feat(pow): add difficulty adjustment calculation
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ecfe556547
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@ -15,6 +15,7 @@ use io::{BufRead, Write};
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use mutagen::mutate;
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use units::parse;
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use crate::block::Header;
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use crate::blockdata::block::BlockHash;
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use crate::consensus::encode::{self, Decodable, Encodable};
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use crate::consensus::Params;
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@ -366,6 +367,82 @@ impl CompactTarget {
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Ok(Self::from_consensus(lock_time))
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}
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/// Computes the [`CompactTarget`] from a difficulty adjustment.
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///
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/// ref: <https://github.com/bitcoin/bitcoin/blob/0503cbea9aab47ec0a87d34611e5453158727169/src/pow.cpp>
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///
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/// Given the previous Target, represented as a [`CompactTarget`], the difficulty is adjusted
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/// by taking the timespan between them, and multipling the current [`CompactTarget`] by a factor
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/// of the net timespan and expected timespan. The [`CompactTarget`] may not adjust by more than
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/// a factor of 4, or adjust beyond the maximum threshold for the network.
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///
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/// # Note
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///
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/// Under the consensus rules, the difference in the number of blocks between the headers does
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/// not equate to the `difficulty_adjustment_interval` of [`Params`]. This is due to an off-by-one
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/// error, and, the expected number of blocks in between headers is `difficulty_adjustment_interval - 1`
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/// when calculating the difficulty adjustment.
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///
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/// Take the example of the first difficulty adjustment. Block 2016 introduces a new [`CompactTarget`],
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/// which takes the net timespan between Block 2015 and Block 0, and recomputes the difficulty.
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///
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/// # Returns
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///
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/// The expected [`CompactTarget`] recalculation.
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pub fn from_next_work_required(
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last: CompactTarget,
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timespan: u64,
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params: impl AsRef<Params>,
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) -> CompactTarget {
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let params = params.as_ref();
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if params.no_pow_retargeting {
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return last;
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}
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// Comments relate to the `pow.cpp` file from Core.
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// ref: <https://github.com/bitcoin/bitcoin/blob/0503cbea9aab47ec0a87d34611e5453158727169/src/pow.cpp>
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let min_timespan = params.pow_target_timespan >> 2; // Lines 56/57
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let max_timespan = params.pow_target_timespan << 2; // Lines 58/59
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let actual_timespan = timespan.clamp(min_timespan, max_timespan);
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let prev_target: Target = last.into();
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let maximum_retarget = prev_target.max_transition_threshold(params); // bnPowLimit
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let retarget = prev_target.0; // bnNew
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let retarget = retarget.mul(actual_timespan.into());
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let retarget = retarget.div(params.pow_target_timespan.into());
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let retarget = Target(retarget);
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if retarget.ge(&maximum_retarget) {
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return maximum_retarget.to_compact_lossy();
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}
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retarget.to_compact_lossy()
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}
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/// Computes the [`CompactTarget`] from a difficulty adjustment,
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/// assuming these are the relevant block headers.
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///
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/// Given two headers, representing the start and end of a difficulty adjustment epoch,
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/// compute the [`CompactTarget`] based on the net time between them and the current
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/// [`CompactTarget`].
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///
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/// # Note
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///
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/// See [`CompactTarget::from_next_work_required`]
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///
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/// For example, to successfully compute the first difficulty adjustment on the Bitcoin network,
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/// one would pass the header for Block 2015 as `current` and the header for Block 0 as
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/// `last_epoch_boundary`.
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///
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/// # Returns
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///
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/// The expected [`CompactTarget`] recalculation.
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pub fn from_header_difficulty_adjustment(
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last_epoch_boundary: Header,
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current: Header,
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params: impl AsRef<Params>,
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) -> CompactTarget {
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let timespan = current.time - last_epoch_boundary.time;
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let bits = current.bits;
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CompactTarget::from_next_work_required(bits, timespan.into(), params)
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}
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/// Creates a [`CompactTarget`] from a consensus encoded `u32`.
