blas: Fix to skip array with too short stride

If we have a matrix of dimension say 5 x 5, BLAS requires the leading
stride to be >= 5. Smaller cases are possible for read-only array views
in ndarray(broadcasting and custom strides).

In this case we mark the array as not BLAS compatible

Author: bluss

src/linalg/impl_linalg.rs

@@ -863,6 +863,7 @@ where
 
 #[cfg(feature = "blas")]
 #[derive(Copy, Clone)]
+#[cfg_attr(test, derive(PartialEq, Eq, Debug))]
 enum MemoryOrder
 {
     C,
@@ -887,24 +888,34 @@ fn is_blas_2d(dim: &Ix2, stride: &Ix2, order: MemoryOrder) -> bool
     let (m, n) = dim.into_pattern();
     let s0 = stride[0] as isize;
     let s1 = stride[1] as isize;
-    let (inner_stride, outer_dim) = match order {
-        MemoryOrder::C => (s1, n),
-        MemoryOrder::F => (s0, m),
+    let (inner_stride, outer_stride, inner_dim, outer_dim) = match order {
+        MemoryOrder::C => (s1, s0, m, n),
+        MemoryOrder::F => (s0, s1, n, m),
     };
+
     if !(inner_stride == 1 || outer_dim == 1) {
         return false;
     }
+
     if s0 < 1 || s1 < 1 {
         return false;
     }
+
     if (s0 > blas_index::MAX as isize || s0 < blas_index::MIN as isize)
         || (s1 > blas_index::MAX as isize || s1 < blas_index::MIN as isize)
     {
         return false;
     }
+
+    // leading stride must >= the dimension (no broadcasting/aliasing)
+    if inner_dim > 1 && (outer_stride as usize) < outer_dim {
+        return false;
+    }
+
     if m > blas_index::MAX as usize || n > blas_index::MAX as usize {
         return false;
     }
+
     true
 }
 
@@ -1068,8 +1079,26 @@ mod blas_tests
     }
 
     #[test]
-    fn test()
+    fn blas_too_short_stride()
     {
-        //WIP test that stride is larger than other dimension
+        // leading stride must be longer than the other dimension
+        // Example, in a 5 x 5 matrix, the leading stride must be >= 5 for BLAS.
+
+        const N: usize = 5;
+        const MAXSTRIDE: usize = N + 2;
+        let mut data = [0; MAXSTRIDE * N];
+        let mut iter = 0..data.len();
+        data.fill_with(|| iter.next().unwrap());
+
+        for stride in 1..=MAXSTRIDE {
+            let m = ArrayView::from_shape((N, N).strides((stride, 1)), &data).unwrap();
+            eprintln!("{:?}", m);
+
+            if stride < N {
+                assert_eq!(get_blas_compatible_layout(&m), None);
+            } else {
+                assert_eq!(get_blas_compatible_layout(&m), Some(MemoryOrder::C));
+            }
+        }
     }
 }