miden-crypto/src/merkle/index.rs

use super::RpoDigest;

// NODE INDEX
// ================================================================================================

/// A Merkle tree address to an arbitrary node.
#[derive(Debug, Default, Copy, Clone, Eq, PartialEq, PartialOrd, Ord, Hash)]
pub struct NodeIndex {
    depth: u8,
    value: u64,
}

impl NodeIndex {
    // CONSTRUCTORS
    // --------------------------------------------------------------------------------------------

    /// Creates a new node index.
    pub const fn new(depth: u8, value: u64) -> Self {
        Self { depth, value }
    }

    /// Creates a new node index pointing to the root of the tree.
    pub const fn root() -> Self {
        Self { depth: 0, value: 0 }
    }

    /// Mutates the instance and returns it, replacing the depth.
    pub const fn with_depth(mut self, depth: u8) -> Self {
        self.depth = depth;
        self
    }

    /// Computes the value of the sibling of the current node.
    pub fn sibling(mut self) -> Self {
        self.value ^= 1;
        self
    }

    // PROVIDERS
    // --------------------------------------------------------------------------------------------

    /// Builds a node to be used as input of a hash function when computing a Merkle path.
    ///
    /// Will evaluate the parity of the current instance to define the result.
    pub const fn build_node(&self, slf: RpoDigest, sibling: RpoDigest) -> [RpoDigest; 2] {
        if self.is_value_odd() {
            [sibling, slf]
        } else {
            [slf, sibling]
        }
    }

    /// Returns the scalar representation of the depth/value pair.
    ///
    /// It is computed as `2^depth + value`.
    pub const fn to_scalar_index(&self) -> u64 {
        (1 << self.depth as u64) + self.value
    }

    /// Returns the depth of the current instance.
    pub const fn depth(&self) -> u8 {
        self.depth
    }

    /// Returns the value of the current depth.
    pub const fn value(&self) -> u64 {
        self.value
    }

    /// Returns true if the current value fits the current depth for a binary tree.
    pub const fn is_valid(&self) -> bool {
        self.value < (1 << self.depth as u64)
    }

    /// Returns true if the current instance points to a right sibling node.
    pub const fn is_value_odd(&self) -> bool {
        (self.value & 1) == 1
    }

    /// Returns `true` if the depth is `0`.
    pub const fn is_root(&self) -> bool {
        self.depth == 0
    }

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

    /// Traverse one level towards the root, decrementing the depth by `1`.
    pub fn move_up(&mut self) -> &mut Self {
        self.depth = self.depth.saturating_sub(1);
        self.value >>= 1;
        self
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use proptest::prelude::*;

    proptest! {
        #[test]
        fn arbitrary_index_wont_panic_on_move_up(
            depth in prop::num::u8::ANY,
            value in prop::num::u64::ANY,
            count in prop::num::u8::ANY,
        ) {
            let mut index = NodeIndex::new(depth, value);
            for _ in 0..count {
                index.move_up();
            }
        }
    }
}