[ENH] Function for converting PAG to MAG

pywhy_graphs/algorithms/pag.py

@@ -6,7 +6,7 @@
 import networkx as nx
 import numpy as np
 
-from pywhy_graphs import PAG, StationaryTimeSeriesPAG
+from pywhy_graphs import CPDAG, PAG, StationaryTimeSeriesPAG
 from pywhy_graphs.algorithms.generic import single_source_shortest_mixed_path
 from pywhy_graphs.typing import Node, TsNode
 
@@ -908,3 +908,49 @@ def _check_ts_node(node):
         )
     if node[1] > 0:
         raise ValueError(f"All lag points should be 0, or less. You passed in {node}.")
+
+
+def pag_to_mag(graph):
+    """Convert an PAG to an MAG.
+
+    Parameters
+    ----------
+    G : Graph
+        The PAG.
+
+    Returns
+    -------
+    mag : Graph
+        The MAG constructed from the PAG.
+    """
+    copy_graph = graph.copy()
+
+    cedges = set(copy_graph.circle_edges)
+    dedges = set(copy_graph.directed_edges)
+
+    temp_cpdag = CPDAG()
+
+    to_remove = []
+    to_reorient = []
+    to_add = []
+
+    for u, v in cedges:
+        if (v, u) in dedges:  # remove the circle end from a 'o-->' edge to make a '-->' edge
+            to_remove.append((u, v))
+        elif (v, u) not in cedges:  # reorient a '--o' edge to '-->'
+            to_reorient.append((u, v))
+        elif (v, u) in cedges and (
+            v,
+            u,
+        ) not in to_add:  # add all 'o--o' edges to the cpdag
+            to_add.append((u, v))
+    for u, v in to_remove:
+        copy_graph.remove_edge(u, v, graph.circle_edge_name)
+    for u, v in to_reorient:
+        copy_graph.orient_uncertain_edge(u, v)
+    for u, v in to_add:
+        temp_cpdag.add_edge(v, u, temp_cpdag.undirected_edge_name)
+
+    # flag = True
+
+    return copy_graph