522 lines
17 KiB
Rust
522 lines
17 KiB
Rust
//! # `incrementalmerkletree`
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//!
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//! Incremental Merkle Trees are fixed-depth Merkle trees with two primary
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//! capabilities: appending (assigning a value to the next unused leaf and
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//! advancing the tree) and obtaining the root of the tree. Importantly the tree
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//! structure attempts to store the least amount of information necessary to
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//! continue to function; other information should be pruned eagerly to avoid
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//! waste when the tree state is encoded.
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//!
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//! ## Witnessing
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//!
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//! Merkle trees are typically used to show that a value exists in the tree via
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//! an authentication path. We need an API that allows us to identify the
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//! current leaf as a value we wish to compute authentication paths for even as
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//! the tree continues to be appended to in the future; this is called
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//! maintaining a witness. When we're later uninterested in such a leaf, we can
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//! prune a witness and remove all unnecessary information from the structure as
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//! a consequence.
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//!
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//! ## Checkpoints and Rollbacks
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//!
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//! The structure is not append-only in the strict sense. It is possible to
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//! identify the current state of the tree as a "checkpoint" and to remove older
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//! checkpoints that we're no longer interested in. It should be possible to
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//! roll back to any previous checkpoint.
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pub trait TreeHasher {
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type Digest: Clone + PartialEq;
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fn empty_leaf() -> Self::Digest;
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fn combine(a: &Self::Digest, b: &Self::Digest) -> Self::Digest;
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}
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pub struct Tree<H: TreeHasher> {
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leaves: Vec<H::Digest>,
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current_position: usize,
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witnesses: Vec<(usize, H::Digest)>,
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checkpoints: Vec<usize>,
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depth: usize,
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}
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impl<H: TreeHasher> Tree<H> {
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/// Creates a new, empty binary tree of specified depth.
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///
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/// # Panics
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///
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/// Panics if the specified depth is zero.
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pub fn new(depth: usize) -> Self {
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if depth == 0 {
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panic!("invalid depth for incremental merkle tree");
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}
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Tree {
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leaves: vec![H::empty_leaf(); 1 << depth],
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current_position: 0,
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witnesses: vec![],
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checkpoints: vec![],
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depth,
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}
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}
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/// Appends a new value to the tree at the next available slot. Returns true
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/// if successful and false if the tree is full.
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pub fn append(&mut self, value: &H::Digest) -> bool {
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if self.current_position == (1 << self.depth) {
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false
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} else {
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self.leaves[self.current_position] = value.clone();
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self.current_position += 1;
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true
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}
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}
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/// Obtains the current root of this Merkle tree.
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pub fn root(&self) -> H::Digest {
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lazy_root::<H>(self.leaves.clone())
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}
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/// Marks the current tree state leaf as a value that we're interested in
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/// witnessing. Returns true if successful and false if the tree is empty.
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pub fn witness(&mut self) -> bool {
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if self.current_position == 0 {
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return false;
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} else {
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let value = self.leaves[self.current_position - 1].clone();
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self.witnesses.push((self.current_position - 1, value));
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true
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}
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}
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/// Obtains an authentication path to the value specified in the tree.
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/// Returns `None` if there is no available authentication path to the
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/// specified value.
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pub fn authentication_path(&self, value: &H::Digest) -> Option<(usize, Vec<H::Digest>)> {
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self.witnesses
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.iter()
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.find(|witness| witness.1 == *value)
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.map(|&(pos, _)| {
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let mut path = vec![];
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let mut index = pos;
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for bit in 0..self.depth {
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index ^= 1 << bit;
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path.push(lazy_root::<H>(self.leaves[index..][0..(1 << bit)].to_vec()));
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index &= usize::MAX << (bit + 1);
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}
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(pos, path)
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})
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}
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/// Marks the specified tree state value as a value we're no longer
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/// interested in maintaining a witness for. Returns true if successful and
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/// false if the value is not a known witness.
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pub fn remove_witness(&mut self, value: &H::Digest) -> bool {
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if let Some((position, _)) = self
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.witnesses
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.iter()
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.enumerate()
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.find(|witness| (witness.1).1 == *value)
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{
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self.witnesses.remove(position);
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true
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} else {
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false
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}
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}
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/// Marks the current tree state as a checkpoint if it is not already a
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/// checkpoint.
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pub fn checkpoint(&mut self) {
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self.checkpoints.push(self.current_position);
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}
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/// Rewinds the tree state to the previous checkpoint. This function will
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/// fail and return false if there is no previous checkpoint or in the event
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/// witness data would be destroyed in the process.
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pub fn rewind(&mut self) -> bool {
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if let Some(checkpoint) = self.checkpoints.pop() {
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if self.witnesses.iter().any(|&(pos, _)| pos >= checkpoint) {
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self.checkpoints.push(checkpoint);
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return false;
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}
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self.current_position = checkpoint;
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if checkpoint != (1 << self.depth) {
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self.leaves[checkpoint..].fill(H::empty_leaf());
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}
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true
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} else {
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false
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}
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}
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/// Removes the oldest checkpoint. Returns true if successful and false if
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/// there are no checkpoints.
