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//! # `bridgetree`
//!
//! This crate provides an implementation of an append-only Merkle tree structure. Individual
//! leaves of the merkle tree may be marked such that witnesses will be maintained for the marked
//! leaves as additional nodes are appended to the tree, but leaf and node data not specifically
//! required to maintain these witnesses is not retained, for space efficiency. The data structure
//! also supports checkpointing of the tree state such that the tree may be reset to a previously
//! checkpointed state, up to a fixed number of checkpoints.
//!
//! The crate also supports using "bridges" containing the minimal possible amount of data to
//! advance witnesses for marked leaves data up to recent checkpoints or the the latest state of
//! the tree without having to append each intermediate leaf individually, given a bridge between
//! the desired states computed by an outside source. The state of the tree is internally
//! represented as a set of such bridges, and the data structure supports fusing and splitting of
//! bridges.
//!
//! ## Marking
//!
//! Merkle trees can be used to show that a value exists in the tree by providing a witness
//! to a leaf value. We provide an API that allows us to mark the current leaf as a value we wish
//! to compute witnesses for even after the tree has been appended to in the future; this is called
//! maintaining a witness. When we're later no longer in a leaf, we can remove the mark and drop
//! the now unnecessary information from the structure.
//!
//! ## Checkpoints and Rollbacks
//!
//! This data structure supports a limited capability to restore previous states of the Merkle
//! tree. It is possible identify the current state of the tree as a "checkpoint" to which the tree
//! can be reset, and later remove checkpoints that we're no longer interested in being able to
//! reset the state to.
//!
//! In this module, the term "ommer" is used as for the sibling of a parent node in a binary tree.
pub use incrementalmerkletree::{
frontier::{Frontier, NonEmptyFrontier},
Address, Hashable, Level, Position, Retention, Source,
};
use std::collections::{BTreeMap, BTreeSet, VecDeque};
use std::fmt::Debug;
use std::ops::Range;
/// Errors that can be discovered during checks that verify the compatibility of adjacent bridges.
#[derive(Clone, Debug, PartialEq, Eq)]
pub enum ContinuityError {
/// Returned when a bridge with no prior position information is
PriorPositionNotFound,
/// Returned when the subsequent bridge's prior position does not match the position of the
/// prior bridge's frontier.
PositionMismatch(Position, Position),
}
/// Errors that can be discovered during the process of attempting to create
/// the witness for a leaf node.
#[derive(Clone, Debug, PartialEq, Eq)]
pub enum WitnessingError {
AuthBaseNotFound,
CheckpointInvalid,
CheckpointTooDeep(usize),
PositionNotMarked(Position),
BridgeFusionError(ContinuityError),
BridgeAddressInvalid(Address),
}
/// 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 (`fuse`d) 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.
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct MerkleBridge<H> {
/// 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.
prior_position: Option<Position>,
/// The set of addresses for which we are waiting to discover the ommers. The values of this
/// set and the keys of the `need` map should always be disjoint. Also, this set should
/// never contain an address for which the sibling value has been discovered; at that point,
/// the address is replaced in this set with its parent and the address/sibling pair is stored
/// in `ommers`.
///
/// Another way to consider the contents of this set is that the values that exist in
/// `ommers`, combined with the values in previous bridges' `ommers` and an original leaf
/// node, already contain all the values needed to compute the value at the given address.
/// Therefore, we are tracking that address as we do not yet have enough information to compute
/// its sibling without filling the sibling subtree with empty nodes.
tracking: BTreeSet<Address>,
/// A map from addresses that were being tracked to the values of their ommers that have been
/// discovered while scanning this bridge's range by adding leaves to the bridge's frontier.
ommers: BTreeMap<Address, H>,
/// The leading edge of the bridge.
frontier: NonEmptyFrontier<H>,
}
impl<H> MerkleBridge<H> {
/// Construct a new Merkle bridge containing only the specified
/// leaf.
pub fn new(value: H) -> Self {
Self {
prior_position: None,
tracking: BTreeSet::new(),
ommers: BTreeMap::new(),
frontier: NonEmptyFrontier::new(value),
}
}
/// Construct a new Merkle bridge from its constituent parts.
