Ignore edge contraction for now (#39)

tests/test_graph.py

@@ -3,6 +3,7 @@
 from pandas.core.frame import DataFrame
 import pytest as pt
 from vsec.graph import COLUMNS, Graph
+from vsec.utils import join_terminal_labels
 
 ATTR = "level"
 IS_FIRST = lambda x: x == "high"
@@ -13,11 +14,13 @@
     ("f", "g_lv"),
     ("g_hv", "g_lv"),
 }
+# There are only two edges left, but sequences might be different.
+EDGES = {("n1", "n2n3"), ("n2n3", "n1"), ("n2n3", "n4"), ("n4", "n2n3")}
 
 
 @pt.mark.usefixtures("case_readme")
 def test_split(case_readme: nx.DiGraph):
-    """Check basic features of ``split`` method in ``GeoGraph``.
+    """Check basic features after one vertex splitting.
 
     Note:
         ``split`` method is not tested thoroughly here.
@@ -52,3 +55,18 @@ def test_split(case_readme: nx.DiGraph):
     assert res.find_vertices_component("g_lv") == {"g_lv", "d", "f"}
 
     assert res.vertices_new == {"g_hv", "g_lv"}
+
+
+@pt.mark.skip(reason="not sure about edge contraction yet")
+@pt.mark.usefixtures("case_simple")
+def test_contract(case_simple: nx.Graph):
+    """Check if edge in a graph contracted and its terminals renamed.
+
+    Args:
+        case_simple: a simple graph with 3 edges.
+    """
+    g = Graph(case_simple)
+    res = g.contract("contraction", join_terminal_labels)
+    print(res.nodes)
+    assert type(res) is Graph
+    assert set(res.edges).difference(EDGES) == set()

vsec/__init__.py

@@ -1,10 +1,9 @@
 """Main module for ``vsec``."""
-from vsec.conversion import contract, split
 from vsec.graph import Graph
+from vsec.utils import join_terminal_labels
 
 __version__ = "0.1.0"
 __all__ = [
-    "contract",
-    "split",
     "Graph",
+    "join_terminal_labels",
 ]

vsec/graph.py

@@ -1,8 +1,14 @@
-"""A class for two operations at the same time."""
+"""A class for two operations at the same time.
+
+Any modification after the initiation should be avoided, or many methods
+and properties will not work as expected.
+"""
 from typing import Callable, Optional, Set, Tuple, Union
 
 from loguru import logger
 import networkx as nx
+from networkx.algorithms.minors import contracted_edge
+from networkx.relabel import relabel_nodes
 import pandas as pd
 from pandas.core.frame import DataFrame
 
@@ -283,3 +289,37 @@ def is_connected_graph(self) -> bool:
             True if this undirected graph is connected.
         """
         return nx.is_connected(self.to_undirected())
+
+    def contract(
+        self, attr: str, naming: Optional[Callable[[Tuple[str, str]], str]],
+    ) -> nx.Graph:
+        """Contract edges with ``attr`` being true in ``Graph``.
+
+        Args:
+            attr: some bool edge attribute indicating contraction if
+                true.
+            naming: a function to name the new node based on labels of
+                two terminals of the contracted edge.
+
+        Returns:
+            A graph with less edge(s).
+        """
+        graph = nx.Graph(self)
+        for u, v, attributes in self.edges(data=True):
+            if attributes[attr]:
+                edge_contracted = (u, v)
+                graph = contracted_edge(
+                    graph, edge_contracted, self_loops=False
+                )
+                logger.debug(f"Edge {edge_contracted} has been contracted.")
+
+                # Find which node is kept.
+                if u in graph.nodes:
+                    mapping = {u: naming(edge_contracted)}
+                else:
+                    mapping = {v: naming(edge_contracted)}
+
+                # Rename the kept node.
+                relabel_nodes(graph, mapping, copy=False)
+
+        return graph