 Feature Name: contextual_difficulty_validation
 Start Date: 20201102
 Design PR: ZcashFoundation/zebra#0000
 Zebra Issue: ZcashFoundation/zebra#1036
Summary
Zcash nodes use a Proof of Work algorithm to reach consensus on the best chain. Valid blocks must reach a difficulty threshold, which is adjusted after every block. The difficulty adjustment calculations depend on the difficulties and times of recent blocks. So Zebra performs contextual validation RFC2 of difficulty adjustments as part of committing blocks to the state.
Motivation
The Zcash block difficulty adjustment is one of the core Zcash consensus rules. Zebra must implement this consensus rule to make sure that its cached chain state is consistent with the consensus of Zcash nodes.
Difficulty adjustment is also a significant part of Zcash's security guarantees. It ensures that the network continues to resist takeover attacks, even as the number of Zcash miners grows.
Difficulty adjustment also ensures that blocks are regularly spaced, which allows users to create and finalise transactions with short, consistent delays. These predictable delays contribute to Zcash's usability.
Definitions
Difficulty:

hash difficulty: An arbitrary ranking of blocks, based on their hashes. Defined as the hash of the block, interpreted as a bigendian 256bit number. Numerically smaller difficulties are harder to generate.

difficulty threshold: The easiest valid hash difficulty for a block. Numerically lower thresholds are harder to satisfy.

difficulty filter: A block passes the difficulty filter if the hash difficulty is less than or equal to the difficulty threshold (based on the block's difficulty field).

block work: The approximate amount of work required for a miner to generate a block hash that passes the difficulty filter. The number of block header attempts and the mining time are proportional to the work value. Numerically higher work values represent longer processing times.

averaging window: The 17 most recent blocks in the relevant chain.

median block span: The 11 most recent blocks from a chosen tip, typically the relevant tip.

target spacing: 150 seconds per block before Blossom activation, 75 seconds per block from Blossom activation onwards.

adjusted difficulty: After each block is mined, the difficulty threshold of the next block is adjusted, to keep the block gap close to the target spacing.

mean target difficulty: The arithmetic mean of the difficulty thresholds of the blocks in the averaging window.

median timespan: The average number of seconds taken to generate the blocks in the averaging window. Calculated using the difference of median block spans in and after the averaging window, then damped and bounded.

target timespan: The target spacing for an averaging window's worth of blocks.
Consensus:

consensus rule: A protocol rule which all nodes must apply consistently, so they can converge on the same chain fork.

structural/semantic/contextual verification: as defined in RFC2.
State:

block chain: A sequence of valid blocks linked by inclusion of the previous block hash in the subsequent block. Chains are rooted at the genesis block and extend to a tip.

relevant chain: The relevant chain for a block starts at the previous block, and extends back to genesis.

relevant tip: The tip of the relevant chain.

nonfinalized state: State data corresponding to blocks above the reorg limit. This data can change in the event of a chain reorg.

finalized state: State data corresponding to blocks below the reorg limit. This data cannot change in the event of a chain reorg.

nonfinalized tips: The highest blocks in each nonfinalized chain. These tips might be at different heights.

