bitcoinbook/code/merkle.cpp

#include <bitcoin/bitcoin.hpp>

bc::hash_digest create_merkle(bc::hash_list& merkle)
{
    // Stop if hash list is empty.
    if (merkle.empty())
        return bc::null_hash;
    else if (merkle.size() == 1)
        return merkle[0];

    // While there is more than 1 hash in the list, keep looping...
    while (merkle.size() > 1)
    {
        // If number of hashes is odd, duplicate last hash in the list.
        if (merkle.size() % 2 != 0)
            merkle.push_back(merkle.back());
        // List size is now even.
        assert(merkle.size() % 2 == 0);

        // New hash list.
        bc::hash_list new_merkle;
        // Loop through hashes 2 at a time.
        for (auto it = merkle.begin(); it != merkle.end(); it += 2)
        {
            // Join both current hashes together (concatenate).
            bc::data_chunk concat_data(bc::hash_size * 2);
            auto concat = bc::make_serializer(concat_data.begin());
            concat.write_hash(*it);
            concat.write_hash(*(it + 1));
            assert(concat.iterator() == concat_data.end());
            // Hash both of the hashes.
            bc::hash_digest new_root = bc::bitcoin_hash(concat_data);
            // Add this to the new list.
            new_merkle.push_back(new_root);
        }
        // This is the new list.
        merkle = new_merkle;

        // DEBUG output -------------------------------------
        std::cout << "Current merkle hash list:" << std::endl;
        for (const auto& hash: merkle)
            std::cout << "  " << bc::encode_hex(hash) << std::endl;
        std::cout << std::endl;
        // --------------------------------------------------
    }
    // Finally we end up with a single item.
    return merkle[0];
}

int main()
{
    // Replace these hashes with ones from a block to reproduce the same merkle root.
    bc::hash_list tx_hashes{{
        bc::hash_literal("0000000000000000000000000000000000000000000000000000000000000000"),
        bc::hash_literal("0000000000000000000000000000000000000000000000000000000000000011"),
        bc::hash_literal("0000000000000000000000000000000000000000000000000000000000000022"),
    }};
    const bc::hash_digest merkle_root = create_merkle(tx_hashes);
    std::cout << "Result: " << bc::encode_hex(merkle_root) << std::endl;
    return 0;
}