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
// 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.

//! > Made with *Substrate*, for *Polkadot*.
//!
//! [![github]](https://github.com/paritytech/substrate/frame/fast-unstake) -
//! [![polkadot]](https://polkadot.network)
//!
//! [polkadot]:
//!     https://img.shields.io/badge/polkadot-E6007A?style=for-the-badge&logo=polkadot&logoColor=white
//! [github]:
//!     https://img.shields.io/badge/github-8da0cb?style=for-the-badge&labelColor=555555&logo=github
//!
//!  # Bags-List Pallet
//!
//! An onchain implementation of a semi-sorted linked list, with permissionless sorting and update
//! operations.
//!
//! ## Pallet API
//!
//! See the [`pallet`] module for more information about the interfaces this pallet exposes,
//! including its configuration trait, dispatchables, storage items, events and errors.
//!
//! This pallet provides an implementation of
//! [`frame_election_provider_support::SortedListProvider`] and it can typically be used by another
//! pallet via this API.
//!
//! ## Overview
//!
//! This pallet splits `AccountId`s into different bags. Within a bag, these `AccountId`s are stored
//! as nodes in a linked-list manner. This pallet then provides iteration over all bags, which
//! basically allows an infinitely large list of items to be kept in a sorted manner.
//!
//! Each bags has a upper and lower range of scores, denoted by [`Config::BagThresholds`]. All nodes
//! within a bag must be within the range of the bag. If not, the permissionless [`Pallet::rebag`]
//! can be used to move any node to the right bag.
//!
//! Once a `rebag` happens, the order within a node is still not enforced. To move a node to the
//! optimal position in a bag, the [`Pallet::put_in_front_of`] or [`Pallet::put_in_front_of_other`]
//! can be used.
//!
//! Additional reading, about how this pallet is used in the context of Polkadot's staking system:
//! <https://polkadot.network/blog/staking-update-september-2021/#bags-list-in-depth>
//!
//! ## Examples
//!
//! See [`example`] for a diagram of `rebag` and `put_in_front_of` operations.
//!
//! ## Low Level / Implementation Details
//!
//! The data structure exposed by this pallet aims to be optimized for:
//!
//! - insertions and removals.
//! - iteration over the top* N items by score, where the precise ordering of items doesn't
//!   particularly matter.
//!
//! ### Further Details
//!
//! - items are kept in bags, which are delineated by their range of score (See
//!   [`Config::BagThresholds`]).
//! - for iteration, bags are chained together from highest to lowest and elements within the bag
//!   are iterated from head to tail.
//! - items within a bag are iterated in order of insertion. Thus removing an item and re-inserting
//!   it will worsen its position in list iteration; this reduces incentives for some types of spam
//!   that involve consistently removing and inserting for better position. Further, ordering
//!   granularity is thus dictated by range between each bag threshold.
//! - if an item's score changes to a value no longer within the range of its current bag the item's
//!   position will need to be updated by an external actor with rebag (update), or removal and
//!   insertion.

#![cfg_attr(not(feature = "std"), no_std)]

#[cfg(doc)]
#[cfg_attr(doc, aquamarine::aquamarine)]
///
/// In this example, assuming each node has an equal id and score (eg. node 21 has a score of 21),
/// the node 22 can be moved from bag 1 to bag 0 with the `rebag` operation.
///
/// Once the whole list is iterated, assuming the above above rebag happens, the order of iteration
/// would be: `25, 21, 22, 12, 22, 5, 7, 3`.
///
/// Moreover, in bag2, node 7 can be moved to the front of node 5 with the `put_in_front_of`, as it
/// has a higher score.
///
/// ```mermaid
/// graph LR
/// 	Bag0 --> Bag1 --> Bag2
///
/// 	subgraph Bag0[Bag 0: 21-30 DOT]
/// 		direction LR
/// 		25 --> 21 --> 22X[22]
/// 	end
///
/// 	subgraph Bag1[Bag 1: 11-20 DOT]
/// 		direction LR
/// 		12 --> 22
/// 	end
///
/// 	subgraph Bag2[Bag 2: 0-10 DOT]
/// 		direction LR
/// 		5 --> 7 --> 3
/// 	end
///
/// 	style 22X stroke-dasharray: 5 5,opacity:50%
/// ```
///
/// The equivalent of this in code would be:
#[doc = docify::embed!("src/tests.rs", examples_work)]
pub mod example {}

