api: apply staggered shift at interpolation replacement before dimens…

…ion is lost

devito/operations/interpolators.py

@@ -188,9 +188,16 @@ def _interpolation_indices(self, variables, offset=0, field_offset=0,
                 condition = sympy.And(lb, ub, evaluate=False)
                 mapper[d] = ConditionalDimension(p.name, self.sfunction._sparse_dim,
                                                  condition=condition, indirect=True)
-
+            # Apply mapper to each variable with origin correction before the
+            # dimension get replaced.
+            vmapper = {}
+            for v in variables:
+                try:
+                    vmapper[v] = v.subs({k: c - v.origin[k] for k, c in mapper.items()})
+                except KeyError:
+                    vmapper[v] = v.subs(mapper)
             # Track Indexed substitutions
-            idx_subs.append(mapper)
+            idx_subs.append(vmapper)
 
         # Temporaries for the position
         temps = [Eq(v, k, implicit_dims=implicit_dims)

devito/types/basic.py

@@ -945,7 +945,8 @@ def origin(self):
         f(x) : origin = 0
         f(x + hx/2) : origin = hx/2
         """
-        return tuple(r - d for d, r in zip(self.dimensions, self.indices_ref))
+        return DimensionTuple(*(r-d for d, r in zip(self.dimensions, self.indices_ref)),
+                              getters=self.dimensions)
 
     @property
     def dimensions(self):

devito/types/dense.py

@@ -1059,7 +1059,7 @@ def __indices_setup__(cls, **kwargs):
             mapper = {d: d for d in dimensions}
             for s in as_tuple(staggered):
                 c, s = s.as_coeff_Mul()
-                mapper.update({s: s + c * s.spacing/2})
+                mapper.update({s: s + c * s.spacing / 2})
             staggered_indices = mapper.values()
 
         return tuple(dimensions), tuple(staggered_indices)