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func ShellSort(items []int) {
	var (
		n    = len(items)
		gaps = []int{1}
		k    = 1
	)

	for {
		gap := element(2, k) + 1
		if gap > n-1 {
			break
		}
		gaps = append([]int{gap}, gaps...)
		k++
	}

	for _, gap := range gaps {
		for i := gap; i < n; i += gap {
			j := i
			for j > 0 {
				if items[j-gap] > items[j] {
					items[j-gap], items[j] = items[j], items[j-gap]
				}
				j = j - gap
			}
		}
	}
}

func element(a, b int) int {
	e := 1
	for b > 0 {
		if b&1 != 0 {
			e *= a
		}
		b >>= 1
		a *= a
	}
	return e
}