From d63df1c1bbc9b963154a656ef94353aa8e5e3f93 Mon Sep 17 00:00:00 2001
From: Nao Pross <np@0hm.ch>
Date: Thu, 9 May 2024 16:22:43 +0200
Subject: Adapt CombinationsExprMixin to work with expression coefficients

---
 polymatrix/expression/impl.py                      |  4 +-
 .../expression/mixins/combinationsexprmixin.py     | 64 ++++++++++++++++++----
 2 files changed, 55 insertions(+), 13 deletions(-)

diff --git a/polymatrix/expression/impl.py b/polymatrix/expression/impl.py
index 429a0b2..e6b021f 100644
--- a/polymatrix/expression/impl.py
+++ b/polymatrix/expression/impl.py
@@ -103,10 +103,10 @@ class CacheExprImpl(CacheExprMixin):
 @dataclassabc.dataclassabc(frozen=True)
 class CombinationsExprImpl(CombinationsExprMixin):
     expression: ExpressionBaseMixin
-    degrees: tuple[int, ...]
+    degrees: ExpressionBaseMixin | tuple[int, ...]
 
     def __str__(self):
-        if len(self.degrees) == 1:
+        if isinstance(self.degrees, tuple) and len(self.degrees) == 1:
             return f"combinations({self.expression}, {self.degrees[0]})"
         return f"combinations({self.expression}, {self.degrees})"
 
diff --git a/polymatrix/expression/mixins/combinationsexprmixin.py b/polymatrix/expression/mixins/combinationsexprmixin.py
index 56a1d64..121d71c 100644
--- a/polymatrix/expression/mixins/combinationsexprmixin.py
+++ b/polymatrix/expression/mixins/combinationsexprmixin.py
@@ -1,5 +1,6 @@
 import abc
 import itertools
+from typing import Iterable
 
 from polymatrix.polymatrix.utils.multiplypolynomial import multiply_polynomial
 from polymatrix.polymatrix.init import init_poly_matrix
@@ -9,35 +10,76 @@ from polymatrix.expression.mixins.expressionbasemixin import ExpressionBaseMixin
 
 
 class CombinationsExprMixin(ExpressionBaseMixin):
+    # FIXME: improve docstring
     """
     combination using degrees=(0, 1, 2, 3):
 
     [[x]]  ->  [[1], [x], [x**2], [x**3]]
     """
-    # NP: example is not great, should be with x_1 and x_2 to show actual
-    # NP: effect of combinations (or am I understanding this wrong?)
 
     @property
     @abc.abstractmethod
-    def expression(self) -> ExpressionBaseMixin: ...
+    def expression(self) -> ExpressionBaseMixin:
+        """ Column vector. """
 
     @property
     @abc.abstractmethod
-    def degrees(self) -> tuple[int, ...]: ...
+    def degrees(self) -> ExpressionBaseMixin | tuple[int, ...]:
+        """
+        Vector or scalar expression, or a list of integers.
+        """
 
-    # overwrites the abstract method of `ExpressionBaseMixin`
     def apply(
         self,
         state: ExpressionState,
     ) -> tuple[ExpressionState, PolyMatrix]:
-        state, poly_matrix = self.expression.apply(state=state)
-
-        assert poly_matrix.shape[1] == 1
+        state, expr_pm = self.expression.apply(state)
+        
+        degrees: Iterable | None = None
+
+        if isinstance(self.degrees, ExpressionBaseMixin):
+            state, deg_pm = self.degrees.apply(state)
+
+            # Check that it is a constant
+            for entry, poly in deg_pm.entries():
+                if not poly.is_constant():
+                    # FIXME: improve error message
+                    raise ValueError("Non-constant exponent resulting from "
+                        f"evaluating {self.degrees}. Exponent must be a constant!")
+            
+            # Scalars are OK
+            nrows, ncols = deg_pm.shape
+            if nrows == 1 and ncols == 1:
+                degrees = (deg_pm.at(0, 0).constant(),)
+
+            # Column vectors are OK
+            elif nrows == 1:
+                degrees = (deg_pm.at(i, 0).constant()
+                           for i in range(nrows))
+
+            # Row vectors are OK
+            elif ncols == 1:
+                degrees = (deg_pm.at(0, i).constant()
+                           for i in range(ncols))
+
+            # Matrices are not OK
+            else:
+                raise ValueError(f"Invalid exponent with shape {deg_pm.shape} resulting from {self.degrees} "
+                                 "Exponent can only be a scalar or a vector of exponents "
+                                 "(multi-index), matrices are not allowed.")
+
+        elif isinstance(self.degrees, tuple):
+            degrees = self.degrees
+
+        # TODO: check that degrees are all integers
+
+        # FIXME: improve error message
+        assert expr_pm.shape[1] == 1
 
         def gen_indices():
-            for degree in self.degrees:
+            for degree in degrees:
                 yield from itertools.combinations_with_replacement(
-                    range(poly_matrix.shape[0]), degree
+                    range(expr_pm.shape[0]), degree
                 )
 
         indices = tuple(gen_indices())
@@ -51,7 +93,7 @@ class CombinationsExprMixin(ExpressionBaseMixin):
                 continue
 
             def acc_product(left, row):
-                right = poly_matrix.get_poly(row, 0)
+                right = expr_pm.get_poly(row, 0)
 
                 if len(left) == 0:
                     return right
-- 
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