miden-crypto/src/merkle/path_set.rs

use super::{BTreeMap, MerkleError, MerklePath, NodeIndex, Rpo256, Vec, Word, ZERO};

// MERKLE PATH SET
// ================================================================================================

/// A set of Merkle paths.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct MerklePathSet {
    root: Word,
    total_depth: u8,
    paths: BTreeMap<u64, MerklePath>,
}

impl MerklePathSet {
    // CONSTRUCTOR
    // --------------------------------------------------------------------------------------------

    /// Returns an empty MerklePathSet.
    pub fn new(depth: u8) -> Result<Self, MerkleError> {
        let root = [ZERO; 4];
        let paths = BTreeMap::new();

        Ok(Self {
            root,
            total_depth: depth,
            paths,
        })
    }

    // PUBLIC ACCESSORS
    // --------------------------------------------------------------------------------------------

    /// Returns the root to which all paths in this set resolve.
    pub const fn root(&self) -> Word {
        self.root
    }

    /// Returns the depth of the Merkle tree implied by the paths stored in this set.
    ///
    /// Merkle tree of depth 1 has two leaves, depth 2 has four leaves etc.
    pub const fn depth(&self) -> u8 {
        self.total_depth
    }

    /// Returns a node at the specified index.
    ///
    /// # Errors
    /// Returns an error if:
    /// * The specified index is not valid for the depth of structure.
    /// * Requested node does not exist in the set.
    pub fn get_node(&self, index: NodeIndex) -> Result<Word, MerkleError> {
        if !index.with_depth(self.total_depth).is_valid() {
            return Err(MerkleError::InvalidIndex(
                index.with_depth(self.total_depth),
            ));
        }
        if index.depth() != self.total_depth {
            return Err(MerkleError::InvalidDepth {
                expected: self.total_depth,
                provided: index.depth(),
            });
        }

        let index_value = index.to_scalar_index();
        let parity = index_value & 1;
        let index_value = index_value / 2;
        self.paths
            .get(&index_value)
            .ok_or(MerkleError::NodeNotInSet(index_value))
            .map(|path| path[parity as usize])
    }

    /// Returns a Merkle path to the node at the specified index. The node itself is
    /// not included in the path.
    ///
    /// # Errors
    /// Returns an error if:
    /// * The specified index is not valid for the depth of structure.
    /// * Node of the requested path does not exist in the set.
    pub fn get_path(&self, index: NodeIndex) -> Result<MerklePath, MerkleError> {
        if !index.with_depth(self.total_depth).is_valid() {
            return Err(MerkleError::InvalidIndex(index));
        }
        if index.depth() != self.total_depth {
            return Err(MerkleError::InvalidDepth {
                expected: self.total_depth,
                provided: index.depth(),
            });
        }

        let index_value = index.to_scalar_index();
        let index = index_value / 2;
        let parity = index_value & 1;
        let mut path = self
            .paths
            .get(&index)
            .cloned()
            .ok_or(MerkleError::NodeNotInSet(index))?;
        path.remove(parity as usize);
        Ok(path)
    }

    // STATE MUTATORS
    // --------------------------------------------------------------------------------------------

    /// Adds the specified Merkle path to this [MerklePathSet]. The `index` and `value` parameters
    /// specify the leaf node at which the path starts.
    ///
    /// # Errors
    /// Returns an error if:
    /// - The specified index is is not valid in the context of this Merkle path set (i.e., the
    ///   index implies a greater depth than is specified for this set).
    /// - The specified path is not consistent with other paths in the set (i.e., resolves to a
    ///   different root).
    pub fn add_path(
        &mut self,
        index_value: u64,
        value: Word,
        mut path: MerklePath,
    ) -> Result<(), MerkleError> {
        let depth = (path.len() + 1) as u8;
        let mut index = NodeIndex::new(depth, index_value);
        if index.depth() != self.total_depth {
            return Err(MerkleError::InvalidDepth {
                expected: self.total_depth,
                provided: index.depth(),
            });
        }

        // update the current path
        let index_value = index.to_scalar_index();
        let upper_index_value = index_value / 2;
        let parity = index_value & 1;
        path.insert(parity as usize, value);

        // traverse to the root, updating the nodes
        let root: Word = Rpo256::merge(&[path[0].into(), path[1].into()]).into();
        let root = path.iter().skip(2).copied().fold(root, |root, hash| {
            index.move_up();
            Rpo256::merge(&index.build_node(root.into(), hash.into())).into()
        });

        // if the path set is empty (the root is all ZEROs), set the root to the root of the added
        // path; otherwise, the root of the added path must be identical to the current root
        if self.root == [ZERO; 4] {
            self.root = root;
        } else if self.root != root {
            return Err(MerkleError::InvalidPath(path));
        }

        // finish updating the path
        self.paths.insert(upper_index_value, path);
        Ok(())
    }

    /// Replaces the leaf at the specified index with the provided value.
    ///
    /// # Errors
    /// Returns an error if:
    /// * Requested node does not exist in the set.
    pub fn update_leaf(&mut self, base_index_value: u64, value: Word) -> Result<(), MerkleError> {
        let depth = self.depth();
        let mut index = NodeIndex::new(depth, base_index_value);
        if !index.is_valid() {
            return Err(MerkleError::InvalidIndex(index));
        }

        let path = match self
            .paths
            .get_mut(&index.clone().move_up().to_scalar_index())
        {
            Some(path) => path,
            None => return Err(MerkleError::NodeNotInSet(base_index_value)),
        };

