fix bilinear basis for miso systems

sysidentpy/basis_function/_bilinear.py

@@ -2,7 +2,7 @@
 
 import warnings
 
-from itertools import combinations_with_replacement
+from itertools import combinations_with_replacement, chain
 from typing import Optional
 import numpy as np
 
@@ -101,19 +101,29 @@ def fit(
                 "In this case, you have a linear polynomial model.",
                 stacklevel=2,
             )
-        else:
-            ny = self.get_max_ylag(ylag)
-            nx = self.get_max_xlag(xlag)
-            combination_ylag = list(
-                combinations_with_replacement(list(range(1, ny + 1)), self.degree)
-            )
-            combination_xlag = list(
+
+        ny = self.get_max_ylag(ylag)
+        combination_ylag = list(
+            combinations_with_replacement(list(range(1, ny + 1)), self.degree)
+        )
+        if isinstance(xlag, int):
+            xlag = [xlag]
+
+        combination_xlag = []
+        ni = 0
+        for lag in xlag:
+            nx = self.get_max_xlag(lag)
+            combination_lag = list(
                 combinations_with_replacement(
-                    list(range(ny + 1, nx + ny + 1)), self.degree
+                    list(range(ny + 1 + ni, nx + ny + 1 + ni)), self.degree
                 )
             )
-            combinations_xy = combination_xlag + combination_ylag
-            combination_list = list(set(combination_list) - set(combinations_xy))
+            combination_xlag.append(combination_lag)
+            ni += nx
+
+        combination_xlag = list(chain.from_iterable(combination_xlag))
+        combinations_xy = combination_xlag + combination_ylag
+        combination_list = list(set(combination_list) - set(combinations_xy))
 
         if predefined_regressors is not None:
             combination_list = [

sysidentpy/basis_function/basis_function_base.py

@@ -56,21 +56,29 @@ def get_max_ylag(self, ylag: int = 1):
         return ny
 
     def get_max_xlag(self, xlag: int = 1):
-        """Get maximum xlag.
+        """Get maximum value from various xlag structures.
 
         Parameters
         ----------
-        xlag : ndarray of int
-            The range of lags according to user definition.
+        xlag : int, list of int, or nested list of int
+            Input that can be a single integer, a list, or a nested list.
 
         Returns
         -------
-        nx : list
-            Maximum value of xlag.
-
+        int
+            Maximum value found.
         """
-        nx = np.max(list(chain.from_iterable([[np.array(xlag, dtype=object)]])))
-        return nx
+        if isinstance(xlag, int):  # Case 1: Single integer
+            return xlag
+
+        if isinstance(xlag, list):
+            # Case 2: Flat list of integers
+            if all(isinstance(i, int) for i in xlag):
+                return max(xlag)
+            # Case 3: Nested list
+            return max(chain.from_iterable(xlag))
+
+        raise ValueError("Unsupported data type for xlag")
 
     def get_iterable_list(
         self, ylag: int = 1, xlag: int = 1, model_type: str = "NARMAX"