Add CPartialMerkleTree

This adds a compact representation for a subset of a merkle tree's
nodes.

src/main.cpp

@@ -2262,6 +2262,127 @@ CMerkleBlock::CMerkleBlock(const CBlock& block, CBloomFilter& filter)
 
 
 
+uint256 CPartialMerkleTree::CalcHash(int height, unsigned int pos, const std::vector<uint256> &vTxid) {
+    if (height == 0) {
+        // hash at height 0 is the txids themself
+        return vTxid[pos];
+    } else {
+        // calculate left hash
+        uint256 left = CalcHash(height-1, pos*2, vTxid), right;
+        // calculate right hash if not beyong the end of the array - copy left hash otherwise1
+        if (pos*2+1 < CalcTreeWidth(height-1))
+            right = CalcHash(height-1, pos*2+1, vTxid);
+        else
+            right = left;
+        // combine subhashes
+        return Hash(BEGIN(left), END(left), BEGIN(right), END(right));
+    }
+}
+
+void CPartialMerkleTree::TraverseAndBuild(int height, unsigned int pos, const std::vector<uint256> &vTxid, const std::vector<bool> &vMatch) {
+    // determine whether this node is the parent of at least one matched txid
+    bool fParentOfMatch = false;
+    for (unsigned int p = pos << height; p < (pos+1) << height && p < nTransactions; p++)
+        fParentOfMatch |= vMatch[p];
+    // store as flag bit
+    vBits.push_back(fParentOfMatch);
+    if (height==0 || !fParentOfMatch) {
+        // if at height 0, or nothing interesting below, store hash and stop
+        vHash.push_back(CalcHash(height, pos, vTxid));
+    } else {
+        // otherwise, don't store any hash, but descend into the subtrees
+        TraverseAndBuild(height-1, pos*2, vTxid, vMatch);
+        if (pos*2+1 < CalcTreeWidth(height-1))
+            TraverseAndBuild(height-1, pos*2+1, vTxid, vMatch);
+    }
+}
+
+uint256 CPartialMerkleTree::TraverseAndExtract(int height, unsigned int pos, unsigned int &nBitsUsed, unsigned int &nHashUsed, std::vector<uint256> &vMatch) {
+    if (nBitsUsed >= vBits.size()) {
+        // overflowed the bits array - failure
+        fBad = true;
+        return 0;
+    }
+    bool fParentOfMatch = vBits[nBitsUsed++];
+    if (height==0 || !fParentOfMatch) {
+        // if at height 0, or nothing interesting below, use stored hash and do not descend
+        if (nHashUsed >= vHash.size()) {
+            // overflowed the hash array - failure
+            fBad = true;
+            return 0;
+        }
+        const uint256 &hash = vHash[nHashUsed++];
+        if (height==0 && fParentOfMatch) // in case of height 0, we have a matched txid
+            vMatch.push_back(hash);
+        return hash;
+    } else {
+        // otherwise, descend into the subtrees to extract matched txids and hashes
+        uint256 left = TraverseAndExtract(height-1, pos*2, nBitsUsed, nHashUsed, vMatch), right;
+        if (pos*2+1 < CalcTreeWidth(height-1))
+            right = TraverseAndExtract(height-1, pos*2+1, nBitsUsed, nHashUsed, vMatch);
+        else
+            right = left;
+        // and combine them before returning
+        return Hash(BEGIN(left), END(left), BEGIN(right), END(right));
+    }
+}
+
+CPartialMerkleTree::CPartialMerkleTree(const std::vector<uint256> &vTxid, const std::vector<bool> &vMatch) : nTransactions(vTxid.size()), fBad(false) {
+    // reset state
+    vBits.clear();
+    vHash.clear();
+
+    // calculate height of tree
+    int nHeight = 0;
+    while (CalcTreeWidth(nHeight) > 1)
+        nHeight++;
+
+    // traverse the partial tree
+    TraverseAndBuild(nHeight, 0, vTxid, vMatch);
+}
+
+CPartialMerkleTree::CPartialMerkleTree() : nTransactions(0), fBad(true) {}
+
+uint256 CPartialMerkleTree::ExtractMatches(std::vector<uint256> &vMatch) {
+    vMatch.clear();
+    // An empty set will not work
+    if (nTransactions == 0)
+        return 0;
+    // check for excessively high numbers of transactions
+    if (nTransactions > MAX_BLOCK_SIZE / 60) // 60 is the lower bound for the size of a serialized CTransaction
+        return 0;
+    // there can never be more hashes provided than one for every txid
+    if (vHash.size() > nTransactions)
+        return 0;
+    // there must be at least one bit per node in the partial tree, and at least one node per hash
+    if (vBits.size() < vHash.size())
+        return 0;
+    // calculate height of tree
+    int nHeight = 0;
+    while (CalcTreeWidth(nHeight) > 1)
+        nHeight++;
+    // traverse the partial tree
+    unsigned int nBitsUsed = 0, nHashUsed = 0;
+    uint256 hashMerkleRoot = TraverseAndExtract(nHeight, 0, nBitsUsed, nHashUsed, vMatch);
+    // verify that no problems occured during the tree traversal
+    if (fBad)
+        return 0;
+    // verify that all bits were consumed (except for the padding caused by serializing it as a byte sequence)
+    if ((nBitsUsed+7)/8 != (vBits.size()+7)/8)
+        return 0;
+    // verify that all hashes were consumed
+    if (nHashUsed != vHash.size())
+        return 0;
+    return hashMerkleRoot;
+}
+
+
+
+
+
+
+
+
 bool CheckDiskSpace(uint64 nAdditionalBytes)
 {
     uint64 nFreeBytesAvailable = filesystem::space(GetDataDir()).available;