FEAT: Add dimension merge function to merge contiguous axes

src/dimension/mod.rs

@@ -759,6 +759,32 @@ where D: Dimension
     }
 }
 
+/// Attempt to merge axes if possible, starting from the back
+///
+/// Given axes [Axis(0), Axis(1), Axis(2), Axis(3)] this attempts
+/// to merge all axes one by one into Axis(3); when/if this fails,
+/// it attempts to merge the rest of the axes together into the next
+/// axis in line, for example a result could be:
+///
+/// [1, Axis(0) + Axis(1), 1, Axis(2) + Axis(3)] where `+` would
+/// mean axes were merged.
+pub(crate) fn merge_axes_from_the_back<D>(dim: &mut D, strides: &mut D)
+where D: Dimension
+{
+    debug_assert_eq!(dim.ndim(), strides.ndim());
+    match dim.ndim() {
+        0 | 1 => {}
+        n => {
+            let mut last = n - 1;
+            for i in (0..last).rev() {
+                if !merge_axes(dim, strides, Axis(i), Axis(last)) {
+                    last = i;
+                }
+            }
+        }
+    }
+}
+
 /// Move the axis which has the smallest absolute stride and a length
 /// greater than one to be the last axis.
 pub fn move_min_stride_axis_to_last<D>(dim: &mut D, strides: &mut D)
@@ -820,6 +846,30 @@ where D: Dimension
     *strides = new_strides;
 }
 
+/// Sort axes to standard/row major order, i.e Axis(0) has biggest stride and Axis(n - 1) least
+/// stride
+///
+/// The axes are sorted according to the .abs() of their stride.
+pub(crate) fn sort_axes_to_standard<D>(dim: &mut D, strides: &mut D)
+where D: Dimension
+{
+    debug_assert!(dim.ndim() > 1);
+    debug_assert_eq!(dim.ndim(), strides.ndim());
+    // bubble sort axes
+    let mut changed = true;
+    while changed {
+        changed = false;
+        for i in 0..dim.ndim() - 1 {
+            // make sure higher stride axes sort before.
+            if strides.get_stride(Axis(i)).abs() < strides.get_stride(Axis(i + 1)).abs() {
+                changed = true;
+                dim.slice_mut().swap(i, i + 1);
+                strides.slice_mut().swap(i, i + 1);
+            }
+        }
+    }
+}
+
 #[cfg(test)]
 mod test
 {
@@ -829,6 +879,7 @@ mod test
         can_index_slice_not_custom,
         extended_gcd,
         max_abs_offset_check_overflow,
+        merge_axes_from_the_back,
         slice_min_max,
         slices_intersect,
         solve_linear_diophantine_eq,
@@ -1213,4 +1264,27 @@ mod test
         assert_eq!(d, dans);
         assert_eq!(s, sans);
     }
+
+    #[test]
+    fn test_merge_axes_from_the_back()
+    {
+        let dyndim = Dim::<&[usize]>;
+
+        let mut d = Dim([3, 4, 5]);
+        let mut s = Dim([20, 5, 1]);
+        merge_axes_from_the_back(&mut d, &mut s);
+        assert_eq!(d, Dim([1, 1, 60]));
+        assert_eq!(s, Dim([20, 5, 1]));
+
+        let mut d = Dim([3, 4, 5, 2]);
+        let mut s = Dim([80, 20, 2, 1]);
+        merge_axes_from_the_back(&mut d, &mut s);
+        assert_eq!(d, Dim([1, 12, 1, 10]));
+        assert_eq!(s, Dim([80, 20, 2, 1]));
+        let mut d = d.into_dyn();
+        let mut s = s.into_dyn();
+        squeeze(&mut d, &mut s);
+        assert_eq!(d, dyndim(&[12, 10]));
+        assert_eq!(s, dyndim(&[20, 1]));
+    }
 }