pub fn from_parts(
prior_position: Option<Position>,
tracking: BTreeSet<Address>,
ommers: BTreeMap<Address, H>,
frontier: NonEmptyFrontier<H>,
) -> Self {
Self {
prior_position,
tracking,
ommers,
frontier,
}
}
/// 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.
pub fn prior_position(&self) -> Option<Position> {
self.prior_position
}
/// Returns the position of the most recently appended leaf.
pub fn position(&self) -> Position {
self.frontier.position()
}
/// Returns the set of internal node addresses that we're searching
/// for the ommers for.
pub fn tracking(&self) -> &BTreeSet<Address> {
&self.tracking
}
/// Returns the set of internal node addresses that we're searching
/// for the ommers for.
pub fn ommers(&self) -> &BTreeMap<Address, H> {
&self.ommers
}
/// Returns the non-empty frontier of this Merkle bridge.
pub fn frontier(&self) -> &NonEmptyFrontier<H> {
&self.frontier
}
/// Returns the value of the most recently appended leaf.
pub fn current_leaf(&self) -> &H {
self.frontier.leaf()
}
/// Checks whether this bridge is a valid successor for the specified
/// bridge.
pub fn check_continuity(&self, next: &Self) -> Result<(), ContinuityError> {
if let Some(pos) = next.prior_position {
if pos == self.frontier.position() {
Ok(())
} else {
Err(ContinuityError::PositionMismatch(
self.frontier.position(),
pos,
))
}
} else {
Err(ContinuityError::PriorPositionNotFound)
}
}
/// Returns the range of positions observed by this bridge.
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 + Clone + Ord + '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, track_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 track_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()
}
})
.map_err(WitnessingError::BridgeAddressInvalid)
}
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)]
pub struct Checkpoint<C> {
/// The unique identifier for this checkpoint.
id: C,
/// The number of bridges that will be retained in a rewind.
bridges_len: usize,
/// 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, we add it to the checkpoint's forgotten set but
/// don't immediately remove it from the `saved` map; that removal occurs when
/// the checkpoint is eventually dropped.
forgotten: BTreeSet<Position>,
}
impl<C> Checkpoint<C> {
/// Creates a new checkpoint from its constituent parts.
pub fn from_parts(
id: C,
bridges_len: usize,
marked: BTreeSet<Position>,
forgotten: BTreeSet<Position>,
) -> Self {
Self {
id,
bridges_len,
marked,
forgotten,
}
}
/// Creates a new empty checkpoint for the specified [`BridgeTree`] state.
pub fn at_length(bridges_len: usize, id: C) -> Self {
Checkpoint {
id,
bridges_len,
marked: BTreeSet::new(),
forgotten: BTreeSet::new(),
}
}
/// The unique identifier for the checkpoint.
pub fn id(&self) -> &C {
&self.id
}
/// 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 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) -> &BTreeSet<Position> {
&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);
}
}
/// 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)]
pub struct BridgeTree<H, C, 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 deque of bridge indices to which it's possible to rewind directly.
/// This deque must be maintained to have a minimum size of 1 and a maximum
/// size of `max_checkpoints` in order to correctly maintain mark & rewind
/// semantics.
checkpoints: VecDeque<Checkpoint<C>>,
/// 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: Debug, C: Debug, const DEPTH: u8> Debug for BridgeTree<H, C, 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, C, const DEPTH: u8> BridgeTree<H, C, DEPTH> {
/// Construct an empty BridgeTree value with the specified maximum number of checkpoints.