use codec::FullCodec;
use frame_election_provider_support::{ScoreProvider, SortedListProvider};
use frame_system::ensure_signed;
use sp_runtime::traits::{AtLeast32BitUnsigned, Bounded, StaticLookup};
use sp_std::prelude::*;

#[cfg(any(test, feature = "try-runtime", feature = "fuzz"))]
use sp_runtime::TryRuntimeError;

#[cfg(any(feature = "runtime-benchmarks", test))]
mod benchmarks;

mod list;
pub mod migrations;
#[cfg(any(test, feature = "fuzz"))]
pub mod mock;
#[cfg(test)]
mod tests;
pub mod weights;

pub use list::{notional_bag_for, Bag, List, ListError, Node};
pub use pallet::*;
pub use weights::WeightInfo;

pub(crate) const LOG_TARGET: &str = "runtime::bags_list";

// syntactic sugar for logging.
#[macro_export]
macro_rules! log {
	($level:tt, $patter:expr $(, $values:expr)* $(,)?) => {
		log::$level!(
			target: crate::LOG_TARGET,
			concat!("[{:?}] 👜 [{}]", $patter),
			<frame_system::Pallet<T>>::block_number(),
			<crate::Pallet::<T, I> as frame_support::traits::PalletInfoAccess>::name()
			$(, $values)*
		)
	};
}

type AccountIdLookupOf<T> = <<T as frame_system::Config>::Lookup as StaticLookup>::Source;

#[frame_support::pallet]
pub mod pallet {
	use super::*;
	use frame_support::pallet_prelude::*;
	use frame_system::pallet_prelude::*;

	#[pallet::pallet]
	pub struct Pallet<T, I = ()>(_);

	#[pallet::config]
	pub trait Config<I: 'static = ()>: frame_system::Config {
		/// The overarching event type.
		type RuntimeEvent: From<Event<Self, I>>
			+ IsType<<Self as frame_system::Config>::RuntimeEvent>;

		/// Weight information for extrinsics in this pallet.
		type WeightInfo: weights::WeightInfo;

		/// Something that provides the scores of ids.
		type ScoreProvider: ScoreProvider<Self::AccountId, Score = Self::Score>;

		/// The list of thresholds separating the various bags.
		///
		/// Ids are separated into unsorted bags according to their score. This specifies the
		/// thresholds separating the bags. An id's bag is the largest bag for which the id's score
		/// is less than or equal to its upper threshold.
		///
		/// When ids are iterated, higher bags are iterated completely before lower bags. This means
		/// that iteration is _semi-sorted_: ids of higher score tend to come before ids of lower
		/// score, but peer ids within a particular bag are sorted in insertion order.
		///
		/// # Expressing the constant
		///
		/// This constant must be sorted in strictly increasing order. Duplicate items are not
		/// permitted.
		///
		/// There is an implied upper limit of `Score::MAX`; that value does not need to be
		/// specified within the bag. For any two threshold lists, if one ends with
		/// `Score::MAX`, the other one does not, and they are otherwise equal, the two
		/// lists will behave identically.
		///
		/// # Calculation
		///
		/// It is recommended to generate the set of thresholds in a geometric series, such that
		/// there exists some constant ratio such that `threshold[k + 1] == (threshold[k] *
		/// constant_ratio).max(threshold[k] + 1)` for all `k`.
		///
		/// The helpers in the `/utils/frame/generate-bags` module can simplify this calculation.
		///
		/// # Examples
		///
		/// - If `BagThresholds::get().is_empty()`, then all ids are put into the same bag, and
		///   iteration is strictly in insertion order.
		/// - If `BagThresholds::get().len() == 64`, and the thresholds are determined according to
		///   the procedure given above, then the constant ratio is equal to 2.
		/// - If `BagThresholds::get().len() == 200`, and the thresholds are determined according to
		///   the procedure given above, then the constant ratio is approximately equal to 1.248.
		/// - If the threshold list begins `[1, 2, 3, ...]`, then an id with score 0 or 1 will fall
		///   into bag 0, an id with score 2 will fall into bag 1, etc.
		///
		/// # Migration
		///
		/// In the event that this list ever changes, a copy of the old bags list must be retained.
		/// With that `List::migrate` can be called, which will perform the appropriate migration.
		#[pallet::constant]
		type BagThresholds: Get<&'static [Self::Score]>;

