g/sample/surface: add region sampling

geom/sample/surface.hpp

@@ -14,6 +14,7 @@
 
 #include "../../coord/fwd.hpp"
 #include "../../extent.hpp"
+#include "../../region.hpp"
 #include "../../random.hpp"
 
 #include <cstddef>
@@ -102,4 +103,35 @@ namespace cruft::geom::sample {
     class surface<extent<2,T>> :
         public volume<extent<2,T>>
     { using volume<extent<2,T>>::volume; };
+
+
+    /// A surface sampler specialisation for 2d regions
+    ///
+    /// We encapsulate an extent sampler and an offset, then mainly pass off
+    /// the work to the extent sampler.
+    template <typename T>
+    class surface<region<2,T>> {
+    public:
+        using shape_type = region<2,T>;
+
+        surface (shape_type const &_shape)
+            : m_extent (_shape.e)
+            , m_offset (_shape.p.template as<vector> ())
+        { ; }
+
+
+        template <typename GeneratorT>
+        decltype(auto)
+        eval (GeneratorT &&gen)
+        {
+            return m_extent.eval (
+                std::forward<GeneratorT> (gen)
+            ) + m_offset;
+        }
+
+
+    private:
+        surface<extent<2,T>> m_extent;
+        cruft::vector<2,T> m_offset;
+    };
 }