Trait bridgetree::Hashable

pub trait Hashable: Sized {
    fn empty_leaf() -> Self;
    fn combine(level: Level, a: &Self, b: &Self) -> Self;

    fn empty_root(level: Level) -> Self { ... }
}

A trait describing the operations that make a type suitable for use as a leaf or node value in a merkle tree.

Required Methods

fn empty_leaf() -> Self

fn combine(level: Level, a: &Self, b: &Self) -> Self

Provided Methods

fn empty_root(level: Level) -> Self

Examples found in repository

src/lib.rs (line 200)

    pub fn root(&self, root_level: Option<Level>) -> H {
        let max_level = root_level.unwrap_or_else(|| self.position.root_level());
        self.position
            .witness_addrs(max_level)
            .fold(
                (self.leaf.clone(), Level::from(0)),
                |(digest, complete_lvl), (addr, source)| {
                    // fold up from complete_lvl to addr.level() pairing with empty roots; if
                    // complete_lvl == addr.level() this is just the complete digest to this point
                    let digest = complete_lvl
                        .iter_to(addr.level())
                        .fold(digest, |d, l| H::combine(l, &d, &H::empty_root(l)));

                    let res_digest = match source {
                        Source::Past(i) => H::combine(addr.level(), &self.ommers[i], &digest),
                        Source::Future => {
                            H::combine(addr.level(), &digest, &H::empty_root(addr.level()))
                        }
                    };

                    (res_digest, addr.level() + 1)
                },
            )
            .0
    }

    /// Constructs a witness for the leaf at the tip of this
    /// frontier, given a source of node values that complement this frontier.
    pub fn witness<F>(&self, depth: u8, bridge_value_at: F) -> Result<Vec<H>, WitnessingError>
    where
        F: Fn(Address) -> Option<H>,
    {
        // construct a complete trailing edge that includes the data from
        // the following frontier not yet included in the trailing edge.
        self.position()
            .witness_addrs(depth.into())
            .map(|(addr, source)| match source {
                Source::Past(i) => Ok(self.ommers[i].clone()),
                Source::Future => {
                    bridge_value_at(addr).ok_or(WitnessingError::BridgeAddressInvalid(addr))
                }
            })
            .collect::<Result<Vec<_>, _>>()
    }
}

/// A possibly-empty Merkle frontier.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct Frontier<H, const DEPTH: u8> {
    frontier: Option<NonEmptyFrontier<H>>,
}

impl<H, const DEPTH: u8> TryFrom<NonEmptyFrontier<H>> for Frontier<H, DEPTH> {
    type Error = FrontierError;
    fn try_from(f: NonEmptyFrontier<H>) -> Result<Self, FrontierError> {
        if f.position.root_level() <= Level::from(DEPTH) {
            Ok(Frontier { frontier: Some(f) })
        } else {
            Err(FrontierError::MaxDepthExceeded {
                depth: f.position.root_level().into(),
            })
        }
    }
}

impl<H, const DEPTH: u8> Frontier<H, DEPTH> {
    /// Constructs a new empty frontier.
    pub fn empty() -> Self {
        Self { frontier: None }
    }

    /// Constructs a new frontier from its constituent parts.
    ///
    /// Returns `None` if the new frontier would exceed the maximum
    /// allowed depth or if the list of ommers provided is not consistent
    /// with the position of the leaf.
    pub fn from_parts(position: Position, leaf: H, ommers: Vec<H>) -> Result<Self, FrontierError> {
        NonEmptyFrontier::from_parts(position, leaf, ommers).and_then(Self::try_from)
    }

    /// Return the wrapped NonEmptyFrontier reference, or None if
    /// the frontier is empty.
    pub fn value(&self) -> Option<&NonEmptyFrontier<H>> {
        self.frontier.as_ref()
    }

    /// Returns the amount of memory dynamically allocated for ommer
    /// values within the frontier.
    pub fn dynamic_memory_usage(&self) -> usize {
        self.frontier.as_ref().map_or(0, |f| {
            size_of::<usize>() + (f.ommers.capacity() + 1) * size_of::<H>()
        })
    }
}

impl<H: Hashable + Clone, const DEPTH: u8> Frontier<H, DEPTH> {
    /// Appends a new value to the frontier at the next available slot.
    /// Returns true if successful and false if the frontier would exceed
    /// the maximum allowed depth.
    pub fn append(&mut self, value: &H) -> bool {
        if let Some(frontier) = self.frontier.as_mut() {
            if frontier.position().is_complete_subtree(DEPTH.into()) {
                false
            } else {
                frontier.append(value.clone());
                true
            }
        } else {
            self.frontier = Some(NonEmptyFrontier::new(value.clone()));
            true
        }
    }

    /// Obtains the current root of this Merkle frontier by hashing
    /// against empty nodes up to the maximum height of the pruned
    /// tree that the frontier represents.
    pub fn root(&self) -> H {
        self.frontier
            .as_ref()
            .map_or(H::empty_root(DEPTH.into()), |frontier| {
                frontier.root(Some(DEPTH.into()))
            })
    }
}

/// 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, Serialize, Deserialize)]
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 + 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,
            &current_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
        }
    }

Implementors