Struct bridgetree::MerkleBridge
source · pub struct MerkleBridge<H> { /* private fields */ }Expand description
The information required to “update” witnesses from one state of a Merkle tree to another.
The witness for a particular leaf of a Merkle tree consists of the siblings of that leaf, plus
the siblings of the parents of that leaf in a path to the root of the tree. When considering a
Merkle tree where leaves are appended to the tree in a linear fashion (rather than being
inserted at arbitrary positions), we often wish to produce a witness for a leaf that was
appended to the tree at some point in the past. A MerkleBridge from one position in the
tree to another position in the tree contains the minimal amount of information necessary to
produce a witness for the leaf at the former position, given that leaves have been subsequently
appended to reach the current position.
MerkleBridge values have a semigroup, such that the sum (fused) value of two successive
bridges, along with a NonEmptyFrontier with its tip at the prior position of the first bridge
being fused, can be used to produce a witness for the leaf at the tip of the prior frontier.
Implementations§
source§impl<H> MerkleBridge<H>
impl<H> MerkleBridge<H>
sourcepub fn new(value: H) -> Self
pub fn new(value: H) -> Self
Construct a new Merkle bridge containing only the specified leaf.
Examples found in repository?
src/lib.rs (line 892)
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pub fn append(&mut self, value: &H) -> bool {
if let Some(bridge) = self.current_bridge.as_mut() {
if bridge
.frontier
.position()
.is_complete_subtree(Level::from(DEPTH))
{
false
} else {
bridge.append(value.clone());
true
}
} else {
self.current_bridge = Some(MerkleBridge::new(value.clone()));
true
}
}sourcepub fn from_parts(
prior_position: Option<Position>,
tracking: BTreeSet<Address>,
ommers: BTreeMap<Address, H>,
frontier: NonEmptyFrontier<H>
) -> Self
pub fn from_parts(
prior_position: Option<Position>,
tracking: BTreeSet<Address>,
ommers: BTreeMap<Address, H>,
frontier: NonEmptyFrontier<H>
) -> Self
Construct a new Merkle bridge from its constituent parts.
Examples found in repository?
src/lib.rs (lines 787-792)
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pub fn from_frontier(max_checkpoints: usize, frontier: NonEmptyFrontier<H>) -> Self {
Self {
prior_bridges: vec![],
current_bridge: Some(MerkleBridge::from_parts(
None,
BTreeSet::new(),
BTreeMap::new(),
frontier,
)),
saved: BTreeMap::new(),
checkpoints: vec![],
max_checkpoints,
}
}sourcepub fn prior_position(&self) -> Option<Position>
pub fn prior_position(&self) -> Option<Position>
Returns the position of the final leaf in the frontier of the bridge that this bridge is the successor of, or None if this is the first bridge in a tree.
sourcepub fn position(&self) -> Position
pub fn position(&self) -> Position
Returns the position of the most recently appended leaf.
Examples found in repository?
src/lib.rs (line 434)
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pub fn position_range(&self) -> Range<Position> {
Range {
start: self.prior_position.unwrap_or_else(|| Position::from(0)),
end: self.position() + 1,
}
}
}
impl<'a, H: Hashable + Ord + Clone + 'a> MerkleBridge<H> {
/// Constructs a new bridge to follow this one. If `mark_current_leaf` is true, the successor
/// will track the information necessary to create a witness for the leaf most
/// recently appended to this bridge's frontier.
#[must_use]
pub fn successor(&self, mark_current_leaf: bool) -> Self {
let mut result = Self {
prior_position: Some(self.frontier.position()),
tracking: self.tracking.clone(),
ommers: BTreeMap::new(),
frontier: self.frontier.clone(),
};
if mark_current_leaf {
result.track_current_leaf();
}
result
}
fn track_current_leaf(&mut self) {
self.tracking
.insert(Address::from(self.frontier.position()).current_incomplete());
}
/// Advances this bridge's frontier by appending the specified node,
/// and updates any auth path ommers being tracked if necessary.
pub fn append(&mut self, value: H) {
self.frontier.append(value);
let mut found = vec![];
for address in self.tracking.iter() {
// We know that there will only ever be one address that we're
// tracking at a given level, because as soon as we find a
// value for the sibling of the address we're tracking, we
// remove the tracked address and replace it the next parent
// of that address for which we need to find a sibling.
if self
.frontier()
.position()
.is_complete_subtree(address.level())
{
let digest = self.frontier.root(Some(address.level()));
self.ommers.insert(address.sibling(), digest);
found.push(*address);
}
}
for address in found {
self.tracking.remove(&address);
// The address of the next incomplete parent note for which
// we need to find a sibling.
let parent = address.next_incomplete_parent();
assert!(!self.ommers.contains_key(&parent));
self.tracking.insert(parent);
}
}
/// Returns a single MerkleBridge that contains the aggregate information
/// of this bridge and `next`, or None if `next` is not a valid successor
/// to this bridge. The resulting Bridge will have the same state as though
/// `self` had had every leaf used to construct `next` appended to it
/// directly.
fn fuse(&self, next: &Self) -> Result<Self, ContinuityError> {
self.check_continuity(next)?;
Ok(Self {
prior_position: self.prior_position,
tracking: next.tracking.clone(),
ommers: self
.ommers
.iter()
.chain(next.ommers.iter())
.map(|(k, v)| (*k, v.clone()))
.collect(),
frontier: next.frontier.clone(),
})
}
/// Returns a single MerkleBridge that contains the aggregate information
/// of all the provided bridges (discarding internal frontiers) or None
/// if the provided iterator is empty. Returns a continuity error if
/// any of the bridges are not valid successors to one another.
fn fuse_all<T: Iterator<Item = &'a Self>>(
mut iter: T,
) -> Result<Option<Self>, ContinuityError> {
let mut fused = iter.next().cloned();
for next in iter {
fused = Some(fused.unwrap().fuse(next)?);
}
Ok(fused)
}
/// If this bridge contains sufficient auth fragment information, construct an authentication
/// path for the specified position by interleaving with values from the prior frontier. This
/// method will panic if the position of the prior frontier does not match this bridge's prior
/// position.
fn witness(
&self,
depth: u8,
prior_frontier: &NonEmptyFrontier<H>,
) -> Result<Vec<H>, WitnessingError> {
assert!(Some(prior_frontier.position()) == self.prior_position);
prior_frontier.witness(depth, |addr| {
let r = addr.position_range();
if self.frontier.position() < r.start {
Some(H::empty_root(addr.level()))
} else if r.contains(&self.frontier.position()) {
Some(self.frontier.root(Some(addr.level())))
} else {
// the frontier's position is after the end of the requested
// range, so the requested value should exist in a stored
// fragment
self.ommers.get(&addr).cloned()
}
})
}
fn retain(&mut self, ommer_addrs: &BTreeSet<Address>) {
// Prune away any ommers & tracking addresses we don't need
self.tracking
.retain(|addr| ommer_addrs.contains(&addr.sibling()));
self.ommers.retain(|addr, _| ommer_addrs.contains(addr));
}
}
/// A data structure used to store the information necessary to "rewind" the state of a
/// [`BridgeTree`] to a particular leaf position.
///
/// This is needed because the [`BridgeTree::marked_indices`] map is a cache of information that
/// crosses [`MerkleBridge`] boundaries, and so it is not sufficient to just truncate the list of
/// bridges; instead, we use [`Checkpoint`] values to be able to rapidly restore the cache to its
/// previous state.
#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
pub struct Checkpoint {
/// The number of bridges that will be retained in a rewind.
bridges_len: usize,
/// A flag indicating whether or not the current state of the tree
/// had been marked at the time the checkpoint was created.
is_marked: bool,
/// A set of the positions that have been marked during the period that this
/// checkpoint is the current checkpoint.
marked: BTreeSet<Position>,
/// When a mark is forgotten, if the index of the forgotten mark is <= bridge_idx we
/// record it in the current checkpoint so that on rollback, we restore the forgotten
/// marks to the BridgeTree's "saved" list. If the mark was newly created since the
/// checkpoint, we don't need to remember when we forget it because both the mark
/// creation and removal will be reverted in the rollback.
forgotten: BTreeMap<Position, usize>,
}
impl Checkpoint {
/// Creates a new checkpoint from its constituent parts.
pub fn from_parts(
bridges_len: usize,
is_marked: bool,
marked: BTreeSet<Position>,
forgotten: BTreeMap<Position, usize>,
) -> Self {
Self {
bridges_len,
is_marked,
marked,
forgotten,
}
}
/// Creates a new empty checkpoint for the specified [`BridgeTree`] state.
pub fn at_length(bridges_len: usize, is_marked: bool) -> Self {
Checkpoint {
bridges_len,
is_marked,
marked: BTreeSet::new(),
forgotten: BTreeMap::new(),
}
}
/// Returns the length of the [`BridgeTree::prior_bridges`] vector of the [`BridgeTree`] to
/// which this checkpoint refers.
