shape: Add trait Sequence

To generalize over Forward/Reverse indexing of dimensions, add simple
traits Sequence/Mut that allow indexing forwards and reversed references
of dimension values. In a future change, they will support strides
(with isize values) too.

Author: bluss

src/dimension/mod.rs

@@ -40,6 +40,7 @@ mod dynindeximpl;
 mod ndindex;
 mod ops;
 mod remove_axis;
+mod sequence;
 
 /// Calculate offset from `Ix` stride converting sign properly
 #[inline(always)]

src/dimension/sequence.rs

@@ -0,0 +1,109 @@
+use std::ops::Index;
+use std::ops::IndexMut;
+
+use crate::dimension::Dimension;
+
+pub(in crate::dimension) struct Forward<D>(pub(crate) D);
+pub(in crate::dimension) struct Reverse<D>(pub(crate) D);
+
+impl<D> Index<usize> for Forward<&D>
+where
+    D: Dimension,
+{
+    type Output = usize;
+
+    #[inline]
+    fn index(&self, index: usize) -> &usize {
+        &self.0[index]
+    }
+}
+
+impl<D> Index<usize> for Forward<&mut D>
+where
+    D: Dimension,
+{
+    type Output = usize;
+
+    #[inline]
+    fn index(&self, index: usize) -> &usize {
+        &self.0[index]
+    }
+}
+
+impl<D> IndexMut<usize> for Forward<&mut D>
+where
+    D: Dimension,
+{
+    #[inline]
+    fn index_mut(&mut self, index: usize) -> &mut usize {
+        &mut self.0[index]
+    }
+}
+
+impl<D> Index<usize> for Reverse<&D>
+where
+    D: Dimension,
+{
+    type Output = usize;
+
+    #[inline]
+    fn index(&self, index: usize) -> &usize {
+        &self.0[self.len() - index - 1]
+    }
+}
+
+impl<D> Index<usize> for Reverse<&mut D>
+where
+    D: Dimension,
+{
+    type Output = usize;
+
+    #[inline]
+    fn index(&self, index: usize) -> &usize {
+        &self.0[self.len() - index - 1]
+    }
+}
+
+impl<D> IndexMut<usize> for Reverse<&mut D>
+where
+    D: Dimension,
+{
+    #[inline]
+    fn index_mut(&mut self, index: usize) -> &mut usize {
+        let len = self.len();
+        &mut self.0[len - index - 1]
+    }
+}
+
+/// Indexable sequence with length
+pub(in crate::dimension) trait Sequence: Index<usize> {
+    fn len(&self) -> usize;
+}
+
+/// Indexable sequence with length (mut)
+pub(in crate::dimension) trait SequenceMut: Sequence + IndexMut<usize> { }
+
+impl<D> Sequence for Forward<&D> where D: Dimension {
+    #[inline]
+    fn len(&self) -> usize { self.0.ndim() }
+}
+
+impl<D> Sequence for Forward<&mut D> where D: Dimension {
+    #[inline]
+    fn len(&self) -> usize { self.0.ndim() }
+}
+
+impl<D> SequenceMut for Forward<&mut D> where D: Dimension { }
+
+impl<D> Sequence for Reverse<&D> where D: Dimension {
+    #[inline]
+    fn len(&self) -> usize { self.0.ndim() }
+}
+
+impl<D> Sequence for Reverse<&mut D> where D: Dimension {
+    #[inline]
+    fn len(&self) -> usize { self.0.ndim() }
+}
+
+impl<D> SequenceMut for Reverse<&mut D> where D: Dimension { }
+