/**
* Copyright 2012 The Bitcoin Developers
* Copyright 2012 Matt Corallo
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.google.devcoin.core;
import java.io.IOException;
import java.io.OutputStream;
import java.util.ArrayList;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Set;
/**
* <p>A data structure that contains proofs of block inclusion for one or more transactions, in an efficient manner.</p>
*
* <p>The encoding works as follows: we traverse the tree in depth-first order, storing a bit for each traversed node,
* signifying whether the node is the parent of at least one matched leaf txid (or a matched txid itself). In case we
* are at the leaf level, or this bit is 0, its merkle node hash is stored, and its children are not explored further.
* Otherwise, no hash is stored, but we recurse into both (or the only) child branch. During decoding, the same
* depth-first traversal is performed, consuming bits and hashes as they were written during encoding.</p>
*
* <p>The serialization is fixed and provides a hard guarantee about the encoded size,
* <tt>SIZE <= 10 + ceil(32.25*N)</tt> where N represents the number of leaf nodes of the partial tree. N itself
* is bounded by:</p>
*
* <p>
* N <= total_transactions<br>
* N <= 1 + matched_transactions*tree_height
* </p>
*
* <p><pre>The serialization format:
* - uint32 total_transactions (4 bytes)
* - varint number of hashes (1-3 bytes)
* - uint256[] hashes in depth-first order (<= 32*N bytes)
* - varint number of bytes of flag bits (1-3 bytes)
* - byte[] flag bits, packed per 8 in a byte, least significant bit first (<= 2*N-1 bits)
* The size constraints follow from this.</pre></p>
*/
public class PartialMerkleTree extends Message {
// the total number of transactions in the block
int transactionCount;
// node-is-parent-of-matched-txid bits
byte[] matchedChildBits;
// txids and internal hashes
List<Sha256Hash> hashes;
public PartialMerkleTree(NetworkParameters params, byte[] payloadBytes, int offset) throws ProtocolException {
super(params, payloadBytes, offset);
}
public void bitcoinSerializeToStream(OutputStream stream) throws IOException {
throw new RuntimeException("Not implemented");
}
@Override
void parse() throws ProtocolException {
transactionCount = (int)readUint32();
int nHashes = (int) readVarInt();
hashes = new ArrayList<Sha256Hash>(nHashes);
for (int i = 0; i < nHashes; i++)
hashes.add(readHash());
int nFlagBytes = (int) readVarInt();
matchedChildBits = readBytes(nFlagBytes);
length = cursor - offset;
}
@Override
protected void parseLite() {
}
// helper function to efficiently calculate the number of nodes at given height in the merkle tree
private int getTreeWidth(int height) {
return (transactionCount+(1 << height)-1) >> height;
}
private static class ValuesUsed {
public int bitsUsed = 0, hashesUsed = 0;
}
// recursive function that traverses tree nodes, consuming the bits and hashes produced by TraverseAndBuild.
// it returns the hash of the respective node.
private Sha256Hash recursiveExtractHashes(int height, int pos, ValuesUsed used, List<Sha256Hash> matchedHashes) throws VerificationException {
if (used.bitsUsed >= matchedChildBits.length*8) {
// overflowed the bits array - failure
throw new VerificationException("CPartialMerkleTree overflowed its bits array");
}
boolean parentOfMatch = Utils.checkBitLE(matchedChildBits, used.bitsUsed++);
if (height == 0 || !parentOfMatch) {
// if at height 0, or nothing interesting below, use stored hash and do not descend
if (used.hashesUsed >= hashes.size()) {
// overflowed the hash array - failure
throw new VerificationException("CPartialMerkleTree overflowed its hash array");
}
if (height == 0 && parentOfMatch) // in case of height 0, we have a matched txid
matchedHashes.add(hashes.get(used.hashesUsed));
return hashes.get(used.hashesUsed++);
} else {
// otherwise, descend into the subtrees to extract matched txids and hashes
byte[] left = recursiveExtractHashes(height-1, pos*2, used, matchedHashes).getBytes(), right;
if (pos*2+1 < getTreeWidth(height-1))
right = recursiveExtractHashes(height-1, pos*2+1, used, matchedHashes).getBytes();
else
right = left;
// and combine them before returning
return new Sha256Hash(Utils.reverseBytes(Utils.doubleDigestTwoBuffers(
Utils.reverseBytes(left), 0, 32,
Utils.reverseBytes(right), 0, 32)));
}
}
/**
* Extracts tx hashes that are in this merkle tree
* and returns the merkle root of this tree.
*
* The returned root should be checked against the
* merkle root contained in the block header for security.
*
* @param matchedHashes A list which will contain the matched txn (will be cleared)
* Required to be a LinkedHashSet in order to retain order or transactions in the block
* @return the merkle root of this merkle tree
* @throws ProtocolException if this partial merkle tree is invalid
*/
public Sha256Hash getTxnHashAndMerkleRoot(List<Sha256Hash> matchedHashes) throws VerificationException {
matchedHashes.clear();
// An empty set will not work
if (transactionCount == 0)
throw new VerificationException("Got a CPartialMerkleTree with 0 transactions");
// check for excessively high numbers of transactions
if (transactionCount > Block.MAX_BLOCK_SIZE / 60) // 60 is the lower bound for the size of a serialized CTransaction
throw new VerificationException("Got a CPartialMerkleTree with more transactions than is possible");
// there can never be more hashes provided than one for every txid
if (hashes.size() > transactionCount)
throw new VerificationException("Got a CPartialMerkleTree with more hashes than transactions");
// there must be at least one bit per node in the partial tree, and at least one node per hash
if (matchedChildBits.length*8 < hashes.size())
throw new VerificationException("Got a CPartialMerkleTree with fewer matched bits than hashes");
// calculate height of tree
int height = 0;
while (getTreeWidth(height) > 1)
height++;
// traverse the partial tree
ValuesUsed used = new ValuesUsed();
Sha256Hash merkleRoot = recursiveExtractHashes(height, 0, used, matchedHashes);
// verify that all bits were consumed (except for the padding caused by serializing it as a byte sequence)
if ((used.bitsUsed+7)/8 != matchedChildBits.length ||
// verify that all hashes were consumed
used.hashesUsed != hashes.size())
throw new VerificationException("Got a CPartialMerkleTree that didn't need all the data it provided");
return merkleRoot;
}
}