///
/// This is the number of bridges that will be retained in the event of a rewind to this
/// checkpoint.
pub fn bridges_len(&self) -> usize {
self.bridges_len
}
/// Returns whether the current state of the tree had been marked at the point that
/// this checkpoint was made.
///
/// In the event of a rewind, the rewind logic will ensure that mark information is
/// properly reconstituted for the checkpointed tree state.
pub fn is_marked(&self) -> bool {
self.is_marked
}
/// Returns a set of the positions that have been marked during the period that this
/// checkpoint is the current checkpoint.
pub fn marked(&self) -> &BTreeSet<Position> {
&self.marked
}
/// Returns the set of previously-marked positions that have had their marks removed
/// during the period that this checkpoint is the current checkpoint.
pub fn forgotten(&self) -> &BTreeMap<Position, usize> {
&self.forgotten
}
// A private convenience method that returns the root of the bridge corresponding to
// this checkpoint at a specified depth, given the slice of bridges from which this checkpoint
// was derived.
fn root<H>(&self, bridges: &[MerkleBridge<H>], level: Level) -> H
where
H: Hashable + Clone + Ord,
{
if self.bridges_len == 0 {
H::empty_root(level)
} else {
bridges[self.bridges_len - 1].frontier().root(Some(level))
}
}
// A private convenience method that returns the position of the bridge corresponding
// to this checkpoint, if the checkpoint is not for the empty bridge.
fn position<H: Ord>(&self, bridges: &[MerkleBridge<H>]) -> Option<Position> {
if self.bridges_len == 0 {
None
} else {
Some(bridges[self.bridges_len - 1].position())
}
}
// A private method that rewrites the indices of each forgotten marked record
// using the specified rewrite function. Used during garbage collection.
fn rewrite_indices<F: Fn(usize) -> usize>(&mut self, f: F) {
self.bridges_len = f(self.bridges_len);
for v in self.forgotten.values_mut() {
*v = f(*v)
}
}
}
/// A sparse representation of a Merkle tree with linear appending of leaves that contains enough
/// information to produce a witness for any `mark`ed leaf.
#[derive(Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct BridgeTree<H, const DEPTH: u8> {
/// The ordered list of Merkle bridges representing the history
/// of the tree. There will be one bridge for each saved leaf.
prior_bridges: Vec<MerkleBridge<H>>,
/// The current (mutable) bridge at the tip of the tree.
current_bridge: Option<MerkleBridge<H>>,
/// A map from positions for which we wish to be able to compute a
/// witness to index in the bridges vector.
saved: BTreeMap<Position, usize>,
/// A stack of bridge indices to which it's possible to rewind directly.
checkpoints: Vec<Checkpoint>,
/// The maximum number of checkpoints to retain. If this number is
/// exceeded, the oldest checkpoint will be dropped when creating
/// a new checkpoint.
max_checkpoints: usize,
}
impl<H: Hashable + Ord + Debug, const DEPTH: u8> Debug for BridgeTree<H, DEPTH> {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> Result<(), std::fmt::Error> {
write!(
f,
"BridgeTree {{\n depth: {:?},\n prior_bridges: {:?},\n current_bridge: {:?},\n saved: {:?},\n checkpoints: {:?},\n max_checkpoints: {:?}\n}}",
DEPTH, self.prior_bridges, self.current_bridge, self.saved, self.checkpoints, self.max_checkpoints
)
}
}
/// Errors that can appear when validating the internal consistency of a `[BridgeTree]`
/// value when constructing a tree from its constituent parts.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum BridgeTreeError {
IncorrectIncompleteIndex,
InvalidMarkIndex(usize),
PositionMismatch { expected: Position, found: Position },
InvalidSavePoints,
Discontinuity(ContinuityError),
CheckpointMismatch,
}
impl<H, const DEPTH: u8> BridgeTree<H, DEPTH> {
/// Construct an empty BridgeTree value with the specified maximum number of checkpoints.
pub fn new(max_checkpoints: usize) -> Self {
Self {
prior_bridges: vec![],
current_bridge: None,
saved: BTreeMap::new(),
checkpoints: vec![],
max_checkpoints,
}
}
/// Removes the oldest checkpoint. Returns true if successful and false if
/// there are no checkpoints.
fn drop_oldest_checkpoint(&mut self) -> bool {
if self.checkpoints.is_empty() {
false
} else {
self.checkpoints.remove(0);
true
}
}
/// Returns the prior bridges that make up this tree
pub fn prior_bridges(&self) -> &[MerkleBridge<H>] {
&self.prior_bridges
}
/// Returns the current bridge at the tip of this tree
pub fn current_bridge(&self) -> &Option<MerkleBridge<H>> {
&self.current_bridge
}
/// Returns the map from leaf positions that have been marked to the index of
/// the bridge whose tip is at that position in this tree's list of bridges.
pub fn marked_indices(&self) -> &BTreeMap<Position, usize> {
&self.saved
}
/// Returns the checkpoints to which this tree may be rewound.
pub fn checkpoints(&self) -> &[Checkpoint] {
&self.checkpoints
}
/// Returns the maximum number of checkpoints that will be maintained
/// by the data structure. When this number of checkpoints is exceeded,
/// the oldest checkpoints are discarded when creating new checkpoints.
pub fn max_checkpoints(&self) -> usize {
self.max_checkpoints
}
/// Returns the bridge's frontier.
pub fn frontier(&self) -> Option<&NonEmptyFrontier<H>> {
self.current_bridge.as_ref().map(|b| b.frontier())
}
}
impl<H: Hashable + Ord + Clone, const DEPTH: u8> BridgeTree<H, DEPTH> {
/// Construct a new BridgeTree that will start recording changes from the state of
/// the specified frontier.
pub fn from_frontier(max_checkpoints: usize, frontier: NonEmptyFrontier<H>) -> Self {
Self {
prior_bridges: vec![],
current_bridge: Some(MerkleBridge::from_parts(
None,
BTreeSet::new(),
BTreeMap::new(),
frontier,
)),
saved: BTreeMap::new(),
checkpoints: vec![],
max_checkpoints,
}
}
/// Construct a new BridgeTree from its constituent parts, checking for internal
/// consistency.
pub fn from_parts(
prior_bridges: Vec<MerkleBridge<H>>,
current_bridge: Option<MerkleBridge<H>>,
saved: BTreeMap<Position, usize>,
checkpoints: Vec<Checkpoint>,
max_checkpoints: usize,
) -> Result<Self, BridgeTreeError> {
Self::check_consistency_internal(
&prior_bridges,
¤t_bridge,
&saved,
&checkpoints,
max_checkpoints,
)?;
Ok(BridgeTree {
prior_bridges,
current_bridge,
saved,
checkpoints,
max_checkpoints,
})
}
fn check_consistency(&self) -> Result<(), BridgeTreeError> {
Self::check_consistency_internal(
&self.prior_bridges,
&self.current_bridge,
&self.saved,
&self.checkpoints,
self.max_checkpoints,
)
}
fn check_consistency_internal(
prior_bridges: &[MerkleBridge<H>],
current_bridge: &Option<MerkleBridge<H>>,
saved: &BTreeMap<Position, usize>,
checkpoints: &[Checkpoint],
max_checkpoints: usize,
) -> Result<(), BridgeTreeError> {
// check that saved values correspond to bridges
for (pos, i) in saved {
if i >= &prior_bridges.len() {
return Err(BridgeTreeError::InvalidMarkIndex(*i));
}
let found = prior_bridges[*i].position();
if &found != pos {
return Err(BridgeTreeError::PositionMismatch {
expected: *pos,
found,
});
}
}
if checkpoints.len() > max_checkpoints
|| checkpoints
.iter()
.any(|c| c.bridges_len > prior_bridges.len())
{
return Err(BridgeTreeError::CheckpointMismatch);
}
for (prev, next) in prior_bridges.iter().zip(prior_bridges.iter().skip(1)) {
prev.check_continuity(next)
.map_err(BridgeTreeError::Discontinuity)?;
}
if let Some((prev, next)) = prior_bridges.last().zip(current_bridge.as_ref()) {
prev.check_continuity(next)
.map_err(BridgeTreeError::Discontinuity)?;
}
Ok(())
}
/// Appends a new value to the tree at the next available slot.