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pub fn pop_checkpoint(&mut self) -> bool {
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if self.checkpoints.is_empty() {
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false
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} else {
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self.checkpoints.remove(0);
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true
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}
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}
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/// Start a recording of append operations performed on a tree.
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pub fn recording(&self) -> Recording<H> {
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Recording {
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start_position: self.current_position,
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current_position: self.current_position,
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depth: self.depth,
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appends: vec![],
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}
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}
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/// Plays a recording of append operations back. Returns true if successful
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/// and false if the recording is incompatible with the current tree state.
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pub fn play(&mut self, recording: &Recording<H>) -> bool {
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if recording.start_position == self.current_position && self.depth == recording.depth {
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for val in recording.appends.iter() {
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self.append(val);
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}
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true
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} else {
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false
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}
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}
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}
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pub struct Recording<H: TreeHasher> {
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start_position: usize,
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current_position: usize,
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depth: usize,
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appends: Vec<H::Digest>,
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}
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impl<H: TreeHasher> Recording<H> {
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/// Appends a new value to the tree at the next available slot. Returns true
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/// if successful and false if the tree is full.
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pub fn append(&mut self, value: &H::Digest) -> bool {
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if self.current_position == (1 << self.depth) {
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false
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} else {
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self.appends.push(value.clone());
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self.current_position += 1;
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true
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}
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}
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/// Plays a recording of append operations back. Returns true if successful
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/// and false if the provided recording is incompatible with `Self`.
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pub fn play(&mut self, recording: &Self) -> bool {
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if self.current_position == recording.start_position && self.depth == recording.depth {
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self.appends.extend_from_slice(&recording.appends);
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self.current_position = recording.current_position;
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true
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} else {
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false
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}
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}
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}
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fn lazy_root<H: TreeHasher>(mut leaves: Vec<H::Digest>) -> H::Digest {
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while leaves.len() != 1 {
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leaves = leaves
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.iter()
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.enumerate()
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.filter(|(i, _)| (i % 2) == 0)
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.map(|(_, a)| a)
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.zip(
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leaves
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.iter()
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.enumerate()
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.filter(|(i, _)| (i % 2) == 1)
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.map(|(_, b)| b),
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)
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.map(|(a, b)| H::combine(a, b))
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.collect();
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}
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leaves[0].clone()
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}
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#[cfg(test)]
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mod tests {
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#![allow(deprecated)]
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use super::*;
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use std::hash::Hasher;
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use std::hash::SipHasher as Hash;
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impl TreeHasher for Hash {
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type Digest = u64;
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fn empty_leaf() -> Self::Digest {
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0
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}
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fn combine(a: &Self::Digest, b: &Self::Digest) -> Self::Digest {
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let mut hasher = Hash::new();
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hasher.write_u64(*a);
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hasher.write_u64(*b);
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hasher.finish()
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}
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}
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fn compute_root_from_auth_path<H: TreeHasher>(
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value: H::Digest,
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position: usize,
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path: &[H::Digest],
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) -> H::Digest {
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let mut cur = value;
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for (i, v) in path
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.iter()
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.enumerate()
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.map(|(i, v)| (((position >> i) & 1) == 1, v))
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{
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if i {
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cur = H::combine(v, &cur);
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} else {
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cur = H::combine(&cur, v);
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}
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}
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cur
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}
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#[test]
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fn test_compute_root_from_auth_path() {
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let expected = Hash::combine(
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&Hash::combine(&Hash::combine(&0, &1), &Hash::combine(&2, &3)),
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&Hash::combine(&Hash::combine(&4, &5), &Hash::combine(&6, &7)),
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);
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assert_eq!(
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compute_root_from_auth_path::<Hash>(
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0,
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0,
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&[
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1,
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Hash::combine(&2, &3),
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Hash::combine(&Hash::combine(&4, &5), &Hash::combine(&6, &7))
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]
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),
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expected
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);
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assert_eq!(
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compute_root_from_auth_path::<Hash>(
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4,
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4,
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&[
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5,
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Hash::combine(&6, &7),
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Hash::combine(&Hash::combine(&0, &1), &Hash::combine(&2, &3))
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]
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),
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expected
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);
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}
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#[test]
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fn correct_empty_root() {
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const DEPTH: usize = 5;
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let mut expected = 0u64;
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for _ in 0..DEPTH {
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expected = Hash::combine(&expected, &expected);
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}
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let tree = Tree::<Hash>::new(DEPTH);
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assert_eq!(tree.root(), expected);
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}
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#[test]
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fn correct_root() {
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const DEPTH: usize = 3;
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let values: Vec<u64> = (0..(1 << DEPTH)).collect();
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let mut tree = Tree::<Hash>::new(DEPTH);
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for value in values.iter() {
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assert!(tree.append(value));
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}
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assert!(!tree.append(&0));
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let expected = Hash::combine(
