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package build
// Subgraph returns a subgraph of g that consists of the vertices in s
// and all edges whose endpoints are in s.
func (g *Virtual) Subgraph(s VertexSet) *Virtual {
n := g.Order()
s = s.And(Range(0, n))
m := s.size()
switch {
case m == 0:
return null
case m == 1:
return singleton()
case m == n:
return g
}
cost := func(v, w int) int64 {
return g.cost(s.get(v), s.get(w))
}
res := generic(m, cost, func(v, w int) bool {
return g.edge(s.get(v), s.get(w))
})
res.visit = func(v int, a int, do func(w int, c int64) bool) (aborted bool) {
if a >= m {
return
}
v0 := s.get(v)
s0 := s.And(Range(s.get(a), n))
for _, in := range s0.set {
if more := false; g.visit(v0, in.a, func(w0 int, c int64) (skip bool) {
switch {
case w0 >= in.b:
more, skip = true, true
return
case do(s.rank(w0), c):
return true
default:
return
}
}) && !more {
return true
}
}
return
}
return res
}
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