private IList<byte[]> BuildMerkleTree()
{
// The Merkle root is based on a tree of hashes calculated from the transactions:
//
// root
// /\
// / \
// A B
// / \ / \
// t1 t2 t3 t4
//
// The tree is represented as a list: t1,t2,t3,t4,A,B,root where each entry is a hash.
//
// The hashing algorithm is double SHA-256. The leaves are a hash of the serialized contents of the
// transaction. The interior nodes are hashes of the concentration of the two child hashes.
//
// This structure allows the creation of proof that a transaction was included into a block without having to
// provide the full block contents. Instead, you can provide only a Merkle branch. For example to prove tx2 was
// in a block you can just provide tx2, the hash(tx1) and B. Now the other party has everything they need to
// derive the root, which can be checked against the block header. These proofs aren't used right now but
// will be helpful later when we want to download partial block contents.
//
// Note that if the number of transactions is not even the last tx is repeated to make it so (see
// tx3 above). A tree with 5 transactions would look like this:
//
// root
// / \
// 1 \
// / \ \
// 2 3 4
// / \ / \ / \
// t1 t2 t3 t4 t5 t5
var tree = new List<byte[]>();
// Start by adding all the hashes of the transactions as leaves of the tree.
foreach (var t in Transactions)
{
tree.Add(t.Hash.Bytes);
}
var levelOffset = 0; // Offset in the list where the currently processed level starts.
// Step through each level, stopping when we reach the root (levelSize == 1).
for (var levelSize = Transactions.Count; levelSize > 1; levelSize = (levelSize + 1)/2)
{
// For each pair of nodes on that level:
for (var left = 0; left < levelSize; left += 2)
{
// The right hand node can be the same as the left hand, in the case where we don't have enough
// transactions.
var right = Math.Min(left + 1, levelSize - 1);
var leftBytes = Utils.ReverseBytes(tree[levelOffset + left]);
var rightBytes = Utils.ReverseBytes(tree[levelOffset + right]);
tree.Add(Utils.ReverseBytes(Utils.DoubleDigestTwoBuffers(leftBytes, 0, 32, rightBytes, 0, 32)));
}
// Move to the next level.
levelOffset += levelSize;
}
return tree;
}
