Bug/428 (#459)

* Fix #428 by considering subtraction of zero column tries to be empty

---------

Co-authored-by: Alex Ivliev <[email protected]>

nemo-physical/src/tabular/operations/subtract.rs

@@ -76,6 +76,11 @@ impl OperationGenerator for GeneratorSubtract {
             return None;
         }
 
+        if tries_subtract.iter().any(|scan| scan.arity() == 0) {
+            // If we subtract a non empty zero column trie the result is defined as empty
+            return None;
+        }
+
         let mut active_scans = Vec::with_capacity(trie_main.arity() + 1);
         active_scans.push(Rc::new(RefCell::new(vec![true; tries_subtract.len()])));
 

resources/testcases/regression/unbound_negation/run.rls

@@ -0,0 +1,15 @@
+%%% Test related to 
+%%% https://github.com/knowsys/nemo/issues/428
+%%%
+%%% Previously, subtracting a zero arity table resulted in a crash 
+
+Input(-1).
+
+Assert(positive) :- Input(?i), ?i > 0.
+Assert(negative) :- Input(?i), ?i < 0.
+
+NonPositive(?r) :- Input(?r), ~Assert(positive).
+NonNegative(?r) :- Input(?r), ~Assert(negative). 
+
+@export NonPositive :- csv {} .
+@export NonNegative :- csv {} .

resources/testcases/regression/unbound_negation/run/NonPositive.csv

@@ -0,0 +1 @@
+-1