zebra_state/service/check/difficulty.rs
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
//! Block difficulty adjustment calculations for contextual validation.
//!
//! This module supports the following consensus rule calculations:
//! * `ThresholdBits` from the Zcash Specification,
//! * the Testnet minimum difficulty adjustment from ZIPs 205 and 208, and
//! * `median-time-past`.
use std::{cmp::max, cmp::min};
use chrono::{DateTime, Duration, Utc};
use zebra_chain::{
block,
block::Block,
parameters::Network,
parameters::NetworkUpgrade,
parameters::POW_AVERAGING_WINDOW,
work::difficulty::{CompactDifficulty, U256},
work::difficulty::{ExpandedDifficulty, ParameterDifficulty as _},
};
/// The median block span for time median calculations.
///
/// `PoWMedianBlockSpan` in the Zcash specification.
pub const POW_MEDIAN_BLOCK_SPAN: usize = 11;
/// The overall block span used for adjusting Zcash block difficulty.
///
/// `PoWAveragingWindow + PoWMedianBlockSpan` in the Zcash specification based on
/// > ActualTimespan(height : N) := MedianTime(height) − MedianTime(height − PoWAveragingWindow)
pub const POW_ADJUSTMENT_BLOCK_SPAN: usize = POW_AVERAGING_WINDOW + POW_MEDIAN_BLOCK_SPAN;
/// The damping factor for median timespan variance.
///
/// `PoWDampingFactor` in the Zcash specification.
pub const POW_DAMPING_FACTOR: i32 = 4;
/// The maximum upward adjustment percentage for median timespan variance.
///
/// `PoWMaxAdjustUp * 100` in the Zcash specification.
pub const POW_MAX_ADJUST_UP_PERCENT: i32 = 16;
/// The maximum downward adjustment percentage for median timespan variance.
///
/// `PoWMaxAdjustDown * 100` in the Zcash specification.
pub const POW_MAX_ADJUST_DOWN_PERCENT: i32 = 32;
/// The maximum number of seconds between the `median-time-past` of a block,
/// and the block's `time` field.
///
/// Part of the block header consensus rules in the Zcash specification.
pub const BLOCK_MAX_TIME_SINCE_MEDIAN: u32 = 90 * 60;
/// Contains the context needed to calculate the adjusted difficulty for a block.
pub(crate) struct AdjustedDifficulty {
/// The `header.time` field from the candidate block
candidate_time: DateTime<Utc>,
/// The coinbase height from the candidate block
///
/// If we only have the header, this field is calculated from the previous
/// block height.
candidate_height: block::Height,
/// The configured network
network: Network,
/// The `header.difficulty_threshold`s from the previous
/// `PoWAveragingWindow + PoWMedianBlockSpan` (28) blocks, in reverse height
/// order.
relevant_difficulty_thresholds: Vec<CompactDifficulty>,
/// The `header.time`s from the previous
/// `PoWAveragingWindow + PoWMedianBlockSpan` (28) blocks, in reverse height
/// order.
///
/// Only the first and last `PoWMedianBlockSpan` times are used. Times
/// `11..=16` are ignored.
relevant_times: Vec<DateTime<Utc>>,
}
impl AdjustedDifficulty {
/// Initialise and return a new `AdjustedDifficulty` using a `candidate_block`,
/// `network`, and a `context`.
///
/// The `context` contains the previous
/// `PoWAveragingWindow + PoWMedianBlockSpan` (28) `difficulty_threshold`s and
/// `time`s from the relevant chain for `candidate_block`, in reverse height
/// order, starting with the previous block.
///
/// Note that the `time`s might not be in reverse chronological order, because
/// block times are supplied by miners.
///
/// # Panics
///
/// If the `context` contains fewer than 28 items.
pub fn new_from_block<C>(
candidate_block: &Block,
network: &Network,
context: C,
) -> AdjustedDifficulty
where
C: IntoIterator<Item = (CompactDifficulty, DateTime<Utc>)>,
{
let candidate_block_height = candidate_block
.coinbase_height()
.expect("semantically valid blocks have a coinbase height");
let previous_block_height = (candidate_block_height - 1)
.expect("contextual validation is never run on the genesis block");
AdjustedDifficulty::new_from_header_time(
candidate_block.header.time,
previous_block_height,
network,
context,
)
}
/// Initialise and return a new [`AdjustedDifficulty`] using a
/// `candidate_header_time`, `previous_block_height`, `network`, and a `context`.
///
/// Designed for use when validating block headers, where the full block has not
/// been downloaded yet.
///
/// See [`Self::new_from_block`] for detailed information about the `context`.
///
/// # Panics
///
/// If the context contains fewer than 28 items.
pub fn new_from_header_time<C>(
candidate_header_time: DateTime<Utc>,
previous_block_height: block::Height,
network: &Network,
context: C,
) -> AdjustedDifficulty
where
C: IntoIterator<Item = (CompactDifficulty, DateTime<Utc>)>,
{
let candidate_height = (previous_block_height + 1).expect("next block height is valid");
let (relevant_difficulty_thresholds, relevant_times) = context
.into_iter()
.take(POW_ADJUSTMENT_BLOCK_SPAN)
.unzip::<_, _, Vec<_>, Vec<_>>();
AdjustedDifficulty {
candidate_time: candidate_header_time,
candidate_height,
network: network.clone(),
relevant_difficulty_thresholds,
relevant_times,
}
}
/// Returns the candidate block's height.
pub fn candidate_height(&self) -> block::Height {
self.candidate_height
}
/// Returns the candidate block's time field.
pub fn candidate_time(&self) -> DateTime<Utc> {
self.candidate_time
}
/// Returns the configured network.
pub fn network(&self) -> Network {
self.network.clone()
}
/// Calculate the expected `difficulty_threshold` for a candidate block, based
/// on the `candidate_time`, `candidate_height`, `network`, and the
/// `difficulty_threshold`s and `time`s from the previous
/// `PoWAveragingWindow + PoWMedianBlockSpan` (28) blocks in the relevant chain.
///
/// Implements `ThresholdBits` from the Zcash specification, and the Testnet
/// minimum difficulty adjustment from ZIPs 205 and 208.
pub fn expected_difficulty_threshold(&self) -> CompactDifficulty {
if NetworkUpgrade::is_testnet_min_difficulty_block(
&self.network,
self.candidate_height,
self.candidate_time,
self.relevant_times[0],
) {
assert!(
self.network.is_a_test_network(),
"invalid network: the minimum difficulty rule only applies on test networks"
);
self.network.target_difficulty_limit().to_compact()
} else {
self.threshold_bits()
}
}
/// Calculate the `difficulty_threshold` for a candidate block, based on the
/// `candidate_height`, `network`, and the relevant `difficulty_threshold`s and
/// `time`s.
///
/// See [`Self::expected_difficulty_threshold`] for details.
///
/// Implements `ThresholdBits` from the Zcash specification. (Which excludes the
/// Testnet minimum difficulty adjustment.)
fn threshold_bits(&self) -> CompactDifficulty {
let averaging_window_timespan = NetworkUpgrade::averaging_window_timespan_for_height(
&self.network,
self.candidate_height,
);
let threshold = (self.mean_target_difficulty() / averaging_window_timespan.num_seconds())
* self.median_timespan_bounded().num_seconds();
let threshold = min(self.network.target_difficulty_limit(), threshold);
threshold.to_compact()
}
/// Calculate the arithmetic mean of the averaging window thresholds: the
/// expanded `difficulty_threshold`s from the previous `PoWAveragingWindow` (17)
/// blocks in the relevant chain.
///
/// Implements `MeanTarget` from the Zcash specification.
fn mean_target_difficulty(&self) -> ExpandedDifficulty {
// In Zebra, contextual validation starts after Canopy activation, so we
// can assume that the relevant chain contains at least 17 blocks.
// Therefore, the `PoWLimit` case of `MeanTarget()` from the Zcash
// specification is unreachable.
let averaging_window_thresholds =
if self.relevant_difficulty_thresholds.len() >= POW_AVERAGING_WINDOW {
&self.relevant_difficulty_thresholds[0..POW_AVERAGING_WINDOW]
} else {
return self.network.target_difficulty_limit();
};
// Since the PoWLimits are `2^251 − 1` for Testnet, and `2^243 − 1` for
// Mainnet, the sum of 17 `ExpandedDifficulty` will be less than or equal
// to: `(2^251 − 1) * 17 = 2^255 + 2^251 - 17`. Therefore, the sum can
// not overflow a u256 value.
let total: ExpandedDifficulty = averaging_window_thresholds
.iter()
.map(|compact| {
compact
.to_expanded()
.expect("difficulty thresholds in previously verified blocks are valid")
})
.sum();
let divisor: U256 = POW_AVERAGING_WINDOW.into();
total / divisor
}
/// Calculate the bounded median timespan. The median timespan is the
/// difference of medians of the timespan times, which are the `time`s from
/// the previous `PoWAveragingWindow + PoWMedianBlockSpan` (28) blocks in the
/// relevant chain.