		/// The type used to dictate a node position relative to other nodes.
		type Score: Clone
			+ Default
			+ PartialEq
			+ Eq
			+ Ord
			+ PartialOrd
			+ sp_std::fmt::Debug
			+ Copy
			+ AtLeast32BitUnsigned
			+ Bounded
			+ TypeInfo
			+ FullCodec
			+ MaxEncodedLen;
	}

	/// A single node, within some bag.
	///
	/// Nodes store links forward and back within their respective bags.
	#[pallet::storage]
	pub(crate) type ListNodes<T: Config<I>, I: 'static = ()> =
		CountedStorageMap<_, Twox64Concat, T::AccountId, list::Node<T, I>>;

	/// A bag stored in storage.
	///
	/// Stores a `Bag` struct, which stores head and tail pointers to itself.
	#[pallet::storage]
	pub(crate) type ListBags<T: Config<I>, I: 'static = ()> =
		StorageMap<_, Twox64Concat, T::Score, list::Bag<T, I>>;

	#[pallet::event]
	#[pallet::generate_deposit(pub(crate) fn deposit_event)]
	pub enum Event<T: Config<I>, I: 'static = ()> {
		/// Moved an account from one bag to another.
		Rebagged { who: T::AccountId, from: T::Score, to: T::Score },
		/// Updated the score of some account to the given amount.
		ScoreUpdated { who: T::AccountId, new_score: T::Score },
	}

	#[pallet::error]
	#[cfg_attr(test, derive(PartialEq))]
	pub enum Error<T, I = ()> {
		/// A error in the list interface implementation.
		List(ListError),
	}

	impl<T, I> From<ListError> for Error<T, I> {
		fn from(t: ListError) -> Self {
			Error::<T, I>::List(t)
		}
	}

	#[pallet::call]
	impl<T: Config<I>, I: 'static> Pallet<T, I> {
		/// Declare that some `dislocated` account has, through rewards or penalties, sufficiently
		/// changed its score that it should properly fall into a different bag than its current
		/// one.
		///
		/// Anyone can call this function about any potentially dislocated account.
		///
		/// Will always update the stored score of `dislocated` to the correct score, based on
		/// `ScoreProvider`.
		///
		/// If `dislocated` does not exists, it returns an error.
		#[pallet::call_index(0)]
		#[pallet::weight(T::WeightInfo::rebag_non_terminal().max(T::WeightInfo::rebag_terminal()))]
		pub fn rebag(origin: OriginFor<T>, dislocated: AccountIdLookupOf<T>) -> DispatchResult {
			ensure_signed(origin)?;
			let dislocated = T::Lookup::lookup(dislocated)?;
			let current_score = T::ScoreProvider::score(&dislocated);
			let _ = Pallet::<T, I>::do_rebag(&dislocated, current_score)
				.map_err::<Error<T, I>, _>(Into::into)?;
			Ok(())
		}

