Algorithm to calculate the root hash given a merkle multi proof

multi_merkle_proof.rs

pub fn calculate_root(proof: Vec<Vec<(usize, [u8; 32])>>) -> [u8; 32] {
    let mut previous_layer = vec![];
    for  layer in proof {
        let mut current_layer = vec![];
        if previous_layer.len() == 0 {
            current_layer = layer;
        } else {
            current_layer.extend(previous_layer.drain(..));
            current_layer.extend(&layer);
            current_layer.sort_by(|(a_i, _), (b_i, _)| a_i.cmp(b_i));
        }

        for index in (0..current_layer.len()).step_by(2) {
            if index + 1 == current_layer.len() {
                let node = current_layer[index].clone();
                previous_layer.push((div_floor(node.0, 2), node.1));
            } else {
                let mut concat = vec![];
                let mut left = current_layer[index].clone();
                let right = current_layer[index + 1].clone();
                concat.extend(&left.1);
                concat.extend(&right.1);
                let hash = keccak256(&concat);

                previous_layer.push((div_floor(left.0, 2), hash));
            }
        }
    }

    previous_layer[0].1
}