Better let the experts (like Todd or Anonymint) have an opinion on this..
No need to take my word for it! These two fragments of code do similar things:
void tree_hash(const char (*hashes)[HASH_SIZE], size_t count, char *root_hash) {
assert(count > 0);
if (count == 1) {
memcpy(root_hash, hashes, HASH_SIZE);
} else if (count == 2) {
cn_fast_hash(hashes, 2 * HASH_SIZE, root_hash);
} else {
size_t i, j;
size_t cnt = count - 1;
char (*ints)[HASH_SIZE];
for (i = 1; i < sizeof(size_t); i <<= 1) {
cnt |= cnt >> i;
}
cnt &= ~(cnt >> 1);
ints = alloca(cnt * HASH_SIZE);
memcpy(ints, hashes, (2 * cnt - count) * HASH_SIZE);
for (i = 2 * cnt - count, j = 2 * cnt - count; j < cnt; i += 2, ++j) {
cn_fast_hash(hashes[i], 64, ints[j]);
}
assert(i == count);
while (cnt > 2) {
cnt >>= 1;
for (i = 0, j = 0; j < cnt; i += 2, ++j) {
cn_fast_hash(ints[i], 64, ints[j]);
}
}
cn_fast_hash(ints[0], 64, root_hash);
}
}
uint256 CBlock::BuildMerkleTree() const
{
vMerkleTree.clear();
BOOST_FOREACH(const CTransaction& tx, vtx)
vMerkleTree.push_back(tx.GetHash());
int j = 0;
for (int nSize = vtx.size(); nSize > 1; nSize = (nSize + 1) / 2)
{
for (int i = 0; i < nSize; i += 2)
{
int i2 = std::min(i+1, nSize-1);
vMerkleTree.push_back(Hash(BEGIN(vMerkleTree[j+i]), END(vMerkleTree[j+i]),
BEGIN(vMerkleTree[j+i2]), END(vMerkleTree[j+i2])));
}
j += nSize;
}
return (vMerkleTree.empty() ? 0 : vMerkleTree.back());
}
The latter is a lot easier to read, and because it's C++ with boost there's no error-prone manual memory management.