/// Returns true if successful and false if the tree would exceed
/// the maximum allowed depth.
pub fn append(&mut self, value: &H) -> bool {
if let Some(bridge) = self.current_bridge.as_mut() {
if bridge
.frontier
.position()
.is_complete_subtree(Level::from(DEPTH))
{
false
} else {
bridge.append(value.clone());
true
}
} else {
self.current_bridge = Some(MerkleBridge::new(value.clone()));
true
}
}
/// Obtains the root of the Merkle tree at the specified checkpoint depth
/// by hashing against empty nodes up to the maximum height of the tree.
/// Returns `None` if there are not enough checkpoints available to reach the
/// requested checkpoint depth.
pub fn root(&self, checkpoint_depth: usize) -> Option<H> {
let root_level = Level::from(DEPTH);
if checkpoint_depth == 0 {
Some(
self.current_bridge
.as_ref()
.map_or(H::empty_root(root_level), |bridge| {
bridge.frontier().root(Some(root_level))
}),
)
} else if self.checkpoints.len() >= checkpoint_depth {
let checkpoint_idx = self.checkpoints.len() - checkpoint_depth;
self.checkpoints
.get(checkpoint_idx)
.map(|c| c.root(&self.prior_bridges, root_level))
} else {
None
}
}
/// Returns the most recently appended leaf value.
pub fn current_position(&self) -> Option<Position> {
self.current_bridge.as_ref().map(|b| b.position())
}
/// Returns the most recently appended leaf value.
pub fn current_leaf(&self) -> Option<&H> {
self.current_bridge.as_ref().map(|b| b.current_leaf())
}
/// Marks the current leaf as one for which we're interested in producing
/// a witness. Returns an optional value containing the
/// current position if successful or if the current value was already
/// marked, or None if the tree is empty.
pub fn mark(&mut self) -> Option<Position> {
match self.current_bridge.take() {
Some(mut cur_b) => {
cur_b.track_current_leaf();
let pos = cur_b.position();
// If the latest bridge is a newly created checkpoint, the last prior
// bridge will have the same position and all we need to do is mark
// the checkpointed leaf as being saved.
if self
.prior_bridges
.last()
.map_or(false, |prior_b| prior_b.position() == cur_b.position())
{
// the current bridge has not been advanced, so we just need to make
// sure that we have are tracking the marked leaf
self.current_bridge = Some(cur_b);
} else {
let successor = cur_b.successor(true);
self.prior_bridges.push(cur_b);
self.current_bridge = Some(successor);
}
self.saved
.entry(pos)
.or_insert(self.prior_bridges.len() - 1);
// mark the position as having been marked in the current checkpoint
if let Some(c) = self.checkpoints.last_mut() {
if !c.is_marked {
c.marked.insert(pos);
}
}
Some(pos)
}
None => None,
}
}
/// Return a set of all the positions for which we have marked.
pub fn marked_positions(&self) -> BTreeSet<Position> {
self.saved.keys().cloned().collect()
}
/// Returns the leaf at the specified position if the tree can produce
/// a witness for it.
pub fn get_marked_leaf(&self, position: Position) -> Option<&H> {
self.saved
.get(&position)
.and_then(|idx| self.prior_bridges.get(*idx).map(|b| b.current_leaf()))
}
/// Marks the value at the specified position as a value we're no longer
/// interested in maintaining a mark for. Returns true if successful and
/// false if we were already not maintaining a mark at this position.
pub fn remove_mark(&mut self, position: Position) -> bool {
if let Some(idx) = self.saved.remove(&position) {
// If the position is one that has *not* just been marked since the last checkpoint,
// then add it to the set of those forgotten during the current checkpoint span so that
// it can be restored on rollback.
if let Some(c) = self.checkpoints.last_mut() {
if !c.marked.contains(&position) {
c.forgotten.insert(position, idx);
}
}
true
} else {
false
}
}
/// Creates a new checkpoint for the current tree state. It is valid to
/// have multiple checkpoints for the same tree state, and each `rewind`
/// call will remove a single checkpoint.
pub fn checkpoint(&mut self) {
match self.current_bridge.take() {
Some(cur_b) => {
let is_marked = self.get_marked_leaf(cur_b.position()).is_some();
// Do not create a duplicate bridge
if self
.prior_bridges
.last()
.map_or(false, |pb| pb.position() == cur_b.position())
{
self.current_bridge = Some(cur_b);
} else {
self.current_bridge = Some(cur_b.successor(false));
self.prior_bridges.push(cur_b);
}
self.checkpoints
.push(Checkpoint::at_length(self.prior_bridges.len(), is_marked));
}
None => {
self.checkpoints.push(Checkpoint::at_length(0, false));
}
}
if self.checkpoints.len() > self.max_checkpoints {
self.drop_oldest_checkpoint();
}
}
/// Rewinds the tree state to the previous checkpoint, and then removes
/// that checkpoint record. If there are multiple checkpoints at a given
/// tree state, the tree state will not be altered until all checkpoints
/// at that tree state have been removed using `rewind`. This function
/// return false and leave the tree unmodified if no checkpoints exist.
pub fn rewind(&mut self) -> bool {
match self.checkpoints.pop() {
Some(mut c) => {
// drop marked values at and above the checkpoint height;
// we will re-mark if necessary.
self.saved.append(&mut c.forgotten);
self.saved.retain(|_, i| *i + 1 < c.bridges_len);
self.prior_bridges.truncate(c.bridges_len);
self.current_bridge = self.prior_bridges.last().map(|b| b.successor(c.is_marked));
if c.is_marked {
self.mark();
}
true
}
None => false,
}
}
/// Obtains a witness to the value at the specified position,
/// as of the tree state corresponding to the given root.
/// Returns `None` if there is no available witness to that
/// position or if the root does not correspond to a checkpointed
/// root of the tree.
pub fn witness(&self, position: Position, as_of_root: &H) -> Option<Vec<H>> {
self.witness_inner(position, as_of_root).ok()
}
fn witness_inner(&self, position: Position, as_of_root: &H) -> Result<Vec<H>, WitnessingError> {
#[derive(Debug)]
enum AuthBase<'a> {
Current,
Checkpoint(usize, &'a Checkpoint),
NotFound,
}
let max_alt = Level::from(DEPTH);
// Find the earliest checkpoint having a matching root, or the current
// root if it matches and there is no earlier matching checkpoint.
let auth_base = self
.checkpoints
.iter()
.enumerate()
.rev()
.take_while(|(_, c)| c.position(&self.prior_bridges) >= Some(position))
.filter(|(_, c)| &c.root(&self.prior_bridges, max_alt) == as_of_root)
.last()
.map(|(i, c)| AuthBase::Checkpoint(i, c))
.unwrap_or_else(|| {
if self.root(0).as_ref() == Some(as_of_root) {
AuthBase::Current
} else {
AuthBase::NotFound
}
});
let saved_idx = self
.saved
.get(&position)
.or_else(|| {
if let AuthBase::Checkpoint(i, _) = auth_base {
// The saved position might have been forgotten since the checkpoint,
// so look for it in each of the subsequent checkpoints' forgotten
// items.
self.checkpoints[i..].iter().find_map(|c| {
// restore the forgotten position, if that position was not also marked
// in the same checkpoint
c.forgotten
.get(&position)
.filter(|_| !c.marked.contains(&position))
})
} else {
None
}
})
.ok_or(WitnessingError::PositionNotMarked(position))?;
let prior_frontier = &self.prior_bridges[*saved_idx].frontier;
// Fuse the following bridges to obtain a bridge that has all
// of the data to the right of the selected value in the tree,
// up to the specified checkpoint depth.
let fuse_from = saved_idx + 1;
let successor = match auth_base {
AuthBase::Current => {
// fuse all the way up to the current tip
MerkleBridge::fuse_all(
self.prior_bridges[fuse_from..]
