// Copyright 2017 The Prometheus Authors
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package index

import (
	"encoding/binary"
	"fmt"
	"math/rand"
	"sort"
	"testing"

	"github.com/stretchr/testify/require"

	"github.com/prometheus/prometheus/pkg/labels"
)

func TestMemPostings_addFor(t *testing.T) {
	p := NewMemPostings()
	p.m[allPostingsKey.Name] = map[string][]uint64{}
	p.m[allPostingsKey.Name][allPostingsKey.Value] = []uint64{1, 2, 3, 4, 6, 7, 8}

	p.addFor(5, allPostingsKey)

	require.Equal(t, []uint64{1, 2, 3, 4, 5, 6, 7, 8}, p.m[allPostingsKey.Name][allPostingsKey.Value])
}

func TestMemPostings_ensureOrder(t *testing.T) {
	p := NewUnorderedMemPostings()
	p.m["a"] = map[string][]uint64{}

	for i := 0; i < 100; i++ {
		l := make([]uint64, 100)
		for j := range l {
			l[j] = rand.Uint64()
		}
		v := fmt.Sprintf("%d", i)

		p.m["a"][v] = l
	}

	p.EnsureOrder()

	for _, e := range p.m {
		for _, l := range e {
			ok := sort.SliceIsSorted(l, func(i, j int) bool {
				return l[i] < l[j]
			})
			if !ok {
				t.Fatalf("postings list %v is not sorted", l)
			}
		}
	}
}

func TestIntersect(t *testing.T) {
	a := newListPostings(1, 2, 3)
	b := newListPostings(2, 3, 4)

	cases := []struct {
		in []Postings

		res Postings
	}{
		{
			in:  []Postings{},
			res: EmptyPostings(),
		},
		{
			in:  []Postings{a, b, EmptyPostings()},
			res: EmptyPostings(),
		},
		{
			in:  []Postings{b, a, EmptyPostings()},
			res: EmptyPostings(),
		},
		{
			in:  []Postings{EmptyPostings(), b, a},
			res: EmptyPostings(),
		},
		{
			in:  []Postings{EmptyPostings(), a, b},
			res: EmptyPostings(),
		},
		{
			in:  []Postings{a, EmptyPostings(), b},
			res: EmptyPostings(),
		},
		{
			in:  []Postings{b, EmptyPostings(), a},
			res: EmptyPostings(),
		},
		{
			in:  []Postings{b, EmptyPostings(), a, a, b, a, a, a},
			res: EmptyPostings(),
		},
		{
			in: []Postings{
				newListPostings(1, 2, 3, 4, 5),
				newListPostings(6, 7, 8, 9, 10),
			},
			res: newListPostings(),
		},
		{
			in: []Postings{
				newListPostings(1, 2, 3, 4, 5),
				newListPostings(4, 5, 6, 7, 8),
			},
			res: newListPostings(4, 5),
		},
		{
			in: []Postings{
				newListPostings(1, 2, 3, 4, 9, 10),
				newListPostings(1, 4, 5, 6, 7, 8, 10, 11),
			},
			res: newListPostings(1, 4, 10),
		},
		{
			in: []Postings{
				newListPostings(1),
				newListPostings(0, 1),
			},
			res: newListPostings(1),
		},
		{
			in: []Postings{
				newListPostings(1),
			},
			res: newListPostings(1),
		},
		{
			in: []Postings{
				newListPostings(1),
				newListPostings(),
			},
			res: newListPostings(),
		},
		{
			in: []Postings{
				newListPostings(),
				newListPostings(),
			},
			res: newListPostings(),
		},
	}

	for _, c := range cases {
		t.Run("", func(t *testing.T) {
			if c.res == nil {
				t.Fatal("intersect result expectancy cannot be nil")
			}

			expected, err := ExpandPostings(c.res)
			require.NoError(t, err)

			i := Intersect(c.in...)

