package q77

var (
	allocBuf []int
	allocP   int
)

func alloc(sz int) []int {
	if allocBuf == nil || allocP+sz > 8192 {
		allocP = 0
		allocBuf = make([]int, 8192)
	}
	ret := allocBuf[allocP : allocP+sz]
	allocP += sz
	return ret
}

func combine(n int, k int) [][]int {
	buf := alloc(k)
	for i := range buf {
		buf[i] = i + 1
	}

	sz := 1
	for i := 1; i <= k; i++ {
		sz = sz * (n - i + 1) / i
	}
	ret := make([][]int, 0, sz)
	for {
		t := alloc(k)
		copy(t, buf)
		ret = append(ret, t)

		if buf[0] == n-k+1 {
			break
		}

		buf[k-1]++
		i := k - 1
		for ; i > 0 && buf[i] > n-(k-1-i); i-- {
			buf[i-1]++
		}
		for i++; i < k; i++ {
			buf[i] = buf[i-1] + 1
		}
	}

	return ret
}

var _ = combine