Implement AdjustedDifficulty creation
Also: * call the difficulty check from `block_is_contextually_valid` * add a stub `difficulty_threshold_is_valid` function
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fa03b83351
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939c2b97a6
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@ -3253,6 +3253,7 @@ dependencies = [
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name = "zebra-state"
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version = "3.0.0-alpha.0"
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dependencies = [
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"chrono",
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"color-eyre",
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"dirs",
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"displaydoc",
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@ -26,6 +26,7 @@ tokio = { version = "0.3", features = ["sync"] }
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displaydoc = "0.1.7"
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rocksdb = "0.15.0"
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tempdir = "0.3.7"
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chrono = "0.4.19"
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[dev-dependencies]
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zebra-chain = { path = "../zebra-chain", features = ["proptest-impl"] }
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@ -6,6 +6,7 @@ use std::{
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time::{Duration, Instant},
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};
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use check::difficulty::{POW_AVERAGING_WINDOW, POW_MEDIAN_BLOCK_SPAN};
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use futures::future::FutureExt;
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use non_finalized_state::{NonFinalizedState, QueuedBlocks};
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use tokio::sync::oneshot;
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@ -184,11 +185,15 @@ impl StateService {
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&mut self,
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prepared: &PreparedBlock,
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) -> Result<(), ValidateContextError> {
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let relevant_chain = self.chain(prepared.block.header.previous_block_hash);
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assert!(relevant_chain.len() >= POW_AVERAGING_WINDOW + POW_MEDIAN_BLOCK_SPAN,
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"contextual validation requires at least 28 (POW_AVERAGING_WINDOW + POW_MEDIAN_BLOCK_SPAN) blocks");
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check::block_is_contextually_valid(
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prepared,
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self.network,
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self.disk.finalized_tip_height(),
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self.chain(prepared.block.header.previous_block_hash),
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relevant_chain,
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)?;
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Ok(())
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@ -1,14 +1,20 @@
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//! Consensus critical contextual checks
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use std::borrow::Borrow;
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use zebra_chain::{
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block::{self, Block},
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parameters::Network,
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work::difficulty::CompactDifficulty,
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};
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use crate::{PreparedBlock, ValidateContextError};
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use super::check;
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pub mod difficulty;
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use difficulty::{AdjustedDifficulty, POW_AVERAGING_WINDOW, POW_MEDIAN_BLOCK_SPAN};
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/// Check that `block` is contextually valid for `network`, based on the
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/// `finalized_tip_height` and `relevant_chain`.
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///
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@ -16,6 +22,9 @@ use super::check;
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/// with its parent block.
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///
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/// Panics if the finalized state is empty.
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///
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/// Skips the difficulty adjustment check if the state contains less than 28
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/// (`POW_AVERAGING_WINDOW + POW_MEDIAN_BLOCK_SPAN`) blocks.
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#[tracing::instrument(
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name = "contextual_validation",
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fields(?network),
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@ -29,30 +38,51 @@ pub(crate) fn block_is_contextually_valid<C>(
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) -> Result<(), ValidateContextError>
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where
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C: IntoIterator,
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C::Item: AsRef<Block>,
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C::Item: Borrow<Block>,
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C::IntoIter: ExactSizeIterator,
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{
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let finalized_tip_height = finalized_tip_height
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.expect("finalized state must contain at least one block to do contextual validation");
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check::block_is_not_orphaned(finalized_tip_height, prepared.height)?;
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let mut relevant_chain = relevant_chain.into_iter();
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// Peek at the first block
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let mut relevant_chain = relevant_chain.into_iter().peekable();
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let parent_block = relevant_chain
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.next()
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.peek()
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.expect("state must contain parent block to do contextual validation");
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let parent_block = parent_block.as_ref();
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let parent_block = parent_block.borrow();
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let parent_height = parent_block
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.coinbase_height()
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.expect("valid blocks have a coinbase height");
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check::height_one_more_than_parent_height(parent_height, prepared.height)?;
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// TODO: validate difficulty adjustment
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// TODO: other contextual validation design and implelentation
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// Note: the difficulty check reads the first 28 blocks from the relevant
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// chain iterator. If you want to use those blocks in other checks, you'll
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// need to clone them here.
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if relevant_chain.len() >= POW_AVERAGING_WINDOW + POW_MEDIAN_BLOCK_SPAN {
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let relevant_data = relevant_chain.map(|block| {
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(
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block.borrow().header.difficulty_threshold,
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block.borrow().header.time,
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)
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});
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// Reads the first 28 blocks from the iterator
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let expected_difficulty =
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AdjustedDifficulty::new_from_block(&prepared.block, network, relevant_data);
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check::difficulty_threshold_is_valid(
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prepared.block.header.difficulty_threshold,
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expected_difficulty,
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)?;
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}
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// TODO: other contextual validation design and implementation
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Ok(())
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}
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/// Returns `ValidateContextError::OrphanedBlock` if the height of the given
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/// block is less than or equal to the finalized tip height.
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pub(super) fn block_is_not_orphaned(
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fn block_is_not_orphaned(
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finalized_tip_height: block::Height,
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height: block::Height,
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) -> Result<(), ValidateContextError> {
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@ -65,7 +95,7 @@ pub(super) fn block_is_not_orphaned(
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/// Returns `ValidateContextError::NonSequentialBlock` if the block height isn't
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/// equal to the parent_height+1.