			if c.res == EmptyPostings() {
				require.Equal(t, EmptyPostings(), i)
				return
			}

			if i == EmptyPostings() {
				t.Fatal("intersect unexpected result: EmptyPostings sentinel")
			}

			res, err := ExpandPostings(i)
			require.NoError(t, err)
			require.Equal(t, expected, res)
		})
	}
}

func TestMultiIntersect(t *testing.T) {
	cases := []struct {
		p   [][]uint64
		res []uint64
	}{
		{
			p: [][]uint64{
				{1, 2, 3, 4, 5, 6, 1000, 1001},
				{2, 4, 5, 6, 7, 8, 999, 1001},
				{1, 2, 5, 6, 7, 8, 1001, 1200},
			},
			res: []uint64{2, 5, 6, 1001},
		},
		// One of the reproducible cases for:
		// https://github.com/prometheus/prometheus/issues/2616
		// The initialisation of intersectPostings was moving the iterator forward
		// prematurely making us miss some postings.
		{
			p: [][]uint64{
				{1, 2},
				{1, 2},
				{1, 2},
				{2},
			},
			res: []uint64{2},
		},
	}

	for _, c := range cases {
		ps := make([]Postings, 0, len(c.p))
		for _, postings := range c.p {
			ps = append(ps, newListPostings(postings...))
		}

		res, err := ExpandPostings(Intersect(ps...))

		require.NoError(t, err)
		require.Equal(t, c.res, res)
	}
}

func BenchmarkIntersect(t *testing.B) {
	t.Run("LongPostings1", func(bench *testing.B) {
		var a, b, c, d []uint64

		for i := 0; i < 10000000; i += 2 {
			a = append(a, uint64(i))
		}
		for i := 5000000; i < 5000100; i += 4 {
			b = append(b, uint64(i))
		}
		for i := 5090000; i < 5090600; i += 4 {
			b = append(b, uint64(i))
		}
		for i := 4990000; i < 5100000; i++ {
			c = append(c, uint64(i))
		}
		for i := 4000000; i < 6000000; i++ {
			d = append(d, uint64(i))
		}

		i1 := newListPostings(a...)
		i2 := newListPostings(b...)
		i3 := newListPostings(c...)
		i4 := newListPostings(d...)

		bench.ResetTimer()
		bench.ReportAllocs()
		for i := 0; i < bench.N; i++ {
			if _, err := ExpandPostings(Intersect(i1, i2, i3, i4)); err != nil {
				bench.Fatal(err)
			}
		}
	})

	t.Run("LongPostings2", func(bench *testing.B) {
		var a, b, c, d []uint64

		for i := 0; i < 12500000; i++ {
			a = append(a, uint64(i))
		}
		for i := 7500000; i < 12500000; i++ {
			b = append(b, uint64(i))
		}
		for i := 9000000; i < 20000000; i++ {
			c = append(c, uint64(i))
		}
		for i := 10000000; i < 12000000; i++ {
			d = append(d, uint64(i))
		}

		i1 := newListPostings(a...)
		i2 := newListPostings(b...)
		i3 := newListPostings(c...)
		i4 := newListPostings(d...)

		bench.ResetTimer()
		bench.ReportAllocs()
		for i := 0; i < bench.N; i++ {
			if _, err := ExpandPostings(Intersect(i1, i2, i3, i4)); err != nil {
				bench.Fatal(err)
			}
		}
	})

	// Many matchers(k >> n).
	t.Run("ManyPostings", func(bench *testing.B) {
		var its []Postings

