checkpoints: support nearest lower checkpoint lookup

src/checkpoints/checkpoints.cpp

@@ -158,6 +158,19 @@ namespace cryptonote
     return m_points.rbegin()->first;
   }
   //---------------------------------------------------------------------------
+  uint64_t checkpoints::get_nearest_checkpoint_height(uint64_t block_height) const
+  {
+    if (m_points.empty())
+      return 0;
+
+    auto it = m_points.upper_bound(block_height);
+    if (it == m_points.begin())
+      return 0;
+
+    --it;
+    return it->first;
+  }
+  //---------------------------------------------------------------------------
   const std::map<uint64_t, crypto::hash>& checkpoints::get_points() const
   {
     return m_points;

src/checkpoints/checkpoints.h

@@ -129,6 +129,14 @@ namespace cryptonote
      */
     uint64_t get_max_height() const;
 
+    /**
+     * @brief gets the highest checkpoint height less than the given block height
+     *
+     * @param block_height the reference block height
+     * @return the nearest checkpoint height below block_height, or 0 if none
+     */
+    uint64_t get_nearest_checkpoint_height(uint64_t block_height) const;
+
     /**
      * @brief gets the checkpoints container
      *