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pub(super) fn height_one_more_than_parent_height(
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fn height_one_more_than_parent_height(
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parent_height: block::Height,
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height: block::Height,
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) -> Result<(), ValidateContextError> {
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@ -76,6 +106,19 @@ pub(super) fn height_one_more_than_parent_height(
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}
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}
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/// Validate the `difficulty_threshold` from a candidate block's header, based
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/// on an `expected_difficulty` for that block.
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///
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/// Uses `expected_difficulty` to calculate the expected `ToCompact(Threshold())`
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/// value, then compares that value to the `difficulty_threshold`.
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/// Returns `Ok(())` if the values are equal.
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fn difficulty_threshold_is_valid(
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_difficulty_threshold: CompactDifficulty,
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_expected_difficulty: AdjustedDifficulty,
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) -> Result<(), ValidateContextError> {
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Ok(())
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}
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#[cfg(test)]
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mod tests {
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use std::sync::Arc;
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@ -0,0 +1,127 @@
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//! Block difficulty adjustment calculations for contextual validation.
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use chrono::{DateTime, Utc};
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use std::convert::TryInto;
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use zebra_chain::{block, block::Block, parameters::Network, work::difficulty::CompactDifficulty};
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/// The averaging window for difficulty threshold arithmetic mean calculations.
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///
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/// `PoWAveragingWindow` in the Zcash specification.
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pub const POW_AVERAGING_WINDOW: usize = 17;
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/// The median block span for time median calculations.
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///
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/// `PoWMedianBlockSpan` in the Zcash specification.
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pub const POW_MEDIAN_BLOCK_SPAN: usize = 11;
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/// Contains the context needed to calculate the adjusted difficulty for a block.
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#[allow(dead_code)]
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pub(super) struct AdjustedDifficulty {
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/// The `header.time` field from the candidate block
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candidate_time: DateTime<Utc>,
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/// The coinbase height from the candidate block
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///
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/// If we only have the header, this field is calculated from the previous
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/// block height.
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candidate_height: block::Height,
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/// The configured network
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network: Network,
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/// The `header.difficulty_threshold`s from the previous
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/// `PoWAveragingWindow + PoWMedianBlockSpan` (28) blocks, in reverse height
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/// order.
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relevant_difficulty_thresholds: [CompactDifficulty; 28],
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/// The `header.time`s from the previous
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/// `PoWAveragingWindow + PoWMedianBlockSpan` (28) blocks, in reverse height
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/// order.
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///
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/// Only the first and last `PoWMedianBlockSpan` times are used. Times
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/// `11..=16` are ignored.
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relevant_times: [DateTime<Utc>; 28],
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}
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impl AdjustedDifficulty {
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/// Initialise and return a new `AdjustedDifficulty` using a `candidate_block`,
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/// `network`, and a `context`.
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///
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/// The `context` contains the previous
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/// `PoWAveragingWindow + PoWMedianBlockSpan` (28) `difficulty_threshold`s and
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/// `time`s from the relevant chain for `candidate_block`, in reverse height
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/// order, starting with the previous block.
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///
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/// Note that the `time`s might not be in reverse chronological order, because
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/// block times are supplied by miners.
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///
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/// Panics:
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/// If the `context` contains fewer than 28 items.
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pub fn new_from_block<C>(
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candidate_block: &Block,
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network: Network,
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context: C,
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) -> AdjustedDifficulty
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where
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C: IntoIterator<Item = (CompactDifficulty, DateTime<Utc>)>,
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{
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let candidate_block_height = candidate_block
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.coinbase_height()
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.expect("semantically valid blocks have a coinbase height");
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let previous_block_height = (candidate_block_height - 1)
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.expect("contextual validation is never run on the genesis block");
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AdjustedDifficulty::new_from_header(
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&candidate_block.header,
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previous_block_height,
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network,
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context,
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)
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}
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/// Initialise and return a new `AdjustedDifficulty` using a
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/// `candidate_header`, `previous_block_height`, `network`, and a `context`.
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///
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/// Designed for use when validating block headers, where the full block has not
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/// been downloaded yet.
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///
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/// See `new_from_block` for detailed information about the `context`.
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///
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/// Panics:
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/// If the context contains fewer than 28 items.
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pub fn new_from_header<C>(
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candidate_header: &block::Header,
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previous_block_height: block::Height,
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network: Network,
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context: C,
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) -> AdjustedDifficulty
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where
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C: IntoIterator<Item = (CompactDifficulty, DateTime<Utc>)>,
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{
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let candidate_height = (previous_block_height + 1).expect("next block height is valid");
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// unzip would be a lot nicer here, but we can't satisfy its trait bounds
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let context: Vec<_> = context
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.into_iter()
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.take(POW_AVERAGING_WINDOW + POW_MEDIAN_BLOCK_SPAN)
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.collect();
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let relevant_difficulty_thresholds = context
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.iter()
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.map(|pair| pair.0)
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.collect::<Vec<_>>()
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.try_into()
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.expect("not enough context: difficulty adjustment needs at least 28 (PoWAveragingWindow + PoWMedianBlockSpan) headers");
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let relevant_times = context
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.iter()
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.map(|pair| pair.1)
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.collect::<Vec<_>>()
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.try_into()
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.expect("not enough context: difficulty adjustment needs at least 28 (PoWAveragingWindow + PoWMedianBlockSpan) headers");
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AdjustedDifficulty {
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candidate_time: candidate_header.time,
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candidate_height,
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network,
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relevant_difficulty_thresholds,
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relevant_times,
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}
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}
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}
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