tx_memory_pool: speedup get_complement() for large requests

Changes complexity from M*N to (2*N+M)*log2(M). The FCMP++ stressnet recently hit mempool sizes of ~55k txs.
If the requesting node's mempool is populated, this results in an average of (55000*55000)/2
(about 1.5 billion) comparisons for the responding node. Under this commit, this would be reduced to
(55000+55000)*log2(55000) comparisons (about 2.6 million), a 99.83% reduction.

1 files changed, 10 insertions, 3 deletions

diff --git a/src/cryptonote_core/tx_pool.cpp b/src/cryptonote_core/tx_pool.cpp
index 40c004da4..3bc508037 100644
--- a/src/cryptonote_core/tx_pool.cpp
+++ b/src/cryptonote_core/tx_pool.cpp
@@ -668,17 +668,24 @@ namespace cryptonote
     return true;
   }
   //---------------------------------------------------------------------------------
-  bool tx_memory_pool::get_complement(const std::vector<crypto::hash> &hashes, std::vector<cryptonote::blobdata> &txes) const
+  bool tx_memory_pool::get_complement(std::vector<crypto::hash> hashes, std::vector<cryptonote::blobdata> &txes) const
   {
     CRITICAL_REGION_LOCAL(m_transactions_lock);
     CRITICAL_REGION_LOCAL1(m_blockchain);
 
+    // Sort so we can do binary search later
+    std::sort(hashes.begin(), hashes.end());
+
     m_blockchain.for_all_txpool_txes([this, &hashes, &txes](const crypto::hash &txid, const txpool_tx_meta_t &meta, const cryptonote::blobdata_ref*) {
       const auto tx_relay_method = meta.get_relay_method();
       if (tx_relay_method != relay_method::block && tx_relay_method != relay_method::fluff)
         return true;
-      const auto i = std::find(hashes.begin(), hashes.end(), txid);
-      if (i == hashes.end())
+
+      // Do binary search for our pool TXID in given list, skip to next if already present
+      const auto hash_it = std::lower_bound(hashes.cbegin(), hashes.cend(), txid);
+      if (hash_it != hashes.cend() && *hash_it == txid)
+        return true;
+
       {
         cryptonote::blobdata bd;
         try