finalized tip: The highest block in the finalized state. The tip of the best chain is usually 100 blocks (the reorg limit) above the finalized tip. But it can be lower during the initial sync, and after a chain reorganization, if the new best chain is at a lower height.
Guidelevel explanation
Zcash's difficulty consensus rules are similar to Bitcoin.
Each block contains a difficulty threshold in its header. The hash of the
block header must be less than this difficulty threshold. (When interpreted
as a 256bit integer in bigendian byte order.) This contextfree semantic
verification check is performed by the BlockVerifier
.
After each block, the difficulty threshold is adjusted so that the block gap is close to the target spacing. On average, harder blocks take longer to mine, and easier blocks take less time.
The adjusted difficulty for the next block is calculated using the difficulty thresholds and times of recent blocks. Zcash uses the most recent 28 blocks in the relevant chain in its difficulty adjustment calculations.
The difficulty adjustment calculations adjust the mean target difficulty, based on the difference between the median timespan and the target timespan. If the median timespan is less than the target timespan, the next block is harder to mine.
The StateService
calculates the adjusted difficulty using the context from the
relevant chain. The difficulty contextual verification check ensures that the
difficulty threshold of the next block is equal to the adjusted difficulty
for its relevant chain.
State service interface changes
Contextual validation accesses recent blocks. So we modify the internal state service interface to provide an abstraction for accessing recent blocks.
The relevant chain
The relevant chain consists of the ancestors of a block, starting with its parent block, and extending back to the genesis block.
In Zebra, recent blocks are part of the nonfinalized state, which can contain multiple chains. Past the reorganization limit, Zebra commits a single chain to the finalized state.
The relevant chain can start at any block in the nonfinalized state, or at the finalized tip. See RFC5 for details.
Contextual validation design
Contextual validation is performed synchronously by the state service, as soon as the state has:
 received the semantically valid next block (via
CommitBlock
), and  committed the previous block.
The difficulty adjustment check calculates the correct adjusted difficulty
threshold value for a candidate block, and ensures that the block's
difficulty_threshold
field is equal to that value.
This check is implemented as follows:
Difficulty adjustment
The block difficulty threshold is adjusted by scaling the mean target difficulty by the median timespan.
On Testnet, if a long time has elapsed since the previous block, the difficulty adjustment is modified to allow minimumdifficulty blocks.
Mean target difficulty
The mean target difficulty is the arithmetic mean of the difficulty
thresholds of the PoWAveragingWindow
(17) most recent blocks in the relevant
chain.
Zcash uses block difficulty thresholds in its difficulty adjustment calculations. (Block hashes are not used for difficulty adjustment.)
Median timespan
The average number of seconds taken to generate the 17 blocks in the averaging window.
The median timespan is calculated by taking the difference of the median times for:
 the relevant tip: the
PoWMedianBlockSpan
(11) most recent blocks, and  the 11 blocks after the 17block
PoWAveragingWindow
: that is, blocks 1828 behind the relevant tip.
The median timespan is damped by the PoWDampingFactor
, and bounded by
PoWMaxAdjustDown
and PoWMaxAdjustUp
.
Test network minimum difficulty blocks
If there is a large gap after a Testnet block, the next block becomes a minimum
difficulty block. Testnet minimum difficulty blocks have their
difficulty_threshold
set to the minimum difficulty for Testnet.
Block difficulty threshold
The block difficulty threshold for the next block is calculated by scaling the mean target difficulty by the ratio between the median timespan and the averaging window timespan.
The result of this calculation is limited by ToCompact(PoWLimit(network))
, a
pernetwork minimum block difficulty. This minimum difficulty is also used when
a Testnet block's time gap exceeds the minimum difficulty gap.
Referencelevel explanation
Contextual validation
Contextual validation is implemented in
StateService::check_contextual_validity
, which calls a separate function for
each contextual validity check.
In Zebra, contextual validation starts after Canopy activation, so we can assume that the relevant chain contains at least 28 blocks on Mainnet and Testnet. (And panic if this assumption does not hold at runtime.)
Fundamental data types
Zebra is free to implement its difficulty calculations in any way that produces
