region: add rotate method

region.hpp

@@ -201,6 +201,25 @@ namespace cruft {
     };
 
 
+    ///////////////////////////////////////////////////////////////////////////
+    template <typename T> using region2 = region<2,T>;
+    template <typename T> using region3 = region<3,T>;
+
+
+    //-------------------------------------------------------------------------
+    using region2u = region2<unsigned>;
+    using region2i = region2<int>;
+    using region2f = region2<float>;
+    using region2d = region2<double>;
+
+
+    //-------------------------------------------------------------------------
+    extern template struct region<2, unsigned>;
+    extern template struct region<2, int>;
+    extern template struct region<2, float>;
+    extern template struct region<2, double>;
+
+
     ///////////////////////////////////////////////////////////////////////////
     /// constructs the minimal region that encompasses a region and a point.
     template <typename T, size_t S>
@@ -324,14 +343,54 @@ namespace cruft {
 
 
     ///////////////////////////////////////////////////////////////////////////
-    template <typename T> using region2 = region<2,T>;
-    template <typename T> using region3 = region<3,T>;
+    /// Returns a region rotated clockwise about the base point in `steps`
+    /// multiples of 90 degrees.
+    ///
+    /// `steps` must lie in the range [0, 4) so we can avoid an expensive
+    /// modulus in the typical case.
+    template <typename T>
+    cruft::region<2,T>
+    rotate (cruft::region<2,T> obj, int steps)
+    {
+        CHECK_LIMIT (steps, 0, 3);
 
+        switch (steps) {
+        case 0:
+            return obj;
 
-    using region2u = region2<unsigned>;
-    using region2i = region2<int>;
-    using region2f = region2<float>;
-    using region2d = region2<double>;
+        case 1:
+            return region2<T> {
+                point2<T> {
+                    obj.p.x - obj.e.h,
+                    obj.p.y
+                },
+                extent2<T> {
+                    obj.e.h,
+                    obj.e.w,
+                }
+            };
+
+        case 2:
+            return region2<T> {
+                point2<T> {
+                    obj.p.x,
+                    obj.p.y - obj.e.h,
+                },
+                obj.e,
+            };
+
+        case 3:
+            return region2<T> {
+                obj.p,
+                extent2<T> {
+                    obj.e.h,
+                    obj.e.w,
+                },
+            };
+        }
+
+        unreachable();
+    }
 
 
     ///////////////////////////////////////////////////////////////////////////

test/region.cpp

@@ -183,5 +183,55 @@ main (int, char **)
             tap.expect_eq (intersection (a, b), res, "region-region interesection: %!", message);
     }
 
+    // Test rotate behaves as expected.
+    //
+    // It's important that the following constraints apply to the test data:
+    //    * no point lies at the origin because we want to catch
+    //      rotate-about-point errors that need a translation.
+    //    * the size is non-square so we can differentiate between rotated
+    //      values
+    //    * use floating point values so we can test for loss of precision.
+    {
+        cruft::point2f  const base { 1, 2 };
+        cruft::extent2f const size { 3, 4 };
+        cruft::region2f const orig { base, size };
+
+        struct {
+            int rotation;
+            cruft::region2f res;
+        } const TESTS[] {
+            {
+                .rotation = 0,
+                .res = orig,
+            },
+            {
+                .rotation = 1,
+                .res = {
+                    cruft::point2f  { -3,  2 },
+                    cruft::extent2f {  4,  3 },
+                },
+            },
+            {
+                .rotation = 2,
+                .res = {
+                    cruft::point2f  {  1, -2 },
+                    cruft::extent2f {  3,  4 },
+                },
+            },
+            {
+                .rotation = 3,
+                .res = {
+                    cruft::point2f  {  1,  2 },
+                    cruft::extent2f {  4,  3 },
+                },
+            }
+        };
+
+        for (auto const &obj: TESTS) {
+            auto const computed = rotate (orig, obj.rotation);
+            tap.expect_eq (computed, obj.res, "%! rotation", obj.rotation);
+        }
+    }
+
     return tap.status ();
 }