referrerpolicy=no-referrer-when-downgrade
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
// 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.

//! ## A static size tracker for the election snapshot data.
//!
//! ### Overview
//!
//! The goal of the size tracker is to provide a static, no-allocation byte tracker to be
//! used by the election data provider when preparing the results of
//! [`ElectionDataProvider::electing_voters`]. The [`StaticTracker`] implementation uses
//! [`codec::Encode::size_hint`] to estimate the SCALE encoded size of the snapshot voters struct
//! as it is being constructed without requiring extra stack allocations.
//!
//! The [`StaticTracker::try_register_voter`] is called to update the static tracker internal
//! state, if It will return an error if the resulting SCALE encoded size (in bytes) is larger than
//! the provided `DataProviderBounds`.
//!
//! ### Example
//!
//! ```ignore
//! use pallet_staking::election_size_tracker::*;
//!
//! // instantiates a new tracker.
//! let mut size_tracker = StaticTracker::<Staking>::default();
//!
//! let voter_bounds = ElectionBoundsBuilder::default().voter_size(1_00.into()).build().voters;
//!
//! let mut sorted_voters = T::VoterList.iter();
//! let mut selected_voters = vec![];
//!
//! // fit as many voters in the vec as the bounds permit.
//! for v in sorted_voters {
//!     let voter = (v, weight_of(&v), targets_of(&v));
//!     if size_tracker.try_register_voter(&voter, &voter_bounds).is_err() {
//!         // voter bounds size exhausted
//!         break;
//!     }
//!     selected_voters.push(voter);
//! }
//!
//! // The SCALE encoded size in bytes of `selected_voters` is guaranteed to be below
//! // `voter_bounds`.
//! debug_assert!(
//!     selected_voters.encoded_size() <=
//!     SizeTracker::<Staking>::final_byte_size_of(size_tracker.num_voters, size_tracker.size)
//! );
//! ```
//!
//! ### Implementation Details
//!
//! The current implementation of the static tracker is tightly coupled with the staking pallet
//! implementation, namely the representation of a voter ([`VoterOf`]). The SCALE encoded byte size
//! is calculated using [`Encode::size_hint`] of each type in the voter tuple. Each voter's byte
//! size is the sum of:
//! - 1 * [`Encode::size_hint`] of the `AccountId` type;
//! - 1 * [`Encode::size_hint`] of the `VoteWeight` type;
//! - `num_votes` * [`Encode::size_hint`] of the `AccountId` type.

use codec::Encode;
use frame_election_provider_support::{
	bounds::{DataProviderBounds, SizeBound},
	ElectionDataProvider, VoterOf,
};

/// Keeps track of the SCALE encoded byte length of the snapshot's voters or targets.
///
/// The tracker calculates the bytes used based on static rules, without requiring any actual
/// encoding or extra allocations.
#[derive(Clone, Copy, Debug)]
pub struct StaticTracker<DataProvider> {
	pub size: usize,
	pub counter: usize,
	_marker: core::marker::PhantomData<DataProvider>,
}

impl<DataProvider> Default for StaticTracker<DataProvider> {
	fn default() -> Self {
		Self { size: 0, counter: 0, _marker: Default::default() }
	}
}

impl<DataProvider> StaticTracker<DataProvider>
where
	DataProvider: ElectionDataProvider,
{
	/// Tries to register a new voter.
	///
	/// If the new voter exhausts the provided bounds, return an error. Otherwise, the internal
	/// state of the tracker is updated with the new registered voter.
	pub fn try_register_voter(
		&mut self,
		voter: &VoterOf<DataProvider>,
		bounds: &DataProviderBounds,
	) -> Result<(), ()> {
		let tracker_size_after = {
			let voter_hint = Self::voter_size_hint(voter);
			Self::final_byte_size_of(self.counter + 1, self.size.saturating_add(voter_hint))
		};

		match bounds.size_exhausted(SizeBound(tracker_size_after as u32)) {
			true => Err(()),
			false => {
				self.size = tracker_size_after;
				self.counter += 1;
				Ok(())
			},
		}
	}

	/// Calculates the size of the voter to register based on [`Encode::size_hint`].
	fn voter_size_hint(voter: &VoterOf<DataProvider>) -> usize {
		let (voter_account, vote_weight, targets) = voter;

		voter_account
			.size_hint()
			.saturating_add(vote_weight.size_hint())
			.saturating_add(voter_account.size_hint().saturating_mul(targets.len()))
	}