.iter()
.chain(&self.current_bridge),
)
.map(|fused| fused.unwrap()) // safe as the iterator being fused is nonempty
.map_err(WitnessingError::BridgeFusionError)
}
AuthBase::Checkpoint(_, checkpoint) if fuse_from < checkpoint.bridges_len => {
// fuse from the provided checkpoint
MerkleBridge::fuse_all(self.prior_bridges[fuse_from..checkpoint.bridges_len].iter())
.map(|fused| fused.unwrap()) // safe as the iterator being fused is nonempty
.map_err(WitnessingError::BridgeFusionError)
}
AuthBase::Checkpoint(_, checkpoint) if fuse_from == checkpoint.bridges_len => {
// The successor bridge should just be the empty successor to the
// checkpointed bridge.
if checkpoint.bridges_len > 0 {
Ok(self.prior_bridges[checkpoint.bridges_len - 1].successor(false))
} else {
Err(WitnessingError::CheckpointInvalid)
}
}
AuthBase::Checkpoint(_, checkpoint) => {
// if the saved index is after the checkpoint, we can't generate
// an auth path
Err(WitnessingError::CheckpointTooDeep(
fuse_from - checkpoint.bridges_len,
))
}
AuthBase::NotFound => {
// we didn't find any suitable auth base
Err(WitnessingError::AuthBaseNotFound)
}
}?;
successor.witness(DEPTH, prior_frontier)
}
/// Remove state from the tree that no longer needs to be maintained
/// because it is associated with checkpoints or marks that
/// have been removed from the tree at positions deeper than those
/// reachable by calls to `rewind`.
pub fn garbage_collect(&mut self) {
// Only garbage collect once we have more bridges than the maximum number of
// checkpoints; we cannot remove information that we might need to restore in
// a rewind.
if self.checkpoints.len() == self.max_checkpoints {
let gc_len = self.checkpoints.first().unwrap().bridges_len;
// Get a list of the leaf positions that we need to retain. This consists of
// all the saved leaves, plus all the leaves that have been forgotten since
// the most distant checkpoint to which we could rewind.
let remember: BTreeSet<Position> = self
.saved
.keys()
.chain(self.checkpoints.iter().flat_map(|c| c.forgotten.keys()))
.cloned()
.collect();
let mut cur: Option<MerkleBridge<H>> = None;
let mut merged = 0;
let mut ommer_addrs: BTreeSet<Address> = BTreeSet::new();
for (i, next_bridge) in std::mem::take(&mut self.prior_bridges)
.into_iter()
.enumerate()
{
if let Some(cur_bridge) = cur {
let pos = cur_bridge.position();
let mut new_cur = if remember.contains(&pos) || i > gc_len {
// We need to remember cur_bridge; update its save index & put next_bridge
// on the chopping block
if let Some(idx) = self.saved.get_mut(&pos) {
*idx -= merged;
}
// Add the elements of the auth path to the set of addresses we should
// continue to track and retain information for
for (addr, source) in cur_bridge
.frontier
.position()
.witness_addrs(Level::from(DEPTH))
{
if source == Source::Future {
ommer_addrs.insert(addr);
}
}
self.prior_bridges.push(cur_bridge);
next_bridge
} else {
// We can fuse these bridges together because we don't need to
// remember next_bridge.
merged += 1;
cur_bridge.fuse(&next_bridge).unwrap()
};
new_cur.retain(&ommer_addrs);
cur = Some(new_cur);
} else {
// this case will only occur for the first bridge
cur = Some(next_bridge);
}
}
// unwrap is safe because we know that prior_bridges was nonempty.
if let Some(last_bridge) = cur {
if let Some(idx) = self.saved.get_mut(&last_bridge.position()) {
*idx -= merged;
}
self.prior_bridges.push(last_bridge);
}
for c in self.checkpoints.iter_mut() {
c.rewrite_indices(|idx| idx - merged);
}
}
if let Err(e) = self.check_consistency() {
panic!(
"Consistency check failed after garbage collection with {:?}",
e
);
}
}sourcepub fn tracking(&self) -> &BTreeSet<Address>
pub fn tracking(&self) -> &BTreeSet<Address>
Returns the set of internal node addresses that we’re searching for the ommers for.
sourcepub fn ommers(&self) -> &BTreeMap<Address, H>
pub fn ommers(&self) -> &BTreeMap<Address, H>
Returns the set of internal node addresses that we’re searching for the ommers for.
sourcepub fn frontier(&self) -> &NonEmptyFrontier<H>
pub fn frontier(&self) -> &NonEmptyFrontier<H>
Returns the non-empty frontier of this Merkle bridge.
Examples found in repository?
src/lib.rs (line 477)
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pub fn append(&mut self, value: H) {
self.frontier.append(value);
let mut found = vec![];
for address in self.tracking.iter() {
// We know that there will only ever be one address that we're
// tracking at a given level, because as soon as we find a
// value for the sibling of the address we're tracking, we
// remove the tracked address and replace it the next parent
// of that address for which we need to find a sibling.
if self
.frontier()
.position()
.is_complete_subtree(address.level())
{
let digest = self.frontier.root(Some(address.level()));
self.ommers.insert(address.sibling(), digest);
found.push(*address);
}
}
for address in found {
self.tracking.remove(&address);
// The address of the next incomplete parent note for which
// we need to find a sibling.
let parent = address.next_incomplete_parent();
assert!(!self.ommers.contains_key(&parent));
self.tracking.insert(parent);
}
}
/// Returns a single MerkleBridge that contains the aggregate information
/// of this bridge and `next`, or None if `next` is not a valid successor
/// to this bridge. The resulting Bridge will have the same state as though
/// `self` had had every leaf used to construct `next` appended to it
/// directly.
fn fuse(&self, next: &Self) -> Result<Self, ContinuityError> {
self.check_continuity(next)?;
Ok(Self {
prior_position: self.prior_position,
tracking: next.tracking.clone(),
ommers: self
.ommers
.iter()
.chain(next.ommers.iter())
.map(|(k, v)| (*k, v.clone()))
.collect(),
frontier: next.frontier.clone(),
})
}
/// Returns a single MerkleBridge that contains the aggregate information
/// of all the provided bridges (discarding internal frontiers) or None
/// if the provided iterator is empty. Returns a continuity error if
/// any of the bridges are not valid successors to one another.
fn fuse_all<T: Iterator<Item = &'a Self>>(
mut iter: T,
) -> Result<Option<Self>, ContinuityError> {
let mut fused = iter.next().cloned();
for next in iter {
fused = Some(fused.unwrap().fuse(next)?);
}
Ok(fused)
}
/// If this bridge contains sufficient auth fragment information, construct an authentication
/// path for the specified position by interleaving with values from the prior frontier. This
/// method will panic if the position of the prior frontier does not match this bridge's prior
/// position.
fn witness(
&self,
depth: u8,
prior_frontier: &NonEmptyFrontier<H>,
) -> Result<Vec<H>, WitnessingError> {
assert!(Some(prior_frontier.position()) == self.prior_position);
prior_frontier.witness(depth, |addr| {
let r = addr.position_range();
if self.frontier.position() < r.start {
Some(H::empty_root(addr.level()))
} else if r.contains(&self.frontier.position()) {
Some(self.frontier.root(Some(addr.level())))
} else {
// the frontier's position is after the end of the requested
// range, so the requested value should exist in a stored
// fragment
self.ommers.get(&addr).cloned()
}
})
}
fn retain(&mut self, ommer_addrs: &BTreeSet<Address>) {
// Prune away any ommers & tracking addresses we don't need
self.tracking
.retain(|addr| ommer_addrs.contains(&addr.sibling()));
self.ommers.retain(|addr, _| ommer_addrs.contains(addr));
}
}
/// A data structure used to store the information necessary to "rewind" the state of a
/// [`BridgeTree`] to a particular leaf position.
///
/// This is needed because the [`BridgeTree::marked_indices`] map is a cache of information that
/// crosses [`MerkleBridge`] boundaries, and so it is not sufficient to just truncate the list of
/// bridges; instead, we use [`Checkpoint`] values to be able to rapidly restore the cache to its
/// previous state.
#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
pub struct Checkpoint {
/// The number of bridges that will be retained in a rewind.
bridges_len: usize,
/// A flag indicating whether or not the current state of the tree
/// had been marked at the time the checkpoint was created.
is_marked: bool,
/// A set of the positions that have been marked during the period that this
/// checkpoint is the current checkpoint.
marked: BTreeSet<Position>,
/// When a mark is forgotten, if the index of the forgotten mark is <= bridge_idx we
/// record it in the current checkpoint so that on rollback, we restore the forgotten
/// marks to the BridgeTree's "saved" list. If the mark was newly created since the
/// checkpoint, we don't need to remember when we forget it because both the mark
/// creation and removal will be reverted in the rollback.
forgotten: BTreeMap<Position, usize>,
}
impl Checkpoint {
/// Creates a new checkpoint from its constituent parts.
pub fn from_parts(
bridges_len: usize,
is_marked: bool,
marked: BTreeSet<Position>,
forgotten: BTreeMap<Position, usize>,
) -> Self {
Self {
bridges_len,
is_marked,
marked,
forgotten,
}
}
/// Creates a new empty checkpoint for the specified [`BridgeTree`] state.
pub fn at_length(bridges_len: usize, is_marked: bool) -> Self {
Checkpoint {
bridges_len,
is_marked,
marked: BTreeSet::new(),
forgotten: BTreeMap::new(),
}
}
/// Returns the length of the [`BridgeTree::prior_bridges`] vector of the [`BridgeTree`] to
/// which this checkpoint refers.
