AP_Param: added set_param_by_name()

this simplifies the GCS_MAVLink code

libraries/AP_Param/AP_Param.cpp

@@ -1161,3 +1161,56 @@ void AP_Param::convert_old_parameters(const struct ConversionInfo *conversion_ta
         convert_old_parameter(&conversion_table[i]);
     }
 }
+
+/*
+  set a parameter by name
+ */
+AP_Param *AP_Param::set_param_by_name(const char *pname, float value, enum ap_var_type *ptype)
+{
+    AP_Param *vp;
+    enum ap_var_type var_type;
+
+    if (isnan(value) || isinf(value)) {
+        return NULL;
+    }
+
+    // find the requested parameter
+    vp = AP_Param::find(pname, &var_type);
+    if (vp == NULL) {
+        return NULL;
+    }
+
+    if (ptype != NULL) {
+        *ptype = var_type;
+    }
+
+    // add a small amount before casting parameter values
+    // from float to integer to avoid truncating to the
+    // next lower integer value.
+    float rounding_addition = 0.01;
+        
+    // handle variables with standard type IDs
+    if (var_type == AP_PARAM_FLOAT) {
+        ((AP_Float *)vp)->set(value);
+    } else if (var_type == AP_PARAM_INT32) {
+        if (value < 0) rounding_addition = -rounding_addition;
+        float v = value+rounding_addition;
+        v = constrain_float(v, -2147483648.0, 2147483647.0);
+        ((AP_Int32 *)vp)->set(v);
+    } else if (var_type == AP_PARAM_INT16) {
+        if (value < 0) rounding_addition = -rounding_addition;
+        float v = value+rounding_addition;
+        v = constrain_float(v, -32768, 32767);
+        ((AP_Int16 *)vp)->set(v);
+    } else if (var_type == AP_PARAM_INT8) {
+        if (value < 0) rounding_addition = -rounding_addition;
+        float v = value+rounding_addition;
+        v = constrain_float(v, -128, 127);
+        ((AP_Int8 *)vp)->set(v);
+    } else {
+        // we don't support mavlink set on this parameter
+        return NULL;
+    }
+    return vp;
+}
+

libraries/AP_Param/AP_Param.h

@@ -196,6 +196,14 @@ public:
     // set a AP_Param variable to a specified value
     static void         set_value(enum ap_var_type type, void *ptr, float def_value);
 
+
+    /*
+      set a parameter by name
+
+      The parameter pointer is returned on success
+    */
+    static AP_Param *set_param_by_name(const char *pname, float value, enum ap_var_type *ptype);
+
     // load default values for scalars in a group
     static void         setup_object_defaults(const void *object_pointer, const struct GroupInfo *group_info);