	/// `resolve_into_existing` or `resolve_creating`.
	///
	/// This is infallible, but doesn't guarantee that the entire `amount` is issued, for example
	/// in the case of overflow.
	fn issue(asset: Self::AssetId, amount: Self::Balance) -> Credit<AccountId, Self> {
		let old = Self::total_issuance(asset.clone());
		let new = old.saturating_add(amount);
		Self::set_total_issuance(asset.clone(), new);
		let delta = new - old;
		Self::done_issue(asset.clone(), delta);
		Imbalance::<Self::AssetId, Self::Balance, Self::OnDropCredit, Self::OnDropDebt>::new(
			asset, delta,
		)
	}

	/// Produce a pair of imbalances that cancel each other out exactly.
	///
	/// This is just the same as burning and issuing the same amount and has no effect on the
	/// total issuance.
	///
	/// This is infallible, but doesn't guarantee that the entire `amount` is used to create the
	/// pair, for example in the case where the amounts would cause overflow or underflow in
	/// [`Balanced::issue`] or [`Balanced::rescind`].
	fn pair(
		asset: Self::AssetId,
		amount: Self::Balance,
	) -> Result<(Debt<AccountId, Self>, Credit<AccountId, Self>), DispatchError> {
		let issued = Self::issue(asset.clone(), amount);
		let rescinded = Self::rescind(asset, amount);
		// Need to check amount in case by some edge case both issued and rescinded are below
		// `amount` by the exact same value
		if issued.peek() != rescinded.peek() || issued.peek() != amount {
			// Issued and rescinded will be dropped automatically
			Err("Failed to issue and rescind equal amounts".into())
		} else {
			Ok((rescinded, issued))
		}
	}

	/// Mints `value` into the account of `who`, creating it as needed.
	///
	/// If `precision` is `BestEffort` and `value` in full could not be minted (e.g. due to
	/// overflow), then the maximum is minted, up to `value`. If `precision` is `Exact`, then
	/// exactly `value` must be minted into the account of `who` or the operation will fail with an
	/// `Err` and nothing will change.
	///
	/// If the operation is successful, this will return `Ok` with a `Debt` of the total value
	/// added to the account.
	fn deposit(
		asset: Self::AssetId,
		who: &AccountId,
		value: Self::Balance,
		precision: Precision,
	) -> Result<Debt<AccountId, Self>, DispatchError> {
		let increase = Self::increase_balance(asset.clone(), who, value, precision)?;
		Self::done_deposit(asset.clone(), who, increase);
		Ok(Imbalance::<Self::AssetId, Self::Balance, Self::OnDropDebt, Self::OnDropCredit>::new(
			asset, increase,
		))
	}

	/// Removes `value` balance from `who` account if possible.
	///
	/// If `precision` is `BestEffort` and `value` in full could not be removed (e.g. due to
	/// underflow), then the maximum is removed, up to `value`. If `precision` is `Exact`, then
	/// exactly `value` must be removed from the account of `who` or the operation will fail with an
	/// `Err` and nothing will change.
	///
	/// If the removal is needed but not possible, then it returns `Err` and nothing is changed.
	/// If the account needed to be deleted, then slightly more than `value` may be removed from the
	/// account owning since up to (but not including) minimum balance may also need to be removed.
	///
	/// If the operation is successful, this will return `Ok` with a `Credit` of the total value
	/// removed from the account.
	fn withdraw(
		asset: Self::AssetId,
		who: &AccountId,
		value: Self::Balance,
		precision: Precision,
		preservation: Preservation,
		force: Fortitude,
	) -> Result<Credit<AccountId, Self>, DispatchError> {
		let decrease =
			Self::decrease_balance(asset.clone(), who, value, precision, preservation, force)?;
		Self::done_withdraw(asset.clone(), who, decrease);
		Ok(Imbalance::<Self::AssetId, Self::Balance, Self::OnDropCredit, Self::OnDropDebt>::new(
			asset, decrease,
		))
	}

