fix negative variable evaluation on formula questions

fixes CNVS-9970
If you use a negative variable as the first element in a formula
expression, formula question operation breaks.  For example, when you
put:

-[x]+5

in the body of the formula question,  and then attempt to put:

-x+5

in the formula definition, the formula question JavaScript does not know
how to parse the expression. This commit adds graceful logic for these cases.

Test plan
- Create a formula question with the initial variable cast as negative
- Formula definitions should be calculated correctly.
Regression
- Confirm that formula questions still work correctly with positive
  variables

Change-Id: I8d2c6f596bb0211e375c5a347dfa8a1ea2870271
Reviewed-on: https://gerrit.instructure.com/33003
Tested-by: Jenkins <[email protected]>
Reviewed-by: Stanley Stuart <[email protected]>
Reviewed-by: Jason Madsen <[email protected]>
QA-Review: Caleb Guanzon <[email protected]>
Product-Review: Josh Simpson <[email protected]>

public/javascripts/calcCmd.js

@@ -60,7 +60,7 @@ define([
         result = syntax[syntaxIndex];
         break;
       case 'subtract':
-        if(syntax[syntaxIndex + 1] && syntax[syntaxIndex + 1].token == 'number') {
+        if(syntax[syntaxIndex + 1] && (syntax[syntaxIndex + 1].token == 'number' || syntax[syntaxIndex + 1].token == 'variable')) {
           syntax[syntaxIndex + 1].value = "-" + syntax[syntaxIndex + 1].value;
           syntaxIndex++;
           result = syntax[syntaxIndex];
@@ -75,8 +75,8 @@ define([
           result.arguments = []
           var ender = 'comma';
           syntaxIndex = syntaxIndex + 2;
-          if(syntax[syntaxIndex].token == 'close_paren') { 
-            ender = 'close_paren'; 
+          if(syntax[syntaxIndex].token == 'close_paren') {
+            ender = 'close_paren';
             syntaxIndex++;
           }
           while(ender == 'comma') {
@@ -109,7 +109,7 @@ define([
     };
     var parseModifier = function(syntax) {
       switch(syntax[syntaxIndex].token) {
-      case 'add':      
+      case 'add':
         return syntax[syntaxIndex++];
         break;
       case 'subtract':
@@ -129,7 +129,7 @@ define([
       var index = (syntax && syntax[syntaxIndex] && syntax[syntaxIndex].newIndex) || 0;
       throw("unexpected " + value + " at " + index);
     };
-    
+
     var parseExpression = function(syntax, enders) {
       var result = {
         token: 'expression',
@@ -264,8 +264,15 @@ define([
         if(tree.value == '_') {
           return lastComputedResult || 0;
         }
-        var value = predefinedVariables && predefinedVariables[tree.value];
-        value = value || (variables && variables[tree.value]);
+        if(tree.value.indexOf("-") == 0) { // the variable is negative, e.g. '-x'
+          var absolute = tree.value.replace(/^\-/, "")
+          var value = predefinedVariables && predefinedVariables[absolute];
+          value = value || (variables && variables[absolute]);
+          value = -value;
+        } else {
+          var value = predefinedVariables && predefinedVariables[tree.value];
+          value = value || (variables && variables[tree.value]);
+        }
         if (value == undefined) {
           throw("undefined variable " + tree.value);
         }
@@ -369,10 +376,10 @@ define([
   (function() {
     var p = function(name, value, description) { calcCmd.addPredefinedVariable(name, value, description); }
     var f = function(name, func, description, example) { calcCmd.addFunction(name, func, description, example); }
-    
+
     p('pi', Math.PI );
     p('e', Math.exp(1));
-    
+
     f('abs', function(val) { return Math.abs(val) }, I18n.t('abs.description', "Returns the absolute value of the given value"), "abs(x)");
     f('asin', function(x) { return Math.asin(x); }, I18n.t('asin.description', "Returns the arcsin of the given value"), "asin(x)");
     f('acos', function(x) { return Math.acos(x); }, I18n.t('acos.description', "Returns the arccos of the given value"), "acos(x)");
@@ -393,7 +400,7 @@ define([
         return args;
       }
     }
-    f('max', function() { 
+    f('max', function() {
       var args = make_list(arguments)
       var max = args[0];
       for(var idx = 0; idx < args.length; idx++) { //in arguments) {
@@ -410,15 +417,15 @@ define([
       return min;
     }, I18n.t('min.description', "Returns the lowest value in the list"), ["min(a,b,c...)", "min(list)"]);
     f('sqrt', function(x) { return Math.sqrt(x); }, I18n.t('sqrt.description', "Returns the square root of the given value"), "sqrt(x)");
-    f('sort', function(x) { 
+    f('sort', function(x) {
       var args = make_list(arguments);
       var list = [];
       for(var idx = 0; idx < args.length; idx++) {
         list.push(args[idx]);
       }
       return list.sort();
     }, I18n.t('sort.description', "Returns the list of values, sorted from lowest to highest"), ["sort(a,b,c...)", "sort(list)"]);
-    f('reverse', function(x) { 
+    f('reverse', function(x) {
       var args = make_list(arguments);
       var list = [];
       for(var idx = 0; idx < args.length; idx++) {
@@ -427,9 +434,9 @@ define([
       return list;
     }, I18n.t('reverse.description', "Reverses the order of the list of values"), ["reverse(a,b,c...)", "reverse(list)"]);
     f('first', function() { return make_list(arguments)[0]; }, I18n.t('first.description', "Returns the first value in the list"), ["first(a,b,c...)", "first(list)"]);
-    f('last', function() { 
+    f('last', function() {
       var args = make_list(arguments);
-      return args[args.length - 1]; 
+      return args[args.length - 1];
     }, I18n.t('last.description', "Returns the last value in the list"), ["last(a,b,c...)", "last(list)"]);
     f('at', function(list, x) { return list[x]; }, I18n.t('at.description', "Returns the indexed value in the given list"), "at(list,index)" );
     f('rand', function(x) { return (Math.random() * (x || 1)); }, I18n.t('rand.description', "Returns a random number between zero and the range specified, or one if no number is given"), "rand(x)");
@@ -443,7 +450,7 @@ define([
       }
       return total;
     }
-    f('mean', function() { 
+    f('mean', function() {
       var args = make_list(arguments);
       return sum(args) / args.length;
     }, I18n.t('mean.description', "Returns the average mean of the values in the list"), ["mean(a,b,c...)", "mean(list)"]);
@@ -460,7 +467,7 @@ define([
         return ((list[Math.round(list.length / 2)] + list[Math.round(list.length / 2) - 1]) / 2);
       }
     }, I18n.t('median.description', "Returns the median for the list of values"), ["median(a,b,c...)", "median(list)"]);
-    f('range', function() { 
+    f('range', function() {
       var args = make_list(arguments);
       var list = [];
       for(var idx = 0; idx < args.length; idx++) {