Function InferTaprootTree

src/script/standard.cpp:524–657

}

std::optional<std::vector<std::tuple<int, CScript, int>>> InferTaprootTree(const TaprootSpendData& spenddata, const XOnlyPubKey& output)
{
    // Verify that the output matches the assumed Merkle root and internal key.
    auto tweak = spenddata.internal_key.CreateTapTweak(spenddata.merkle_root.IsNull() ? nullptr : &spenddata.merkle_root);
    if (!tweak || tweak->first != output) return std::nullopt;
    // If the Merkle root is 0, the tree is empty, and we're done.
    std::vector<std::tuple<int, CScript, int>> ret;
    if (spenddata.merkle_root.IsNull()) return ret;

    /** Data structure to represent the nodes of the tree we're going to build. */
    struct TreeNode {
        /** Hash of this node, if known; 0 otherwise. */
        uint256 hash;
        /** The left and right subtrees (note that their order is irrelevant). */
        std::unique_ptr<TreeNode> sub[2];
        /** If this is known to be a leaf node, a pointer to the (script, leaf_ver) pair.
         *  nullptr otherwise. */
        const std::pair<CScript, int>* leaf = nullptr;
        /** Whether or not this node has been explored (is known to be a leaf, or known to have children). */
        bool explored = false;
        /** Whether or not this node is an inner node (unknown until explored = true). */
        bool inner;
        /** Whether or not we have produced output for this subtree. */
        bool done = false;
    };

    // Build tree from the provided branches.
    TreeNode root;
    root.hash = spenddata.merkle_root;
    for (const auto& [key, control_blocks] : spenddata.scripts) {
        const auto& [script, leaf_ver] = key;
        for (const auto& control : control_blocks) {
            // Skip script records with nonsensical leaf version.
            if (leaf_ver < 0 || leaf_ver >= 0x100 || leaf_ver & 1) continue;
            // Skip script records with invalid control block sizes.
            if (control.size() < TAPROOT_CONTROL_BASE_SIZE || control.size() > TAPROOT_CONTROL_MAX_SIZE ||
                ((control.size() - TAPROOT_CONTROL_BASE_SIZE) % TAPROOT_CONTROL_NODE_SIZE) != 0) continue;
            // Skip script records that don't match the control block.
            if ((control[0] & TAPROOT_LEAF_MASK) != leaf_ver) continue;
            // Skip script records that don't match the provided Merkle root.
            const uint256 leaf_hash = ComputeTapleafHash(leaf_ver, script);
            const uint256 merkle_root = ComputeTaprootMerkleRoot(control, leaf_hash);
            if (merkle_root != spenddata.merkle_root) continue;

            TreeNode* node = &root;
            size_t levels = (control.size() - TAPROOT_CONTROL_BASE_SIZE) / TAPROOT_CONTROL_NODE_SIZE;
            for (size_t depth = 0; depth < levels; ++depth) {
                // Can't descend into a node which we already know is a leaf.
                if (node->explored && !node->inner) return std::nullopt;

                // Extract partner hash from Merkle branch in control block.
                uint256 hash;
                std::copy(control.begin() + TAPROOT_CONTROL_BASE_SIZE + (levels - 1 - depth) * TAPROOT_CONTROL_NODE_SIZE,
                          control.begin() + TAPROOT_CONTROL_BASE_SIZE + (levels - depth) * TAPROOT_CONTROL_NODE_SIZE,
                          hash.begin());

                if (node->sub[0]) {
                    // Descend into the existing left or right branch.