		// 100000 matchers(k=100000).
		for i := 0; i < 100000; i++ {
			var temp []uint64
			for j := 1; j < 100; j++ {
				temp = append(temp, uint64(j))
			}
			its = append(its, newListPostings(temp...))
		}

		bench.ResetTimer()
		bench.ReportAllocs()
		for i := 0; i < bench.N; i++ {
			if _, err := ExpandPostings(Intersect(its...)); err != nil {
				bench.Fatal(err)
			}
		}
	})
}

func TestMultiMerge(t *testing.T) {
	i1 := newListPostings(1, 2, 3, 4, 5, 6, 1000, 1001)
	i2 := newListPostings(2, 4, 5, 6, 7, 8, 999, 1001)
	i3 := newListPostings(1, 2, 5, 6, 7, 8, 1001, 1200)

	res, err := ExpandPostings(Merge(i1, i2, i3))
	require.NoError(t, err)
	require.Equal(t, []uint64{1, 2, 3, 4, 5, 6, 7, 8, 999, 1000, 1001, 1200}, res)
}

func TestMergedPostings(t *testing.T) {
	cases := []struct {
		in []Postings

		res Postings
	}{
		{
			in:  []Postings{},
			res: EmptyPostings(),
		},
		{
			in: []Postings{
				newListPostings(),
				newListPostings(),
			},
			res: EmptyPostings(),
		},
		{
			in: []Postings{
				newListPostings(),
			},
			res: newListPostings(),
		},
		{
			in: []Postings{
				EmptyPostings(),
				EmptyPostings(),
				EmptyPostings(),
				EmptyPostings(),
			},
			res: EmptyPostings(),
		},
		{
			in: []Postings{
				newListPostings(1, 2, 3, 4, 5),
				newListPostings(6, 7, 8, 9, 10),
			},
			res: newListPostings(1, 2, 3, 4, 5, 6, 7, 8, 9, 10),
		},
		{
			in: []Postings{
				newListPostings(1, 2, 3, 4, 5),
				newListPostings(4, 5, 6, 7, 8),
			},
			res: newListPostings(1, 2, 3, 4, 5, 6, 7, 8),
		},
		{
			in: []Postings{
				newListPostings(1, 2, 3, 4, 9, 10),
				newListPostings(1, 4, 5, 6, 7, 8, 10, 11),
			},
			res: newListPostings(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11),
		},
		{
			in: []Postings{
				newListPostings(1, 2, 3, 4, 9, 10),
				EmptyPostings(),
				newListPostings(1, 4, 5, 6, 7, 8, 10, 11),
			},
			res: newListPostings(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11),
		},
		{
			in: []Postings{
				newListPostings(1, 2),
				newListPostings(),
			},
			res: newListPostings(1, 2),
		},
		{
			in: []Postings{
				newListPostings(1, 2),
				EmptyPostings(),
			},
			res: newListPostings(1, 2),
		},
	}

	for _, c := range cases {
		t.Run("", func(t *testing.T) {
			if c.res == nil {
				t.Fatal("merge result expectancy cannot be nil")
			}

			expected, err := ExpandPostings(c.res)
			require.NoError(t, err)

			m := Merge(c.in...)

			if c.res == EmptyPostings() {
				require.Equal(t, EmptyPostings(), m)
				return
			}

			if m == EmptyPostings() {
				t.Fatal("merge unexpected result: EmptyPostings sentinel")
			}

			res, err := ExpandPostings(m)
			require.NoError(t, err)
			require.Equal(t, expected, res)
		})
	}
}

func TestMergedPostingsSeek(t *testing.T) {
	cases := []struct {
		a, b []uint64

		seek    uint64
		success bool
		res     []uint64
	}{
		{
			a: []uint64{2, 3, 4, 5},
			b: []uint64{6, 7, 8, 9, 10},

			seek:    1,
			success: true,
			res:     []uint64{2, 3, 4, 5, 6, 7, 8, 9, 10},
		},
		{
			a: []uint64{1, 2, 3, 4, 5},
			b: []uint64{6, 7, 8, 9, 10},

			seek:    2,
			success: true,
			res:     []uint64{2, 3, 4, 5, 6, 7, 8, 9, 10},
		},
		{
			a: []uint64{1, 2, 3, 4, 5},
			b: []uint64{4, 5, 6, 7, 8},

			seek:    9,
			success: false,
			res:     nil,
		},
		{
			a: []uint64{1, 2, 3, 4, 9, 10},
			b: []uint64{1, 4, 5, 6, 7, 8, 10, 11},

			seek:    10,
			success: true,
			res:     []uint64{10, 11},
		},
	}

	for _, c := range cases {
		a := newListPostings(c.a...)
		b := newListPostings(c.b...)

