opentofu/internal/dag/dag.go
2023-05-02 15:33:06 +00:00

373 lines
9.4 KiB
Go

// Copyright (c) HashiCorp, Inc.
// SPDX-License-Identifier: MPL-2.0
package dag
import (
"fmt"
"sort"
"strings"
"github.com/hashicorp/terraform/internal/tfdiags"
"github.com/hashicorp/go-multierror"
)
// AcyclicGraph is a specialization of Graph that cannot have cycles.
type AcyclicGraph struct {
Graph
}
// WalkFunc is the callback used for walking the graph.
type WalkFunc func(Vertex) tfdiags.Diagnostics
// DepthWalkFunc is a walk function that also receives the current depth of the
// walk as an argument
type DepthWalkFunc func(Vertex, int) error
func (g *AcyclicGraph) DirectedGraph() Grapher {
return g
}
// Returns a Set that includes every Vertex yielded by walking down from the
// provided starting Vertex v.
func (g *AcyclicGraph) Ancestors(v Vertex) (Set, error) {
s := make(Set)
memoFunc := func(v Vertex, d int) error {
s.Add(v)
return nil
}
if err := g.DepthFirstWalk(g.downEdgesNoCopy(v), memoFunc); err != nil {
return nil, err
}
return s, nil
}
// Returns a Set that includes every Vertex yielded by walking up from the
// provided starting Vertex v.
func (g *AcyclicGraph) Descendents(v Vertex) (Set, error) {
s := make(Set)
memoFunc := func(v Vertex, d int) error {
s.Add(v)
return nil
}
if err := g.ReverseDepthFirstWalk(g.upEdgesNoCopy(v), memoFunc); err != nil {
return nil, err
}
return s, nil
}
// Root returns the root of the DAG, or an error.
//
// Complexity: O(V)
func (g *AcyclicGraph) Root() (Vertex, error) {
roots := make([]Vertex, 0, 1)
for _, v := range g.Vertices() {
if g.upEdgesNoCopy(v).Len() == 0 {
roots = append(roots, v)
}
}
if len(roots) > 1 {
// TODO(mitchellh): make this error message a lot better
return nil, fmt.Errorf("multiple roots: %#v", roots)
}
if len(roots) == 0 {
return nil, fmt.Errorf("no roots found")
}
return roots[0], nil
}
// TransitiveReduction performs the transitive reduction of graph g in place.
// The transitive reduction of a graph is a graph with as few edges as
// possible with the same reachability as the original graph. This means
// that if there are three nodes A => B => C, and A connects to both
// B and C, and B connects to C, then the transitive reduction is the
// same graph with only a single edge between A and B, and a single edge
// between B and C.
//
// The graph must be free of cycles for this operation to behave properly.
//
// Complexity: O(V(V+E)), or asymptotically O(VE)
func (g *AcyclicGraph) TransitiveReduction() {
// For each vertex u in graph g, do a DFS starting from each vertex
// v such that the edge (u,v) exists (v is a direct descendant of u).
//
// For each v-prime reachable from v, remove the edge (u, v-prime).
for _, u := range g.Vertices() {
uTargets := g.downEdgesNoCopy(u)
g.DepthFirstWalk(g.downEdgesNoCopy(u), func(v Vertex, d int) error {
shared := uTargets.Intersection(g.downEdgesNoCopy(v))
for _, vPrime := range shared {
g.RemoveEdge(BasicEdge(u, vPrime))
}
return nil
})
}
}
// Validate validates the DAG. A DAG is valid if it has a single root
// with no cycles.
func (g *AcyclicGraph) Validate() error {
if _, err := g.Root(); err != nil {
return err
}
// Look for cycles of more than 1 component
var err error
cycles := g.Cycles()
if len(cycles) > 0 {
for _, cycle := range cycles {
cycleStr := make([]string, len(cycle))
for j, vertex := range cycle {
cycleStr[j] = VertexName(vertex)
}
err = multierror.Append(err, fmt.Errorf(
"Cycle: %s", strings.Join(cycleStr, ", ")))
}
}
// Look for cycles to self
for _, e := range g.Edges() {
if e.Source() == e.Target() {
err = multierror.Append(err, fmt.Errorf(
"Self reference: %s", VertexName(e.Source())))
}
}
return err
}
// Cycles reports any cycles between graph nodes.
// Self-referencing nodes are not reported, and must be detected separately.
func (g *AcyclicGraph) Cycles() [][]Vertex {
var cycles [][]Vertex
for _, cycle := range StronglyConnected(&g.Graph) {
if len(cycle) > 1 {
cycles = append(cycles, cycle)
}
}
return cycles
}
// Walk walks the graph, calling your callback as each node is visited.
