[polygons/osm] Connecting OSM polygons from their constituent ways is an instance of finding strongly connected components in a graph, adding implementation

scripts/geodata/graph/scc.py

@@ -0,0 +1,34 @@
+VISIT, VISIT_EDGE, POST_VISIT = range(3)
+
+
+def strongly_connected_components(graph):
+    identified = set()
+    stack = []
+    index = {}
+    boundaries = []
+
+    for v in graph:
+        if v not in index:
+            todo = [(VISIT, v)]
+            while todo:
+                op, v = todo.pop()
+                if op == VISIT:
+                    index[v] = len(stack)
+                    stack.append(v)
+                    boundaries.append(index[v])
+                    todo.append((POST_VISIT, v))
+                    todo.extend([(VISIT_EDGE, w) for w in graph[v]])
+                elif op == VISIT_EDGE:
+                    if v not in index:
+                        todo.append((VISIT, v))
+                    elif v not in identified:
+                        while index[v] < boundaries[-1]:
+                            boundaries.pop()
+                else:
+                    # op == POST_VISIT
+                    if boundaries[-1] == index[v]:
+                        boundaries.pop()
+                        scc = stack[index[v]:]
+                        del stack[index[v]:]
+                        identified.update(scc)
+                        yield scc