// Copyright 2021 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 chunkenc

import (
	"testing"

	"github.com/stretchr/testify/require"

	"github.com/prometheus/prometheus/model/histogram"
	"github.com/prometheus/prometheus/tsdb/tsdbutil"
)

type result struct {
	t  int64
	h  *histogram.Histogram
	fh *histogram.FloatHistogram
}

func TestFirstHistogramExplicitCounterReset(t *testing.T) {
	tests := map[string]struct {
		hint      histogram.CounterResetHint
		expHeader CounterResetHeader
	}{
		"CounterReset": {
			hint:      histogram.CounterReset,
			expHeader: CounterReset,
		},
		"NotCounterReset": {
			hint:      histogram.NotCounterReset,
			expHeader: UnknownCounterReset,
		},
		"UnknownCounterReset": {
			hint:      histogram.UnknownCounterReset,
			expHeader: UnknownCounterReset,
		},
		"Gauge": {
			hint:      histogram.GaugeType,
			expHeader: GaugeType,
		},
	}
	for name, test := range tests {
		t.Run(name, func(t *testing.T) {
			h := &histogram.Histogram{
				CounterResetHint: test.hint,
			}
			chk := NewHistogramChunk()
			app, err := chk.Appender()
			require.NoError(t, err)
			newChk, recoded, newApp, err := app.AppendHistogram(nil, 0, h, false)
			require.NoError(t, err)
			require.Nil(t, newChk)
			require.False(t, recoded)
			require.Equal(t, app, newApp)
			require.Equal(t, test.expHeader, chk.GetCounterResetHeader())
		})
	}
}

func TestHistogramChunkSameBuckets(t *testing.T) {
	c := NewHistogramChunk()
	var exp []result

	// Create fresh appender and add the first histogram.
	app, err := c.Appender()
	require.NoError(t, err)
	require.Equal(t, 0, c.NumSamples())

	ts := int64(1234567890)
	h := &histogram.Histogram{
		Count:         15,
		ZeroCount:     2,
		Sum:           18.4,
		ZeroThreshold: 1e-100,
		Schema:        1,
		PositiveSpans: []histogram.Span{
			{Offset: 0, Length: 2},
			{Offset: 1, Length: 2},
		},
		PositiveBuckets: []int64{1, 1, -1, 0}, // counts: 1, 2, 1, 1 (total 5)
		NegativeSpans: []histogram.Span{
			{Offset: 1, Length: 1},
			{Offset: 2, Length: 3},
		},
		NegativeBuckets: []int64{2, 1, -1, -1}, // counts: 2, 3, 2, 1 (total 8)
	}
	chk, _, app, err := app.AppendHistogram(nil, ts, h, false)
	require.NoError(t, err)
	require.Nil(t, chk)
	exp = append(exp, result{t: ts, h: h, fh: h.ToFloat(nil)})
	require.Equal(t, 1, c.NumSamples())

	// Add an updated histogram.
	ts += 16
	h = h.Copy()
	h.Count = 32
	h.ZeroCount++
	h.Sum = 24.4
	h.PositiveBuckets = []int64{5, -2, 1, -2} // counts: 5, 3, 4, 2 (total 14)
	h.NegativeBuckets = []int64{4, -1, 1, -1} // counts: 4, 3, 4, 4 (total 15)
	chk, _, _, err = app.AppendHistogram(nil, ts, h, false)
	require.NoError(t, err)
	require.Nil(t, chk)
	hExp := h.Copy()
	hExp.CounterResetHint = histogram.NotCounterReset
	exp = append(exp, result{t: ts, h: hExp, fh: hExp.ToFloat(nil)})
	require.Equal(t, 2, c.NumSamples())

	// Add update with new appender.
	app, err = c.Appender()
	require.NoError(t, err)

	ts += 14
	h = h.Copy()
	h.Count = 54
	h.ZeroCount += 2
	h.Sum = 24.4
	h.PositiveBuckets = []int64{6, 1, -3, 6} // counts: 6, 7, 4, 10 (total 27)
	h.NegativeBuckets = []int64{5, 1, -2, 3} // counts: 5, 6, 4, 7 (total 22)
	chk, _, _, err = app.AppendHistogram(nil, ts, h, false)
	require.NoError(t, err)
	require.Nil(t, chk)
	hExp = h.Copy()
	hExp.CounterResetHint = histogram.NotCounterReset
	exp = append(exp, result{t: ts, h: hExp, fh: hExp.ToFloat(nil)})
	require.Equal(t, 3, c.NumSamples())

