Merge pull request #574 from jturner314/optimize-fold

Improve performance of .fold()

Author: bluss

src/dimension/axes.rs

@@ -75,6 +75,15 @@ impl<'a, D> Iterator for Axes<'a, D>
         }
     }
 
+    fn fold<B, F>(self, init: B, f: F) -> B
+    where
+        F: FnMut(B, AxisDescription) -> B,
+    {
+        (self.start..self.end)
+            .map(move |i| AxisDescription(Axis(i), self.dim[i], self.strides[i] as isize))
+            .fold(init, f)
+    }
+
     fn size_hint(&self) -> (usize, Option<usize>) {
         let len = self.end - self.start;
         (len, Some(len))

src/impl_methods.rs

@@ -1854,13 +1854,25 @@ where
         } else {
             let mut v = self.view();
             // put the narrowest axis at the last position
-            if v.ndim() > 1 {
-                let last = v.ndim() - 1;
-                let narrow_axis = v.axes()
-                                   .filter(|ax| ax.len() > 1)
-                                   .min_by_key(|ax| ax.stride().abs())
-                                   .map_or(last, |ax| ax.axis().index());
-                v.swap_axes(last, narrow_axis);
+            match v.ndim() {
+                0 | 1 => {}
+                2 => {
+                    if self.len_of(Axis(1)) <= 1
+                        || self.len_of(Axis(0)) > 1
+                            && self.stride_of(Axis(0)).abs() < self.stride_of(Axis(1)).abs()
+                    {
+                        v.swap_axes(0, 1);
+                    }
+                }
+                n => {
+                    let last = n - 1;
+                    let narrow_axis = v
+                        .axes()
+                        .filter(|ax| ax.len() > 1)
+                        .min_by_key(|ax| ax.stride().abs())
+                        .map_or(last, |ax| ax.axis().index());
+                    v.swap_axes(last, narrow_axis);
+                }
             }
             v.into_elements_base().fold(init, f)
         }