Fix ranking to make spec return [ ] if answerSet is empty (#183)

src/ranking/ranking.ts

@@ -37,8 +37,12 @@ export function rank(group: SpecQueryModelGroup, query: Query, schema: Schema, l
     if (query.orderBy || query.chooseBy) {
       group.items.sort(comparator(query.orderBy || query.chooseBy, schema, query.config));
       if (query.chooseBy) {
-        // for chooseBy -- only keep the top-item
-        group.items = [group.items[0]];
+        if (group.items.length > 0) {
+          // for chooseBy -- only keep the top-item
+          group.items = [group.items[0]];
+        } else { // except when the group is empty
+          group.items = [];
+        }
       }
     }
   } else {

test/ranking/ranking.test.ts

@@ -0,0 +1,34 @@
+import {Channel} from 'vega-lite/src/channel';
+import {Mark} from 'vega-lite/src/mark';
+import {Type} from 'vega-lite/src/type';
+
+
+import {schema} from '../fixture';
+import {rank} from '../../src/ranking/ranking';
+
+import {SpecQueryModelGroup} from '../../src/modelgroup';
+
+import {assert} from 'chai';
+
+
+describe('ranking', () => {
+  describe('rank', () => {
+    it('should return an empty group if the input group is empty', () => {
+      let group: SpecQueryModelGroup = rank(
+        {name: '', path: '', items: []},
+        {
+          spec: {
+            mark: Mark.BAR,
+            encodings: [
+              {channel: Channel.SHAPE, field: 'N', type: Type.NOMINAL},
+            ]
+          },
+          chooseBy: 'effectiveness'
+        },
+        schema,
+        0
+      );
+      assert.deepEqual(group.items, []);
+    });
+  });
+});

tsconfig.json

@@ -60,6 +60,7 @@
         "test/ranking/effectiveness/effectiveness.test.ts",
         "test/ranking/effectiveness/channel.test.ts",
         "test/ranking/effectiveness/mark.test.ts",
+        "test/ranking/ranking.test.ts",
         "test/enumerator.test.ts",
         "test/enumspec.test.ts",
         "test/enumspecindex.test.ts",