	// 1. Expand iterator in simple case.
	it := c.Iterator(nil)
	require.NoError(t, it.Err())
	var act []result
	for it.Next() == ValHistogram {
		ts, h := it.AtHistogram(nil)
		fts, fh := it.AtFloatHistogram(nil)
		require.Equal(t, ts, fts)
		act = append(act, result{t: ts, h: h, fh: fh})
	}
	require.NoError(t, it.Err())
	require.Equal(t, exp, act)

	// 2. Expand second iterator while reusing first one.
	it2 := c.Iterator(it)
	var act2 []result
	for it2.Next() == ValHistogram {
		ts, h := it2.AtHistogram(nil)
		fts, fh := it2.AtFloatHistogram(nil)
		require.Equal(t, ts, fts)
		act2 = append(act2, result{t: ts, h: h, fh: fh})
	}
	require.NoError(t, it2.Err())
	require.Equal(t, exp, act2)

	// 3. Now recycle an iterator that was never used to access anything.
	itX := c.Iterator(nil)
	for itX.Next() == ValHistogram {
		// Just iterate through without accessing anything.
	}
	it3 := c.iterator(itX)
	var act3 []result
	for it3.Next() == ValHistogram {
		ts, h := it3.AtHistogram(nil)
		fts, fh := it3.AtFloatHistogram(nil)
		require.Equal(t, ts, fts)
		act3 = append(act3, result{t: ts, h: h, fh: fh})
	}
	require.NoError(t, it3.Err())
	require.Equal(t, exp, act3)

	// 4. Test iterator Seek.
	mid := len(exp) / 2
	it4 := c.Iterator(nil)
	var act4 []result
	require.Equal(t, ValHistogram, it4.Seek(exp[mid].t))
	// Below ones should not matter.
	require.Equal(t, ValHistogram, it4.Seek(exp[mid].t))
	require.Equal(t, ValHistogram, it4.Seek(exp[mid].t))
	ts, h = it4.AtHistogram(nil)
	fts, fh := it4.AtFloatHistogram(nil)
	require.Equal(t, ts, fts)
	act4 = append(act4, result{t: ts, h: h, fh: fh})
	for it4.Next() == ValHistogram {
		ts, h := it4.AtHistogram(nil)
		fts, fh := it4.AtFloatHistogram(nil)
		require.Equal(t, ts, fts)
		act4 = append(act4, result{t: ts, h: h, fh: fh})
	}
	require.NoError(t, it4.Err())
	require.Equal(t, exp[mid:], act4)
	require.Equal(t, ValNone, it4.Seek(exp[len(exp)-1].t+1))
}

// Mimics the scenario described for expandSpansForward.
func TestHistogramChunkBucketChanges(t *testing.T) {
	c := Chunk(NewHistogramChunk())

	// Create fresh appender and add the first histogram.
	app, err := c.Appender()
	require.NoError(t, err)
	require.Equal(t, 0, c.NumSamples())

	ts1 := int64(1234567890)
	h1 := &histogram.Histogram{
		Count:         27,
		ZeroCount:     2,
		Sum:           18.4,
		ZeroThreshold: 1e-125,
		Schema:        1,
		PositiveSpans: []histogram.Span{
			{Offset: 0, Length: 2},
			{Offset: 2, Length: 1},
			{Offset: 3, Length: 2},
			{Offset: 3, Length: 1},
			{Offset: 1, Length: 1},
		},
		PositiveBuckets: []int64{6, -3, 0, -1, 2, 1, -4}, // counts: 6, 3, 3, 2, 4, 5, 1 (total 24)
		NegativeSpans:   []histogram.Span{{Offset: 1, Length: 1}},
		NegativeBuckets: []int64{1},
	}

	chk, _, app, err := app.AppendHistogram(nil, ts1, h1, false)
	require.NoError(t, err)
	require.Nil(t, chk)
	require.Equal(t, 1, c.NumSamples())