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pub fn from_consensus(bits: u32) -> Self { Self(bits) }
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@ -1689,6 +1766,113 @@ mod tests {
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assert_eq!(format!("{:08X}", CompactTarget(0x01d0f456)), "01D0F456");
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}
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#[test]
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fn compact_target_from_upwards_difficulty_adjustment() {
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let params = Params::new(crate::Network::Signet);
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let starting_bits = CompactTarget::from_consensus(503543726); // Genesis compact target on Signet
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let start_time: u64 = 1598918400; // Genesis block unix time
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let end_time: u64 = 1599332177; // Block 2015 unix time
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let timespan = end_time - start_time; // Faster than expected
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let adjustment = CompactTarget::from_next_work_required(starting_bits, timespan, ¶ms);
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let adjustment_bits = CompactTarget::from_consensus(503394215); // Block 2016 compact target
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assert_eq!(adjustment, adjustment_bits);
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}
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#[test]
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fn compact_target_from_downwards_difficulty_adjustment() {
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let params = Params::new(crate::Network::Signet);
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let starting_bits = CompactTarget::from_consensus(503394215); // Block 2016 compact target
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let start_time: u64 = 1599332844; // Block 2016 unix time
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let end_time: u64 = 1600591200; // Block 4031 unix time
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let timespan = end_time - start_time; // Slower than expected
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let adjustment = CompactTarget::from_next_work_required(starting_bits, timespan, ¶ms);
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let adjustment_bits = CompactTarget::from_consensus(503397348); // Block 4032 compact target
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assert_eq!(adjustment, adjustment_bits);
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}
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#[test]
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fn compact_target_from_upwards_difficulty_adjustment_using_headers() {
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use crate::{block::Version, constants::genesis_block, TxMerkleNode};
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use hashes::Hash;
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let params = Params::new(crate::Network::Signet);
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let epoch_start = genesis_block(¶ms).header;
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// Block 2015, the only information used are `bits` and `time`
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let current = Header {
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version: Version::ONE,
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prev_blockhash: BlockHash::all_zeros(),
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merkle_root: TxMerkleNode::all_zeros(),
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time: 1599332177,
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bits: epoch_start.bits,
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nonce: epoch_start.nonce
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};
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let adjustment = CompactTarget::from_header_difficulty_adjustment(epoch_start, current, params);
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let adjustment_bits = CompactTarget::from_consensus(503394215); // Block 2016 compact target
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assert_eq!(adjustment, adjustment_bits);
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}
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#[test]
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fn compact_target_from_downwards_difficulty_adjustment_using_headers() {
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use crate::{block::Version, TxMerkleNode};
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use hashes::Hash;
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let params = Params::new(crate::Network::Signet);
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let starting_bits = CompactTarget::from_consensus(503394215); // Block 2016 compact target
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// Block 2016, the only information used is `time`
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let epoch_start = Header {
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version: Version::ONE,
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prev_blockhash: BlockHash::all_zeros(),
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merkle_root: TxMerkleNode::all_zeros(),
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time: 1599332844,
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bits: starting_bits,
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nonce: 0
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};
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// Block 4031, the only information used are `bits` and `time`
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let current = Header {
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version: Version::ONE,
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prev_blockhash: BlockHash::all_zeros(),
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merkle_root: TxMerkleNode::all_zeros(),
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time: 1600591200,
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bits: starting_bits,
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nonce: 0
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};
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let adjustment = CompactTarget::from_header_difficulty_adjustment(epoch_start, current, params);
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let adjustment_bits = CompactTarget::from_consensus(503397348); // Block 4032 compact target
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assert_eq!(adjustment, adjustment_bits);
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}
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#[test]
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fn compact_target_from_maximum_upward_difficulty_adjustment() {
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let params = Params::new(crate::Network::Signet);
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let starting_bits = CompactTarget::from_consensus(503403001);
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let timespan = (0.2 * params.pow_target_timespan as f64) as u64;
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let got = CompactTarget::from_next_work_required(starting_bits, timespan, params);
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let want = Target::from_compact(starting_bits)
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.min_transition_threshold()
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.to_compact_lossy();
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assert_eq!(got, want);
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}
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#[test]
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fn compact_target_from_minimum_downward_difficulty_adjustment() {
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let params = Params::new(crate::Network::Signet);
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let starting_bits = CompactTarget::from_consensus(403403001); // High difficulty for Signet
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let timespan = 5 * params.pow_target_timespan; // Really slow.
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let got = CompactTarget::from_next_work_required(starting_bits, timespan, ¶ms);
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let want = Target::from_compact(starting_bits)
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.max_transition_threshold(params)
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.to_compact_lossy();
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assert_eq!(got, want);
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}
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#[test]
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fn compact_target_from_adjustment_is_max_target() {
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let params = Params::new(crate::Network::Signet);
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let starting_bits = CompactTarget::from_consensus(503543726); // Genesis compact target on Signet
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let timespan = 5 * params.pow_target_timespan; // Really slow.
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let got = CompactTarget::from_next_work_required(starting_bits, timespan, ¶ms);
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let want = params.max_attainable_target.to_compact_lossy();
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assert_eq!(got, want);
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}
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#[test]
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fn target_from_compact() {
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// (nBits, target)
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