equivalent results to zcashd
and the Zcash specification.
Difficulty
In Zcash block headers, difficulty thresholds are stored as a "compact" nBits
value, which uses a custom 32bit floatingpoint encoding. Zebra calls this type
CompactDifficulty
.
In Zcash, difficulty threshold calculations are performed using unsigned 256bit
integers. Rust has no standard u256
type, but there are a number of crates
available which implement the required operations on 256bit integers. Zebra
abstracts over the chosen u256
implementation using its ExpandedDifficulty
type.
Time
In Zcash, time values are unsigned 32bit integers. But the difficulty adjustment
calculations include time subtractions which could overflow an unsigned type, so
they are performed using signed 64bit integers in zcashd
.
Zebra parses the header.time
field into a DateTime<Utc>
. Conveniently, the
chrono::DateTime<_>::timestamp()
function returns i64
values. So Zebra can do
its signed time calculations using i64
values internally.
Note: i32
is an unsuitable type for signed time calculations. It is
theoretically possible for the time gap between blocks to be larger than
i32::MAX
, because those times are provided by miners. Even if the median time
gap is that large, the bounds and minimum difficulty in Zcash's difficulty
adjustment algorithm will preserve a reasonable difficulty threshold. So Zebra
must support this edge case.
ConsensusCritical Operations
The order of operations and overflow semantics for 256bit integers can be consensuscritical.
For example:
 dividing before multiplying discards lowerorder bits, but
 multiplying before dividing can cause overflow.
Zebra's implementation should try to match zcashd's order of operations and overflow handling as closely as possible.
Difficulty adjustment check
The difficulty adjustment check calculates the correct difficulty threshold
value for a candidate block, and ensures that the block's
difficulty_threshold
field is equal to that value.
Context data type
The difficulty adjustment functions use a context consisting of the difficulties and times from the previous 28 blocks in the relevant chain.
These functions also use the candidate block's height
and network
.
To make these functions more ergonomic, we create a AdjustedDifficulty
type, and implement the difficulty adjustment calculations as methods on that
type.
#![allow(unused)] fn main() { /// The averaging window for difficulty threshold arithmetic mean calculations. /// /// `PoWAveragingWindow` in the Zcash specification. pub const POW_AVERAGING_WINDOW: usize = 17; /// The median block span for time median calculations. /// /// `PoWMedianBlockSpan` in the Zcash specification. pub const POW_MEDIAN_BLOCK_SPAN: usize = 11; /// Contains the context needed to calculate the adjusted difficulty for a block. struct AdjustedDifficulty { candidate_time: DateTime<Utc>, candidate_height: block::Height, network: Network, relevant_difficulty_thresholds: [CompactDifficulty; POW_AVERAGING_WINDOW + POW_MEDIAN_BLOCK_SPAN], relevant_times: [DateTime<Utc>; POW_AVERAGING_WINDOW + POW_MEDIAN_BLOCK_SPAN], } }
We implement some initialiser methods on AdjustedDifficulty
for convenience.
We might want to validate downloaded headers in future, so we include a
new_from_header
initialiser.
#![allow(unused)] fn main() { /// 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>)>, { ... } /// Initialise and return a new `AdjustedDifficulty` using a /// `candidate_header`, `previous_block_height`, `network`, and a `context`. /// /// Designed for use when validating block headers, where the full block has not /// been downloaded yet. /// /// See `new_from_block` for detailed information about the `context`. /// /// Panics: /// If the context contains fewer than 28 items. pub fn new_from_header<C>(candidate_header: &block::Header, previous_block_height: block::Height, network: Network, context: C) > AdjustedDifficulty where C: IntoIterator<Item = (CompactDifficulty, DateTime<Utc>)>, { ... } }
Memory usage note
Copying CompactDifficulty
values into the AdjustedDifficulty
struct uses
less memory than borrowing those values. CompactDifficulty
values are 32 bits,
but pointers are 64bit on most modern machines. (And since they all come from
different blocks, we need a pointer to each individual value.)
Borrowing DateTime<Utc>
values might use slightly less memory than copying
them  but that depends on the exact way that Rust stores associated types
derived from a generic argument.
In any case, the overall size of each AdjustedDifficulty
is only a few
hundred bytes. If it turns up in profiles, we can look at borrowing the block
header data.
Difficulty adjustment check implementation
The difficulty adjustment check ensures that the
candidate_difficulty_threshold
is equal to the difficulty_threshold
value
calculated using AdjustedDifficulty::adjusted_difficulty_threshold
