Initial QSfera import

Server/vendor/github.com/mna/pigeon/builder/scc.go

@@ -0,0 +1,151 @@
+package builder
+
+import (
+	"errors"
+	"fmt"
+)
+
+// ErrInvalidParameters is parameters error.
+var ErrInvalidParameters = errors.New("invalid parameters passed to function")
+
+func min(a1 int, a2 int) int {
+	if a1 <= a2 {
+		return a1
+	}
+	return a2
+}
+
+// StronglyConnectedComponents compute strongly сonnected сomponents of a graph.
+// Tarjan's strongly connected components algorithm.
+func StronglyConnectedComponents(
+	vertices []string, edges map[string]map[string]struct{},
+) []map[string]struct{} {
+	// Tarjan's strongly connected components algorithm
+	var (
+		identified = map[string]struct{}{}
+		stack      = []string{}
+		index      = map[string]int{}
+		lowlink    = map[string]int{}
+		dfs        func(v string) []map[string]struct{}
+	)
+
+	dfs = func(vertex string) []map[string]struct{} {
+		index[vertex] = len(stack)
+		stack = append(stack, vertex)
+		lowlink[vertex] = index[vertex]
+
+		sccs := []map[string]struct{}{}
+		for w := range edges[vertex] {
+			if _, ok := index[w]; !ok {
+				sccs = append(sccs, dfs(w)...)
+				lowlink[vertex] = min(lowlink[vertex], lowlink[w])
+			} else if _, ok := identified[w]; !ok {
+				lowlink[vertex] = min(lowlink[vertex], lowlink[w])
+			}
+		}
+
+		if lowlink[vertex] == index[vertex] {
+			scc := map[string]struct{}{}
+			for _, v := range stack[index[vertex]:] {
+				scc[v] = struct{}{}
+			}
+			stack = stack[:index[vertex]]
+			for v := range scc {
+				identified[v] = struct{}{}
+			}
+			sccs = append(sccs, scc)
+		}
+		return sccs
+	}
+
+	sccs := []map[string]struct{}{}
+	for _, v := range vertices {
+		if _, ok := index[v]; !ok {
+			sccs = append(sccs, dfs(v)...)
+		}
+	}
+	return sccs
+}
+
+func contains(s []string, e string) bool {
+	for _, a := range s {
+		if a == e {
+			return true
+		}
+	}
+	return false
+}
+
+func reduceGraph(
+	graph map[string]map[string]struct{}, scc map[string]struct{},
+) map[string]map[string]struct{} {
+	reduceGraph := map[string]map[string]struct{}{}
+	for src, dsts := range graph {
+		if _, ok := scc[src]; !ok {
+			continue
+		}
+		reduceGraph[src] = map[string]struct{}{}
+		for dst := range dsts {
+			if _, ok := scc[dst]; !ok {
+				continue
+			}
+			reduceGraph[src][dst] = struct{}{}
+		}
+	}
+	return reduceGraph
+}
+
+// FindCyclesInSCC find cycles in SCC emanating from start.
+// Yields lists of the form ['A', 'B', 'C', 'A'], which means there's
+// a path from A -> B -> C -> A.  The first item is always the start
+// argument, but the last item may be another element, e.g.  ['A',
+// 'B', 'C', 'B'] means there's a path from A to B and there's a
+// cycle from B to C and back.
+func FindCyclesInSCC(
+	graph map[string]map[string]struct{}, scc map[string]struct{}, start string,
+) ([][]string, error) {
+	// Basic input checks.
+	if _, ok := scc[start]; !ok {
+		return nil, fmt.Errorf(
+			"%w: scc %v does not contain %q", ErrInvalidParameters, scc, start)
+	}
+	extravertices := []string{}
+	for k := range scc {
+		if _, ok := graph[k]; !ok {
+			extravertices = append(extravertices, k)
+		}
+	}
+	if len(extravertices) != 0 {
+		return nil, fmt.Errorf(
+			"%w: graph does not contain scc. %v",
+			ErrInvalidParameters, extravertices)
+	}
+
+	// Reduce the graph to nodes in the SCC.
+	graph = reduceGraph(graph, scc)
+	if _, ok := graph[start]; !ok {
+		return nil, fmt.Errorf(
+			"%w: graph %v does not contain %q",
+			ErrInvalidParameters, graph, start)
+	}
+
+	// Recursive helper that yields cycles.
+	var dfs func(node string, path []string) [][]string
+	dfs = func(node string, path []string) [][]string {
+		ret := [][]string{}
+		if contains(path, node) {
+			t := make([]string, 0, len(path)+1)
+			t = append(t, path...)
+			t = append(t, node)
+			ret = append(ret, t)
+			return ret
+		}
+		path = append(path, node) // TODO: Make this not quadratic.
+		for child := range graph[node] {
+			ret = append(ret, dfs(child, path)...)
+		}
+		return ret
+	}
+
+	return dfs(start, []string{}), nil
+}