[Battery] Potential improvement for battery assignment (#7163)

The behaviour change in the battery was copied over from the assign_missing _instruments php script. This is not the ideal implementation but is sufficient to serve as a proposal up for discussion. Alternate solutions can include

    completely separate function to populate vs create a battery
    only allow to "fill in" battery if already existing instruments are surveys (not a prepopulated battery of instruments)
    behaviour could be added uniquely to next stage (although this limits the potential to allow users to have a button au run "assign missing instruments from the front end")
    ... other combinations...

Fixes #7162

Author: ridz1208

php/libraries/NDB_BVL_Battery.class.inc

@@ -135,13 +135,6 @@ class NDB_BVL_Battery
             );
         }
 
-        // assert that the current battery is empty
-        if (count($currentTests) > 0) {
-            throw new Exception(
-                "Battery is not empty - will not clobber an existing battery."
-            );
-        }
-
         // determine the list of instruments to register
         $neededTests = $this->lookupBattery(
             $this->age,
@@ -152,13 +145,34 @@ class NDB_BVL_Battery
             $firstVisit
         );
 
+        // assert that the current battery is empty
+        if (count($currentTests) > 0) {
+            $neededTests = $this->getBatteryDifference($currentTests, $neededTests);
+        }
+
         // loop over the list of instruments
         foreach ($neededTests AS $testName) {
             // add the instrument
             $this->addInstrument($testName);
         } // end looping over the list of instruments
     } // end createBattery()
 
+    /**
+     * Computes the difference between the existing battery and the
+     * theoretical battery and returns only the missing tests as an array
+     *
+     * @param array $currentBattery Existing battery at the specified timepoint
+     * @param array $lookupBattery  Theoretical battery from the test_battery table
+     *
+     * @return array
+     */
+    function getBatteryDifference(
+        array $currentBattery=[],
+        array $lookupBattery=[]
+    ) : array {
+        return array_unique(array_diff($lookupBattery, $currentBattery));
+    }
+
     /**
      * Adds the specified instrument to the current battery, so long
      * as it is not already there (singleton model)