speedup get_output_histogram for all amounts when min_count > 0

This skips the vast majority of "dust" output amounts with just
one instance on the chain. Clocks in at 0.15% of the original
time on testnet.

src/blockchain_db/blockchain_db.h

@@ -1490,10 +1490,11 @@ public:
    * @param amounts optional set of amounts to lookup
    * @param unlocked whether to restrict count to unlocked outputs
    * @param recent_cutoff timestamp to determine whether an output is recent
+   * @param min_count return only amounts with at least that many instances
    *
    * @return a set of amount/instances
    */
-  virtual std::map<uint64_t, std::tuple<uint64_t, uint64_t, uint64_t>> get_output_histogram(const std::vector<uint64_t> &amounts, bool unlocked, uint64_t recent_cutoff) const = 0;
+  virtual std::map<uint64_t, std::tuple<uint64_t, uint64_t, uint64_t>> get_output_histogram(const std::vector<uint64_t> &amounts, bool unlocked, uint64_t recent_cutoff, uint64_t min_count) const = 0;
 
   /**
    * @brief is BlockchainDB in read-only mode?

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -3086,7 +3086,7 @@ void BlockchainLMDB::get_output_tx_and_index(const uint64_t& amount, const std::
   LOG_PRINT_L3("db3: " << db3);
 }
 
-std::map<uint64_t, std::tuple<uint64_t, uint64_t, uint64_t>> BlockchainLMDB::get_output_histogram(const std::vector<uint64_t> &amounts, bool unlocked, uint64_t recent_cutoff) const
+std::map<uint64_t, std::tuple<uint64_t, uint64_t, uint64_t>> BlockchainLMDB::get_output_histogram(const std::vector<uint64_t> &amounts, bool unlocked, uint64_t recent_cutoff, uint64_t min_count) const
 {
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);
   check_open();
@@ -3112,6 +3112,7 @@ std::map<uint64_t, std::tuple<uint64_t, uint64_t, uint64_t>> BlockchainLMDB::get
       mdb_size_t num_elems = 0;
       mdb_cursor_count(m_cur_output_amounts, &num_elems);
       uint64_t amount = *(const uint64_t*)k.mv_data;
+      if (num_elems >= min_count)
         histogram[amount] = std::make_tuple(num_elems, 0, 0);
     }
   }
@@ -3123,12 +3124,14 @@ std::map<uint64_t, std::tuple<uint64_t, uint64_t, uint64_t>> BlockchainLMDB::get
       int ret = mdb_cursor_get(m_cur_output_amounts, &k, &v, MDB_SET);
       if (ret == MDB_NOTFOUND)
       {
+        if (0 >= min_count)
           histogram[amount] = std::make_tuple(0, 0, 0);
       }
       else if (ret == MDB_SUCCESS)
       {
         mdb_size_t num_elems = 0;
         mdb_cursor_count(m_cur_output_amounts, &num_elems);
+        if (num_elems >= min_count)
           histogram[amount] = std::make_tuple(num_elems, 0, 0);
       }
       else

src/blockchain_db/lmdb/db_lmdb.h

@@ -287,10 +287,11 @@ public:
    * @param amounts optional set of amounts to lookup
    * @param unlocked whether to restrict count to unlocked outputs
    * @param recent_cutoff timestamp to determine which outputs are recent
+   * @param min_count return only amounts with at least that many instances
    *
    * @return a set of amount/instances
    */
-  std::map<uint64_t, std::tuple<uint64_t, uint64_t, uint64_t>> get_output_histogram(const std::vector<uint64_t> &amounts, bool unlocked, uint64_t recent_cutoff) const;
+  std::map<uint64_t, std::tuple<uint64_t, uint64_t, uint64_t>> get_output_histogram(const std::vector<uint64_t> &amounts, bool unlocked, uint64_t recent_cutoff, uint64_t min_count) const;
 
 private:
   void do_resize(uint64_t size_increase=0);

src/cryptonote_core/blockchain.cpp

@@ -4322,9 +4322,9 @@ uint64_t Blockchain::get_difficulty_target() const
   return get_current_hard_fork_version() < 2 ? DIFFICULTY_TARGET_V1 : DIFFICULTY_TARGET_V2;
 }
 
-std::map<uint64_t, std::tuple<uint64_t, uint64_t, uint64_t>> Blockchain:: get_output_histogram(const std::vector<uint64_t> &amounts, bool unlocked, uint64_t recent_cutoff) const
+std::map<uint64_t, std::tuple<uint64_t, uint64_t, uint64_t>> Blockchain:: get_output_histogram(const std::vector<uint64_t> &amounts, bool unlocked, uint64_t recent_cutoff, uint64_t min_count) const
 {
-  return m_db->get_output_histogram(amounts, unlocked, recent_cutoff);
+  return m_db->get_output_histogram(amounts, unlocked, recent_cutoff, min_count);
 }
 
 std::list<std::pair<Blockchain::block_extended_info,uint64_t>> Blockchain::get_alternative_chains() const

src/cryptonote_core/blockchain.h

@@ -827,10 +827,11 @@ namespace cryptonote
      * @param amounts optional set of amounts to lookup
      * @param unlocked whether to restrict instances to unlocked ones
      * @param recent_cutoff timestamp to consider outputs as recent
+     * @param min_count return only amounts with at least that many instances
      *
      * @return a set of amount/instances
      */
-    std::map<uint64_t, std::tuple<uint64_t, uint64_t, uint64_t>> get_output_histogram(const std::vector<uint64_t> &amounts, bool unlocked, uint64_t recent_cutoff) const;
+    std::map<uint64_t, std::tuple<uint64_t, uint64_t, uint64_t>> get_output_histogram(const std::vector<uint64_t> &amounts, bool unlocked, uint64_t recent_cutoff, uint64_t min_count = 0) const;
 
     /**
      * @brief perform a check on all key images in the blockchain

src/rpc/core_rpc_server.cpp

@@ -1728,7 +1728,7 @@ namespace cryptonote
     std::map<uint64_t, std::tuple<uint64_t, uint64_t, uint64_t>> histogram;
     try
     {
-      histogram = m_core.get_blockchain_storage().get_output_histogram(req.amounts, req.unlocked, req.recent_cutoff);
+      histogram = m_core.get_blockchain_storage().get_output_histogram(req.amounts, req.unlocked, req.recent_cutoff, req.min_count);
     }
     catch (const std::exception &e)
     {