///
/// Panics if `max_checkpoints < 1` because mark/rewind logic depends upon the presence
/// of checkpoints to function.
pub fn new(max_checkpoints: usize) -> Self {
assert!(max_checkpoints >= 1);
Self {
prior_bridges: vec![],
current_bridge: None,
saved: BTreeMap::new(),
checkpoints: VecDeque::new(),
max_checkpoints,
}
}
/// Removes the oldest checkpoint if there are more than `max_checkpoints`. Returns true if
/// successful and false if there are not enough checkpoints.
fn drop_oldest_checkpoint(&mut self) -> bool {
if self.checkpoints.len() > self.max_checkpoints {
let c = self
.checkpoints
.pop_front()
.expect("Checkpoints deque is known to be non-empty.");
for pos in c.forgotten.iter() {
self.saved.remove(pos);
}
true
} else {
false
}
}
/// 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) -> &VecDeque<Checkpoint<C>> {
&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 + Clone + Ord, C: Clone + Ord, const DEPTH: u8> BridgeTree<H, C, 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: VecDeque::new(),
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: VecDeque<Checkpoint<C>>,
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: &VecDeque<Checkpoint<C>>,
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);
true
}
} else {
self.current_bridge = Some(MerkleBridge::new(value));
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) => {
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
cur_b.track_current_leaf();
self.current_bridge = Some(cur_b);
} else {
// the successor(true) call will ensure that the marked leaf is tracked
let successor = cur_b.successor(true);
self.prior_bridges.push(cur_b);
self.current_bridge = Some(successor);
}
// mark the position as having been marked in the current checkpoint
if let std::collections::btree_map::Entry::Vacant(e) = self.saved.entry(pos) {
if let Some(c) = self.checkpoints.back_mut() {
c.marked.insert(pos);
}
e.insert(self.prior_bridges.len() - 1);
}
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 self.saved.contains_key(&position) {
if let Some(c) = self.checkpoints.back_mut() {
c.forgotten.insert(position);
} else {
self.saved.remove(&position);
}
true
} else {
false
}
}
/// Creates a new checkpoint for the current tree state, with the given identifier.
///
/// It is valid to have multiple checkpoints for the same tree state, and each `rewind` call
/// will remove a single checkpoint. Successive checkpoint identifiers must always be provided
/// in increasing order.
pub fn checkpoint(&mut self, id: C) -> bool {
if Some(&id) > self.checkpoints.back().map(|c| &c.id) {
match self.current_bridge.take() {
Some(cur_b) => {
// 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_back(Checkpoint::at_length(self.prior_bridges.len(), id));
}
None => {
self.checkpoints.push_back(Checkpoint::at_length(0, id));
}
}
if self.checkpoints.len() > self.max_checkpoints {
self.drop_oldest_checkpoint();
}
true
} else {
false
}
}
/// 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 {
if let Some(c) = self.checkpoints.pop_back() {
// Remove marks for positions that were marked during the lifetime of this checkpoint.
for pos in c.marked {
self.saved.remove(&pos);
}
self.prior_bridges.truncate(c.bridges_len);
self.current_bridge = self
.prior_bridges
.last()
.map(|b| b.successor(self.saved.contains_key(&b.position())));
true
} else {
false
}
}
/// Obtains a witness for the value at the specified leaf position, as of the tree state at the
/// given checkpoint depth. Returns `None` if there is no witness information for the requested
/// position or if no checkpoint is available at the specified depth.
pub fn witness(
&self,
position: Position,
checkpoint_depth: usize,
) -> Result<Vec<H>, WitnessingError> {
#[derive(Debug)]
enum AuthBase<'a, C> {
Current,
Checkpoint(usize, &'a Checkpoint<C>),
}
// 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 = if checkpoint_depth == 0 {
Ok(AuthBase::Current)
} else if self.checkpoints.len() >= checkpoint_depth {
let c_idx = self.checkpoints.len() - checkpoint_depth;
if self
.checkpoints
.iter()
.skip(c_idx)
.take_while(|c| {
c.position(&self.prior_bridges)
.iter()
.any(|p| p <= &position)
})
.any(|c| c.marked.contains(&position))
{
// The mark had not yet been established at the point the checkpoint was
// created, so we can't treat it as marked.
