Fix a few inconsistencies around negative strides in constructors

src/impl_views/constructors.rs

@@ -13,6 +13,7 @@ use crate::error::ShapeError;
 use crate::extension::nonnull::nonnull_debug_checked_from_ptr;
 use crate::imp_prelude::*;
 use crate::{is_aligned, StrideShape};
+use crate::dimension::offset_from_ptr_to_memory;
 
 /// Methods for read-only array views.
 impl<'a, A, D> ArrayView<'a, A, D>
@@ -29,6 +30,7 @@ where
     /// use ndarray::arr3;
     /// use ndarray::ShapeBuilder;
     ///
+    /// // advanced example where we are even specifying exact strides to use (which is optional).
     /// let s = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12];
     /// let a = ArrayView::from_shape((2, 3, 2).strides((1, 4, 2)),
     ///                               &s).unwrap();
@@ -55,7 +57,7 @@ where
         let dim = shape.dim;
         dimension::can_index_slice_with_strides(xs, &dim, &shape.strides)?;
         let strides = shape.strides.strides_for_dim(&dim);
-        unsafe { Ok(Self::new_(xs.as_ptr(), dim, strides)) }
+        unsafe { Ok(Self::new_(xs.as_ptr().offset(-offset_from_ptr_to_memory(&dim, &strides)), dim, strides)) }
     }
 
     /// Create an `ArrayView<A, D>` from shape information and a raw pointer to
@@ -152,7 +154,7 @@ where
         let dim = shape.dim;
         dimension::can_index_slice_with_strides(xs, &dim, &shape.strides)?;
         let strides = shape.strides.strides_for_dim(&dim);
-        unsafe { Ok(Self::new_(xs.as_mut_ptr(), dim, strides)) }
+        unsafe { Ok(Self::new_(xs.as_mut_ptr().offset(-offset_from_ptr_to_memory(&dim, &strides)), dim, strides)) }
     }
 
     /// Create an `ArrayViewMut<A, D>` from shape information and a

tests/array.rs

@@ -1827,6 +1827,27 @@ fn map_memory_order() {
     assert_eq!(amap.strides(), v.strides());
 }
 
+#[test]
+fn test_view_from_shape() {
+    let s = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11];
+    let a = ArrayView::from_shape((2, 3, 2), &s).unwrap();
+    let mut answer = Array::from(s.to_vec()).into_shape((2, 3, 2)).unwrap();
+    assert_eq!(a, answer);
+
+    // custom strides (row major)
+    let a = ArrayView::from_shape((2, 3, 2).strides((6, 2, 1)), &s).unwrap();
+    assert_eq!(a, answer);
+
+    // custom strides (col major)
+    let a = ArrayView::from_shape((2, 3, 2).strides((1, 2, 6)), &s).unwrap();
+    assert_eq!(a, answer.t());
+
+    // negative strides
+    let a = ArrayView::from_shape((2, 3, 2).strides((6, (-2isize) as usize, 1)), &s).unwrap();
+    answer.invert_axis(Axis(1));
+    assert_eq!(a, answer);
+}
+
 #[test]
 fn test_contiguous() {
     let c = arr3(&[[[1, 2, 3], [4, 5, 6]], [[4, 5, 6], [7, 7, 7]]]);
@@ -1973,6 +1994,46 @@ fn test_view_from_shape_ptr() {
     assert_eq!(view, aview2(&[[0, 0, 2], [3, 4, 6]]));
 }
 
+#[should_panic(expected = "Unsupported")]
+#[cfg(debug_assertions)]
+#[test]
+fn test_view_from_shape_ptr_deny_neg_strides() {
+    let data = [0, 1, 2, 3, 4, 5];
+    let _view = unsafe {
+        ArrayView::from_shape_ptr((2, 3).strides((-3isize as usize, 1)), data.as_ptr())
+    };
+}
+
+#[should_panic(expected = "Unsupported")]
+#[cfg(debug_assertions)]
+#[test]
+fn test_view_mut_from_shape_ptr_deny_neg_strides() {
+    let mut data = [0, 1, 2, 3, 4, 5];
+    let _view = unsafe {
+        ArrayViewMut::from_shape_ptr((2, 3).strides((-3isize as usize, 1)), data.as_mut_ptr())
+    };
+}
+
+#[should_panic(expected = "Unsupported")]
+#[cfg(debug_assertions)]
+#[test]
+fn test_raw_view_from_shape_ptr_deny_neg_strides() {
+    let data = [0, 1, 2, 3, 4, 5];
+    let _view = unsafe {
+        RawArrayView::from_shape_ptr((2, 3).strides((-3isize as usize, 1)), data.as_ptr())
+    };
+}
+
+#[should_panic(expected = "Unsupported")]
+#[cfg(debug_assertions)]
+#[test]
+fn test_raw_view_mut_from_shape_ptr_deny_neg_strides() {
+    let mut data = [0, 1, 2, 3, 4, 5];
+    let _view = unsafe {
+        RawArrayViewMut::from_shape_ptr((2, 3).strides((-3isize as usize, 1)), data.as_mut_ptr())
+    };
+}
+
 #[test]
 fn test_default() {
     let a = <Array<f32, Ix2> as Default>::default();