Make is_constant for MonomialIndex and VariableIndex static methods

also fix an error in VariableIndex.is_constant

1 files changed, 9 insertions, 7 deletions

diff --git a/polymatrix/polymatrix/typing.py b/polymatrix/polymatrix/typing.py
index 06b65cd..ce33e67 100644
--- a/polymatrix/polymatrix/typing.py
+++ b/polymatrix/polymatrix/typing.py
@@ -28,9 +28,10 @@ class VariableIndex(NamedTuple):
     def constant() -> VariableIndex:
         return cast(VariableIndex, tuple())
 
-    def is_constant(self) -> bool:
+    @staticmethod
+    def is_constant(index: VariableIndex) -> bool:
         """ Returns true if it is indexing a constant the unit '1' variable. """
-        return len(self) > 1
+        return not bool(index) # empty tuple is falsy
 
 
 class MonomialIndex(tuple[VariableIndex]):
@@ -62,11 +63,12 @@ class MonomialIndex(tuple[VariableIndex]):
         """ Get the placeholder for constant terms. """
         return MonomialIndex((VariableIndex.constant(),))
 
-    def is_constant(self) -> bool:
+    @staticmethod
+    def is_constant(index: MonomialIndex) -> bool:
         """ Returns true if it is indexing a constant monomial. """
-        if len(self) > 1:
+        if len(index) > 1:
             return False
-        return self[0].is_constant()
+        return VariableIndex.is_constant(index[0])
 
     @staticmethod
     def sort(index: MonomialIndex) -> MonomialIndex:
@@ -81,10 +83,10 @@ class MonomialIndex(tuple[VariableIndex]):
         For example if left is the index of :math:`xy` and right is the index
         of :math:`y^2` this functions returns the index of :math:`xy^3`.
         """
-        if left.is_constant():
+        if MonomialIndex.is_constant(left):
             return right
 
-        if right.is_constant():
+        if MonomialIndex.is_constant(right):
             return left
 
         # Compute the product of each non-constant term in left with each