1 files changed, 62 insertions, 0 deletions
diff --git a/polymatrix/expression/mixins/legendreseriesmixin.py b/polymatrix/expression/mixins/legendreseriesmixin.py
new file mode 100644
index 0000000..3232207
--- /dev/null
+++ b/polymatrix/expression/mixins/legendreseriesmixin.py
@@ -0,0 +1,62 @@
+import abc
+
+from polymatrix.utils.getstacklines import FrameSummary
+from polymatrix.polymatrix.polymatrix import PolyMatrix
+from polymatrix.polymatrix.init.initpolymatrix import init_poly_matrix
+from polymatrix.expressionstate.expressionstate import ExpressionState
+from polymatrix.expression.mixins.expressionbasemixin import ExpressionBaseMixin
+
+
+class LegendreSeriesMixin(ExpressionBaseMixin):
+    @property
+    @abc.abstractmethod
+    def underlying(self) -> tuple[ExpressionBaseMixin]:
+        ...
+
+    @property
+    @abc.abstractmethod
+    def degrees(self) -> tuple[int, ...] | None:
+        ...
+
+    @property
+    @abc.abstractmethod
+    def stack(self) -> tuple[FrameSummary]:
+        ...
+
+    # overwrites abstract method of `ExpressionBaseMixin`
+    def apply(
+        self, 
+        state: ExpressionState,
+    ) -> tuple[ExpressionState, PolyMatrix]:
+        
+        state, underlying = self.underlying.apply(state)
+
+        if self.degrees is None:
+            degrees = range(underlying.shape[0])
+        else:
+            degrees = self.degrees
+        
+        terms = {}
+
+        for degree in degrees:
+        # for degree in self.degree:
+            poly = underlying.get_poly(degree, 0)
+
+            terms[degree, 0] = dict(poly)
+
+            if 2 <= degree:
+                poly = underlying.get_poly(degree - 2, 0)
+                factor = - (degree - 1) / (degree + 1)
+
+                for m, v in poly.items():
+                    if m in terms[degree, 0]:
+                        terms[degree, 0][m] += v*factor
+                    else:
+                        terms[degree, 0][m] = v*factor
+
+        poly_matrix = init_poly_matrix(
+            terms=terms,
+            shape=(len(terms), 1),
+        )
+
+        return state, poly_matrix