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58 lines
1.5 KiB
58 lines
1.5 KiB
// Copyright 2019 The sortutil Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package sortutil // import "modernc.org/sortutil"
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// TopologicalSortNode represents a node of a graph for TopologicalSort.
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// Implementations of TopologicalSortNode must be comparable.
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type TopologicalSortNode interface {
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// Edges return the list of nodes this node points to.
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Edges() []TopologicalSortNode
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}
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// TopologicalSort returns a reversed topological ordering of a directed
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// acyclic graph or nil if graph is not a DAG.
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//
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// It implements the Depth-first search algorithm:
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//
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// https://en.wikipedia.org/wiki/Topological_sorting#Depth-first_search
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func TopologicalSort(graph []TopologicalSortNode) []TopologicalSortNode {
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l := make([]TopologicalSortNode, 0, len(graph))
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noPermanentMark := make(map[TopologicalSortNode]struct{}, len(graph))
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for _, n := range graph {
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noPermanentMark[n] = struct{}{}
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}
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temporaryMark := make(map[TopologicalSortNode]struct{}, len(graph))
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var visit func(TopologicalSortNode) bool
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visit = func(n TopologicalSortNode) bool {
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if _, ok := noPermanentMark[n]; !ok {
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return true
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}
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if _, ok := temporaryMark[n]; ok {
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return false
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}
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temporaryMark[n] = struct{}{}
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for _, m := range n.Edges() {
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visit(m)
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}
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delete(temporaryMark, n)
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delete(noPermanentMark, n)
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l = append(l, n)
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return true
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}
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for len(noPermanentMark) != 0 {
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for n := range noPermanentMark {
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if !visit(n) {
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return nil // Not a DAG
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}
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}
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}
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return l
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}
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