Err(WitnessingError::PositionNotMarked(position))
} else {
Ok(AuthBase::Checkpoint(c_idx, &self.checkpoints[c_idx]))
}
} else {
Err(WitnessingError::CheckpointInvalid)
}?;
let saved_idx = self
.saved
.get(&position)
.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,
))
}
}?;
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.front().unwrap().bridges_len;
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 self.saved.contains_key(&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
);
}
}
}
#[cfg(test)]
mod tests {
use proptest::prelude::*;
use std::fmt::Debug;
use super::*;
use incrementalmerkletree::{
testing::{
self, apply_operation, arb_operation, check_checkpoint_rewind, check_operations,
check_remove_mark, check_rewind_remove_mark, check_root_hashes, check_witnesses,
complete_tree::CompleteTree, CombinedTree, SipHashable,
},
Hashable,
};
impl<H: Hashable + Clone + Ord, const DEPTH: u8> testing::Tree<H, usize>
for BridgeTree<H, usize, DEPTH>
{
fn append(&mut self, value: H, retention: Retention<usize>) -> bool {
let appended = BridgeTree::append(self, value);
if appended {
if retention.is_marked() {
BridgeTree::mark(self);
}
if let Retention::Checkpoint { id, .. } = retention {
BridgeTree::checkpoint(self, id);
}
}
appended
}
fn depth(&self) -> u8 {
DEPTH
}
fn current_position(&self) -> Option<Position> {
BridgeTree::current_position(self)
}
fn get_marked_leaf(&self, position: Position) -> Option<H> {
BridgeTree::get_marked_leaf(self, position).cloned()
}
fn marked_positions(&self) -> BTreeSet<Position> {
BridgeTree::marked_positions(self)
}
fn root(&self, checkpoint_depth: usize) -> Option<H> {
BridgeTree::root(self, checkpoint_depth)
}
fn witness(&self, position: Position, checkpoint_depth: usize) -> Option<Vec<H>> {
BridgeTree::witness(self, position, checkpoint_depth).ok()
}
fn remove_mark(&mut self, position: Position) -> bool {
BridgeTree::remove_mark(self, position)
}
fn checkpoint(&mut self, id: usize) -> bool {
BridgeTree::checkpoint(self, id)
}
fn rewind(&mut self) -> bool {
BridgeTree::rewind(self)
}
}
#[test]
fn tree_depth() {
let mut tree = BridgeTree::<String, usize, 3>::new(100);
for c in 'a'..'i' {
assert!(tree.append(c.to_string()))
}
assert!(!tree.append('i'.to_string()));
}
fn check_garbage_collect<H: Hashable + Clone + Ord + Debug, const DEPTH: u8>(
mut tree: BridgeTree<H, usize, DEPTH>,
) {
// Add checkpoints until we're sure everything that can be gc'ed will be gc'ed
for i in 0..tree.max_checkpoints {
tree.checkpoint(i + 1);
}
let mut tree_mut = tree.clone();
tree_mut.garbage_collect();
for pos in tree.saved.keys() {
assert_eq!(tree.witness(*pos, 0), tree_mut.witness(*pos, 0));
}
}
fn arb_bridgetree<G: Strategy + Clone>(
item_gen: G,
max_count: usize,
) -> impl Strategy<Value = BridgeTree<G::Value, usize, 8>>
where
G::Value: Hashable + Clone + Ord + Debug + 'static,
{
let pos_gen = (0..max_count).prop_map(|p| Position::try_from(p).unwrap());
proptest::collection::vec(arb_operation(item_gen, pos_gen), 0..max_count).prop_map(|ops| {
let mut tree: BridgeTree<G::Value, usize, 8> = BridgeTree::new(10);
for (i, op) in ops.into_iter().enumerate() {
apply_operation(&mut tree, op.map_checkpoint_id(|_| i));
}
tree
})
}
proptest! {
#[test]
fn bridgetree_from_parts(
tree in arb_bridgetree((97u8..123).prop_map(|c| char::from(c).to_string()), 100)
) {
assert_eq!(
BridgeTree::from_parts(
tree.prior_bridges.clone(),
tree.current_bridge.clone(),
