plonky2/src/hash/merkle_tree.rs

use rayon::prelude::*;
use serde::{Deserialize, Serialize};

use crate::field::field_types::Field;
use crate::hash::hash_types::HashOut;
use crate::hash::hashing::{compress, hash_or_noop};
use crate::hash::merkle_proofs::MerkleProof;

/// The Merkle cap of height `h` of a Merkle tree is the `h`-th layer (from the root) of the tree.
/// It can be used in place of the root to verify Merkle paths, which are `h` elements shorter.
#[derive(Clone, Debug, Serialize, Deserialize)]
#[serde(bound = "")]
pub struct MerkleCap<F: Field>(pub Vec<HashOut<F>>);

impl<F: Field> MerkleCap<F> {
    pub fn flatten(&self) -> Vec<F> {
        self.0.iter().flat_map(|h| h.elements).collect()
    }
}

#[derive(Clone, Debug)]
pub struct MerkleTree<F: Field> {
    /// The data in the leaves of the Merkle tree.
    pub leaves: Vec<Vec<F>>,

    /// The layers of hashes in the tree. The first layer is the one at the bottom.
    pub layers: Vec<Vec<HashOut<F>>>,

    /// The Merkle cap.
    pub cap: MerkleCap<F>,
}

impl<F: Field> MerkleTree<F> {
    pub fn new(leaves: Vec<Vec<F>>, cap_height: usize) -> Self {
        let mut layers = vec![leaves
            .par_iter()
            .map(|l| hash_or_noop(l.clone()))
            .collect::<Vec<_>>()];
        while let Some(l) = layers.last() {
            if l.len() == 1 << cap_height {
                break;
            }
            let next_layer = l
                .par_chunks(2)
                .map(|chunk| compress(chunk[0], chunk[1]))
                .collect::<Vec<_>>();
            layers.push(next_layer);
        }
        let cap = layers.pop().unwrap();
        Self {
            leaves,
            layers,
            cap: MerkleCap(cap),
        }
    }

    pub fn get(&self, i: usize) -> &[F] {
        &self.leaves[i]
    }

    /// Create a Merkle proof from a leaf index.
    pub fn prove(&self, leaf_index: usize) -> MerkleProof<F> {
        MerkleProof {
            siblings: self
                .layers
                .iter()
                .scan(leaf_index, |acc, layer| {
                    let index = *acc ^ 1;
                    *acc >>= 1;
                    Some(layer[index])
                })
                .collect(),
        }
    }
}

#[cfg(test)]
mod tests {
    use anyhow::Result;

    use super::*;
    use crate::field::crandall_field::CrandallField;
    use crate::hash::merkle_proofs::verify_merkle_proof;

    fn random_data<F: Field>(n: usize, k: usize) -> Vec<Vec<F>> {
        (0..n).map(|_| F::rand_vec(k)).collect()
    }

    fn verify_all_leaves<F: Field>(
        leaves: Vec<Vec<F>>,
        n: usize,
        reverse_bits: bool,
    ) -> Result<()> {
        let tree = MerkleTree::new(leaves.clone(), 1);
        for i in 0..n {
            let proof = tree.prove(i);
            verify_merkle_proof(leaves[i].clone(), i, &tree.cap, &proof, reverse_bits)?;
        }
        Ok(())
    }

    #[test]
    fn test_merkle_trees() -> Result<()> {
        type F = CrandallField;

        let log_n = 8;
        let n = 1 << log_n;
        let leaves = random_data::<F>(n, 7);

        verify_all_leaves(leaves, n, false)?;

        Ok(())
    }
}