rpc: speed up the common get_output_distribution case while syncing

src/rpc/rpc_handler.cpp

@@ -43,8 +43,25 @@ namespace rpc
 
       std::vector<std::uint64_t> distribution;
       std::uint64_t start_height, base;
-      if (!f(amount, from_height, to_height, start_height, distribution, base))
-        return boost::none;
+
+      // see if we can extend the cache - a common case
+      if (d.cached && amount == 0 && d.cached_from == from_height && to_height > d.cached_to)
+      {
+        std::vector<std::uint64_t> new_distribution;
+        if (!f(amount, d.cached_to + 1, to_height, start_height, new_distribution, base))
+          return boost::none;
+        distribution = d.cached_distribution;
+        distribution.reserve(distribution.size() + new_distribution.size());
+        for (const auto &e: new_distribution)
+          distribution.push_back(e);
+        start_height = d.cached_start_height;
+        base = d.cached_base;
+      }
+      else
+      {
+        if (!f(amount, from_height, to_height, start_height, distribution, base))
+          return boost::none;
+      }
 
       if (to_height > 0 && to_height >= from_height)
       {