AP_Param: add method to find key of top level group variable by pointer

libraries/AP_Param/AP_Param.cpp

@@ -973,6 +973,23 @@ bool AP_Param::find_key_by_pointer(const void *ptr, uint16_t &key)
     return false;
 }
 
+/*
+  Find key to top level group parameters by pointer
+*/
+bool AP_Param::find_top_level_key_by_pointer(const void *ptr, uint16_t &key)
+{
+    for (uint16_t i=0; i<_num_vars; i++) {
+        if (_var_info[i].type != AP_PARAM_GROUP) {
+            continue;
+        }
+        if (ptr == (void **)_var_info[i].ptr) {
+            key = _var_info[i].key;
+            return true;
+        }
+    }
+    return false;
+}
+
 
 // Find a object by name.
 //

libraries/AP_Param/AP_Param.h

@@ -323,6 +323,14 @@ public:
                                           ptrdiff_t offset, uint16_t &key);
     static bool find_key_by_pointer(const void *ptr, uint16_t &key);
 
+    /// Find key of top level group variable by pointer
+    ///
+    ///
+    /// @param  p               Pointer to variable
+    /// @return                 key for variable
+    static bool find_top_level_key_by_pointer(const void *ptr, uint16_t &key);
+
+
     /// Find a object in the top level var_info table
     ///
     /// If the variable has no name, it cannot be found by this interface.