///
/// This is the number of bridges that will be retained in the event of a rewind to this
/// checkpoint.
pub fn bridges_len(&self) -> usize {
self.bridges_len
}
/// Returns whether the current state of the tree had been marked at the point that
/// this checkpoint was made.
///
/// In the event of a rewind, the rewind logic will ensure that mark information is
/// properly reconstituted for the checkpointed tree state.
pub fn is_marked(&self) -> bool {
self.is_marked
}
/// Returns a set of the positions that have been marked during the period that this
/// checkpoint is the current checkpoint.
pub fn marked(&self) -> &BTreeSet<Position> {
&self.marked
}
/// Returns the set of previously-marked positions that have had their marks removed
/// during the period that this checkpoint is the current checkpoint.
pub fn forgotten(&self) -> &BTreeMap<Position, usize> {
&self.forgotten
}
// A private convenience method that returns the root of the bridge corresponding to
// this checkpoint at a specified depth, given the slice of bridges from which this checkpoint
// was derived.
fn root<H>(&self, bridges: &[MerkleBridge<H>], level: Level) -> H
where
H: Hashable + Clone + Ord,
{
if self.bridges_len == 0 {
H::empty_root(level)
} else {
bridges[self.bridges_len - 1].frontier().root(Some(level))
}
}
// A private convenience method that returns the position of the bridge corresponding
// to this checkpoint, if the checkpoint is not for the empty bridge.
fn position<H: Ord>(&self, bridges: &[MerkleBridge<H>]) -> Option<Position> {
if self.bridges_len == 0 {
None
} else {
Some(bridges[self.bridges_len - 1].position())
}
}
// A private method that rewrites the indices of each forgotten marked record
// using the specified rewrite function. Used during garbage collection.
fn rewrite_indices<F: Fn(usize) -> usize>(&mut self, f: F) {
self.bridges_len = f(self.bridges_len);
for v in self.forgotten.values_mut() {
*v = f(*v)
}
}
}
/// A sparse representation of a Merkle tree with linear appending of leaves that contains enough
/// information to produce a witness for any `mark`ed leaf.
#[derive(Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct BridgeTree<H, const DEPTH: u8> {
/// The ordered list of Merkle bridges representing the history
/// of the tree. There will be one bridge for each saved leaf.
prior_bridges: Vec<MerkleBridge<H>>,
/// The current (mutable) bridge at the tip of the tree.
current_bridge: Option<MerkleBridge<H>>,
/// A map from positions for which we wish to be able to compute a
/// witness to index in the bridges vector.
saved: BTreeMap<Position, usize>,
/// A stack of bridge indices to which it's possible to rewind directly.
checkpoints: Vec<Checkpoint>,
/// The maximum number of checkpoints to retain. If this number is
/// exceeded, the oldest checkpoint will be dropped when creating
/// a new checkpoint.
max_checkpoints: usize,
}
impl<H: Hashable + Ord + Debug, const DEPTH: u8> Debug for BridgeTree<H, DEPTH> {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> Result<(), std::fmt::Error> {
write!(
f,
"BridgeTree {{\n depth: {:?},\n prior_bridges: {:?},\n current_bridge: {:?},\n saved: {:?},\n checkpoints: {:?},\n max_checkpoints: {:?}\n}}",
DEPTH, self.prior_bridges, self.current_bridge, self.saved, self.checkpoints, self.max_checkpoints
)
}
}
/// Errors that can appear when validating the internal consistency of a `[BridgeTree]`
/// value when constructing a tree from its constituent parts.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum BridgeTreeError {
IncorrectIncompleteIndex,
InvalidMarkIndex(usize),
PositionMismatch { expected: Position, found: Position },
InvalidSavePoints,
Discontinuity(ContinuityError),
CheckpointMismatch,
}
impl<H, const DEPTH: u8> BridgeTree<H, DEPTH> {
/// Construct an empty BridgeTree value with the specified maximum number of checkpoints.
pub fn new(max_checkpoints: usize) -> Self {
Self {
prior_bridges: vec![],
current_bridge: None,
saved: BTreeMap::new(),
checkpoints: vec![],
max_checkpoints,
}
}
/// Removes the oldest checkpoint. Returns true if successful and false if
/// there are no checkpoints.
fn drop_oldest_checkpoint(&mut self) -> bool {
if self.checkpoints.is_empty() {
false
} else {
self.checkpoints.remove(0);
true
}
}
/// Returns the prior bridges that make up this tree
pub fn prior_bridges(&self) -> &[MerkleBridge<H>] {
&self.prior_bridges
}
/// Returns the current bridge at the tip of this tree
pub fn current_bridge(&self) -> &Option<MerkleBridge<H>> {
&self.current_bridge
}
/// Returns the map from leaf positions that have been marked to the index of
/// the bridge whose tip is at that position in this tree's list of bridges.
pub fn marked_indices(&self) -> &BTreeMap<Position, usize> {
&self.saved
}
/// Returns the checkpoints to which this tree may be rewound.
pub fn checkpoints(&self) -> &[Checkpoint] {
&self.checkpoints
}
/// Returns the maximum number of checkpoints that will be maintained
/// by the data structure. When this number of checkpoints is exceeded,
/// the oldest checkpoints are discarded when creating new checkpoints.
pub fn max_checkpoints(&self) -> usize {
self.max_checkpoints
}
/// Returns the bridge's frontier.
pub fn frontier(&self) -> Option<&NonEmptyFrontier<H>> {
self.current_bridge.as_ref().map(|b| b.frontier())
}
}
impl<H: Hashable + Ord + Clone, const DEPTH: u8> BridgeTree<H, DEPTH> {
/// Construct a new BridgeTree that will start recording changes from the state of
/// the specified frontier.
pub fn from_frontier(max_checkpoints: usize, frontier: NonEmptyFrontier<H>) -> Self {
Self {
prior_bridges: vec![],
current_bridge: Some(MerkleBridge::from_parts(
None,
BTreeSet::new(),
BTreeMap::new(),
frontier,
)),
saved: BTreeMap::new(),
checkpoints: vec![],
max_checkpoints,
}
}
/// Construct a new BridgeTree from its constituent parts, checking for internal
/// consistency.
pub fn from_parts(
prior_bridges: Vec<MerkleBridge<H>>,
current_bridge: Option<MerkleBridge<H>>,
saved: BTreeMap<Position, usize>,
checkpoints: Vec<Checkpoint>,
max_checkpoints: usize,
) -> Result<Self, BridgeTreeError> {
Self::check_consistency_internal(
&prior_bridges,
¤t_bridge,
&saved,
&checkpoints,
max_checkpoints,
)?;
Ok(BridgeTree {
prior_bridges,
current_bridge,
saved,
checkpoints,
max_checkpoints,
})
}
fn check_consistency(&self) -> Result<(), BridgeTreeError> {
Self::check_consistency_internal(
&self.prior_bridges,
&self.current_bridge,
&self.saved,
&self.checkpoints,
self.max_checkpoints,
)
}
fn check_consistency_internal(
prior_bridges: &[MerkleBridge<H>],
current_bridge: &Option<MerkleBridge<H>>,
saved: &BTreeMap<Position, usize>,
checkpoints: &[Checkpoint],
max_checkpoints: usize,
) -> Result<(), BridgeTreeError> {
// check that saved values correspond to bridges
for (pos, i) in saved {
if i >= &prior_bridges.len() {
return Err(BridgeTreeError::InvalidMarkIndex(*i));
}
let found = prior_bridges[*i].position();
if &found != pos {
return Err(BridgeTreeError::PositionMismatch {
expected: *pos,
found,
});
}
}
if checkpoints.len() > max_checkpoints
|| checkpoints
.iter()
.any(|c| c.bridges_len > prior_bridges.len())
{
return Err(BridgeTreeError::CheckpointMismatch);
}
for (prev, next) in prior_bridges.iter().zip(prior_bridges.iter().skip(1)) {
prev.check_continuity(next)
.map_err(BridgeTreeError::Discontinuity)?;
}
if let Some((prev, next)) = prior_bridges.last().zip(current_bridge.as_ref()) {
prev.check_continuity(next)
.map_err(BridgeTreeError::Discontinuity)?;
}
Ok(())
}
/// Appends a new value to the tree at the next available slot.
