Add AssignmentCollection.has_exclusive_writes

This properties checks whether all writes of this AC are exclusive,
i.e. can be safely parallelized. AC not fulfilling this property are
malformed for pystencils since this property is implicitly assumed.

pystencils/simp/assignment_collection.py

@@ -3,6 +3,7 @@ from copy import copy
 from typing import List, Optional, Dict, Any, Set, Sequence, Iterator, Iterable, Union
 from pystencils.assignment import Assignment
 from pystencils.sympyextensions import fast_subs, count_operations, sort_assignments_topologically
+import pystencils
 
 
 class AssignmentCollection:
@@ -360,6 +361,25 @@ class AssignmentCollection:
         self.sub_expressions = [Assignment(k, v)
                                 for k, v in sub_expressions_dict.items()]
 
+    def has_exclusive_writes(self):
+        """
+        Simple check for exclusive (non-overlapping) writes.
+        I.e. AssignmentCollection can be executed safely in parallel without caring about race conditions.
+        """
+
+        assignments = self.main_assignments
+        write_field_accesses = [a.lhs for a in assignments if isinstance(a.lhs, pystencils.Field.Access)]
+
+        exclusive_writes = set()
+        for a in write_field_accesses:
+
+            if (a.field, a.index) in exclusive_writes:
+                return False
+            else:
+                exclusive_writes.add((a.field, a.index))
+
+        return True
+
 
 class SymbolGen:
     """Default symbol generator producing number symbols ζ_0, ζ_1, ..."""