///
/// Uses the candidate block's `height' and `network` to calculate the
/// `AveragingWindowTimespan` for that block.
///
/// The median timespan is damped by the `PoWDampingFactor`, and bounded by
/// `PoWMaxAdjustDown` and `PoWMaxAdjustUp`.
///
/// Implements `ActualTimespanBounded` from the Zcash specification.
///
/// Note: This calculation only uses `PoWMedianBlockSpan` (11) times at the
/// start and end of the timespan times. timespan times `[11..=16]` are ignored.
fn median_timespan_bounded(&self) -> Duration {
let averaging_window_timespan = NetworkUpgrade::averaging_window_timespan_for_height(
&self.network,
self.candidate_height,
);
// This value is exact, but we need to truncate its nanoseconds component
let damped_variance =
(self.median_timespan() - averaging_window_timespan) / POW_DAMPING_FACTOR;
// num_seconds truncates negative values towards zero, matching the Zcash specification
let damped_variance = Duration::seconds(damped_variance.num_seconds());
// `ActualTimespanDamped` in the Zcash specification
let median_timespan_damped = averaging_window_timespan + damped_variance;
// `MinActualTimespan` and `MaxActualTimespan` in the Zcash spec
let min_median_timespan =
averaging_window_timespan * (100 - POW_MAX_ADJUST_UP_PERCENT) / 100;
let max_median_timespan =
averaging_window_timespan * (100 + POW_MAX_ADJUST_DOWN_PERCENT) / 100;
// `ActualTimespanBounded` in the Zcash specification
max(
min_median_timespan,
min(max_median_timespan, median_timespan_damped),
)
}
/// Calculate the median timespan. The median timespan is the difference of
/// medians of the timespan times, which are the `time`s from the previous
/// `PoWAveragingWindow + PoWMedianBlockSpan` (28) blocks in the relevant chain.
///
/// Implements `ActualTimespan` from the Zcash specification.
///
/// See [`Self::median_timespan_bounded`] for details.
fn median_timespan(&self) -> Duration {
let newer_median = self.median_time_past();
// MedianTime(height : N) := median([ nTime(𝑖) for 𝑖 from max(0, height − PoWMedianBlockSpan) up to max(0, height − 1) ])
let older_median = if self.relevant_times.len() > POW_AVERAGING_WINDOW {
let older_times: Vec<_> = self
.relevant_times
.iter()
.skip(POW_AVERAGING_WINDOW)
.cloned()
.take(POW_MEDIAN_BLOCK_SPAN)
.collect();
AdjustedDifficulty::median_time(older_times)
} else {
self.relevant_times
.last()
.cloned()
.expect("there must be a Genesis block")
};
// `ActualTimespan` in the Zcash specification
newer_median - older_median
}
/// Calculate the median of the `time`s from the previous
/// `PoWMedianBlockSpan` (11) blocks in the relevant chain.
///
/// Implements `median-time-past` and `MedianTime(candidate_height)` from the
/// Zcash specification. (These functions are identical, but they are
/// specified in slightly different ways.)
pub fn median_time_past(&self) -> DateTime<Utc> {
let median_times: Vec<DateTime<Utc>> = self
.relevant_times
.iter()
.take(POW_MEDIAN_BLOCK_SPAN)
.cloned()
.collect();
AdjustedDifficulty::median_time(median_times)
}
/// Calculate the median of the `median_block_span_times`: the `time`s from a
/// Vec of `PoWMedianBlockSpan` (11) or fewer blocks in the relevant chain.
///
/// Implements `MedianTime` from the Zcash specification.
///
/// # Panics
///
/// If provided an empty Vec
pub(crate) fn median_time(mut median_block_span_times: Vec<DateTime<Utc>>) -> DateTime<Utc> {
median_block_span_times.sort_unstable();
// > median(𝑆) := sorted(𝑆)_{ceiling((length(𝑆)+1)/2)}
// <https://zips.z.cash/protocol/protocol.pdf>, section 7.7.3, Difficulty Adjustment (p. 132)
let median_idx = median_block_span_times.len() / 2;
median_block_span_times[median_idx]
}
}