/// Returns true if successful and false if the tree would exceed
/// the maximum allowed depth.
pub fn append(&mut self, value: &H) -> bool {
if let Some(bridge) = self.current_bridge.as_mut() {
if bridge
.frontier
.position()
.is_complete_subtree(Level::from(DEPTH))
{
false
} else {
bridge.append(value.clone());
true
}
} else {
self.current_bridge = Some(MerkleBridge::new(value.clone()));
true
}
}
/// Obtains the root of the Merkle tree at the specified checkpoint depth
/// by hashing against empty nodes up to the maximum height of the tree.
/// Returns `None` if there are not enough checkpoints available to reach the
/// requested checkpoint depth.
pub fn root(&self, checkpoint_depth: usize) -> Option<H> {
let root_level = Level::from(DEPTH);
if checkpoint_depth == 0 {
Some(
self.current_bridge
.as_ref()
.map_or(H::empty_root(root_level), |bridge| {
bridge.frontier().root(Some(root_level))
}),
)
} else if self.checkpoints.len() >= checkpoint_depth {
let checkpoint_idx = self.checkpoints.len() - checkpoint_depth;
self.checkpoints
.get(checkpoint_idx)
.map(|c| c.root(&self.prior_bridges, root_level))
} else {
None
}
}sourcepub fn current_leaf(&self) -> &H
pub fn current_leaf(&self) -> &H
Returns the value of the most recently appended leaf.
Examples found in repository?
src/lib.rs (line 928)
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pub fn current_leaf(&self) -> Option<&H> {
self.current_bridge.as_ref().map(|b| b.current_leaf())
}
/// Marks the current leaf as one for which we're interested in producing
/// a witness. Returns an optional value containing the
/// current position if successful or if the current value was already
/// marked, or None if the tree is empty.
pub fn mark(&mut self) -> Option<Position> {
match self.current_bridge.take() {
Some(mut cur_b) => {
cur_b.track_current_leaf();
let pos = cur_b.position();
// If the latest bridge is a newly created checkpoint, the last prior
// bridge will have the same position and all we need to do is mark
// the checkpointed leaf as being saved.
if self
.prior_bridges
.last()
.map_or(false, |prior_b| prior_b.position() == cur_b.position())
{
// the current bridge has not been advanced, so we just need to make
// sure that we have are tracking the marked leaf
self.current_bridge = Some(cur_b);
} else {
let successor = cur_b.successor(true);
self.prior_bridges.push(cur_b);
self.current_bridge = Some(successor);
}
self.saved
.entry(pos)
.or_insert(self.prior_bridges.len() - 1);
// mark the position as having been marked in the current checkpoint
if let Some(c) = self.checkpoints.last_mut() {
if !c.is_marked {
c.marked.insert(pos);
}
}
Some(pos)
}
None => None,
}
}
/// Return a set of all the positions for which we have marked.
pub fn marked_positions(&self) -> BTreeSet<Position> {
self.saved.keys().cloned().collect()
}
/// Returns the leaf at the specified position if the tree can produce
/// a witness for it.
pub fn get_marked_leaf(&self, position: Position) -> Option<&H> {
self.saved
.get(&position)
.and_then(|idx| self.prior_bridges.get(*idx).map(|b| b.current_leaf()))
}sourcepub fn check_continuity(&self, next: &Self) -> Result<(), ContinuityError>
pub fn check_continuity(&self, next: &Self) -> Result<(), ContinuityError>
Checks whether this bridge is a valid successor for the specified bridge.
Examples found in repository?
src/lib.rs (line 504)
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fn fuse(&self, next: &Self) -> Result<Self, ContinuityError> {
self.check_continuity(next)?;
Ok(Self {
prior_position: self.prior_position,
tracking: next.tracking.clone(),
ommers: self
.ommers
.iter()
.chain(next.ommers.iter())
.map(|(k, v)| (*k, v.clone()))
.collect(),
frontier: next.frontier.clone(),
})
}
/// Returns a single MerkleBridge that contains the aggregate information
/// of all the provided bridges (discarding internal frontiers) or None
/// if the provided iterator is empty. Returns a continuity error if
/// any of the bridges are not valid successors to one another.
fn fuse_all<T: Iterator<Item = &'a Self>>(
mut iter: T,
) -> Result<Option<Self>, ContinuityError> {
let mut fused = iter.next().cloned();
for next in iter {
fused = Some(fused.unwrap().fuse(next)?);
}
Ok(fused)
}
/// If this bridge contains sufficient auth fragment information, construct an authentication
/// path for the specified position by interleaving with values from the prior frontier. This
/// method will panic if the position of the prior frontier does not match this bridge's prior
/// position.
fn witness(
&self,
depth: u8,
prior_frontier: &NonEmptyFrontier<H>,
) -> Result<Vec<H>, WitnessingError> {
assert!(Some(prior_frontier.position()) == self.prior_position);
prior_frontier.witness(depth, |addr| {
let r = addr.position_range();
if self.frontier.position() < r.start {
Some(H::empty_root(addr.level()))
} else if r.contains(&self.frontier.position()) {
Some(self.frontier.root(Some(addr.level())))
} else {
// the frontier's position is after the end of the requested
// range, so the requested value should exist in a stored
// fragment
self.ommers.get(&addr).cloned()
}
})
}
fn retain(&mut self, ommer_addrs: &BTreeSet<Address>) {
// Prune away any ommers & tracking addresses we don't need
self.tracking
.retain(|addr| ommer_addrs.contains(&addr.sibling()));
self.ommers.retain(|addr, _| ommer_addrs.contains(addr));
}
}
/// A data structure used to store the information necessary to "rewind" the state of a
/// [`BridgeTree`] to a particular leaf position.
///
/// This is needed because the [`BridgeTree::marked_indices`] map is a cache of information that
/// crosses [`MerkleBridge`] boundaries, and so it is not sufficient to just truncate the list of
/// bridges; instead, we use [`Checkpoint`] values to be able to rapidly restore the cache to its
/// previous state.
#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
pub struct Checkpoint {
/// The number of bridges that will be retained in a rewind.
bridges_len: usize,
/// A flag indicating whether or not the current state of the tree
/// had been marked at the time the checkpoint was created.
is_marked: bool,
/// A set of the positions that have been marked during the period that this
/// checkpoint is the current checkpoint.
marked: BTreeSet<Position>,
/// When a mark is forgotten, if the index of the forgotten mark is <= bridge_idx we
/// record it in the current checkpoint so that on rollback, we restore the forgotten
/// marks to the BridgeTree's "saved" list. If the mark was newly created since the
/// checkpoint, we don't need to remember when we forget it because both the mark
/// creation and removal will be reverted in the rollback.
forgotten: BTreeMap<Position, usize>,
}
impl Checkpoint {
/// Creates a new checkpoint from its constituent parts.
pub fn from_parts(
bridges_len: usize,
is_marked: bool,
marked: BTreeSet<Position>,
forgotten: BTreeMap<Position, usize>,
) -> Self {
Self {
bridges_len,
is_marked,
marked,
forgotten,
}
}
/// Creates a new empty checkpoint for the specified [`BridgeTree`] state.
pub fn at_length(bridges_len: usize, is_marked: bool) -> Self {
Checkpoint {
bridges_len,
is_marked,
marked: BTreeSet::new(),
forgotten: BTreeMap::new(),
}
}
/// Returns the length of the [`BridgeTree::prior_bridges`] vector of the [`BridgeTree`] to
/// which this checkpoint refers.
///
/// This is the number of bridges that will be retained in the event of a rewind to this
/// checkpoint.
pub fn bridges_len(&self) -> usize {
self.bridges_len
}
/// Returns whether the current state of the tree had been marked at the point that
/// this checkpoint was made.
///
/// In the event of a rewind, the rewind logic will ensure that mark information is
/// properly reconstituted for the checkpointed tree state.
pub fn is_marked(&self) -> bool {
self.is_marked
}
/// Returns a set of the positions that have been marked during the period that this
/// checkpoint is the current checkpoint.
pub fn marked(&self) -> &BTreeSet<Position> {
&self.marked
}
/// Returns the set of previously-marked positions that have had their marks removed
/// during the period that this checkpoint is the current checkpoint.
pub fn forgotten(&self) -> &BTreeMap<Position, usize> {
&self.forgotten
}
// A private convenience method that returns the root of the bridge corresponding to
// this checkpoint at a specified depth, given the slice of bridges from which this checkpoint
// was derived.
fn root<H>(&self, bridges: &[MerkleBridge<H>], level: Level) -> H
where
H: Hashable + Clone + Ord,
{
if self.bridges_len == 0 {
H::empty_root(level)
} else {
bridges[self.bridges_len - 1].frontier().root(Some(level))
}
}
// A private convenience method that returns the position of the bridge corresponding
// to this checkpoint, if the checkpoint is not for the empty bridge.
fn position<H: Ord>(&self, bridges: &[MerkleBridge<H>]) -> Option<Position> {
if self.bridges_len == 0 {
None
} else {
Some(bridges[self.bridges_len - 1].position())
}
}
// A private method that rewrites the indices of each forgotten marked record
// using the specified rewrite function. Used during garbage collection.
fn rewrite_indices<F: Fn(usize) -> usize>(&mut self, f: F) {
self.bridges_len = f(self.bridges_len);
for v in self.forgotten.values_mut() {
*v = f(*v)
}
}
}
/// A sparse representation of a Merkle tree with linear appending of leaves that contains enough
/// information to produce a witness for any `mark`ed leaf.