.
We implement this function:
#![allow(unused)] fn main() { /// Validate the `difficulty_threshold` from a candidate block's header, based /// on an `expected_difficulty` for that block. /// /// Uses `expected_difficulty` to calculate the expected `ToCompact(Threshold())` /// value, then compares that value to the `difficulty_threshold`. Returns /// `Ok(())` if the values are equal. pub fn difficulty_threshold_is_valid(difficulty_threshold: CompactDifficulty, expected_difficulty: AdjustedDifficulty) > Result<(), ValidateContextError> { ... } }
Mean target difficulty calculation
The mean target difficulty is the arithmetic mean of the difficulty
thresholds of the PoWAveragingWindow
(17) most recent blocks in the relevant
chain.
We implement this method on AdjustedDifficulty
:
#![allow(unused)] fn main() { /// 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 { ... } }
Implementation notes
Since the PoWLimit
s are 2^251 − 1
for Testnet, and 2^243 − 1
for Mainnet,
the sum of these difficulty thresholds will be less than or equal to
(2^251 − 1)*17 = 2^255 + 2^251  17
. Therefore, this calculation can not
overflow a u256
value. So the function is infalliable.
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()
in the Zcash specification is unreachable.
Median timespan calculation
The median timespan is the difference of the median times for:
 the relevant tip: the
PoWMedianBlockSpan
(11) most recent blocks, and  the 11 blocks after the 17block
PoWAveragingWindow
: that is, blocks 1828 behind the relevant tip.
(The median timespan is known as the ActualTimespan
in the Zcash specification,
but this terminology is confusing, because it is a difference of medians, rather
than any "actual" elapsed time.)
Zebra implements the median timespan using the following methods on
AdjustedDifficulty
:
#![allow(unused)] fn main() { /// 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 { ... } /// 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 `median_timespan_bounded` for details. fn median_timespan(&self) > Duration { ... } /// Calculate the median of the `median_block_span_times`: the `time`s from a /// slice of `PoWMedianBlockSpan` (11) blocks in the relevant chain. /// /// Implements `MedianTime` from the Zcash specification. fn median_time(mut median_block_span_times: [DateTime<Utc>; POW_MEDIAN_BLOCK_SPAN]) > DateTime<Utc> { ... } }
Zebra implements the AveragingWindowTimespan
using the following methods on
NetworkUpgrade
:
#![allow(unused)] fn main() { impl NetworkUpgrade { /// Returns the `AveragingWindowTimespan` for the network upgrade. pub fn averaging_window_timespan(&self) > Duration { ... } /// Returns the `AveragingWindowTimespan` for `network` and `height`. pub fn averaging_window_timespan_for_height(network: Network, height: block::Height) > Duration { ... } } }
Implementation notes
In Zebra, contextual validation starts after Canopy activation, so we can assume that the relevant chain contains at least 28 blocks. Therefore:
max(0, height − PoWMedianBlockSpan)
in theMedianTime()
calculation simplifies toheight − PoWMedianBlockSpan
, and there is always an odd number of blocks in
MedianTime()
, so the median is always the exact middle of the sequence.
Therefore, the function is infalliable.
Test network minimum difficulty calculation
A block is a Testnet minimum difficulty block if:
 the block is a Testnet block,
 the block's height is 299188 or greater, and
 the time gap from the previous block is greater than the Testnet minimum difficulty gap, which is 6 times the target spacing for the block's height. (The target spacing was halved from the Blossom network upgrade onwards.)
The difficulty adjustment is modified for Testnet minimum difficulty blocks as follows:
 the difficulty threshold in the block header is set to the Testnet minimum
difficulty threshold,
ToCompact(PoWLimit(network))
.
Since the new difficulty changes the block header, Testnet blocks can only satisfy one of the alternate difficulty adjustment rules:
 if the time gap is less than or equal to the Testnet minimum difficulty gap: the difficulty threshold is calculated using the default difficulty adjustment rule,
 if the time gap is greater than the Testnet minimum difficulty gap: the difficulty threshold is the Testnet minimum difficulty threshold.
See ZIP208 for details.
Note: some older versions of ZIPs 205 and 208 incorrectly said that:
 the time gap threshold uses an "at least" check (it is strictly greater than),
 the minimum difficulty threshold value was
PoWLimit
(it isToCompact(PoWLimit)
),  the
difficulty_threshold
(nBits
) field is not modified in Testnet minimum difficulty blocks (the field is modified), and  the Testnet minimum difficulty value is not used to calculate future difficulty adjustments (the modified value is used in future adjustments).