		p := Merge(a, b)

		require.Equal(t, c.success, p.Seek(c.seek))

		// After Seek(), At() should be called.
		if c.success {
			start := p.At()
			lst, err := ExpandPostings(p)
			require.NoError(t, err)

			lst = append([]uint64{start}, lst...)
			require.Equal(t, c.res, lst)
		}
	}
}

func TestRemovedPostings(t *testing.T) {
	cases := []struct {
		a, b []uint64
		res  []uint64
	}{
		{
			a:   nil,
			b:   nil,
			res: []uint64(nil),
		},
		{
			a:   []uint64{1, 2, 3, 4},
			b:   nil,
			res: []uint64{1, 2, 3, 4},
		},
		{
			a:   nil,
			b:   []uint64{1, 2, 3, 4},
			res: []uint64(nil),
		},
		{
			a:   []uint64{1, 2, 3, 4, 5},
			b:   []uint64{6, 7, 8, 9, 10},
			res: []uint64{1, 2, 3, 4, 5},
		},
		{
			a:   []uint64{1, 2, 3, 4, 5},
			b:   []uint64{4, 5, 6, 7, 8},
			res: []uint64{1, 2, 3},
		},
		{
			a:   []uint64{1, 2, 3, 4, 9, 10},
			b:   []uint64{1, 4, 5, 6, 7, 8, 10, 11},
			res: []uint64{2, 3, 9},
		},
		{
			a:   []uint64{1, 2, 3, 4, 9, 10},
			b:   []uint64{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11},
			res: []uint64(nil),
		},
	}

	for _, c := range cases {
		a := newListPostings(c.a...)
		b := newListPostings(c.b...)

		res, err := ExpandPostings(newRemovedPostings(a, b))
		require.NoError(t, err)
		require.Equal(t, c.res, res)
	}
}

func TestRemovedNextStackoverflow(t *testing.T) {
	var full []uint64
	var remove []uint64

	var i uint64
	for i = 0; i < 1e7; i++ {
		full = append(full, i)
		remove = append(remove, i)
	}

	flp := newListPostings(full...)
	rlp := newListPostings(remove...)
	rp := newRemovedPostings(flp, rlp)
	gotElem := false
	for rp.Next() {
		gotElem = true
	}

	require.NoError(t, rp.Err())
	require.False(t, gotElem)
}

func TestRemovedPostingsSeek(t *testing.T) {
	cases := []struct {
		a, b []uint64

		seek    uint64
		success bool
		res     []uint64
	}{
		{
			a: []uint64{2, 3, 4, 5},
			b: []uint64{6, 7, 8, 9, 10},

			seek:    1,
			success: true,
			res:     []uint64{2, 3, 4, 5},
		},
		{
			a: []uint64{1, 2, 3, 4, 5},
			b: []uint64{6, 7, 8, 9, 10},

			seek:    2,
			success: true,
			res:     []uint64{2, 3, 4, 5},
		},
		{
			a: []uint64{1, 2, 3, 4, 5},
			b: []uint64{4, 5, 6, 7, 8},

			seek:    9,
			success: false,
			res:     nil,
		},
		{
			a: []uint64{1, 2, 3, 4, 9, 10},
			b: []uint64{1, 4, 5, 6, 7, 8, 10, 11},

			seek:    10,
			success: false,
			res:     nil,
		},
		{
			a: []uint64{1, 2, 3, 4, 9, 10},
			b: []uint64{1, 4, 5, 6, 7, 8, 11},

			seek:    4,
			success: true,
			res:     []uint64{9, 10},
		},
		{
			a: []uint64{1, 2, 3, 4, 9, 10},
			b: []uint64{1, 4, 5, 6, 7, 8, 11},

			seek:    5,
			success: true,
			res:     []uint64{9, 10},
		},
		{
			a: []uint64{1, 2, 3, 4, 9, 10},
			b: []uint64{1, 4, 5, 6, 7, 8, 11},

			seek:    10,
			success: true,
			res:     []uint64{10},
		},
	}

	for _, c := range cases {
		a := newListPostings(c.a...)
		b := newListPostings(c.b...)

