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use core::convert::Infallible;
use core::fmt;
use std::collections::{BTreeMap, BTreeSet};
use serde::{Deserialize, Serialize};
use tiny_keccak::{Hasher, Sha3};
use crate::traits::{CmRDT, CvRDT};
/// The hash of a node
pub type Hash = [u8; 32];
/// A node in the Merkle DAG
#[derive(Debug, Clone, PartialEq, Eq, Hash, PartialOrd, Ord, Serialize, Deserialize)]
pub struct Node<T> {
/// The child nodes, addressed by their hash.
pub children: BTreeSet<Hash>,
/// The value stored at this node.
pub value: T,
}
impl<T: Sha3Hash> Node<T> {
/// Compute the hash name of this node.
///
/// hash = sha3_256(child1 <> child2 <> .. <> childN <> value)
///
/// Where children are ordered lexigraphically.
pub fn hash(&self) -> Hash {
let mut sha3 = Sha3::v256();
self.children.iter().for_each(|c| sha3.update(c));
self.value.hash(&mut sha3);
let mut hash = [0u8; 32];
sha3.finalize(&mut hash);
hash
}
}
/// The contents of a MerkleReg.
///
/// Usually this is retrieved through a call to `MerkleReg::read`
pub struct Content<'a, T> {
nodes: BTreeMap<Hash, &'a Node<T>>,
}
impl<'a, T> Content<'a, T> {
/// Checks if the contents is empty
pub fn is_empty(&self) -> bool {
self.nodes.is_empty()
}
/// Iterate over the content values
pub fn values(&self) -> impl Iterator<Item = &T> {
self.nodes.values().map(|n| &n.value)
}
/// Iterate over the Merkle DAG nodes holding the content values.
pub fn nodes(&self) -> impl Iterator<Item = &Node<T>> {
self.nodes.values().copied()
}
/// Iterate over the hashes of the content values.
pub fn hashes(&self) -> BTreeSet<Hash> {
self.nodes.keys().copied().collect()
}
/// Iterate over the hashes of the content values.
pub fn hashes_and_nodes(&self) -> impl Iterator<Item = (Hash, &Node<T>)> {
self.nodes.iter().map(|(hash, node)| (*hash, *node))
}
}
/// The MerkleReg is a Register CRDT that uses the Merkle DAG
/// structure to track the current value(s) held by this register.
/// The roots of the Merkle DAG are the current concurrent values.
#[derive(Debug, Clone, PartialEq, Eq, Hash, PartialOrd, Ord, Serialize, Deserialize)]
pub struct MerkleReg<T> {
roots: BTreeSet<Hash>,
dag: BTreeMap<Hash, Node<T>>,
orphans: BTreeMap<Hash, Node<T>>,
}
impl<T> Default for MerkleReg<T> {
fn default() -> Self {
Self {
roots: Default::default(),
dag: Default::default(),
orphans: Default::default(),
}
}
}
impl<T> MerkleReg<T> {
/// Return a new instance of the MerkleReg
pub fn new() -> Self {
Default::default()
}
/// Read the current values held by the register
pub fn read(&self) -> Content<T> {
Content {
nodes: self
.roots
.iter()
.copied()
.filter_map(|root| self.dag.get(&root).map(|node| (root, node)))
.collect(),
}
}
/// Write the given value on top of the given children.
pub fn write(&self, value: T, children: BTreeSet<Hash>) -> Node<T> {
Node { children, value }
}
/// Retrieve a node in the Merkle DAG by it's hash.
///
/// Traverse the history of the register by pairing this method
/// with the children of the nodes retrieved in Content::nodes().
pub fn node(&self, hash: Hash) -> Option<&Node<T>> {
self.dag.get(&hash).or_else(|| self.orphans.get(&hash))
}
/// Iterator over all the nodes in the Merkle DAG.
pub fn all_nodes(&self) -> impl Iterator<Item = &Node<T>> {
self.dag.values()
}
/// Returns the children of a node
pub fn children(&self, hash: Hash) -> Content<T> {
let nodes = self.dag.get(&hash).map(|node| {
node.children
.iter()
.copied()
.filter_map(|child| self.dag.get(&child).map(|node| (child, node)))
.collect()
});
Content {
nodes: nodes.unwrap_or_default(),
}
}
/// Returns the parents of a node
pub fn parents(&self, hash: Hash) -> Content<T> {
let parents = self
.dag
.iter()
.filter_map(|(h, node)| {
if node.children.contains(&hash) {
Some((*h, node))
} else {
None
}
})
.collect();
Content { nodes: parents }
}
/// Returns the number of nodes who are visible, i.e. their children have been seen.
