Fix _fastest_varying_stride_order to always be a stable sort order

This adds a test that ensures order indices are unique even when strides
are equal.

Author: bluss

src/dimension.rs

@@ -585,10 +585,11 @@ unsafe impl Dimension for (Ix, Ix, Ix) {
             }
         }
         {
+            // stable sorting network for 3 elements
             let order = order.slice_mut();
             let strides = stride.slice_mut();
+            swap![strides, order, 1, 2];
             swap![strides, order, 0, 1];
-            swap![strides, order, 0, 2];
             swap![strides, order, 1, 2];
         }
         order

tests/dimension.rs

@@ -51,5 +51,11 @@ fn fastest_varying_order() {
     assert_eq!((1, 3)._fastest_varying_stride_order(), (0, 1));
     assert_eq!((7, 2)._fastest_varying_stride_order(), (1, 0));
     assert_eq!((6, 1, 3)._fastest_varying_stride_order(), (1, 2, 0));
+
+    // it's important that it produces distinct indices. Prefer the stable order
+    // where 0 is before 1 when they are equal.
+    assert_eq!((2, 2)._fastest_varying_stride_order(), (0, 1));
+    assert_eq!((2, 2, 1)._fastest_varying_stride_order(), (2, 0, 1));
+    assert_eq!((2, 2, 3, 1, 2)._fastest_varying_stride_order(), (3, 0, 1, 4, 2));
 }