		p := newRemovedPostings(a, b)

		require.Equal(t, c.success, p.Seek(c.seek))

		// After Seek(), At() should be called.
		if c.success {
			start := p.At()
			lst, err := ExpandPostings(p)
			require.NoError(t, err)

			lst = append([]uint64{start}, lst...)
			require.Equal(t, c.res, lst)
		}
	}
}

func TestBigEndian(t *testing.T) {
	num := 1000
	// mock a list as postings
	ls := make([]uint32, num)
	ls[0] = 2
	for i := 1; i < num; i++ {
		ls[i] = ls[i-1] + uint32(rand.Int31n(25)) + 2
	}

	beLst := make([]byte, num*4)
	for i := 0; i < num; i++ {
		b := beLst[i*4 : i*4+4]
		binary.BigEndian.PutUint32(b, ls[i])
	}

	t.Run("Iteration", func(t *testing.T) {
		bep := newBigEndianPostings(beLst)
		for i := 0; i < num; i++ {
			require.True(t, bep.Next())
			require.Equal(t, uint64(ls[i]), bep.At())
		}

		require.False(t, bep.Next())
		require.NoError(t, bep.Err())
	})

	t.Run("Seek", func(t *testing.T) {
		table := []struct {
			seek  uint32
			val   uint32
			found bool
		}{
			{
				ls[0] - 1, ls[0], true,
			},
			{
				ls[4], ls[4], true,
			},
			{
				ls[500] - 1, ls[500], true,
			},
			{
				ls[600] + 1, ls[601], true,
			},
			{
				ls[600] + 1, ls[601], true,
			},
			{
				ls[600] + 1, ls[601], true,
			},
			{
				ls[0], ls[601], true,
			},
			{
				ls[600], ls[601], true,
			},
			{
				ls[999], ls[999], true,
			},
			{
				ls[999] + 10, ls[999], false,
			},
		}

		bep := newBigEndianPostings(beLst)

		for _, v := range table {
			require.Equal(t, v.found, bep.Seek(uint64(v.seek)))
			require.Equal(t, uint64(v.val), bep.At())
			require.NoError(t, bep.Err())
		}
	})
}

func TestIntersectWithMerge(t *testing.T) {
	// One of the reproducible cases for:
	// https://github.com/prometheus/prometheus/issues/2616
	a := newListPostings(21, 22, 23, 24, 25, 30)

	b := Merge(
		newListPostings(10, 20, 30),
		newListPostings(15, 26, 30),
	)

	p := Intersect(a, b)
	res, err := ExpandPostings(p)

	require.NoError(t, err)
	require.Equal(t, []uint64{30}, res)
}

func TestWithoutPostings(t *testing.T) {
	cases := []struct {
		base Postings
		drop Postings

		res Postings
	}{
		{
			base: EmptyPostings(),
			drop: EmptyPostings(),

			res: EmptyPostings(),
		},
		{
			base: EmptyPostings(),
			drop: newListPostings(1, 2),

			res: EmptyPostings(),
		},
		{
			base: newListPostings(1, 2),
			drop: EmptyPostings(),

			res: newListPostings(1, 2),
		},
		{
			base: newListPostings(),
			drop: newListPostings(),

			res: newListPostings(),
		},
		{
			base: newListPostings(1, 2, 3),
			drop: newListPostings(),

			res: newListPostings(1, 2, 3),
		},
		{
			base: newListPostings(1, 2, 3),
			drop: newListPostings(4, 5, 6),