// This will walk nodes in parallel if it can. The resulting diagnostics
// contains problems from all graphs visited, in no particular order.
func (g *AcyclicGraph) Walk(cb WalkFunc) tfdiags.Diagnostics {
w := &Walker{Callback: cb, Reverse: true}
w.Update(g)
return w.Wait()
}
// simple convenience helper for converting a dag.Set to a []Vertex
func AsVertexList(s Set) []Vertex {
vertexList := make([]Vertex, 0, len(s))
for _, raw := range s {
vertexList = append(vertexList, raw.(Vertex))
}
return vertexList
}
type vertexAtDepth struct {
Vertex Vertex
Depth int
}
// TopologicalOrder returns a topological sort of the given graph, with source
// vertices ordered before the targets of their edges. The nodes are not sorted,
// and any valid order may be returned. This function will panic if it
// encounters a cycle.
func (g *AcyclicGraph) TopologicalOrder() []Vertex {
return g.topoOrder(upOrder)
}
// ReverseTopologicalOrder returns a topological sort of the given graph, with
// target vertices ordered before the sources of their edges. The nodes are not
// sorted, and any valid order may be returned. This function will panic if it
// encounters a cycle.
func (g *AcyclicGraph) ReverseTopologicalOrder() []Vertex {
return g.topoOrder(downOrder)
}
func (g *AcyclicGraph) topoOrder(order walkType) []Vertex {
// Use a dfs-based sorting algorithm, similar to that used in
// TransitiveReduction.
sorted := make([]Vertex, 0, len(g.vertices))
// tmp track the current working node to check for cycles
tmp := map[Vertex]bool{}
// perm tracks completed nodes to end the recursion
perm := map[Vertex]bool{}
var visit func(v Vertex)
visit = func(v Vertex) {
if perm[v] {
return
}
if tmp[v] {
panic("cycle found in dag")
}
tmp[v] = true
var next Set
switch {
case order&downOrder != 0:
next = g.downEdgesNoCopy(v)
case order&upOrder != 0:
next = g.upEdgesNoCopy(v)
default:
panic(fmt.Sprintln("invalid order", order))
}
for _, u := range next {
visit(u)
}
tmp[v] = false
perm[v] = true
sorted = append(sorted, v)
}
for _, v := range g.Vertices() {
visit(v)
}
return sorted
}
type walkType uint64
const (
depthFirst walkType = 1 << iota
breadthFirst
downOrder
upOrder
)
// DepthFirstWalk does a depth-first walk of the graph starting from
// the vertices in start.
func (g *AcyclicGraph) DepthFirstWalk(start Set, f DepthWalkFunc) error {
return g.walk(depthFirst|downOrder, false, start, f)
}
// ReverseDepthFirstWalk does a depth-first walk _up_ the graph starting from
// the vertices in start.
func (g *AcyclicGraph) ReverseDepthFirstWalk(start Set, f DepthWalkFunc) error {
return g.walk(depthFirst|upOrder, false, start, f)
}
// BreadthFirstWalk does a breadth-first walk of the graph starting from
// the vertices in start.
func (g *AcyclicGraph) BreadthFirstWalk(start Set, f DepthWalkFunc) error {
return g.walk(breadthFirst|downOrder, false, start, f)
}
// ReverseBreadthFirstWalk does a breadth-first walk _up_ the graph starting from
// the vertices in start.
func (g *AcyclicGraph) ReverseBreadthFirstWalk(start Set, f DepthWalkFunc) error {
return g.walk(breadthFirst|upOrder, false, start, f)
}
// Setting test to true will walk sets of vertices in sorted order for
// deterministic testing.
func (g *AcyclicGraph) walk(order walkType, test bool, start Set, f DepthWalkFunc) error {
seen := make(map[Vertex]struct{})
frontier := make([]vertexAtDepth, 0, len(start))
for _, v := range start {
frontier = append(frontier, vertexAtDepth{
Vertex: v,
Depth: 0,
})
}
if test {
testSortFrontier(frontier)
}
for len(frontier) > 0 {
// Pop the current vertex
var current vertexAtDepth
switch {
case order&depthFirst != 0:
// depth first, the frontier is used like a stack
n := len(frontier)
current = frontier[n-1]
frontier = frontier[:n-1]
case order&breadthFirst != 0:
// breadth first, the frontier is used like a queue
current = frontier[0]
frontier = frontier[1:]
default:
panic(fmt.Sprint("invalid visit order", order))
}
// Check if we've seen this already and return...
if _, ok := seen[current.Vertex]; ok {
continue
}
seen[current.Vertex] = struct{}{}
// Visit the current node
if err := f(current.Vertex, current.Depth); err != nil {
return err
}
var edges Set
switch {
case order&downOrder != 0:
edges = g.downEdgesNoCopy(current.Vertex)
case order&upOrder != 0:
edges = g.upEdgesNoCopy(current.Vertex)
default:
panic(fmt.Sprint("invalid walk order", order))
}
if test {
frontier = testAppendNextSorted(frontier, edges, current.Depth+1)
} else {
frontier = appendNext(frontier, edges, current.Depth+1)
}
}
return nil
}
func appendNext(frontier []vertexAtDepth, next Set, depth int) []vertexAtDepth {
for _, v := range next {
frontier = append(frontier, vertexAtDepth{
Vertex: v,
Depth: depth,
})
}
return frontier
}
func testAppendNextSorted(frontier []vertexAtDepth, edges Set, depth int) []vertexAtDepth {
var newEdges []vertexAtDepth
for _, v := range edges {
newEdges = append(newEdges, vertexAtDepth{
Vertex: v,
Depth: depth,
})
}
testSortFrontier(newEdges)
return append(frontier, newEdges...)
}
func testSortFrontier(f []vertexAtDepth) {
sort.Slice(f, func(i, j int) bool {
return VertexName(f[i].Vertex) < VertexName(f[j].Vertex)
})
}