Merge pull request RustPython#422 from HomerMcMillan/range-reversed

Fix range reversed RustPython#421

Author: rmliddle

tests/snippets/builtin_range.py

@@ -61,3 +61,4 @@ def assert_raises(expr, exc_type):
 # __reversed__
 assert list(reversed(range(5))) == [4, 3, 2, 1, 0]
 assert list(reversed(range(5, 0, -1))) == [1, 2, 3, 4, 5]
+assert list(reversed(range(1,10,5))) == [6, 1]

vm/src/obj/objrange.rs

@@ -95,14 +95,23 @@ impl RangeType {
 
     #[inline]
     pub fn reversed(&self) -> Self {
+        // compute the last element that is actually contained within the range
+        // this is the new start
+        let remainder = ((&self.end - &self.start) % &self.step).abs();
+        let start = if remainder.is_zero() {
+            &self.end - &self.step
+        } else {
+            &self.end - &remainder
+        };
+
         match self.step.sign() {
             Sign::Plus => RangeType {
-                start: &self.end - 1,
+                start,
                 end: &self.start - 1,
                 step: -&self.step,
             },
             Sign::Minus => RangeType {
-                start: &self.end + 1,
+                start,
                 end: &self.start + 1,
                 step: -&self.step,
             },