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feat(chain): refactor merge_chains
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`merge_chains` now returns a tuple of the resultant checkpoint AND
changeset. This is arguably a more readable/understandable setup.

To do this, we had to create `CheckPoint::apply_changeset` which is kept
as a private method.

Thank you @ValuedMammal for the suggestion.

Co-authored-by: valuedvalued mammal <[email protected]>
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@evanlinjin @ValuedMammal
evanlinjin and ValuedMammal committed Apr 22, 2024
1 parent 2f22987 commit 96a9aa6
Showing 1 changed file with 74 additions and 61 deletions.
135 changes: 74 additions & 61 deletions crates/chain/src/local_chain.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -213,6 +213,46 @@ impl CheckPoint {
base.extend(core::iter::once(block_id).chain(tail.into_iter().rev()))
.expect("tail is in order")
}

/// Apply `changeset` to the checkpoint.
fn apply_changeset(mut self, changeset: &ChangeSet) -> Result<CheckPoint, MissingGenesisError> {
if let Some(start_height) = changeset.keys().next().cloned() {
// changes after point of agreement
let mut extension = BTreeMap::default();
// point of agreement
let mut base: Option<CheckPoint> = None;

for cp in self.iter() {
if cp.height() >= start_height {
extension.insert(cp.height(), cp.hash());
} else {
base = Some(cp);
break;
}
}

for (&height, &hash) in changeset {
match hash {
Some(hash) => {
extension.insert(height, hash);
}
None => {
extension.remove(&height);
}
};
}

let new_tip = match base {
Some(base) => base
.extend(extension.into_iter().map(BlockId::from))
.expect("extension is strictly greater than base"),
None => LocalChain::from_blocks(extension)?.tip(),
};
self = new_tip;
}

Ok(self)
}
}

/// Iterates over checkpoints backwards.
Expand Down Expand Up @@ -348,32 +388,22 @@ impl LocalChain {

/// Applies the given `update` to the chain.
///
/// The method returns [`ChangeSet`] on success. This represents the applied changes to `self`.
/// The method returns [`ChangeSet`] on success. This represents the changes applied to `self`.
///
/// There must be no ambiguity about which of the existing chain's blocks are still valid and
/// which are now invalid. That is, the new chain must implicitly connect to a definite block in
/// the existing chain and invalidate the block after it (if it exists) by including a block at
/// the same height but with a different hash to explicitly exclude it as a connection point.
///
/// Additionally, a chain with a single block can have it's block invalidated by an update
/// chain with a block at the same height but different hash.
///
/// # Errors
///
/// An error will occur if the update does not correctly connect with `self`.
///
/// [module-level documentation]: crate::local_chain
pub fn apply_update(&mut self, update: CheckPoint) -> Result<ChangeSet, CannotConnectError> {
let (changeset, can_replace) = merge_chains(self.tip.clone(), update.clone())?;
if can_replace {
self.tip = update;
} else {
// `._check_changeset_is_applied` is called in `.apply_changeset`
self.apply_changeset(&changeset)
.map_err(|_| CannotConnectError {
try_include_height: 0,
})?;
}
let (new_tip, changeset) = merge_chains(self.tip.clone(), update)?;
self.tip = new_tip;
self._check_changeset_is_applied(&changeset);
Ok(changeset)
}

Expand Down Expand Up @@ -465,43 +495,10 @@ impl LocalChain {

/// Apply the given `changeset`.
pub fn apply_changeset(&mut self, changeset: &ChangeSet) -> Result<(), MissingGenesisError> {
if let Some(start_height) = changeset.keys().next().cloned() {
// changes after point of agreement
let mut extension = BTreeMap::default();
// point of agreement
let mut base: Option<CheckPoint> = None;

for cp in self.iter_checkpoints() {
if cp.height() >= start_height {
extension.insert(cp.height(), cp.hash());
} else {
base = Some(cp);
break;
}
}

for (&height, &hash) in changeset {
match hash {
Some(hash) => {
extension.insert(height, hash);
}
None => {
extension.remove(&height);
}
};
}

let new_tip = match base {
Some(base) => base
.extend(extension.into_iter().map(BlockId::from))
.expect("extension is strictly greater than base"),
None => LocalChain::from_blocks(extension)?.tip(),
};
self.tip = new_tip;

debug_assert!(self._check_changeset_is_applied(changeset));
}

let old_tip = self.tip.clone();
let new_tip = old_tip.apply_changeset(changeset)?;
self.tip = new_tip;
debug_assert!(self._check_changeset_is_applied(changeset));
Ok(())
}

Expand Down Expand Up @@ -731,10 +728,10 @@ impl std::error::Error for ApplyHeaderError {}
fn merge_chains(
original_tip: CheckPoint,
update_tip: CheckPoint,
) -> Result<(ChangeSet, bool), CannotConnectError> {
) -> Result<(CheckPoint, ChangeSet), CannotConnectError> {
let mut changeset = ChangeSet::default();
let mut orig = original_tip.into_iter();
let mut update = update_tip.into_iter();
let mut orig = original_tip.iter();
let mut update = update_tip.iter();
let mut curr_orig = None;
let mut curr_update = None;
let mut prev_orig: Option<CheckPoint> = None;
Expand All @@ -743,10 +740,11 @@ fn merge_chains(
let mut prev_orig_was_invalidated = false;
let mut potentially_invalidated_heights = vec![];

// Flag to set if heights are removed from original chain. If no heights are removed, and we
// have a matching node pointer between the two chains, we can conclude that the update tip can
// just replace the original tip.
let mut has_removed_heights = false;
// If we can, we want to return the update tip as the new tip because this allows checkpoints
// in multiple locations to keep the same `Arc` pointers when they are being updated from each
// other using this function. We can do this as long as long as the update contains every
// block's height of the original chain.
let mut is_update_height_superset_of_original = true;

// To find the difference between the new chain and the original we iterate over both of them
// from the tip backwards in tandem. We always dealing with the highest one from either chain
Expand All @@ -773,7 +771,7 @@ fn merge_chains(
prev_orig_was_invalidated = false;
prev_orig = curr_orig.take();

has_removed_heights = true;
is_update_height_superset_of_original = false;

// OPTIMIZATION: we have run out of update blocks so we don't need to continue
// iterating because there's no possibility of adding anything to changeset.
Expand All @@ -800,7 +798,17 @@ fn merge_chains(
// OPTIMIZATION 2 -- if we have the same underlying pointer at this point, we
// can guarantee that no older blocks are introduced.
if Arc::as_ptr(&o.0) == Arc::as_ptr(&u.0) {
return Ok((changeset, !has_removed_heights));
if is_update_height_superset_of_original {
return Ok((update_tip, changeset));
} else {
let new_tip =
original_tip.apply_changeset(&changeset).map_err(|_| {
CannotConnectError {
try_include_height: 0,
}
})?;
return Ok((new_tip, changeset));
}
}
} else {
// We have an invalidation height so we set the height to the updated hash and
Expand Down Expand Up @@ -834,5 +842,10 @@ fn merge_chains(
}
}

Ok((changeset, false))
let new_tip = original_tip
.apply_changeset(&changeset)
.map_err(|_| CannotConnectError {
try_include_height: 0,
})?;
Ok((new_tip, changeset))
}

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