		/// Move the caller's Id directly in front of `lighter`.
		///
		/// The dispatch origin for this call must be _Signed_ and can only be called by the Id of
		/// the account going in front of `lighter`. Fee is payed by the origin under all
		/// circumstances.
		///
		/// Only works if:
		///
		/// - both nodes are within the same bag,
		/// - and `origin` has a greater `Score` than `lighter`.
		#[pallet::call_index(1)]
		#[pallet::weight(T::WeightInfo::put_in_front_of())]
		pub fn put_in_front_of(
			origin: OriginFor<T>,
			lighter: AccountIdLookupOf<T>,
		) -> DispatchResult {
			let heavier = ensure_signed(origin)?;
			let lighter = T::Lookup::lookup(lighter)?;
			List::<T, I>::put_in_front_of(&lighter, &heavier)
				.map_err::<Error<T, I>, _>(Into::into)
				.map_err::<DispatchError, _>(Into::into)
		}

		/// Same as [`Pallet::put_in_front_of`], but it can be called by anyone.
		///
		/// Fee is paid by the origin under all circumstances.
		#[pallet::call_index(2)]
		#[pallet::weight(T::WeightInfo::put_in_front_of())]
		pub fn put_in_front_of_other(
			origin: OriginFor<T>,
			heavier: AccountIdLookupOf<T>,
			lighter: AccountIdLookupOf<T>,
		) -> DispatchResult {
			let _ = ensure_signed(origin)?;
			let lighter = T::Lookup::lookup(lighter)?;
			let heavier = T::Lookup::lookup(heavier)?;
			List::<T, I>::put_in_front_of(&lighter, &heavier)
				.map_err::<Error<T, I>, _>(Into::into)
				.map_err::<DispatchError, _>(Into::into)
		}
	}

	#[pallet::hooks]
	impl<T: Config<I>, I: 'static> Hooks<BlockNumberFor<T>> for Pallet<T, I> {
		fn integrity_test() {
			// ensure they are strictly increasing, this also implies that duplicates are detected.
			assert!(
				T::BagThresholds::get().windows(2).all(|window| window[1] > window[0]),
				"thresholds must strictly increase, and have no duplicates",
			);
		}

		#[cfg(feature = "try-runtime")]
		fn try_state(_: BlockNumberFor<T>) -> Result<(), TryRuntimeError> {
			<Self as SortedListProvider<T::AccountId>>::try_state()
		}
	}
}

#[cfg(any(test, feature = "try-runtime", feature = "fuzz"))]
impl<T: Config<I>, I: 'static> Pallet<T, I> {
	pub fn do_try_state() -> Result<(), TryRuntimeError> {
		List::<T, I>::do_try_state()
	}
}

impl<T: Config<I>, I: 'static> Pallet<T, I> {
	/// Move an account from one bag to another, depositing an event on success.
	///
	/// If the account changed bags, returns `Ok(Some((from, to)))`.
	pub fn do_rebag(
		account: &T::AccountId,
		new_score: T::Score,
	) -> Result<Option<(T::Score, T::Score)>, ListError> {
		// If no voter at that node, don't do anything. the caller just wasted the fee to call this.
		let node = list::Node::<T, I>::get(&account).ok_or(ListError::NodeNotFound)?;
		let maybe_movement = List::update_position_for(node, new_score);
		if let Some((from, to)) = maybe_movement {
			Self::deposit_event(Event::<T, I>::Rebagged { who: account.clone(), from, to });
		};
		Self::deposit_event(Event::<T, I>::ScoreUpdated { who: account.clone(), new_score });
		Ok(maybe_movement)
	}

