blockchain: simple cache for the long term block weights

src/cryptonote_core/blockchain.cpp

@@ -178,6 +178,7 @@ Blockchain::Blockchain(tx_memory_pool& tx_pool) :
   m_enforce_dns_checkpoints(false), m_max_prepare_blocks_threads(4), m_db_sync_on_blocks(true), m_db_sync_threshold(1), m_db_sync_mode(db_async), m_db_default_sync(false), m_fast_sync(true), m_show_time_stats(false), m_sync_counter(0), m_bytes_to_sync(0), m_cancel(false),
   m_long_term_block_weights_window(CRYPTONOTE_LONG_TERM_BLOCK_WEIGHT_WINDOW_SIZE),
   m_long_term_effective_median_block_weight(0),
+  m_long_term_block_weights_cache_tip_hash(crypto::null_hash),
   m_difficulty_for_next_block_top_hash(crypto::null_hash),
   m_difficulty_for_next_block(1),
   m_btc_valid(false)
@@ -1279,7 +1280,28 @@ void Blockchain::get_long_term_block_weights(std::vector<uint64_t>& weights, uin
   LOG_PRINT_L3("Blockchain::" << __func__);
   CRITICAL_REGION_LOCAL(m_blockchain_lock);
 
+  if (count == 0)
+    return;
+
+  bool cached = false;
+  uint64_t blockchain_height = m_db->height();
+  uint64_t tip_height = start_height + count - 1;
+  crypto::hash tip_hash = crypto::null_hash;
+  if (tip_height < blockchain_height && count == m_long_term_block_weights_cache.size())
+  {
+    tip_hash = m_db->get_block_hash_from_height(tip_height);
+    cached = tip_hash == m_long_term_block_weights_cache_tip_hash;
+  }
+
+  if (cached)
+  {
+    weights = m_long_term_block_weights_cache;
+    return;
+  }
+
   weights = m_db->get_long_term_block_weights(start_height, count);
+  m_long_term_block_weights_cache = weights;
+  m_long_term_block_weights_cache_tip_hash = tip_hash;
 }
 //------------------------------------------------------------------
 uint64_t Blockchain::get_current_cumulative_block_weight_limit() const

src/cryptonote_core/blockchain.h

@@ -1060,6 +1060,8 @@ namespace cryptonote
     uint64_t m_timestamps_and_difficulties_height;
     uint64_t m_long_term_block_weights_window;
     uint64_t m_long_term_effective_median_block_weight;
+    mutable crypto::hash m_long_term_block_weights_cache_tip_hash;
+    mutable std::vector<uint64_t> m_long_term_block_weights_cache;
 
     epee::critical_section m_difficulty_lock;
     crypto::hash m_difficulty_for_next_block_top_hash;

tests/block_weight/block_weight.cpp

@@ -84,6 +84,11 @@ public:
     while (count-- && start_height < blocks.size()) ret.push_back(blocks[start_height++].long_term_weight);
     return ret;
   }
+  virtual crypto::hash get_block_hash_from_height(const uint64_t &height) const override {
+    crypto::hash hash = crypto::null_hash;
+    *(uint64_t*)&hash = height;
+    return hash;
+  }
   virtual crypto::hash top_block_hash(uint64_t *block_height = NULL) const override {
     uint64_t h = height();
     crypto::hash top = crypto::null_hash;