	/// Tries to register a new target.
	///
	/// If the new target exhausts the provided bounds, return an error. Otherwise, the internal
	/// state of the tracker is updated with the new registered target.
	pub fn try_register_target(
		&mut self,
		target: DataProvider::AccountId,
		bounds: &DataProviderBounds,
	) -> Result<(), ()> {
		let tracker_size_after = Self::final_byte_size_of(
			self.counter + 1,
			self.size.saturating_add(target.size_hint()),
		);

		match bounds.size_exhausted(SizeBound(tracker_size_after as u32)) {
			true => Err(()),
			false => {
				self.size = tracker_size_after;
				self.counter += 1;
				Ok(())
			},
		}
	}

	/// Size of the SCALE encoded prefix with a given length.
	#[inline]
	fn length_prefix(len: usize) -> usize {
		use codec::{Compact, CompactLen};
		Compact::<u32>::compact_len(&(len as u32))
	}

	/// Calculates the final size in bytes of the SCALE encoded snapshot voter struct.
	fn final_byte_size_of(num_voters: usize, size: usize) -> usize {
		Self::length_prefix(num_voters).saturating_add(size)
	}
}

#[cfg(test)]
mod tests {
	use super::*;
	use crate::{
		mock::{AccountId, Staking, Test},
		BoundedVec, MaxNominationsOf,
	};
	use frame_election_provider_support::bounds::ElectionBoundsBuilder;
	use sp_core::bounded_vec;

	type Voters = BoundedVec<AccountId, MaxNominationsOf<Test>>;

	#[test]
	pub fn election_size_tracker_works() {
		let mut voters: Vec<(u64, u64, Voters)> = vec![];
		let mut size_tracker = StaticTracker::<Staking>::default();
		let voter_bounds = ElectionBoundsBuilder::default().voters_size(1_50.into()).build().voters;

		// register 1 voter with 1 vote.
		let voter = (1, 10, bounded_vec![2]);
		assert!(size_tracker.try_register_voter(&voter, &voter_bounds).is_ok());
		voters.push(voter);

		assert_eq!(
			StaticTracker::<Staking>::final_byte_size_of(size_tracker.counter, size_tracker.size),
			voters.encoded_size()
		);

		// register another voter, now with 3 votes.
		let voter = (2, 20, bounded_vec![3, 4, 5]);
		assert!(size_tracker.try_register_voter(&voter, &voter_bounds).is_ok());
		voters.push(voter);

		assert_eq!(
			StaticTracker::<Staking>::final_byte_size_of(size_tracker.counter, size_tracker.size),
			voters.encoded_size()
		);

		// register noop vote (unlikely to happen).
		let voter = (3, 30, bounded_vec![]);
		assert!(size_tracker.try_register_voter(&voter, &voter_bounds).is_ok());
		voters.push(voter);

		assert_eq!(
			StaticTracker::<Staking>::final_byte_size_of(size_tracker.counter, size_tracker.size),
			voters.encoded_size()
		);
	}

	#[test]
	pub fn election_size_tracker_bounds_works() {
		let mut voters: Vec<(u64, u64, Voters)> = vec![];
		let mut size_tracker = StaticTracker::<Staking>::default();
		let voter_bounds = ElectionBoundsBuilder::default().voters_size(1_00.into()).build().voters;

		let voter = (1, 10, bounded_vec![2]);
		assert!(size_tracker.try_register_voter(&voter, &voter_bounds).is_ok());
		voters.push(voter);

		assert_eq!(
			StaticTracker::<Staking>::final_byte_size_of(size_tracker.counter, size_tracker.size),
			voters.encoded_size()
		);

		assert!(size_tracker.size > 0 && size_tracker.size < 1_00);
		let size_before_overflow = size_tracker.size;

		// try many voters that will overflow the tracker's buffer.
		let voter = (2, 10, bounded_vec![2, 3, 4, 5, 6, 7, 8, 9]);
		voters.push(voter.clone());

		assert!(size_tracker.try_register_voter(&voter, &voter_bounds).is_err());
		assert!(size_tracker.size > 0 && size_tracker.size < 1_00);

		// size of the tracker did not update when trying to register votes failed.
		assert_eq!(size_tracker.size, size_before_overflow);
	}

	#[test]
	fn len_prefix_works() {
		let length_samples =
			vec![0usize, 1, 62, 63, 64, 16383, 16384, 16385, 1073741822, 1073741823, 1073741824];

		for s in length_samples {
			// the encoded size of a vector of n bytes should be n + the length prefix
			assert_eq!(vec![1u8; s].encoded_size(), StaticTracker::<Staking>::length_prefix(s) + s);
		}
	}
}