github.com/gavinw2006/hashicorp-terraform@v0.11.12-beta1/dag/tarjan.go (about)

     1  package dag
     2  
     3  // StronglyConnected returns the list of strongly connected components
     4  // within the Graph g. This information is primarily used by this package
     5  // for cycle detection, but strongly connected components have widespread
     6  // use.
     7  func StronglyConnected(g *Graph) [][]Vertex {
     8  	vs := g.Vertices()
     9  	acct := sccAcct{
    10  		NextIndex:   1,
    11  		VertexIndex: make(map[Vertex]int, len(vs)),
    12  	}
    13  	for _, v := range vs {
    14  		// Recurse on any non-visited nodes
    15  		if acct.VertexIndex[v] == 0 {
    16  			stronglyConnected(&acct, g, v)
    17  		}
    18  	}
    19  	return acct.SCC
    20  }
    21  
    22  func stronglyConnected(acct *sccAcct, g *Graph, v Vertex) int {
    23  	// Initial vertex visit
    24  	index := acct.visit(v)
    25  	minIdx := index
    26  
    27  	for _, raw := range g.DownEdges(v).List() {
    28  		target := raw.(Vertex)
    29  		targetIdx := acct.VertexIndex[target]
    30  
    31  		// Recurse on successor if not yet visited
    32  		if targetIdx == 0 {
    33  			minIdx = min(minIdx, stronglyConnected(acct, g, target))
    34  		} else if acct.inStack(target) {
    35  			// Check if the vertex is in the stack
    36  			minIdx = min(minIdx, targetIdx)
    37  		}
    38  	}
    39  
    40  	// Pop the strongly connected components off the stack if
    41  	// this is a root vertex
    42  	if index == minIdx {
    43  		var scc []Vertex
    44  		for {
    45  			v2 := acct.pop()
    46  			scc = append(scc, v2)
    47  			if v2 == v {
    48  				break
    49  			}
    50  		}
    51  
    52  		acct.SCC = append(acct.SCC, scc)
    53  	}
    54  
    55  	return minIdx
    56  }
    57  
    58  func min(a, b int) int {
    59  	if a <= b {
    60  		return a
    61  	}
    62  	return b
    63  }
    64  
    65  // sccAcct is used ot pass around accounting information for
    66  // the StronglyConnectedComponents algorithm
    67  type sccAcct struct {
    68  	NextIndex   int
    69  	VertexIndex map[Vertex]int
    70  	Stack       []Vertex
    71  	SCC         [][]Vertex
    72  }
    73  
    74  // visit assigns an index and pushes a vertex onto the stack
    75  func (s *sccAcct) visit(v Vertex) int {
    76  	idx := s.NextIndex
    77  	s.VertexIndex[v] = idx
    78  	s.NextIndex++
    79  	s.push(v)
    80  	return idx
    81  }
    82  
    83  // push adds a vertex to the stack
    84  func (s *sccAcct) push(n Vertex) {
    85  	s.Stack = append(s.Stack, n)
    86  }
    87  
    88  // pop removes a vertex from the stack
    89  func (s *sccAcct) pop() Vertex {
    90  	n := len(s.Stack)
    91  	if n == 0 {
    92  		return nil
    93  	}
    94  	vertex := s.Stack[n-1]
    95  	s.Stack = s.Stack[:n-1]
    96  	return vertex
    97  }
    98  
    99  // inStack checks if a vertex is in the stack
   100  func (s *sccAcct) inStack(needle Vertex) bool {
   101  	for _, n := range s.Stack {
   102  		if n == needle {
   103  			return true
   104  		}
   105  	}
   106  	return false
   107  }