ZIP 205 and 208 were fixed on 14 November 2020, see ZIP PR 417 and ZIP commit 806076c for details.
Test network minimum difficulty implementation
The Testnet minimum difficulty calculation uses the existing
NetworkUpgrade::minimum_difficulty_spacing_for_height
function to calculate the
minimum difficulty gap.
We implement this method on NetworkUpgrade
:
#![allow(unused)] fn main() { /// Returns true if the gap between `block_time` and `previous_block_time` is /// greater than the Testnet minimum difficulty time gap. This time gap /// depends on the `network` and `block_height`. /// /// Returns false on Mainnet, when `block_height` is less than the minimum /// difficulty start height, and when the time gap is too small. /// /// `block_time` can be less than, equal to, or greater than /// `previous_block_time`, because block times are provided by miners. /// /// Implements the Testnet minimum difficulty adjustment from ZIPs 205 and 208. /// /// Spec Note: Some parts of ZIPs 205 and 208 previously specified an incorrect /// check for the time gap. This function implements the correct "greater than" /// check. pub fn is_testnet_min_difficulty_block( network: Network, block_height: block::Height, block_time: DateTime<Utc>, previous_block_time: DateTime<Utc>, ) > bool { ... } }
Implementation notes
In Zcash, the Testnet minimum difficulty rule starts at block 299188, and in Zebra, contextual validation starts after Canopy activation. So we can assume that there is always a previous block.
Therefore, this function is infalliable.
Block difficulty threshold calculation
The block difficulty threshold for the next block is calculated by scaling the mean target difficulty by the ratio between the median timespan and the averaging window timespan.
The result of the scaled threshold calculation is limited by
ToCompact(PoWLimit(network))
, a pernetwork minimum block difficulty. This
minimum difficulty is also used when a Testnet block's time gap exceeds the
minimum difficulty gap. We use the existing
ExpandedDifficulty::target_difficulty_limit
function to calculate the value of
ToCompact(PoWLimit(network))
.
In Zebra, contextual validation starts after Canopy activation, so the genesis
case of Threshold()
in the Zcash specification is unreachable.
Block difficulty threshold implementation
We implement these methods on AdjustedDifficulty
:
#![allow(unused)] fn main() { /// 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 { ... } /// Calculate the `difficulty_threshold` for a candidate block, based on the /// `candidate_height`, `network`, and the relevant `difficulty_threshold`s and /// `time`s. /// /// See `expected_difficulty_threshold` for details. /// /// Implements `ThresholdBits` from the Zcash specification. (Which excludes the /// Testnet minimum difficulty adjustment.) fn threshold_bits(&self) > CompactDifficulty { ... } }
Implementation notes
Since:
 the
PoWLimit
s are2^251 − 1
for Testnet, and2^243 − 1
for Mainnet,  the
ActualTimespanBounded
can be at mostMaxActualTimespan
, which isfloor(PoWAveragingWindow * PoWTargetSpacing * (1 + PoWMaxAdjustDown))
orfloor(17 * 150 * (1 + 32/100)) = 3366
, AveragingWindowTimespan
is at most17 * 150 = 2250
, andMeanTarget
is at mostPoWLimit
, ...
The maximum scaled value inside the Threshold()
calculation is:
floor(PoWLimit / 2250) * 3366
, which equalsfloor((2^251 − 1) / 2250) * 3366
, which equals(2^251 − 1) * 132/100
, which is less than
2^252
.
Therefore, this calculation can not overflow a u256
value. (And even if it did
overflow, it would be constrained to a valid value by the PoWLimit
minimum.)
Note that the multiplication by ActualTimespanBounded
must happen after the
division by AveragingWindowTimespan
. Performing the multiplication first
could overflow.
If implemented in this way, the function is infalliable.
zcashd
truncates the MeanTarget
after the mean calculation, and
after dividing by AveragingWindowTimespan
. But as long as there is no overflow,
this is equivalent to the single truncation of the final result in the Zcash
specification. However, Zebra should follow the order of operations in zcashd
,
and use repeated divisions, because that can't overflow. See the relevant
comment in the zcashd souce code.
Module Structure
The structs and functions in this RFC are implemented in a new
zebra_state::service::check::difficulty
module.
This module has two entry points:
DifficultyAdjustment::new_from_block
difficulty_threshold_is_valid
These entry points are both called from
StateService::check_contextual_validity
.
Test Plan
Explain how the feature will be tested, including:
 tests for consensuscritical functionality
 existing test vectors, if available
 Zcash blockchain block test vectors (specify the network upgrade, feature, or block height and network)
 property testing or fuzzing
The tests should cover:

positive cases: make sure the feature accepts valid inputs
 using block test vectors for each network upgrade provides some coverage of valid inputs

negative cases: make sure the feature rejects invalid inputs
 make sure there is a test case for each error condition in the code
 if there are lots of potential errors, prioritise:
 consensuscritical errors
 securitycritical errors, and
 likely errors
 edge cases: make sure that boundary conditions are correctly handled
Drawbacks
Why should we not do this?
Alternate consensus parameters
Any alternate consensus parameters or regtest
mode would have to respect the constraints set by this design.
In particular:
 the
PoWLimit
must be less than or equal to(2^256  1) / PoWAveragingWindow
(approximately2^251
) to avoid overflow,  the
PoWAveragingWindow
andPoWMedianBlockSpan
are fixed by function argument types (at least until Rust gets stable const generics), and  the design eliminates a significant number of edge cases by assuming that difficulty adjustments aren't
validated for the first
PoWAveragingWindow + PoWMedianBlockSpan
(28) blocks in the chain.
Rationale and alternatives
Is this design a good basis for later designs or implementations?
The design includes specific methods for a future headeronly validation design.
What other designs have been considered and what is the rationale for not choosing them?
A previous version of the RFC did not have the AdjustedDifficulty
struct and
methods. That design was easy to misuse, because each function had a complicated
argument list.
What is the impact of not doing this?
Zebra could accept invalid, lowdifficulty blocks from arbitrary miners. That would be a security issue.
Prior art
zcashd
 the Zcash specification
 Bitcoin
Unresolved questions

What parts of the design do you expect to resolve through the implementation of this feature before stabilization?
 Guidelevel examples
 Referencelevel examples
 Corner case examples
 Testing

What related issues do you consider out of scope for this RFC that could be addressed in the future independently of the solution that comes out of this RFC?
 Monitoring and maintainence
Future possibilities
Reusing the relevant chain API in other contextual checks
The relevant chain iterator can be reused to implement other contextual validation checks.
For example, responding to peer requests for block locators, which means
implementing relevant chain hash queries as a StateService
request
Headeronly difficulty adjustment validation
Implementing headeronly difficulty adjustment validation as a StateService
request.
Caching difficulty calculations
Difficulty calculations use u256
could be a bit expensive, particularly if we
get a flood of lowdifficulty blocks. To reduce the impact of this kind of DoS,
we could cache the value returned by threshold_bits
for each block in the
nonfinalized state, and for the finalized tip. This value could be used to
quickly calculate the difficulties for any child blocks of these blocks.
There's no need to persist this cache, or prefill it. (Minimumdifficulty
Testnet blocks don't call threshold_bits
, and some sidechain blocks will
never have a next block.)
This caching is only worth implementing if these calculations show up in zebrad
profiles.