terraform/dag/tarjan.go

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2015-01-24 01:42:06 +01:00
package dag
// StronglyConnected returns the list of strongly connected components
// within the Graph g. This information is primarily used by this package
// for cycle detection, but strongly connected components have widespread
// use.
func StronglyConnected(g *Graph) [][]Vertex {
vs := g.Vertices()
data := tarjanData{
index: make(map[interface{}]int),
stack: make([]*tarjanVertex, 0, len(vs)),
vertices: make([]*tarjanVertex, 0, len(vs)),
}
for _, v := range vs {
if _, ok := data.index[v]; !ok {
strongConnect(g, v, &data)
}
}
return data.result
}
type tarjanData struct {
index map[interface{}]int
result [][]Vertex
stack []*tarjanVertex
vertices []*tarjanVertex
}
type tarjanVertex struct {
V Vertex
Lowlink int
Index int
Stack bool
}
func strongConnect(g *Graph, v Vertex, data *tarjanData) *tarjanVertex {
index := len(data.index)
data.index[v] = index
tv := &tarjanVertex{V: v, Lowlink: index, Index: index, Stack: true}
data.stack = append(data.stack, tv)
data.vertices = append(data.vertices, tv)
for _, raw := range g.downEdges[v].List() {
target := raw.(Vertex)
if idx, ok := data.index[target]; !ok {
if tv2 := strongConnect(g, target, data); tv2.Lowlink < tv.Lowlink {
tv.Lowlink = tv2.Lowlink
}
} else if data.vertices[idx].Stack {
if idx < tv.Lowlink {
tv.Lowlink = idx
}
}
}
if tv.Lowlink == index {
vs := make([]Vertex, 0, 2)
for i := len(data.stack) - 1; ; i-- {
v := data.stack[i]
data.stack[i] = nil
data.stack = data.stack[:i]
data.vertices[data.index[v]].Stack = false
vs = append(vs, v.V)
if data.index[v] == i {
break
}
}
data.result = append(data.result, vs)
}
return tv
}