topo: add PathExistsIn helper function

topo/topo.go

@@ -38,6 +38,16 @@ func IsPathIn(g graph.Graph, path []graph.Node) bool {
 	}
 }
 
+// PathExistsIn returns whether there is a path in g starting at from extending
+// to to.
+//
+// PathExistsIn exists as a helper function. If many tests for path existence
+// are being performed, other approaches will be more efficient.
+func PathExistsIn(g graph.Graph, from, to graph.Node) bool {
+	var t traverse.BreadthFirst
+	return t.Walk(g, from, func(n graph.Node, _ int) bool { return n.ID() == to.ID() }) != nil
+}
+
 // ConnectedComponents returns the connected components of the undirected graph g.
 func ConnectedComponents(g graph.Undirected) [][]graph.Node {
 	var (

topo/topo_test.go

@@ -54,6 +54,81 @@ func TestIsPath(t *testing.T) {
 	}
 }
 
+var pathExistsInUndirectedTests = []struct {
+	g        []intset
+	from, to int
+	want     bool
+}{
+	{g: batageljZaversnikGraph, from: 0, to: 0, want: true},
+	{g: batageljZaversnikGraph, from: 0, to: 1, want: false},
+	{g: batageljZaversnikGraph, from: 1, to: 2, want: true},
+	{g: batageljZaversnikGraph, from: 2, to: 1, want: true},
+	{g: batageljZaversnikGraph, from: 2, to: 12, want: false},
+	{g: batageljZaversnikGraph, from: 20, to: 6, want: true},
+}
+
+func TestPathExistsInUndirected(t *testing.T) {
+	for i, test := range pathExistsInUndirectedTests {
+		g := simple.NewUndirectedGraph(0, math.Inf(1))
+
+		for u, e := range test.g {
+			if !g.Has(simple.Node(u)) {
+				g.AddNode(simple.Node(u))
+			}
+			for v := range e {
+				if !g.Has(simple.Node(v)) {
+					g.AddNode(simple.Node(v))
+				}
+				g.SetEdge(simple.Edge{F: simple.Node(u), T: simple.Node(v)})
+			}
+		}
+
+		got := PathExistsIn(g, simple.Node(test.from), simple.Node(test.to))
+		if got != test.want {
+			t.Errorf("unexpected result for path existance in test %d: got:%t want %t", i, got, test.want)
+		}
+	}
+}
+
+var pathExistsInDirectedTests = []struct {
+	g        []intset
+	from, to int
+	want     bool
+}{
+	// The graph definition is such that from node IDs are
+	// less than to node IDs.
+	{g: batageljZaversnikGraph, from: 0, to: 0, want: true},
+	{g: batageljZaversnikGraph, from: 0, to: 1, want: false},
+	{g: batageljZaversnikGraph, from: 1, to: 2, want: true},
+	{g: batageljZaversnikGraph, from: 2, to: 1, want: false},
+	{g: batageljZaversnikGraph, from: 2, to: 12, want: false},
+	{g: batageljZaversnikGraph, from: 20, to: 6, want: false},
+	{g: batageljZaversnikGraph, from: 6, to: 20, want: true},
+}
+
+func TestPathExistsInDirected(t *testing.T) {
+	for i, test := range pathExistsInDirectedTests {
+		g := simple.NewDirectedGraph(0, math.Inf(1))
+
+		for u, e := range test.g {
+			if !g.Has(simple.Node(u)) {
+				g.AddNode(simple.Node(u))
+			}
+			for v := range e {
+				if !g.Has(simple.Node(v)) {
+					g.AddNode(simple.Node(v))
+				}
+				g.SetEdge(simple.Edge{F: simple.Node(u), T: simple.Node(v)})
+			}
+		}
+
+		got := PathExistsIn(g, simple.Node(test.from), simple.Node(test.to))
+		if got != test.want {
+			t.Errorf("unexpected result for path existance in test %d: got:%t want %t", i, got, test.want)
+		}
+	}
+}
+
 var connectedComponentTests = []struct {
 	g    []intset
 	want [][]int