dsl: tweak to staggered indices generation for intuitive mapping of c…

…ustom coefficcients onto stencils

devito/finite_differences/tools.py

@@ -248,8 +248,22 @@ def generate_indices_staggered(func, dim, order, side=None, x0=None):
     except AttributeError:
         ind0 = start
     if start != ind0:
-        ind = [start - diff/2 - i * diff for i in range(0, order//2)][::-1]
-        ind += [start + diff/2 + i * diff for i in range(0, order//2)]
+        # Need to extend range by one depending on stagger direction
+        # This is for symbolic coefficients only to ensure intutitive mapping
+        # of user-specified coefficients onto stencils
+        if func.coefficients == 'symbolic':
+            stagger_dir = np.sign((start-ind0).subs(diff, 1))
+            # If start - ind0 is negative then need to add one to upper end
+            if stagger_dir == -1:
+                ind = [start - diff/2 - i * diff for i in range(0, order//2)][::-1]
+                ind += [start + diff/2 + i * diff for i in range(0, order//2+1)]
+            # If start - ind0 is positive then need to add one to lower end
+            elif stagger_dir == 1:
+                ind = [start - diff/2 - i * diff for i in range(0, order//2+1)][::-1]
+                ind += [start + diff/2 + i * diff for i in range(0, order//2)]
+        else:
+            ind = [start - diff/2 - i * diff for i in range(0, order//2)][::-1]
+            ind += [start + diff/2 + i * diff for i in range(0, order//2)]
         if order < 2:
             ind = [start - diff/2, start + diff/2]
     else: