Source File
	dag.go

Belonging Package
	github.com/dominikbraun/graph

package graph

import (
	"errors"
	"fmt"
	"sort"
)

// TopologicalSort runs a topological sort on a given directed graph and returns
// the vertex hashes in topological order. The topological order is a non-unique
// order of vertices in a directed graph where an edge from vertex A to vertex B
// implies that vertex A appears before vertex B.
//
// Note that TopologicalSort doesn't make any guarantees about the order. If there
// are multiple valid topological orderings, an arbitrary one will be returned.
// To make the output deterministic, use [StableTopologicalSort].
//
// TopologicalSort only works for directed acyclic graphs. This implementation
// works non-recursively and utilizes Kahn's algorithm.
func TopologicalSort[K comparable, T any](g Graph[K, T]) ([]K, error) {
	if !g.Traits().IsDirected {
		return nil, fmt.Errorf("topological sort cannot be computed on undirected graph")
	}

	gOrder, err := g.Order()
	if err != nil {
		return nil, fmt.Errorf("failed to get graph order: %w", err)
	}

	predecessorMap, err := g.PredecessorMap()
	if err != nil {
		return nil, fmt.Errorf("failed to get predecessor map: %w", err)
	}

	queue := make([]K, 0)

	for vertex, predecessors := range predecessorMap {
		if len(predecessors) == 0 {
			queue = append(queue, vertex)
		}
	}

	order := make([]K, 0, gOrder)
	visited := make(map[K]struct{}, gOrder)

	for len(queue) > 0 {
		currentVertex := queue[0]
		queue = queue[1:]

		if _, ok := visited[currentVertex]; ok {
			continue
		}

		order = append(order, currentVertex)
		visited[currentVertex] = struct{}{}

		for vertex, predecessors := range predecessorMap {
			delete(predecessors, currentVertex)

			if len(predecessors) == 0 {
				queue = append(queue, vertex)
			}
		}
	}

	if len(order) != gOrder {
		return nil, errors.New("topological sort cannot be computed on graph with cycles")
	}

	return order, nil
}

// StableTopologicalSort does the same as [TopologicalSort], but takes a function
// for comparing (and then ordering) two given vertices. This allows for a stable
// and deterministic output even for graphs with multiple topological orderings.
func StableTopologicalSort[K comparable, T any](g Graph[K, T], less func(K, K) bool) ([]K, error) {
	if !g.Traits().IsDirected {
		return nil, fmt.Errorf("topological sort cannot be computed on undirected graph")
	}

	predecessorMap, err := g.PredecessorMap()
	if err != nil {
		return nil, fmt.Errorf("failed to get predecessor map: %w", err)
	}

	queue := make([]K, 0)
	queued := make(map[K]struct{})

	for vertex, predecessors := range predecessorMap {
		if len(predecessors) == 0 {
			queue = append(queue, vertex)
			queued[vertex] = struct{}{}
		}
	}

	order := make([]K, 0, len(predecessorMap))
	visited := make(map[K]struct{})

	sort.Slice(queue, func(i, j int) bool {
		return less(queue[i], queue[j])
	})

	for len(queue) > 0 {
		currentVertex := queue[0]
		queue = queue[1:]

		if _, ok := visited[currentVertex]; ok {
			continue
		}

		order = append(order, currentVertex)
		visited[currentVertex] = struct{}{}

		frontier := make([]K, 0)

		for vertex, predecessors := range predecessorMap {
			delete(predecessors, currentVertex)

			if len(predecessors) != 0 {
				continue
			}

			if _, ok := queued[vertex]; ok {
				continue
			}

			frontier = append(frontier, vertex)
			queued[vertex] = struct{}{}
		}

		sort.Slice(frontier, func(i, j int) bool {
			return less(frontier[i], frontier[j])
		})

		queue = append(queue, frontier...)
	}

	gOrder, err := g.Order()
	if err != nil {
		return nil, fmt.Errorf("failed to get graph order: %w", err)
	}

	if len(order) != gOrder {
		return nil, errors.New("topological sort cannot be computed on graph with cycles")
	}

	return order, nil
}

// TransitiveReduction returns a new graph with the same vertices and the same
// reachability as the given graph, but with as few edges as possible. The graph
// must be a directed acyclic graph.
//
// TransitiveReduction is a very expensive operation scaling with O(V(V+E)).
func TransitiveReduction[K comparable, T any](g Graph[K, T]) (Graph[K, T], error) {
	if !g.Traits().IsDirected {
		return nil, fmt.Errorf("transitive reduction cannot be performed on undirected graph")
	}

	transitiveReduction, err := g.Clone()
	if err != nil {
		return nil, fmt.Errorf("failed to clone the graph: %w", err)
	}

	adjacencyMap, err := transitiveReduction.AdjacencyMap()
	if err != nil {
		return nil, fmt.Errorf("failed to get adajcency map: %w", err)
	}

	// For each vertex in the graph, run a depth-first search from each direct
	// successor of that vertex. Then, for each vertex visited within the DFS,
	// inspect all of its edges. Remove the edges that also appear in the edge
	// set of the top-level vertex and target the current vertex. These edges
	// are redundant because their targets apparently are not only reachable
	// from the top-level vertex, but also through a DFS.
	for vertex, successors := range adjacencyMap {
		tOrder, err := transitiveReduction.Order()
		if err != nil {
			return nil, fmt.Errorf("failed to get graph order: %w", err)
		}
		for successor := range successors {
			stack := make([]K, 0, tOrder)
			visited := make(map[K]struct{}, tOrder)
			onStack := make(map[K]bool, tOrder)

			stack = append(stack, successor)

			for len(stack) > 0 {
				current := stack[len(stack)-1]
				stack = stack[:len(stack)-1]

				if _, ok := visited[current]; ok {
					onStack[current] = false
					continue
				}

				visited[current] = struct{}{}
				onStack[current] = true
				stack = append(stack, current)

				if len(adjacencyMap[current]) == 0 {
					onStack[current] = false
				}

				for adjacency := range adjacencyMap[current] {
					if _, ok := visited[adjacency]; ok {
						if onStack[adjacency] {
							// If the current adjacency is both on the stack and
							// has already been visited, there is a cycle.
							return nil, fmt.Errorf("transitive reduction cannot be performed on graph with cycle")
						}
						continue
					}

					if _, ok := adjacencyMap[vertex][adjacency]; ok {
						_ = transitiveReduction.RemoveEdge(vertex, adjacency)
					}
					stack = append(stack, adjacency)
				}
			}
		}
	}

	return transitiveReduction, nil
}

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