pgraph: Print cycles on error

I'm a terrible algorithmist, so who knows if this is correct, but it
seems to work in my cursory testing.

lang/interpret/interpret.go

@@ -291,7 +291,14 @@ func (obj *Interpreter) Interpret(ast interfaces.Stmt, table map[interfaces.Func
 
 	// ensure that we have a DAG!
 	if _, err := graph.TopologicalSort(); err != nil {
-		// TODO: print information on the cycles
+		errNotAcyclic, ok := err.(*pgraph.ErrNotAcyclic)
+		if !ok {
+			return nil, err // programming error
+		}
+		obj.Logf("%s", err)
+		for _, vertex := range errNotAcyclic.Cycle {
+			obj.Logf("* %s", vertex)
+		}
 		return nil, errwrap.Wrapf(err, "resource graph has cycles")
 	}
 

pgraph/pgraph.go

@@ -31,7 +31,6 @@
 package pgraph
 
 import (
-	"errors"
 	"fmt"
 	"sort"
 	"strings"
@@ -40,7 +39,15 @@ import (
 )
 
 // ErrNotAcyclic specifies that a particular graph was not found to be a dag.
-var ErrNotAcyclic = errors.New("not a dag")
+type ErrNotAcyclic struct {
+	Cycle []Vertex
+}
+
+// Error lets this satisfy the error interface.
+func (obj *ErrNotAcyclic) Error() string {
+	//return fmt.Sprintf("not a dag: %v", obj.Cycle)
+	return "not a dag"
+}
 
 // Graph is the graph structure in this library. The graph abstract data type
 // (ADT) is defined as follows:
@@ -667,7 +674,12 @@ func (g *Graph) TopologicalSort() ([]Vertex, error) { // kahn's algorithm
 		if in > 0 {
 			for n := range g.adjacency[c] {
 				if remaining[n] > 0 {
-					return nil, ErrNotAcyclic
+					cycle := g.findCycleDFS(c)
+					if len(cycle) == 0 {
+						// Hopefully this doesn't happen!
+						return nil, fmt.Errorf("programming error")
+					}
+					return nil, &ErrNotAcyclic{Cycle: cycle}
 				}
 			}
 		}
@@ -676,6 +688,61 @@ func (g *Graph) TopologicalSort() ([]Vertex, error) { // kahn's algorithm
 	return L, nil
 }
 
+// findCycleDFS is a helper for the TopologicalSort functions.
+// XXX: A professional should look over this function and try and find issues.
+func (g *Graph) findCycleDFS(start Vertex) []Vertex {
+	visited := make(map[Vertex]bool)
+	stack := make(map[Vertex]bool)
+	var path []Vertex
+	var result []Vertex
+	found := false
+
+	var dfs func(Vertex) bool
+	dfs = func(v Vertex) bool {
+		if found {
+			return true
+		}
+		visited[v] = true
+		stack[v] = true
+		path = append(path, v)
+
+		for n := range g.adjacency[v] {
+			if !visited[n] {
+				if dfs(n) {
+					return true
+				}
+			} else if stack[n] {
+				// cycle detected
+				idx := len(path) - 1
+				for idx >= 0 && path[idx] != n {
+					idx--
+				}
+				if idx >= 0 {
+					result = append([]Vertex{}, path[idx:]...)
+					result = append(result, n) // close the cycle
+					found = true
+					return true
+				}
+			}
+		}
+
+		stack[v] = false
+		path = path[:len(path)-1]
+		return false
+	}
+
+	// run DFS from all potentially cyclic nodes
+	for v := range g.adjacency {
+		if !visited[v] {
+			if dfs(v) {
+				break
+			}
+		}
+	}
+
+	return result
+}
+
 // DeterministicTopologicalSort returns the sort of graph vertices in a stable
 // topological sort order. It's slower than the TopologicalSort implementation,
 // but guarantees that two identical graphs produce the same sort each time.
@@ -731,7 +798,12 @@ func (g *Graph) DeterministicTopologicalSort() ([]Vertex, error) { // kahn's alg
 		if in > 0 {
 			for n := range g.adjacency[c] {
 				if remaining[n] > 0 {
-					return nil, ErrNotAcyclic
+					cycle := g.findCycleDFS(c)
+					if len(cycle) == 0 {
+						// Hopefully this doesn't happen!
+						return nil, fmt.Errorf("programming error")
+					}
+					return nil, &ErrNotAcyclic{Cycle: cycle}
 				}
 			}
 		}

pgraph/pgraph_test.go

@@ -464,6 +464,56 @@ func TestTopoSort2(t *testing.T) {
 	}
 }
 
+func TestTopoSort3(t *testing.T) {
+	G, _ := NewGraph("g11")
+	v1 := NV("v1")
+	v2 := NV("v2")
+	v3 := NV("v3")
+	v4 := NV("v4")
+	v5 := NV("v5")
+	v6 := NV("v6")
+	e1 := NE("e1")
+	e2 := NE("e2")
+	e3 := NE("e3")
+	e4 := NE("e4")
+	e5 := NE("e5")
+	e6 := NE("e6")
+	G.AddEdge(v1, v2, e1)
+	G.AddEdge(v2, v3, e2)
+	G.AddEdge(v3, v4, e3)
+	G.AddEdge(v4, v5, e4)
+	G.AddEdge(v5, v6, e5)
+	G.AddEdge(v4, v2, e6) // cycle
+
+	G.ExecGraphviz("/tmp/g.dot")
+
+	_, err := G.TopologicalSort()
+	if err == nil {
+		t.Errorf("topological sort passed, but graph is cyclic")
+		return
+	}
+	errNotAcyclic, ok := err.(*ErrNotAcyclic)
+	if !ok {
+		t.Errorf("wrong kind of error, got: %v", err)
+		return
+	}
+	cycle := errNotAcyclic.Cycle
+
+	t.Logf("cycle: %v", cycle)
+	if len(cycle) < 2 {
+		t.Errorf("cycle is too short")
+	}
+	cycle1 := []Vertex{v2, v3, v4, v2}
+	cycle2 := []Vertex{v3, v4, v2, v3}
+	cycle3 := []Vertex{v4, v2, v3, v4}
+	b1 := reflect.DeepEqual(cycle, cycle1)
+	b2 := reflect.DeepEqual(cycle, cycle2)
+	b3 := reflect.DeepEqual(cycle, cycle3)
+	if !b1 && !b2 && !b3 {
+		t.Errorf("cycle didn't match")
+	}
+}
+
 // empty
 func TestReachability0(t *testing.T) {
 	{