PID_ADD_EXTRUSION_RATE based on MarlinKimbra

Marlin/temperature.cpp

 
         pid_output = pTerm[e] + iTerm[e] - dTerm[e];
 
+        #if ENABLED(SINGLENOZZLE)
+          #define _NOZZLE_TEST     true
+          #define _NOZZLE_EXTRUDER active_extruder
+          #define _CTERM_INDEX     0
+        #else
+          #define _NOZZLE_TEST     e == active_extruder
+          #define _NOZZLE_EXTRUDER e
+          #define _CTERM_INDEX     e
+        #endif
+
         #if ENABLED(PID_ADD_EXTRUSION_RATE)
-          cTerm[e] = 0;
-          if (e == active_extruder) {
+          cTerm[_CTERM_INDEX] = 0;
+          if (_NOZZLE_TEST) {
             long e_position = stepper.position(E_AXIS);
-            if (e_position > last_position[e]) {
-              lpq[lpq_ptr++] = e_position - last_position[e];
-              last_position[e] = e_position;
+            if (e_position > last_position[_NOZZLE_EXTRUDER]) {
+              lpq[lpq_ptr++] = e_position - last_position[_NOZZLE_EXTRUDER];
+              last_position[_NOZZLE_EXTRUDER] = e_position;
             }
             else {
               lpq[lpq_ptr++] = 0;
             }
             if (lpq_ptr >= lpq_len) lpq_ptr = 0;
-            cTerm[e] = (lpq[lpq_ptr] / planner.axis_steps_per_unit[E_AXIS]) * PID_PARAM(Kc, e);
+            cTerm[_CTERM_INDEX] = (lpq[lpq_ptr] / planner.axis_steps_per_unit[E_AXIS]) * PID_PARAM(Kc, e);
             pid_output += cTerm[e];
           }
         #endif //PID_ADD_EXTRUSION_RATE