			res: newListPostings(1, 2, 3),
		},
		{
			base: newListPostings(1, 2, 3),
			drop: newListPostings(3, 4, 5),

			res: newListPostings(1, 2),
		},
	}

	for _, c := range cases {
		t.Run("", func(t *testing.T) {
			if c.res == nil {
				t.Fatal("without result expectancy cannot be nil")
			}

			expected, err := ExpandPostings(c.res)
			require.NoError(t, err)

			w := Without(c.base, c.drop)

			if c.res == EmptyPostings() {
				require.Equal(t, EmptyPostings(), w)
				return
			}

			if w == EmptyPostings() {
				t.Fatal("without unexpected result: EmptyPostings sentinel")
			}

			res, err := ExpandPostings(w)
			require.NoError(t, err)
			require.Equal(t, expected, res)
		})
	}
}

func BenchmarkPostings_Stats(b *testing.B) {
	p := NewMemPostings()

	var seriesID uint64

	createPostingsLabelValues := func(name, valuePrefix string, count int) {
		for n := 1; n < count; n++ {
			value := fmt.Sprintf("%s-%d", valuePrefix, n)
			p.Add(seriesID, labels.FromStrings(name, value))
			seriesID++
		}
	}
	createPostingsLabelValues("__name__", "metrics_name_can_be_very_big_and_bad", 1e3)
	for i := 0; i < 20; i++ {
		createPostingsLabelValues(fmt.Sprintf("host-%d", i), "metrics_name_can_be_very_big_and_bad", 1e3)
		createPostingsLabelValues(fmt.Sprintf("instance-%d", i), "10.0.IP.", 1e3)
		createPostingsLabelValues(fmt.Sprintf("job-%d", i), "Small_Job_name", 1e3)
		createPostingsLabelValues(fmt.Sprintf("err-%d", i), "avg_namespace-", 1e3)
		createPostingsLabelValues(fmt.Sprintf("team-%d", i), "team-", 1e3)
		createPostingsLabelValues(fmt.Sprintf("container_name-%d", i), "pod-", 1e3)
		createPostingsLabelValues(fmt.Sprintf("cluster-%d", i), "newcluster-", 1e3)
		createPostingsLabelValues(fmt.Sprintf("uid-%d", i), "123412312312312311-", 1e3)
		createPostingsLabelValues(fmt.Sprintf("area-%d", i), "new_area_of_work-", 1e3)
		createPostingsLabelValues(fmt.Sprintf("request_id-%d", i), "owner_name_work-", 1e3)
	}
	b.ResetTimer()
	for n := 0; n < b.N; n++ {
		p.Stats("__name__")
	}
}

func TestMemPostings_Delete(t *testing.T) {
	p := NewMemPostings()
	p.Add(1, labels.FromStrings("lbl1", "a"))
	p.Add(2, labels.FromStrings("lbl1", "b"))
	p.Add(3, labels.FromStrings("lbl2", "a"))

	before := p.Get(allPostingsKey.Name, allPostingsKey.Value)
	p.Delete(map[uint64]struct{}{
		2: {},
	})
	after := p.Get(allPostingsKey.Name, allPostingsKey.Value)

	// Make sure postings gotten before the delete have the old data when
	// iterated over.
	expanded, err := ExpandPostings(before)
	require.NoError(t, err)
	require.Equal(t, []uint64{1, 2, 3}, expanded)

