cuprate_consensus_context/difficulty.rs
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//! Difficulty Module
//!
//! This module handles keeping track of the data required to calculate block difficulty.
//! This data is currently the cumulative difficulty of each block and its timestamp.
//!
//! The timestamps are also used in other consensus rules so instead of duplicating the same
//! data in a different cache, the timestamps needed are retrieved from here.
//!
use std::{collections::VecDeque, ops::Range};
use tower::ServiceExt;
use tracing::instrument;
use cuprate_helper::num::median;
use cuprate_types::{
blockchain::{BlockchainReadRequest, BlockchainResponse},
Chain,
};
use crate::{ContextCacheError, Database, HardFork};
/// The amount of blocks we account for to calculate difficulty
const DIFFICULTY_WINDOW: usize = 720;
/// The proportion of blocks we remove from the [`DIFFICULTY_WINDOW`]. When the window
/// if 720 this means that 60 blocks are removed from the ends of the window so 120
/// blocks removed in total.
const DIFFICULTY_CUT: usize = 60;
/// The amount of blocks we add onto the window before doing any calculations so that the
/// difficulty lags by this amount of blocks
const DIFFICULTY_LAG: usize = 15;
/// Configuration for the difficulty cache.
///
#[derive(Debug, Clone, Copy, Eq, PartialEq)]
pub struct DifficultyCacheConfig {
pub window: usize,
pub cut: usize,
pub lag: usize,
}
impl DifficultyCacheConfig {
/// Create a new difficulty cache config.
///
/// # Notes
/// You probably do not need this, use [`DifficultyCacheConfig::main_net`] instead.
pub const fn new(window: usize, cut: usize, lag: usize) -> Self {
Self { window, cut, lag }
}
/// Returns the total amount of blocks we need to track to calculate difficulty
pub const fn total_block_count(&self) -> usize {
self.window + self.lag
}
/// The amount of blocks we account for after removing the outliers.
pub const fn accounted_window_len(&self) -> usize {
self.window - 2 * self.cut
}
/// Returns the config needed for [`Mainnet`](cuprate_helper::network::Network::Mainnet). This is also the
/// config for all other current networks.
pub const fn main_net() -> Self {
Self {
window: DIFFICULTY_WINDOW,
cut: DIFFICULTY_CUT,
lag: DIFFICULTY_LAG,
}
}
}
/// This struct is able to calculate difficulties from blockchain information.
///
#[derive(Debug, Clone, Eq, PartialEq)]
pub struct DifficultyCache {
/// The list of timestamps in the window.
pub timestamps: VecDeque<u64>,
/// The current cumulative difficulty of the chain.
pub cumulative_difficulties: VecDeque<u128>,
/// The last height we accounted for.
pub last_accounted_height: usize,
/// The config
pub config: DifficultyCacheConfig,
}
impl DifficultyCache {
/// Initialize the difficulty cache from the specified chain height.
#[instrument(name = "init_difficulty_cache", level = "info", skip(database, config))]
pub async fn init_from_chain_height<D: Database + Clone>(
chain_height: usize,
config: DifficultyCacheConfig,
database: D,
chain: Chain,
) -> Result<Self, ContextCacheError> {
tracing::info!("Initializing difficulty cache this may take a while.");
let mut block_start = chain_height.saturating_sub(config.total_block_count());
// skip the genesis block.
if block_start == 0 {
block_start = 1;
}
let (timestamps, cumulative_difficulties) =
get_blocks_in_pow_info(database.clone(), block_start..chain_height, chain).await?;
debug_assert_eq!(timestamps.len(), chain_height - block_start);
tracing::info!(
"Current chain height: {}, accounting for {} blocks timestamps",
chain_height,
timestamps.len()
);
let diff = Self {
timestamps,
cumulative_difficulties,
last_accounted_height: chain_height - 1,
config,
};
Ok(diff)
}
/// Pop some blocks from the top of the cache.
///
/// The cache will be returned to the state it would have been in `numb_blocks` ago.