tree.saved.clone(),
tree.checkpoints.clone(),
tree.max_checkpoints
),
Ok(tree),
);
}
#[test]
fn prop_garbage_collect(
tree in arb_bridgetree((97u8..123).prop_map(|c| char::from(c).to_string()), 100)
) {
check_garbage_collect(tree);
}
}
#[test]
fn root_hashes() {
check_root_hashes(BridgeTree::<String, usize, 4>::new);
}
#[test]
fn witness() {
check_witnesses(BridgeTree::<String, usize, 4>::new);
}
#[test]
fn checkpoint_rewind() {
check_checkpoint_rewind(|max_checkpoints| {
BridgeTree::<String, usize, 4>::new(max_checkpoints)
});
}
#[test]
fn rewind_remove_mark() {
check_rewind_remove_mark(|max_checkpoints| {
BridgeTree::<String, usize, 4>::new(max_checkpoints)
});
}
#[test]
fn garbage_collect() {
let mut tree: BridgeTree<String, usize, 7> = BridgeTree::new(1000);
let empty_root = tree.root(0);
tree.append("a".to_string());
for i in 0..100 {
tree.checkpoint(i + 1);
}
tree.garbage_collect();
assert!(tree.root(0) != empty_root);
tree.rewind();
assert!(tree.root(0) != empty_root);
let mut t = BridgeTree::<String, usize, 7>::new(10);
let mut to_unmark = vec![];
let mut has_witness = vec![];
for i in 0u64..100 {
let elem: String = format!("{},", i);
assert!(t.append(elem), "Append should succeed.");
if i % 5 == 0 {
t.checkpoint(usize::try_from(i).unwrap() + 1);
}
if i % 7 == 0 {
t.mark();
if i > 0 && i % 2 == 0 {
to_unmark.push(Position::from(i));
} else {
has_witness.push(Position::from(i));
}
}
if i % 11 == 0 && !to_unmark.is_empty() {
let pos = to_unmark.remove(0);
t.remove_mark(pos);
}
}
// 32 = 20 (checkpointed) + 14 (marked) - 2 (marked & checkpointed)
assert_eq!(t.prior_bridges().len(), 20 + 14 - 2);
let witness = has_witness
.iter()
.map(|pos| match t.witness(*pos, 0) {
Ok(path) => path,
Err(e) => panic!("Failed to get auth path: {:?}", e),
})
.collect::<Vec<_>>();
t.garbage_collect();
// 20 = 32 - 10 (removed checkpoints) + 1 (not removed due to mark) - 3 (removed marks)
assert_eq!(t.prior_bridges().len(), 32 - 10 + 1 - 3);
let retained_witness = has_witness
.iter()
.map(|pos| t.witness(*pos, 0).expect("Must be able to get auth path"))
.collect::<Vec<_>>();
assert_eq!(witness, retained_witness);
}
// Combined tree tests
fn new_combined_tree<H: Hashable + Clone + Ord + Debug>(
max_checkpoints: usize,
) -> CombinedTree<H, usize, CompleteTree<H, usize, 4>, BridgeTree<H, usize, 4>> {
CombinedTree::new(
CompleteTree::<H, usize, 4>::new(max_checkpoints),
BridgeTree::<H, usize, 4>::new(max_checkpoints),
)
}
#[test]
fn combined_remove_mark() {
check_remove_mark(new_combined_tree);
}
#[test]
fn combined_rewind_remove_mark() {
check_rewind_remove_mark(new_combined_tree);
}
proptest! {
#![proptest_config(ProptestConfig::with_cases(100000))]
#[test]
fn check_randomized_u64_ops(
ops in proptest::collection::vec(
arb_operation(
(0..32u64).prop_map(SipHashable),
(0u64..100).prop_map(Position::from)
),
1..100
)
) {
let tree = new_combined_tree(100);
let indexed_ops = ops.iter().enumerate().map(|(i, op)| op.map_checkpoint_id(|_| i + 1)).collect::<Vec<_>>();
check_operations(tree, &indexed_ops)?;
}
#[test]
fn check_randomized_str_ops(
ops in proptest::collection::vec(
arb_operation(
(97u8..123).prop_map(|c| char::from(c).to_string()),
(0u64..100).prop_map(Position::from)
),
1..100
)
) {
let tree = new_combined_tree(100);
let indexed_ops = ops.iter().enumerate().map(|(i, op)| op.map_checkpoint_id(|_| i + 1)).collect::<Vec<_>>();
check_operations(tree, &indexed_ops)?;
}
}
}