	/// The balance of `who` is increased in order to counter `credit`. If the whole of `credit`
	/// cannot be countered, then nothing is changed and the original `credit` is returned in an
	/// `Err`.
	///
	/// Please note: If `credit.peek()` is less than `Self::minimum_balance()`, then `who` must
	/// already exist for this to succeed.
	fn resolve(
		who: &AccountId,
		credit: Credit<AccountId, Self>,
	) -> Result<(), Credit<AccountId, Self>> {
		let v = credit.peek();
		let debt = match Self::deposit(credit.asset(), who, v, Exact) {
			Err(_) => return Err(credit),
			Ok(d) => d,
		};
		if let Ok(result) = credit.offset(debt) {
			let result = result.try_drop();
			debug_assert!(result.is_ok(), "ok deposit return must be equal to credit value; qed");
		} else {
			debug_assert!(false, "debt.asset is credit.asset; qed");
		}
		Ok(())
	}

	/// The balance of `who` is decreased in order to counter `debt`. If the whole of `debt`
	/// cannot be countered, then nothing is changed and the original `debt` is returned in an
	/// `Err`.
	fn settle(
		who: &AccountId,
		debt: Debt<AccountId, Self>,
		preservation: Preservation,
	) -> Result<Credit<AccountId, Self>, Debt<AccountId, Self>> {
		let amount = debt.peek();
		let asset = debt.asset();
		let credit = match Self::withdraw(asset.clone(), who, amount, Exact, preservation, Polite) {
			Err(_) => return Err(debt),
			Ok(d) => d,
		};
		match credit.offset(debt) {
			Ok(SameOrOther::None) => Ok(Credit::<AccountId, Self>::zero(asset)),
			Ok(SameOrOther::Same(dust)) => Ok(dust),
			Ok(SameOrOther::Other(rest)) => {
				debug_assert!(false, "ok withdraw return must be at least debt value; qed");
				Err(rest)
			},
			Err(_) => {
				debug_assert!(false, "debt.asset is credit.asset; qed");
				Ok(Credit::<AccountId, Self>::zero(asset))
			},
		}
	}

	fn done_rescind(_asset: Self::AssetId, _amount: Self::Balance) {}
	fn done_issue(_asset: Self::AssetId, _amount: Self::Balance) {}
	fn done_deposit(_asset: Self::AssetId, _who: &AccountId, _amount: Self::Balance) {}
	fn done_withdraw(_asset: Self::AssetId, _who: &AccountId, _amount: Self::Balance) {}
}

/// Dummy implementation of [`Inspect`]
#[cfg(feature = "std")]
impl<AccountId> Inspect<AccountId> for () {
	type AssetId = u32;
	type Balance = u32;
	fn total_issuance(_: Self::AssetId) -> Self::Balance {
		0
	}
	fn minimum_balance(_: Self::AssetId) -> Self::Balance {
		0
	}
	fn total_balance(_: Self::AssetId, _: &AccountId) -> Self::Balance {
		0
	}
	fn balance(_: Self::AssetId, _: &AccountId) -> Self::Balance {
		0
	}
	fn reducible_balance(
		_: Self::AssetId,
		_: &AccountId,
		_: Preservation,
		_: Fortitude,
	) -> Self::Balance {
		0
	}
	fn can_deposit(
		_: Self::AssetId,
		_: &AccountId,
		_: Self::Balance,
		_: Provenance,
	) -> DepositConsequence {
		DepositConsequence::Success
	}
	fn can_withdraw(
		_: Self::AssetId,
		_: &AccountId,
		_: Self::Balance,
	) -> WithdrawConsequence<Self::Balance> {
		WithdrawConsequence::Success
	}
	fn asset_exists(_: Self::AssetId) -> bool {
		false
	}
}

/// Dummy implementation of [`Unbalanced`]
#[cfg(feature = "std")]
impl<AccountId> Unbalanced<AccountId> for () {
	fn handle_dust(_: Dust<AccountId, Self>) {}
	fn write_balance(
		_: Self::AssetId,
		_: &AccountId,
		_: Self::Balance,
	) -> Result<Option<Self::Balance>, DispatchError> {
		Ok(None)
	}
	fn set_total_issuance(_: Self::AssetId, _: Self::Balance) {}
}

/// Dummy implementation of [`Mutate`]
#[cfg(feature = "std")]
impl<AccountId: Eq> Mutate<AccountId> for () {}