///
/// # Invariant
///
/// This _must_ only be used on a main-chain cache.
#[instrument(name = "pop_blocks_diff_cache", skip_all, fields(numb_blocks = numb_blocks))]
pub async fn pop_blocks_main_chain<D: Database + Clone>(
&mut self,
numb_blocks: usize,
database: D,
) -> Result<(), ContextCacheError> {
let Some(retained_blocks) = self.timestamps.len().checked_sub(numb_blocks) else {
// More blocks to pop than we have in the cache, so just restart a new cache.
*self = Self::init_from_chain_height(
self.last_accounted_height - numb_blocks + 1,
self.config,
database,
Chain::Main,
)
.await?;
return Ok(());
};
let current_chain_height = self.last_accounted_height + 1;
let mut new_start_height = current_chain_height
.saturating_sub(self.config.total_block_count())
.saturating_sub(numb_blocks);
// skip the genesis block.
if new_start_height == 0 {
new_start_height = 1;
}
let (mut timestamps, mut cumulative_difficulties) = get_blocks_in_pow_info(
database,
new_start_height
// current_chain_height - self.timestamps.len() blocks are already in the cache.
..(current_chain_height - self.timestamps.len()),
Chain::Main,
)
.await?;
self.timestamps.drain(retained_blocks..);
self.cumulative_difficulties.drain(retained_blocks..);
timestamps.append(&mut self.timestamps);
cumulative_difficulties.append(&mut self.cumulative_difficulties);
self.timestamps = timestamps;
self.cumulative_difficulties = cumulative_difficulties;
self.last_accounted_height -= numb_blocks;
assert_eq!(self.timestamps.len(), self.cumulative_difficulties.len());
Ok(())
}
/// Add a new block to the difficulty cache.
pub fn new_block(&mut self, height: usize, timestamp: u64, cumulative_difficulty: u128) {
assert_eq!(self.last_accounted_height + 1, height);
self.last_accounted_height += 1;
tracing::debug!(
"Accounting for new blocks timestamp ({timestamp}) and cumulative_difficulty ({cumulative_difficulty})",
);
self.timestamps.push_back(timestamp);
self.cumulative_difficulties
.push_back(cumulative_difficulty);
if self.timestamps.len() > self.config.total_block_count() {
self.timestamps.pop_front();
self.cumulative_difficulties.pop_front();
}
}
/// Returns the required difficulty for the next block.
///
/// See: <https://cuprate.github.io/monero-book/consensus_rules/blocks/difficulty.html#calculating-difficulty>
pub fn next_difficulty(&self, hf: HardFork) -> u128 {
next_difficulty(
&self.config,
&self.timestamps,
&self.cumulative_difficulties,
hf,
)
}
/// Returns the difficulties for multiple next blocks, using the provided timestamps and hard-forks when needed.
///
/// The first difficulty will be the same as the difficulty from [`DifficultyCache::next_difficulty`] after that the
/// first timestamp and hf will be applied to the cache and the difficulty from that will be added to the list.
///
/// After all timestamps and hfs have been dealt with the cache will be returned back to its original state and the
/// difficulties will be returned.
pub fn next_difficulties(
&self,
blocks: Vec<(u64, HardFork)>,
current_hf: HardFork,
) -> Vec<u128> {
let mut timestamps = self.timestamps.clone();
let mut cumulative_difficulties = self.cumulative_difficulties.clone();
let mut difficulties = Vec::with_capacity(blocks.len() + 1);
difficulties.push(self.next_difficulty(current_hf));
for (new_timestamp, hf) in blocks {
timestamps.push_back(new_timestamp);
let last_cum_diff = cumulative_difficulties.back().copied().unwrap_or(1);
cumulative_difficulties.push_back(last_cum_diff + *difficulties.last().unwrap());
if timestamps.len() > self.config.total_block_count() {
timestamps.pop_front().unwrap();
cumulative_difficulties.pop_front().unwrap();
}
difficulties.push(next_difficulty(
&self.config,
×tamps,
&cumulative_difficulties,
hf,
));
}
difficulties
}
/// Returns the median timestamp over the last `numb_blocks`, including the genesis block if the block height is low enough.