#[derive(Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct BridgeTree<H, const DEPTH: u8> {
/// The ordered list of Merkle bridges representing the history
/// of the tree. There will be one bridge for each saved leaf.
prior_bridges: Vec<MerkleBridge<H>>,
/// The current (mutable) bridge at the tip of the tree.
current_bridge: Option<MerkleBridge<H>>,
/// A map from positions for which we wish to be able to compute a
/// witness to index in the bridges vector.
saved: BTreeMap<Position, usize>,
/// A stack of bridge indices to which it's possible to rewind directly.
checkpoints: Vec<Checkpoint>,
/// The maximum number of checkpoints to retain. If this number is
/// exceeded, the oldest checkpoint will be dropped when creating
/// a new checkpoint.
max_checkpoints: usize,
}
impl<H: Hashable + Ord + Debug, const DEPTH: u8> Debug for BridgeTree<H, DEPTH> {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> Result<(), std::fmt::Error> {
write!(
f,
"BridgeTree {{\n depth: {:?},\n prior_bridges: {:?},\n current_bridge: {:?},\n saved: {:?},\n checkpoints: {:?},\n max_checkpoints: {:?}\n}}",
DEPTH, self.prior_bridges, self.current_bridge, self.saved, self.checkpoints, self.max_checkpoints
)
}
}
/// Errors that can appear when validating the internal consistency of a `[BridgeTree]`
/// value when constructing a tree from its constituent parts.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum BridgeTreeError {
IncorrectIncompleteIndex,
InvalidMarkIndex(usize),
PositionMismatch { expected: Position, found: Position },
InvalidSavePoints,
Discontinuity(ContinuityError),
CheckpointMismatch,
}
impl<H, const DEPTH: u8> BridgeTree<H, DEPTH> {
/// Construct an empty BridgeTree value with the specified maximum number of checkpoints.
pub fn new(max_checkpoints: usize) -> Self {
Self {
prior_bridges: vec![],
current_bridge: None,
saved: BTreeMap::new(),
checkpoints: vec![],
max_checkpoints,
}
}
/// Removes the oldest checkpoint. Returns true if successful and false if
/// there are no checkpoints.
fn drop_oldest_checkpoint(&mut self) -> bool {
if self.checkpoints.is_empty() {
false
} else {
self.checkpoints.remove(0);
true
}
}
/// Returns the prior bridges that make up this tree
pub fn prior_bridges(&self) -> &[MerkleBridge<H>] {
&self.prior_bridges
}
/// Returns the current bridge at the tip of this tree
pub fn current_bridge(&self) -> &Option<MerkleBridge<H>> {
&self.current_bridge
}
/// Returns the map from leaf positions that have been marked to the index of
/// the bridge whose tip is at that position in this tree's list of bridges.
pub fn marked_indices(&self) -> &BTreeMap<Position, usize> {
&self.saved
}
/// Returns the checkpoints to which this tree may be rewound.
pub fn checkpoints(&self) -> &[Checkpoint] {
&self.checkpoints
}
/// Returns the maximum number of checkpoints that will be maintained
/// by the data structure. When this number of checkpoints is exceeded,
/// the oldest checkpoints are discarded when creating new checkpoints.
pub fn max_checkpoints(&self) -> usize {
self.max_checkpoints
}
/// Returns the bridge's frontier.
pub fn frontier(&self) -> Option<&NonEmptyFrontier<H>> {
self.current_bridge.as_ref().map(|b| b.frontier())
}
}
impl<H: Hashable + Ord + Clone, const DEPTH: u8> BridgeTree<H, DEPTH> {
/// Construct a new BridgeTree that will start recording changes from the state of
/// the specified frontier.
pub fn from_frontier(max_checkpoints: usize, frontier: NonEmptyFrontier<H>) -> Self {
Self {
prior_bridges: vec![],
current_bridge: Some(MerkleBridge::from_parts(
None,
BTreeSet::new(),
BTreeMap::new(),
frontier,
)),
saved: BTreeMap::new(),
checkpoints: vec![],
max_checkpoints,
}
}
/// Construct a new BridgeTree from its constituent parts, checking for internal
/// consistency.
pub fn from_parts(
prior_bridges: Vec<MerkleBridge<H>>,
current_bridge: Option<MerkleBridge<H>>,
saved: BTreeMap<Position, usize>,
checkpoints: Vec<Checkpoint>,
max_checkpoints: usize,
) -> Result<Self, BridgeTreeError> {
Self::check_consistency_internal(
&prior_bridges,
¤t_bridge,
&saved,
&checkpoints,
max_checkpoints,
)?;
Ok(BridgeTree {
prior_bridges,
current_bridge,
saved,
checkpoints,
max_checkpoints,
})
}
fn check_consistency(&self) -> Result<(), BridgeTreeError> {
Self::check_consistency_internal(
&self.prior_bridges,
&self.current_bridge,
&self.saved,
&self.checkpoints,
self.max_checkpoints,
)
}
fn check_consistency_internal(
prior_bridges: &[MerkleBridge<H>],
current_bridge: &Option<MerkleBridge<H>>,
saved: &BTreeMap<Position, usize>,
checkpoints: &[Checkpoint],
max_checkpoints: usize,
) -> Result<(), BridgeTreeError> {
// check that saved values correspond to bridges
for (pos, i) in saved {
if i >= &prior_bridges.len() {
return Err(BridgeTreeError::InvalidMarkIndex(*i));
}
let found = prior_bridges[*i].position();
if &found != pos {
return Err(BridgeTreeError::PositionMismatch {
expected: *pos,
found,
});
}
}
if checkpoints.len() > max_checkpoints
|| checkpoints
.iter()
.any(|c| c.bridges_len > prior_bridges.len())
{
return Err(BridgeTreeError::CheckpointMismatch);
}
for (prev, next) in prior_bridges.iter().zip(prior_bridges.iter().skip(1)) {
prev.check_continuity(next)
.map_err(BridgeTreeError::Discontinuity)?;
}
if let Some((prev, next)) = prior_bridges.last().zip(current_bridge.as_ref()) {
prev.check_continuity(next)
.map_err(BridgeTreeError::Discontinuity)?;
}
Ok(())
}sourcepub fn position_range(&self) -> Range<Position>
pub fn position_range(&self) -> Range<Position>
Returns the range of positions observed by this bridge.
source§impl<'a, H: Hashable + Ord + Clone + 'a> MerkleBridge<H>
impl<'a, H: Hashable + Ord + Clone + 'a> MerkleBridge<H>
sourcepub fn successor(&self, mark_current_leaf: bool) -> Self
pub fn successor(&self, mark_current_leaf: bool) -> Self
Constructs a new bridge to follow this one. If mark_current_leaf is true, the successor
will track the information necessary to create a witness for the leaf most
recently appended to this bridge’s frontier.
