Support XOR expressions (#2699)

Replace XOR by OR (-> #2685).

Author: dweindl

python/sdist/amici/import_utils.py

@@ -531,6 +531,11 @@ def _parse_heaviside_trigger(trigger: sp.Expr) -> sp.Expr:
             # y >= x => not(x < y) => not(x - y < 0) => not r < 0
             return sp.Heaviside(root)
 
+    # rewrite n-ary XOR to OR to be handled below:
+    trigger = trigger.replace(sp.Xor, _xor_to_or)
+
+    # TODO: x == y
+
     # or(x,y) = not(and(not(x),not(y))
     if isinstance(trigger, sp.Or):
         return sp.Integer(1) - sp.Mul(
@@ -540,8 +545,6 @@ def _parse_heaviside_trigger(trigger: sp.Expr) -> sp.Expr:
             ]
         )
 
-    # TODO: x XOR y = (A & ~B) | (~A & B)
-    # TODO: x == y
     if isinstance(trigger, sp.And):
         return sp.Mul(*[_parse_heaviside_trigger(arg) for arg in trigger.args])
 
@@ -551,6 +554,25 @@ def _parse_heaviside_trigger(trigger: sp.Expr) -> sp.Expr:
     )
 
 
+def _xor_to_or(*args):
+    """
+    Replace XOR by OR expression.
+
+    ``xor(x, y, ...) = (x & ~y & ...) | (~x & y & ...) | ...``.
+
+    to be used in ``trigger = trigger.replace(sp.Xor, _xor_to_or)``.
+    """
+    res = sp.false
+    for i in range(len(args)):
+        res = sp.Or(
+            res,
+            sp.And(
+                *(arg if i == j else sp.Not(arg) for j, arg in enumerate(args))
+            ),
+        )
+    return res.simplify()
+
+
 def grouper(
     iterable: Iterable, n: int, fillvalue: Any = None
 ) -> Iterable[tuple[Any]]:

python/sdist/amici/sbml_import.py

@@ -52,6 +52,7 @@
     _default_simplify,
     generate_flux_symbol,
     _parse_piecewise_to_heaviside,
+    _xor_to_or,
 )
 from .logging import get_logger, log_execution_time, set_log_level
 from .sbml_utils import SBMLException
@@ -2976,11 +2977,6 @@ def subs_locals(expr: sp.Basic) -> sp.Basic:
                 f"Unsupported input: {var_or_math}, type: {type(var_or_math)}"
             )
 
-        if expr.has(sp.Xor):
-            raise SBMLException(
-                "Xor is currently not supported as logical operation."
-            )
-
         try:
             _check_unsupported_functions_sbml(expr, expression_type=ele_name)
         except SBMLException:
@@ -3218,6 +3214,9 @@ def _parse_event_trigger(trigger: sp.Expr) -> sp.Expr:
             # y >= x or y > x
             return root
 
+    # rewrite n-ary XOR to OR to be handled below:
+    trigger = trigger.replace(sp.Xor, _xor_to_or)
+
     # or(x,y): any of {x,y} is > 0: sp.Max(x, y)
     if isinstance(trigger, sp.Or):
         return sp.Max(*[_parse_event_trigger(arg) for arg in trigger.args])