Fixed massive writes to creature.variables on calc

Now only writes changed variables, preventing oplog from being
polluted with massive updates

app/imports/api/engine/computation/writeComputation/writeScope.js

@@ -1,10 +1,31 @@
 import Creatures from '/imports/api/creature/creatures/Creatures.js';
+import { EJSON } from 'meteor/ejson';
 
-export default function writeScope(creatureId, scope){
-  // Remove large properties that aren't likely to be accessed
+export default function writeScope(creatureId, computation) {
+  const scope = computation.scope;
+  const variables = computation.creature.variables || {};
+  let $set;
   for (const key in scope){
+    // Remove large properties that aren't likely to be accessed
     delete scope[key].parent;
     delete scope[key].ancestors;
+    
+    // Remove empty keys
+    for (const subKey in scope[key]) {
+      if (scope[key][subKey] === undefined) {
+        delete scope[key][subKey]
+      }
+    }
+    
+    // Only update changed fields
+    if (!EJSON.equals(variables[key], scope[key])) {
+      if (!$set) $set = {};
+
+      // Set the changed key in the creature variables
+      $set[`variables.${key}`] = scope[key];
+    }
+  }
+  if ($set) {
+    Creatures.update(creatureId, {$set});
   }
-  Creatures.update(creatureId, {$set: {variables: scope}});
 }