Examples found in repository?
src/lib.rs (line 952)
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pub fn mark(&mut self) -> Option<Position> {
match self.current_bridge.take() {
Some(mut cur_b) => {
cur_b.track_current_leaf();
let pos = cur_b.position();
// If the latest bridge is a newly created checkpoint, the last prior
// bridge will have the same position and all we need to do is mark
// the checkpointed leaf as being saved.
if self
.prior_bridges
.last()
.map_or(false, |prior_b| prior_b.position() == cur_b.position())
{
// the current bridge has not been advanced, so we just need to make
// sure that we have are tracking the marked leaf
self.current_bridge = Some(cur_b);
} else {
let successor = cur_b.successor(true);
self.prior_bridges.push(cur_b);
self.current_bridge = Some(successor);
}
self.saved
.entry(pos)
.or_insert(self.prior_bridges.len() - 1);
// mark the position as having been marked in the current checkpoint
if let Some(c) = self.checkpoints.last_mut() {
if !c.is_marked {
c.marked.insert(pos);
}
}
Some(pos)
}
None => None,
}
}
/// Return a set of all the positions for which we have marked.
pub fn marked_positions(&self) -> BTreeSet<Position> {
self.saved.keys().cloned().collect()
}
/// Returns the leaf at the specified position if the tree can produce
/// a witness for it.
pub fn get_marked_leaf(&self, position: Position) -> Option<&H> {
self.saved
.get(&position)
.and_then(|idx| self.prior_bridges.get(*idx).map(|b| b.current_leaf()))
}
/// Marks the value at the specified position as a value we're no longer
/// interested in maintaining a mark for. Returns true if successful and
/// false if we were already not maintaining a mark at this position.
pub fn remove_mark(&mut self, position: Position) -> bool {
if let Some(idx) = self.saved.remove(&position) {
// If the position is one that has *not* just been marked since the last checkpoint,
// then add it to the set of those forgotten during the current checkpoint span so that
// it can be restored on rollback.
if let Some(c) = self.checkpoints.last_mut() {
if !c.marked.contains(&position) {
c.forgotten.insert(position, idx);
}
}
true
} else {
false
}
}
/// Creates a new checkpoint for the current tree state. It is valid to
/// have multiple checkpoints for the same tree state, and each `rewind`
/// call will remove a single checkpoint.
pub fn checkpoint(&mut self) {
match self.current_bridge.take() {
Some(cur_b) => {
let is_marked = self.get_marked_leaf(cur_b.position()).is_some();
// Do not create a duplicate bridge
if self
.prior_bridges
.last()
.map_or(false, |pb| pb.position() == cur_b.position())
{
self.current_bridge = Some(cur_b);
} else {
self.current_bridge = Some(cur_b.successor(false));
self.prior_bridges.push(cur_b);
}
self.checkpoints
.push(Checkpoint::at_length(self.prior_bridges.len(), is_marked));
}
None => {
self.checkpoints.push(Checkpoint::at_length(0, false));
}
}
if self.checkpoints.len() > self.max_checkpoints {
self.drop_oldest_checkpoint();
}
}
/// Rewinds the tree state to the previous checkpoint, and then removes
/// that checkpoint record. If there are multiple checkpoints at a given
/// tree state, the tree state will not be altered until all checkpoints
/// at that tree state have been removed using `rewind`. This function
/// return false and leave the tree unmodified if no checkpoints exist.
pub fn rewind(&mut self) -> bool {
match self.checkpoints.pop() {
Some(mut c) => {
// drop marked values at and above the checkpoint height;
// we will re-mark if necessary.
self.saved.append(&mut c.forgotten);
self.saved.retain(|_, i| *i + 1 < c.bridges_len);
self.prior_bridges.truncate(c.bridges_len);
self.current_bridge = self.prior_bridges.last().map(|b| b.successor(c.is_marked));
if c.is_marked {
self.mark();
}
true
}
None => false,
}
}
/// Obtains a witness to the value at the specified position,
/// as of the tree state corresponding to the given root.
/// Returns `None` if there is no available witness to that
/// position or if the root does not correspond to a checkpointed
/// root of the tree.
pub fn witness(&self, position: Position, as_of_root: &H) -> Option<Vec<H>> {
self.witness_inner(position, as_of_root).ok()
}
fn witness_inner(&self, position: Position, as_of_root: &H) -> Result<Vec<H>, WitnessingError> {
#[derive(Debug)]
enum AuthBase<'a> {
Current,
Checkpoint(usize, &'a Checkpoint),
NotFound,
}
let max_alt = Level::from(DEPTH);
// Find the earliest checkpoint having a matching root, or the current
// root if it matches and there is no earlier matching checkpoint.
let auth_base = self
.checkpoints
.iter()
.enumerate()
.rev()
.take_while(|(_, c)| c.position(&self.prior_bridges) >= Some(position))
.filter(|(_, c)| &c.root(&self.prior_bridges, max_alt) == as_of_root)
.last()
.map(|(i, c)| AuthBase::Checkpoint(i, c))
.unwrap_or_else(|| {
if self.root(0).as_ref() == Some(as_of_root) {
AuthBase::Current
} else {
AuthBase::NotFound
}
});
let saved_idx = self
.saved
.get(&position)
.or_else(|| {
if let AuthBase::Checkpoint(i, _) = auth_base {
// The saved position might have been forgotten since the checkpoint,
// so look for it in each of the subsequent checkpoints' forgotten
// items.
self.checkpoints[i..].iter().find_map(|c| {
// restore the forgotten position, if that position was not also marked
// in the same checkpoint
c.forgotten
.get(&position)
.filter(|_| !c.marked.contains(&position))
})
} else {
None
}
})
.ok_or(WitnessingError::PositionNotMarked(position))?;
let prior_frontier = &self.prior_bridges[*saved_idx].frontier;
// Fuse the following bridges to obtain a bridge that has all
// of the data to the right of the selected value in the tree,
// up to the specified checkpoint depth.
let fuse_from = saved_idx + 1;
let successor = match auth_base {
AuthBase::Current => {
// fuse all the way up to the current tip
MerkleBridge::fuse_all(
self.prior_bridges[fuse_from..]
.iter()
.chain(&self.current_bridge),
)
.map(|fused| fused.unwrap()) // safe as the iterator being fused is nonempty
.map_err(WitnessingError::BridgeFusionError)
}
AuthBase::Checkpoint(_, checkpoint) if fuse_from < checkpoint.bridges_len => {
// fuse from the provided checkpoint
MerkleBridge::fuse_all(self.prior_bridges[fuse_from..checkpoint.bridges_len].iter())
.map(|fused| fused.unwrap()) // safe as the iterator being fused is nonempty
.map_err(WitnessingError::BridgeFusionError)
}
AuthBase::Checkpoint(_, checkpoint) if fuse_from == checkpoint.bridges_len => {
// The successor bridge should just be the empty successor to the
// checkpointed bridge.
if checkpoint.bridges_len > 0 {
Ok(self.prior_bridges[checkpoint.bridges_len - 1].successor(false))
} else {
Err(WitnessingError::CheckpointInvalid)
}
}
AuthBase::Checkpoint(_, checkpoint) => {
// if the saved index is after the checkpoint, we can't generate
// an auth path
Err(WitnessingError::CheckpointTooDeep(
fuse_from - checkpoint.bridges_len,
))
}
AuthBase::NotFound => {
// we didn't find any suitable auth base
Err(WitnessingError::AuthBaseNotFound)
}
}?;
successor.witness(DEPTH, prior_frontier)
}sourcepub fn append(&mut self, value: H)
pub fn append(&mut self, value: H)
Advances this bridge’s frontier by appending the specified node, and updates any auth path ommers being tracked if necessary.
Examples found in repository?
src/lib.rs (line 888)
879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895
pub fn append(&mut self, value: &H) -> bool {
if let Some(bridge) = self.current_bridge.as_mut() {
if bridge
.frontier
.position()
.is_complete_subtree(Level::from(DEPTH))
{
false
} else {
bridge.append(value.clone());
true
}
} else {
self.current_bridge = Some(MerkleBridge::new(value.clone()));
true
}
}Trait Implementations§
source§impl<H: Clone> Clone for MerkleBridge<H>
impl<H: Clone> Clone for MerkleBridge<H>
source§fn clone(&self) -> MerkleBridge<H>
fn clone(&self) -> MerkleBridge<H>
Returns a copy of the value. Read more
1.0.0 · source§fn clone_from(&mut self, source: &Self)
fn clone_from(&mut self, source: &Self)
Performs copy-assignment from
source. Read moresource§impl<H: Debug> Debug for MerkleBridge<H>
impl<H: Debug> Debug for MerkleBridge<H>
source§impl<'de, H> Deserialize<'de> for MerkleBridge<H>where
H: Deserialize<'de>,
impl<'de, H> Deserialize<'de> for MerkleBridge<H>where
H: Deserialize<'de>,
source§fn deserialize<__D>(__deserializer: __D) -> Result<Self, __D::Error>where
__D: Deserializer<'de>,
fn deserialize<__D>(__deserializer: __D) -> Result<Self, __D::Error>where
__D: Deserializer<'de>,
Deserialize this value from the given Serde deserializer. Read more
source§impl<H: PartialEq> PartialEq<MerkleBridge<H>> for MerkleBridge<H>
impl<H: PartialEq> PartialEq<MerkleBridge<H>> for MerkleBridge<H>
source§fn eq(&self, other: &MerkleBridge<H>) -> bool
fn eq(&self, other: &MerkleBridge<H>) -> bool
This method tests for
self and other values to be equal, and is used
by ==.