        // Fill old_hashes vector -----------------------------------------------------------------
        let mut current_index = index;
        let mut old_hashes = Vec::with_capacity(path.len().saturating_sub(2));
        let mut root: Word = Rpo256::merge(&[path[0].into(), path[1].into()]).into();
        for hash in path.iter().skip(2).copied() {
            old_hashes.push(root);
            current_index.move_up();
            let input = current_index.build_node(hash.into(), root.into());
            root = Rpo256::merge(&input).into();
        }

        // Fill new_hashes vector -----------------------------------------------------------------
        path[index.is_value_odd() as usize] = value;

        let mut new_hashes = Vec::with_capacity(path.len().saturating_sub(2));
        let mut new_root: Word = Rpo256::merge(&[path[0].into(), path[1].into()]).into();
        for path_hash in path.iter().skip(2).copied() {
            new_hashes.push(new_root);
            index.move_up();
            let input = current_index.build_node(path_hash.into(), new_root.into());
            new_root = Rpo256::merge(&input).into();
        }

        self.root = new_root;

        // update paths ---------------------------------------------------------------------------
        for path in self.paths.values_mut() {
            for i in (0..old_hashes.len()).rev() {
                if path[i + 2] == old_hashes[i] {
                    path[i + 2] = new_hashes[i];
                    break;
                }
            }
        }

        Ok(())
    }
}

// TESTS
// ================================================================================================

#[cfg(test)]
mod tests {
    use super::*;
    use crate::merkle::int_to_node;

    #[test]
    fn get_root() {
        let leaf0 = int_to_node(0);
        let leaf1 = int_to_node(1);
        let leaf2 = int_to_node(2);
        let leaf3 = int_to_node(3);

        let parent0 = calculate_parent_hash(leaf0, 0, leaf1);
        let parent1 = calculate_parent_hash(leaf2, 2, leaf3);

        let root_exp = calculate_parent_hash(parent0, 0, parent1);

        let mut set = super::MerklePathSet::new(3).unwrap();

        set.add_path(0, leaf0, vec![leaf1, parent1].into()).unwrap();

        assert_eq!(set.root(), root_exp);
    }

    #[test]
    fn add_and_get_path() {
        let path_6 = vec![int_to_node(7), int_to_node(45), int_to_node(123)];
        let hash_6 = int_to_node(6);
        let index = 6_u64;
        let depth = 4_u8;
        let mut set = super::MerklePathSet::new(depth).unwrap();

        set.add_path(index, hash_6, path_6.clone().into()).unwrap();
        let stored_path_6 = set.get_path(NodeIndex::new(depth, index)).unwrap();

        assert_eq!(path_6, *stored_path_6);
        assert!(set.get_path(NodeIndex::new(depth, 15_u64)).is_err())
    }

    #[test]
    fn get_node() {
        let path_6 = vec![int_to_node(7), int_to_node(45), int_to_node(123)];
        let hash_6 = int_to_node(6);
        let index = 6_u64;
        let depth = 4_u8;
        let mut set = MerklePathSet::new(depth).unwrap();

        set.add_path(index, hash_6, path_6.into()).unwrap();

        assert_eq!(
            int_to_node(6u64),
            set.get_node(NodeIndex::new(depth, index)).unwrap()
        );
        assert!(set.get_node(NodeIndex::new(depth, 15_u64)).is_err());
    }

    #[test]
    fn update_leaf() {
        let hash_4 = int_to_node(4);
        let hash_5 = int_to_node(5);
        let hash_6 = int_to_node(6);
        let hash_7 = int_to_node(7);
        let hash_45 = calculate_parent_hash(hash_4, 12u64, hash_5);
        let hash_67 = calculate_parent_hash(hash_6, 14u64, hash_7);

        let hash_0123 = int_to_node(123);

        let path_6 = vec![hash_7, hash_45, hash_0123];
        let path_5 = vec![hash_4, hash_67, hash_0123];
        let path_4 = vec![hash_5, hash_67, hash_0123];

        let index_6 = 6_u64;
        let index_5 = 5_u64;
        let index_4 = 4_u64;
        let depth = 4_u8;
        let mut set = MerklePathSet::new(depth).unwrap();

        set.add_path(index_6, hash_6, path_6.into()).unwrap();
        set.add_path(index_5, hash_5, path_5.into()).unwrap();
        set.add_path(index_4, hash_4, path_4.into()).unwrap();

        let new_hash_6 = int_to_node(100);
        let new_hash_5 = int_to_node(55);

        set.update_leaf(index_6, new_hash_6).unwrap();
        let new_path_4 = set.get_path(NodeIndex::new(depth, index_4)).unwrap();
        let new_hash_67 = calculate_parent_hash(new_hash_6, 14_u64, hash_7);
        assert_eq!(new_hash_67, new_path_4[1]);

        set.update_leaf(index_5, new_hash_5).unwrap();
        let new_path_4 = set.get_path(NodeIndex::new(depth, index_4)).unwrap();
        let new_path_6 = set.get_path(NodeIndex::new(depth, index_6)).unwrap();
        let new_hash_45 = calculate_parent_hash(new_hash_5, 13_u64, hash_4);
        assert_eq!(new_hash_45, new_path_6[1]);
        assert_eq!(new_hash_5, new_path_4[0]);
    }

    // HELPER FUNCTIONS
    // --------------------------------------------------------------------------------------------

    const fn is_even(pos: u64) -> bool {
        pos & 1 == 0
    }

    /// Calculates the hash of the parent node by two sibling ones
    /// - node — current node
    /// - node_pos — position of the current node
    /// - sibling — neighboring vertex in the tree
    fn calculate_parent_hash(node: Word, node_pos: u64, sibling: Word) -> Word {
        if is_even(node_pos) {
            Rpo256::merge(&[node.into(), sibling.into()]).into()
        } else {
            Rpo256::merge(&[sibling.into(), node.into()]).into()
        }
    }
}