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&Hash::combine(&Hash::combine(&0, &1), &Hash::combine(&2, &3)),
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&Hash::combine(&Hash::combine(&4, &5), &Hash::combine(&6, &7)),
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);
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assert_eq!(tree.root(), expected);
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}
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#[test]
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fn correct_auth_path() {
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const DEPTH: usize = 3;
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let values: Vec<u64> = (0..(1 << DEPTH)).collect();
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let mut tree = Tree::<Hash>::new(DEPTH);
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for value in values.iter() {
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assert!(tree.append(value));
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tree.witness();
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}
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assert!(!tree.append(&0));
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let expected = Hash::combine(
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&Hash::combine(&Hash::combine(&0, &1), &Hash::combine(&2, &3)),
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&Hash::combine(&Hash::combine(&4, &5), &Hash::combine(&6, &7)),
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);
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assert_eq!(tree.root(), expected);
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for i in 0..(1 << DEPTH) {
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println!("value: {}", i);
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let (position, path) = tree.authentication_path(&i).unwrap();
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assert_eq!(
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compute_root_from_auth_path::<Hash>(i, position, &path),
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expected
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);
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}
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}
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use proptest::prelude::*;
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#[derive(Clone, Debug)]
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enum Operation {
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Append(u64),
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Witness,
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Unwitness(u64),
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Checkpoint,
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Rewind,
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PopCheckpoint,
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Authpath(u64),
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}
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use Operation::*;
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prop_compose! {
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fn arb_operation()
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(
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opid in (0..7),
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item in (0..32u64),
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)
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-> Operation
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{
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match opid {
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0 => Append(item),
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1 => Witness,
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2 => Unwitness(item),
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3 => Checkpoint,
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4 => Rewind,
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5 => PopCheckpoint,
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6 => Authpath(item),
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_ => unimplemented!()
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}
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}
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}
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proptest! {
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#[test]
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fn do_stuff(ops in proptest::collection::vec(arb_operation(), 1..200)) {
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const DEPTH: usize = 4;
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let mut tree = Tree::<Hash>::new(DEPTH);
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let mut tree_size = 0;
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let mut tree_values = vec![];
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let mut tree_checkpoints = vec![];
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let mut tree_witnesses: Vec<(usize, u64)> = vec![];
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for op in ops {
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assert_eq!(tree_size, tree_values.len());
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match op {
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Append(value) => {
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if tree.append(&value) {
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assert!(tree_size < (1 << DEPTH));
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tree_size += 1;
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tree_values.push(value);
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} else {
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assert!(tree_size == (1 << DEPTH));
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}
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}
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Witness => {
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if tree.witness() {
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assert!(tree_size != 0);
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tree_witnesses.push((tree_size - 1, *tree_values.last().unwrap()));
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} else {
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assert!(tree_size == 0);
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}
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}
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Unwitness(value) => {
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if tree.remove_witness(&value) {
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if let Some((i, _)) = tree_witnesses.iter().enumerate().find(|v| (v.1).1 == value) {
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tree_witnesses.remove(i);
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} else {
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panic!("witness should not have been removed");
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}
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} else {
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if tree_witnesses.iter().find(|v| v.1 == value).is_some() {
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panic!("witness should have been removed");
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}
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}
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}
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Checkpoint => {
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tree_checkpoints.push(tree_size);
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tree.checkpoint();
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}
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Rewind => {
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if tree.rewind() {
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assert!(tree_checkpoints.len() > 0);
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let checkpoint_location = tree_checkpoints.pop().unwrap();
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for &(index, _) in tree_witnesses.iter() {
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// index is the index in tree_values
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// checkpoint_location is the size of the tree
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// at the time of the checkpoint
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// index should always be strictly smaller or
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// else a witness would be erased!
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assert!(index < checkpoint_location);
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}
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tree_values.truncate(checkpoint_location);
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tree_size = checkpoint_location;
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} else {
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if tree_checkpoints.len() != 0 {
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let checkpoint_location = *tree_checkpoints.last().unwrap();
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assert!(tree_witnesses.iter().any(|&(index, _)| index >= checkpoint_location));
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}
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}
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}
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PopCheckpoint => {
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if tree.pop_checkpoint() {
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assert!(tree_checkpoints.len() > 0);
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tree_checkpoints.remove(0);
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} else {
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assert!(tree_checkpoints.len() == 0);
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}
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}
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Authpath(value) => {
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if let Some((position, path)) = tree.authentication_path(&value) {
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// must be the case that value was a witness
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assert!(tree_witnesses.iter().any(|&(_, witness)| witness == value));
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let mut extended_tree_values = tree_values.clone();
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extended_tree_values.resize(1 << DEPTH, Hash::empty_leaf());
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let expected_root = lazy_root::<Hash>(extended_tree_values);
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let tree_root = tree.root();
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assert_eq!(tree_root, expected_root);
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assert_eq!(
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compute_root_from_auth_path::<Hash>(value, position, &path),
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expected_root
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);
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} else {
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// must be the case that value wasn't a witness
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for &(_, witness) in tree_witnesses.iter() {
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assert!(witness != value);
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}
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}
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}
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}
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}
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}
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}
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}
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