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package build
// Cycle returns a virtual cycle graph with n vertices and
// the edges {0, 1}, {1, 2}, {2, 3},... , {n-1, 0}.
func Cycle(n int) *Virtual {
switch {
case n < 0:
return nil
case n == 0:
return null
case n == 1:
return singleton()
case n == 2:
return edge()
}
g := generic0(n, func(v, w int) (edge bool) {
switch v - w {
case 1 - n, -1, 1, n - 1:
edge = true
}
return
})
// Precondition : n ≥ 3.
g.degree = func(v int) int { return 2 }
// Precondition : n ≥ 3.
g.visit = func(v int, a int, do func(w int, c int64) bool) (aborted bool) {
var w [2]int
switch v {
case 0:
w = [2]int{1, n - 1}
case n - 1:
w = [2]int{0, n - 2}
default:
w = [2]int{v - 1, v + 1}
}
for _, w := range w {
if w >= a && do(w, 0) {
return true
}
}
return
}
return g
}
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