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// Copyright (C) 2019-2020 Aleo Systems Inc.
// This file is part of the snarkOS library.

// The snarkOS library is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.

// The snarkOS library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with the snarkOS library. If not, see <https://www.gnu.org/licenses/>.

use snarkos_algorithms::crh::double_sha256;

#[derive(Clone, Debug, Eq, PartialEq)]
pub struct MerkleTreeRootHash([u8; 32]);

fn merkle_round(hashes: &[Vec<u8>]) -> Vec<Vec<u8>> {
    let mut pairs = vec![];

    for i in (0..hashes.len() - 1).step_by(2) {
        pairs.push((&hashes[i], &hashes[i + 1]));
    }

    // Duplicate the last element if there are an odd number of leaves
    if hashes.len() % 2 == 1 {
        let last = &hashes[hashes.len() - 1];
        pairs.push((last, last));
    }

    let result: Vec<Vec<u8>> = pairs.iter().map(|x| merkle_hash(x.0, x.1)).collect();

    result
}

/// Calculates a Merkle root and also returns the subroots at a desired depth. If the tree is too
/// shallow to have subroots at that depth, returns the root as a single subroot.
pub fn merkle_root_with_subroots(hashes: &[Vec<u8>], subroots_depth: usize) -> (Vec<u8>, Vec<Vec<u8>>) {
    merkle_root_with_subroots_inner(hashes, &[], subroots_depth)
}

fn merkle_root_with_subroots_inner(
    hashes: &[Vec<u8>],
    subroots: &[Vec<u8>],
    subroots_depth: usize,
) -> (Vec<u8>, Vec<Vec<u8>>) {
    if hashes.len() == 1 {
        // Tree was too shallow.
        let root = hashes[0].clone();
        let subroots = if subroots.len() == 0 {
            vec![root.clone()]
        } else {
            subroots.to_vec()
        };
        return (root, subroots);
    }

    let result = merkle_round(hashes);
    if result.len() == 1 << subroots_depth {
        merkle_root_with_subroots_inner(&result, &result, subroots_depth)
    } else {
        merkle_root_with_subroots_inner(&result, subroots, subroots_depth)
    }
}

/// Calculates the root of the Merkle tree
pub fn merkle_root(hashes: &[Vec<u8>]) -> Vec<u8> {
    if hashes.len() == 1 {
        return hashes[0].clone();
    }

    let result = merkle_round(hashes);

    merkle_root(&result)
}

/// Calculate the Merkle tree hash by concatenating the left and right children nodes.
pub fn merkle_hash(left: &Vec<u8>, right: &Vec<u8>) -> Vec<u8> {
    let mut result = [0u8; 64];
    result[0..32].copy_from_slice(&left);
    result[32..64].copy_from_slice(&right);
    double_sha256(&result)
}

#[cfg(test)]
mod tests {
    use super::merkle_root;

    // block 80_000
    // https://blockchain.info/block/000000000043a8c0fd1d6f726790caa2a406010d19efd2780db27bdbbd93baf6
    #[test]
    fn test_merkle_root_2_hashes() {
        let mut tx1 = hex::decode("c06fbab289f723c6261d3030ddb6be121f7d2508d77862bb1e484f5cd7f92b25").unwrap();
        let mut tx2 = hex::decode("5a4ebf66822b0b2d56bd9dc64ece0bc38ee7844a23ff1d7320a88c5fdb2ad3e2").unwrap();

        tx1.reverse();
        tx2.reverse();

        let result = merkle_root(&[tx1, tx2]);

        let mut expected = hex::decode("8fb300e3fdb6f30a4c67233b997f99fdd518b968b9a3fd65857bfe78b2600719").unwrap();
        expected.reverse();

        assert_eq!(result, expected);
    }

    #[test]
    fn test_merkle_root_5_hashes() {
        let tx1 = hex::decode("1da63abbc8cc611334a753c4c31de14d19839c65b2b284202eaf3165861fb58d").unwrap();
        let tx2 = hex::decode("26c6a6f18d13d2f0787c1c0f3c5e23cf5bc8b3de685dd1923ae99f44c5341c0c").unwrap();
        let tx3 = hex::decode("513507fa209db823541caf7b9742bb9999b4a399cf604ba8da7037f3acced649").unwrap();
        let tx4 = hex::decode("6bf5d2e02b8432d825c5dff692d435b6c5f685d94efa6b3d8fb818f2ecdcfb66").unwrap();
        let tx5 = hex::decode("8a5ad423bc54fb7c76718371fd5a73b8c42bf27beaf2ad448761b13bcafb8895").unwrap();

        let vec: Vec<Vec<u8>> = vec![tx1, tx2, tx3, tx4, tx5]
            .iter()
            .map(|tx| {
                let mut tx = tx.clone();
                tx.reverse();
                tx
            })
            .collect();

        let result = merkle_root(&vec);

        let mut expected = hex::decode("3a432cd416ea05b1be4ec1e72d7952d08670eaa5505b6794a186ddb253aa62e6").unwrap();
        expected.reverse();

        assert_eq!(result, expected);
    }
}