Implement an equivalent to numpy.roll

Fix #1766

Author: serge-sans-paille

include/xtensor/xmanipulation.hpp

@@ -608,6 +608,32 @@ namespace xt
 
         return rot90_impl<n>()(std::forward<E>(e), norm_axes);
     }
+
+    template<class E>
+    inline auto roll(E&& e, std::ptrdiff_t shift) {
+      decltype(auto) cpy = empty_like(std::forward<E>(e));
+      auto eflat_view = flatten(e);
+      auto cpyflat_view = flatten(cpy);
+      if(shift == 0) {
+        std::copy(eflat_view.begin(), eflat_view.end(), cpyflat_view.begin());
+      }
+      else if(shift < 0) {
+        shift = -shift;
+        if(shift > eflat_view.size())
+          shift %= eflat_view.size();
+        std::vector<typename std::decay_t<E>::value_type> tmp{eflat_view.begin(), eflat_view.begin() + shift};
+        std::copy(tmp.begin(), tmp.end(),
+                  std::copy(eflat_view.begin() + shift, eflat_view.end(), cpyflat_view.begin()));
+      }
+      else {
+        if(shift > eflat_view.size())
+          shift %= eflat_view.size();
+        std::vector<typename std::decay_t<E>::value_type> tmp{eflat_view.end() - shift, eflat_view.end()};
+        std::copy(eflat_view.begin(), eflat_view.end() - shift,
+                  std::copy(tmp.begin(), tmp.end(), cpyflat_view.begin()));
+      }
+      return cpy;
+    }
 }
 
 #endif

test/test_xmanipulation.cpp

@@ -334,4 +334,20 @@ namespace xt
         xarray<double> expected5 = {{{1, 3}, {0, 2}}, {{5, 7}, {4, 6}}};
         ASSERT_EQ(expected5, xt::rot90(e3, {1, 2}));
     }
+
+    TEST(xmanipulation, roll)
+    {
+        xarray<double> e1 = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
+
+        ASSERT_EQ(e1, xt::roll(e1, 0));
+
+        xarray<double> expected1 = {{2, 3, 4}, {5, 6, 7}, {8, 9, 1}};
+        ASSERT_EQ(expected1, xt::roll(e1, -1));
+
+        xarray<double> expected2 = {{8, 9, 1}, {2, 3, 4}, {5, 6, 7}};
+        ASSERT_EQ(expected2, xt::roll(e1, 2));
+
+        xarray<double> expected3 = {{8, 9, 1}, {2, 3, 4}, {5, 6, 7}};
+        ASSERT_EQ(expected3, xt::roll(e1, 11));
+    }
 }