implement problem.enabled config values, fix repeated appearance of same problem through retrying

src/arithmetic.js

@@ -236,7 +236,7 @@ export default {
                             style: 'is-danger'
                         }
                     ],
-                    method: self.methods.square,
+                    method: self.methods.square_root,
                     enabled: true,
                     current: 0
                 }

src/problems.js

@@ -2,14 +2,32 @@ import arithmetic from "@/arithmetic";
 import utils from "@/utils";
 
 export default {
+    computed: {
+      availableProblems() {
+          return this.problems.filter(problem => problem.enabled);
+      }
+    },
     methods: {
         getProblem() {
-            let problem = this.problems[utils.methods.getRandomInt(0, this.problems.length)];
+            let problemType = this.availableProblems[utils.methods.getRandomInt(0, this.availableProblems.length)];
-            return problem.method(problem.difficulties[problem.current].options);
+            let problem = null;
+
+            // Begin looking for a 'unique'ish problem
+            for (let i = 0; i < 5; i++) {
+                // Generate a problem
+                problem = problemType.method(problemType.difficulties[problemType.current].options);
+                // Check that there was a previous problem and that the text doesn't match
+                if (this.previousProblem == null || problem.text !== this.previousProblem.text)
+                    break;
+            }
+
+            this.previousProblem = problem;
+            return problem;
         }
     },
     data() {
         return {
+            previousProblem: null,
             problems: arithmetic.data().problems
         }
     }