pub fn num_nodes(&self) -> usize {
self.dag.len()
}
/// Returns the number of nodes who are not visible due to missing children.
pub fn num_orphans(&self) -> usize {
self.orphans.len()
}
fn all_hashes_seen(&self, hashes: &BTreeSet<Hash>) -> bool {
hashes.iter().all(|h| self.dag.contains_key(h))
}
}
/// Validation errors that may occur when applying or merging MerkleReg
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum ValidationError {
/// The Op is attempting to insert a node with a child we
/// haven't seen yet.
MissingChild(Hash),
}
impl fmt::Display for ValidationError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Debug::fmt(self, f)
}
}
impl std::error::Error for ValidationError {}
impl<T: Sha3Hash> CmRDT for MerkleReg<T> {
type Op = Node<T>;
type Validation = ValidationError;
fn validate_op(&self, op: &Self::Op) -> Result<(), Self::Validation> {
for child in op.children.iter() {
if !self.dag.contains_key(child) {
return Err(ValidationError::MissingChild(*child));
}
}
Ok(())
}
fn apply(&mut self, node: Self::Op) {
let node_hash = node.hash();
if self.dag.contains_key(&node_hash) || self.orphans.contains_key(&node_hash) {
return;
}
if self.all_hashes_seen(&node.children) {
// Any children who happen to be roots will no longer be roots
// after this node is inserted.
for child in node.children.iter() {
self.roots.remove(child);
}
// Since we have never seen this node before, it's guaranteed to be a root.
self.roots.insert(node_hash);
// It is now safe to insert this node into the DAG since we've seen its children.
self.dag.insert(node_hash, node);
// Now check if inserting this node resolves any orphans nodes.
// TODO: replace this logic with BTreeMap::drain_filter once it's stable.
let hashes_that_are_now_ready_to_apply = self
.orphans
.iter()
.filter(|(_, node)| self.all_hashes_seen(&node.children))
.map(|(hash, _)| hash)
.copied()
.collect::<Vec<_>>();
let mut nodes_to_apply = Vec::new();
for hash in hashes_that_are_now_ready_to_apply {
// Remove the previously orphaned nodes that are now
// ready to apply before we recurse, else we risk an
// exponential growth in memory.
if let Some(node) = self.orphans.remove(&hash) {
nodes_to_apply.push(node);
}
}
for node in nodes_to_apply {
self.apply(node);
}
} else {
self.orphans.insert(node_hash, node);
}
}
}
impl<T: Sha3Hash> CvRDT for MerkleReg<T> {
type Validation = Infallible;
fn validate_merge(&self, _: &Self) -> Result<(), Self::Validation> {
Ok(())
}
fn merge(&mut self, other: Self) {
let MerkleReg { dag, orphans, .. } = other;
for (_, node) in dag {
self.apply(node);
}
for (_, node) in orphans {
self.apply(node);
}
}
}
/// Values in the MerkleReg must be hasheable
/// with tiny_keccak::Sha3.
pub trait Sha3Hash {
/// Update the hasher with self's data
fn hash(&self, hasher: &mut Sha3);
}
// Blanket implementation for anything that can be converted to &[u8]
impl<T: AsRef<[u8]>> Sha3Hash for T {
fn hash(&self, hasher: &mut Sha3) {
hasher.update(self.as_ref());
}
}
#[cfg(feature = "quickcheck")]
use quickcheck::{Arbitrary, Gen};
#[cfg(feature = "quickcheck")]
impl<T: Arbitrary + Sha3Hash> Arbitrary for MerkleReg<T> {
fn arbitrary(g: &mut Gen) -> Self {
let mut reg = MerkleReg::new();
let mut nodes: Vec<Node<_>> = Vec::new();
let n_nodes = u8::arbitrary(g) % 12;
for _ in 0..n_nodes {
let value = T::arbitrary(g);
let mut children = BTreeSet::new();
if !nodes.is_empty() {
let n_children = u8::arbitrary(g) % 12;
for _ in 0..n_children {
children.insert(nodes[usize::arbitrary(g) % nodes.len()].hash());
}
}
let op = reg.write(value, children);
nodes.push(op.clone());
reg.apply(op)
}
reg
}
}