feat(orderBy): Stable sort the input

Stable sort the input array

Closes angular#12408
Fixes angular#12405

Author: lgalfaso

src/ng/filter/orderBy.js

@@ -183,6 +183,10 @@ function orderByFilter($parse) {
     if (sortPredicate.length === 0) { sortPredicate = ['+']; }
 
     var predicates = processPredicates(sortPredicate, reverseOrder);
+    // Add a predicate at the end that evaluates to the element index. This makes the
+    // sort stable as it works as a tie-breaker when all the input predicates cannot
+    // distinguish between two elements.
+    predicates.push({ get: function() { return {}; }, descending: reverseOrder ? -1 : 1});
 
     // The next three lines are a version of a Swartzian Transform idiom from Perl
     // (sometimes called the Decorate-Sort-Undecorate idiom)

test/ng/filter/orderBySpec.js

@@ -194,6 +194,35 @@ describe('Filter: orderBy', function() {
     it('should sort mixed array of objects and values in a stable way', function() {
       expect(orderBy([{foo: 2}, {foo: {}}, {foo: 3}, {foo: 4}], 'foo')).toEqualData([{foo: 2}, {foo: 3}, {foo: 4}, {foo: {}}]);
     });
+
+
+    it('should perform a stable sort', function() {
+      expect(orderBy([
+          {foo: 2, bar: 1}, {foo: 1, bar: 2}, {foo: 2, bar: 3},
+          {foo: 2, bar: 4}, {foo: 1, bar: 5}, {foo: 2, bar: 6},
+          {foo: 2, bar: 7}, {foo: 1, bar: 8}, {foo: 2, bar: 9},
+          {foo: 1, bar: 10}, {foo: 2, bar: 11}, {foo: 1, bar: 12}
+        ], 'foo'))
+          .toEqualData([
+          {foo: 1, bar: 2}, {foo: 1, bar: 5}, {foo: 1, bar: 8},
+          {foo: 1, bar: 10}, {foo: 1, bar: 12}, {foo: 2, bar: 1},
+          {foo: 2, bar: 3}, {foo: 2, bar: 4}, {foo: 2, bar: 6},
+          {foo: 2, bar: 7}, {foo: 2, bar: 9}, {foo: 2, bar: 11}
+          ]);
+
+      expect(orderBy([
+          {foo: 2, bar: 1}, {foo: 1, bar: 2}, {foo: 2, bar: 3},
+          {foo: 2, bar: 4}, {foo: 1, bar: 5}, {foo: 2, bar: 6},
+          {foo: 2, bar: 7}, {foo: 1, bar: 8}, {foo: 2, bar: 9},
+          {foo: 1, bar: 10}, {foo: 2, bar: 11}, {foo: 1, bar: 12}
+        ], 'foo', true))
+          .toEqualData([
+          {foo: 2, bar: 11}, {foo: 2, bar: 9}, {foo: 2, bar: 7},
+          {foo: 2, bar: 6}, {foo: 2, bar: 4}, {foo: 2, bar: 3},
+          {foo: 2, bar: 1}, {foo: 1, bar: 12}, {foo: 1, bar: 10},
+          {foo: 1, bar: 8}, {foo: 1, bar: 5}, {foo: 1, bar: 2}
+          ]);
+    });
   });