	// Make sure postings gotten after the delete have the new data when
	// iterated over.
	expanded, err = ExpandPostings(after)
	require.NoError(t, err)
	require.Equal(t, []uint64{1, 3}, expanded)

	deleted := p.Get("lbl1", "b")
	expanded, err = ExpandPostings(deleted)
	require.NoError(t, err)
	require.Equal(t, 0, len(expanded), "expected empty postings, got %v", expanded)
}

func TestPostingsCloner(t *testing.T) {
	for _, tc := range []struct {
		name  string
		check func(testing.TB, *PostingsCloner)
	}{
		{
			name: "seek beyond highest value of postings, then other clone seeks higher",
			check: func(t testing.TB, pc *PostingsCloner) {
				p1 := pc.Clone()
				require.False(t, p1.Seek(9))
				require.Equal(t, uint64(0), p1.At())

				p2 := pc.Clone()
				require.False(t, p2.Seek(10))
				require.Equal(t, uint64(0), p2.At())
			},
		},
		{
			name: "seek beyond highest value of postings, then other clone seeks lower",
			check: func(t testing.TB, pc *PostingsCloner) {
				p1 := pc.Clone()
				require.False(t, p1.Seek(9))
				require.Equal(t, uint64(0), p1.At())

				p2 := pc.Clone()
				require.True(t, p2.Seek(2))
				require.Equal(t, uint64(2), p2.At())
			},
		},
		{
			name: "seek to posting with value 3 or higher",
			check: func(t testing.TB, pc *PostingsCloner) {
				p := pc.Clone()
				require.True(t, p.Seek(3))
				require.Equal(t, uint64(4), p.At())
				require.True(t, p.Seek(4))
				require.Equal(t, uint64(4), p.At())
			},
		},
		{
			name: "seek alternatively on different postings",
			check: func(t testing.TB, pc *PostingsCloner) {
				p1 := pc.Clone()
				require.True(t, p1.Seek(1))
				require.Equal(t, uint64(1), p1.At())

				p2 := pc.Clone()
				require.True(t, p2.Seek(2))
				require.Equal(t, uint64(2), p2.At())

				p3 := pc.Clone()
				require.True(t, p3.Seek(4))
				require.Equal(t, uint64(4), p3.At())

				p4 := pc.Clone()
				require.True(t, p4.Seek(5))
				require.Equal(t, uint64(8), p4.At())

				require.True(t, p1.Seek(3))
				require.Equal(t, uint64(4), p1.At())
				require.True(t, p1.Seek(4))
				require.Equal(t, uint64(4), p1.At())
			},
		},
		{
			name: "iterate through the postings",
			check: func(t testing.TB, pc *PostingsCloner) {
				p1 := pc.Clone()
				p2 := pc.Clone()

				// both one step
				require.True(t, p1.Next())
				require.Equal(t, uint64(1), p1.At())
				require.True(t, p2.Next())
				require.Equal(t, uint64(1), p2.At())

				require.True(t, p1.Next())
				require.Equal(t, uint64(2), p1.At())
				require.True(t, p1.Next())
				require.Equal(t, uint64(4), p1.At())
				require.True(t, p1.Next())
				require.Equal(t, uint64(8), p1.At())
				require.False(t, p1.Next())

				require.True(t, p2.Next())
				require.Equal(t, uint64(2), p2.At())
				require.True(t, p2.Next())
				require.Equal(t, uint64(4), p2.At())
			},
		},
		{
			name: "at before call of next shouldn't panic",
			check: func(t testing.TB, pc *PostingsCloner) {
				p := pc.Clone()
				require.Equal(t, uint64(0), p.At())
			},
		},
		{
			name: "ensure a failed seek doesn't allow more next calls",
			check: func(t testing.TB, pc *PostingsCloner) {
				p := pc.Clone()
				require.False(t, p.Seek(9))
				require.Equal(t, uint64(0), p.At())
				require.False(t, p.Next())
				require.Equal(t, uint64(0), p.At())
			},
		},
	} {
		t.Run(tc.name, func(t *testing.T) {
			pc := NewPostingsCloner(newListPostings(1, 2, 4, 8))
			tc.check(t, pc)
		})
	}

	t.Run("cloning an err postings", func(t *testing.T) {
		expectedErr := fmt.Errorf("foobar")
		pc := NewPostingsCloner(ErrPostings(expectedErr))
		p := pc.Clone()
		require.False(t, p.Next())
		require.Equal(t, expectedErr, p.Err())
	})
}