	/// Equivalent to `ListBags::get`, but public. Useful for tests in outside of this crate.
	#[cfg(feature = "std")]
	pub fn list_bags_get(score: T::Score) -> Option<list::Bag<T, I>> {
		ListBags::get(score)
	}
}

impl<T: Config<I>, I: 'static> SortedListProvider<T::AccountId> for Pallet<T, I> {
	type Error = ListError;
	type Score = T::Score;

	fn iter() -> Box<dyn Iterator<Item = T::AccountId>> {
		Box::new(List::<T, I>::iter().map(|n| n.id().clone()))
	}

	fn iter_from(
		start: &T::AccountId,
	) -> Result<Box<dyn Iterator<Item = T::AccountId>>, Self::Error> {
		let iter = List::<T, I>::iter_from(start)?;
		Ok(Box::new(iter.map(|n| n.id().clone())))
	}

	fn count() -> u32 {
		ListNodes::<T, I>::count()
	}

	fn contains(id: &T::AccountId) -> bool {
		List::<T, I>::contains(id)
	}

	fn on_insert(id: T::AccountId, score: T::Score) -> Result<(), ListError> {
		List::<T, I>::insert(id, score)
	}

	fn get_score(id: &T::AccountId) -> Result<T::Score, ListError> {
		List::<T, I>::get_score(id)
	}

	fn on_update(id: &T::AccountId, new_score: T::Score) -> Result<(), ListError> {
		Pallet::<T, I>::do_rebag(id, new_score).map(|_| ())
	}

	fn on_remove(id: &T::AccountId) -> Result<(), ListError> {
		List::<T, I>::remove(id)
	}

	fn unsafe_regenerate(
		all: impl IntoIterator<Item = T::AccountId>,
		score_of: Box<dyn Fn(&T::AccountId) -> T::Score>,
	) -> u32 {
		// NOTE: This call is unsafe for the same reason as SortedListProvider::unsafe_regenerate.
		// I.e. because it can lead to many storage accesses.
		// So it is ok to call it as caller must ensure the conditions.
		List::<T, I>::unsafe_regenerate(all, score_of)
	}

	#[cfg(feature = "try-runtime")]
	fn try_state() -> Result<(), TryRuntimeError> {
		Self::do_try_state()
	}

	fn unsafe_clear() {
		// NOTE: This call is unsafe for the same reason as SortedListProvider::unsafe_clear.
		// I.e. because it can lead to many storage accesses.
		// So it is ok to call it as caller must ensure the conditions.
		List::<T, I>::unsafe_clear()
	}

	frame_election_provider_support::runtime_benchmarks_enabled! {
		fn score_update_worst_case(who: &T::AccountId, is_increase: bool) -> Self::Score {
			use frame_support::traits::Get as _;
			let thresholds = T::BagThresholds::get();
			let node = list::Node::<T, I>::get(who).unwrap();
			let current_bag_idx = thresholds
				.iter()
				.chain(sp_std::iter::once(&T::Score::max_value()))
				.position(|w| w == &node.bag_upper)
				.unwrap();

			if is_increase {
				let next_threshold_idx = current_bag_idx + 1;
				assert!(thresholds.len() > next_threshold_idx);
				thresholds[next_threshold_idx]
			} else {
				assert!(current_bag_idx != 0);
				let prev_threshold_idx = current_bag_idx - 1;
				thresholds[prev_threshold_idx]
			}
		}
	}
}

impl<T: Config<I>, I: 'static> ScoreProvider<T::AccountId> for Pallet<T, I> {
	type Score = <Pallet<T, I> as SortedListProvider<T::AccountId>>::Score;

	fn score(id: &T::AccountId) -> T::Score {
		Node::<T, I>::get(id).map(|node| node.score()).unwrap_or_default()
	}

	frame_election_provider_support::runtime_benchmarks_fuzz_or_std_enabled! {
		fn set_score_of(id: &T::AccountId, new_score: T::Score) {
			ListNodes::<T, I>::mutate(id, |maybe_node| {
				if let Some(node) = maybe_node.as_mut() {
					node.score = new_score;
				} else {
					panic!("trying to mutate {:?} which does not exists", id);
				}
			})
		}
	}
}