///
/// Will return [`None`] if there aren't enough blocks.
pub fn median_timestamp(&self, numb_blocks: usize) -> Option<u64> {
let mut timestamps = if self.last_accounted_height + 1 == numb_blocks {
// if the chain height is equal to `numb_blocks` add the genesis block.
// otherwise if the chain height is less than `numb_blocks` None is returned
// and if it's more it would be excluded from calculations.
let mut timestamps = self.timestamps.clone();
// all genesis blocks have a timestamp of 0.
// https://cuprate.github.io/monero-book/consensus_rules/genesis_block.html
timestamps.push_front(0);
timestamps.into()
} else {
self.timestamps
.range(self.timestamps.len().checked_sub(numb_blocks)?..)
.copied()
.collect::<Vec<_>>()
};
timestamps.sort_unstable();
debug_assert_eq!(timestamps.len(), numb_blocks);
Some(median(×tamps))
}
/// Returns the cumulative difficulty of the chain.
pub fn cumulative_difficulty(&self) -> u128 {
// the genesis block has a difficulty of 1
self.cumulative_difficulties.back().copied().unwrap_or(1)
}
/// Returns the top block's timestamp, returns [`None`] if the top block is the genesis block.
pub fn top_block_timestamp(&self) -> Option<u64> {
self.timestamps.back().copied()
}
}
/// Calculates the next difficulty with the inputted `config/timestamps/cumulative_difficulties`.
fn next_difficulty(
config: &DifficultyCacheConfig,
timestamps: &VecDeque<u64>,
cumulative_difficulties: &VecDeque<u128>,
hf: HardFork,
) -> u128 {
if timestamps.len() <= 1 {
return 1;
}
let mut timestamps = timestamps.clone();
if timestamps.len() > config.window {
// remove the lag.
timestamps.drain(config.window..);
};
let timestamps_slice = timestamps.make_contiguous();
let (window_start, window_end) = get_window_start_and_end(
timestamps_slice.len(),
config.accounted_window_len(),
config.window,
);
// We don't sort the whole timestamp list
let mut time_span = u128::from(
*timestamps_slice.select_nth_unstable(window_end - 1).1
- *timestamps_slice.select_nth_unstable(window_start).1,
);
let windowed_work =
cumulative_difficulties[window_end - 1] - cumulative_difficulties[window_start];
if time_span == 0 {
time_span = 1;
}
// TODO: do checked operations here and unwrap so we don't silently overflow?
(windowed_work * u128::from(hf.block_time().as_secs()) + time_span - 1) / time_span
}
/// Get the start and end of the window to calculate difficulty.
fn get_window_start_and_end(
window_len: usize,
accounted_window: usize,
window: usize,
) -> (usize, usize) {
debug_assert!(window > accounted_window);
let window_len = if window_len > window {
window
} else {
window_len
};
if window_len <= accounted_window {
(0, window_len)
} else {
let start = (window_len - (accounted_window) + 1) / 2;
(start, start + accounted_window)
}
}
/// Returns the timestamps and cumulative difficulty for the blocks with heights in the specified range.
#[instrument(name = "get_blocks_timestamps", skip(database), level = "info")]
async fn get_blocks_in_pow_info<D: Database + Clone>(
database: D,
block_heights: Range<usize>,
chain: Chain,
) -> Result<(VecDeque<u64>, VecDeque<u128>), ContextCacheError> {
tracing::info!("Getting blocks timestamps");
let BlockchainResponse::BlockExtendedHeaderInRange(ext_header) = database
.oneshot(BlockchainReadRequest::BlockExtendedHeaderInRange(
block_heights,
chain,
))
.await?
else {
panic!("Database sent incorrect response");
};
Ok(ext_header
.into_iter()
.map(|info| (info.timestamp, info.cumulative_difficulty))
.unzip())
}