	// Add a new histogram that has expanded buckets.
	ts2 := ts1 + 16
	h2 := h1.Copy()
	h2.PositiveSpans = []histogram.Span{
		{Offset: 0, Length: 3},
		{Offset: 1, Length: 1},
		{Offset: 1, Length: 4},
		{Offset: 3, Length: 3},
	}
	h2.NegativeSpans = []histogram.Span{{Offset: 0, Length: 2}}
	h2.Count = 35
	h2.ZeroCount++
	h2.Sum = 30
	// Existing histogram should get values converted from the above to:
	//   6 3 0 3 0 0 2 4 5 0 1 (previous values with some new empty buckets in between)
	// so the new histogram should have new counts >= these per-bucket counts, e.g.:
	h2.PositiveBuckets = []int64{7, -2, -4, 2, -2, -1, 2, 3, 0, -5, 1} // 7 5 1 3 1 0 2 5 5 0 1 (total 30)
	// Existing histogram should get values converted from the above to:
	//   0 1 (previous values with some new empty buckets in between)
	// so the new histogram should have new counts >= these per-bucket counts, e.g.:
	h2.NegativeBuckets = []int64{2, -1} // 2 1 (total 3)
	// This is how span changes will be handled.
	hApp, _ := app.(*HistogramAppender)
	posInterjections, negInterjections, backwardPositiveInserts, backwardNegativeInserts, ok, cr := hApp.appendable(h2)
	require.NotEmpty(t, posInterjections)
	require.NotEmpty(t, negInterjections)
	require.Empty(t, backwardPositiveInserts)
	require.Empty(t, backwardNegativeInserts)
	require.True(t, ok) // Only new buckets came in.
	require.False(t, cr)
	c, app = hApp.recode(posInterjections, negInterjections, h2.PositiveSpans, h2.NegativeSpans)
	chk, _, _, err = app.AppendHistogram(nil, ts2, h2, false)
	require.NoError(t, err)
	require.Nil(t, chk)

	require.Equal(t, 2, c.NumSamples())

	// Because the 2nd histogram has expanded buckets, we should expect all
	// histograms (in particular the first) to come back using the new spans
	// metadata as well as the expanded buckets.
	h1.PositiveSpans = h2.PositiveSpans
	h1.PositiveBuckets = []int64{6, -3, -3, 3, -3, 0, 2, 2, 1, -5, 1}
	h1.NegativeSpans = h2.NegativeSpans
	h1.NegativeBuckets = []int64{0, 1}
	hExp := h2.Copy()
	hExp.CounterResetHint = histogram.NotCounterReset
	exp := []result{
		{t: ts1, h: h1, fh: h1.ToFloat(nil)},
		{t: ts2, h: hExp, fh: hExp.ToFloat(nil)},
	}
	it := c.Iterator(nil)
	var act []result
	for it.Next() == ValHistogram {
		ts, h := it.AtHistogram(nil)
		fts, fh := it.AtFloatHistogram(nil)
		require.Equal(t, ts, fts)
		act = append(act, result{t: ts, h: h, fh: fh})
	}
	require.NoError(t, it.Err())
	require.Equal(t, exp, act)
}

func TestHistogramChunkAppendable(t *testing.T) {
	eh := &histogram.Histogram{
		Count:         5,
		ZeroCount:     2,
		Sum:           18.4,
		ZeroThreshold: 1e-125,
		Schema:        1,
		PositiveSpans: []histogram.Span{
			{Offset: 0, Length: 2},
			{Offset: 2, Length: 1},
			{Offset: 3, Length: 2},
			{Offset: 3, Length: 1},
			{Offset: 1, Length: 1},
		},
		PositiveBuckets: []int64{6, -3, 0, -1, 2, 1, -4}, // counts: 6, 3, 3, 2, 4, 5, 1 (total 24)
	}

	cbh := &histogram.Histogram{
		Count:  24,
		Sum:    18.4,
		Schema: histogram.CustomBucketsSchema,
		PositiveSpans: []histogram.Span{
			{Offset: 0, Length: 2},
			{Offset: 2, Length: 1},
			{Offset: 3, Length: 2},
			{Offset: 3, Length: 1},
			{Offset: 1, Length: 1},
		},
		PositiveBuckets: []int64{6, -3, 0, -1, 2, 1, -4}, // counts: 6, 3, 3, 2, 4, 5, 1 (total 24)
		CustomValues:    []float64{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20},
	}

	setup := func(h *histogram.Histogram) (Chunk, *HistogramAppender, int64, *histogram.Histogram) {
		c := Chunk(NewHistogramChunk())

		// Create fresh appender and add the first histogram.
		app, err := c.Appender()
		require.NoError(t, err)
		require.Equal(t, 0, c.NumSamples())

		ts := int64(1234567890)

		chk, _, app, err := app.AppendHistogram(nil, ts, h.Copy(), false)
		require.NoError(t, err)
		require.Nil(t, chk)
		require.Equal(t, 1, c.NumSamples())
		require.Equal(t, UnknownCounterReset, c.(*HistogramChunk).GetCounterResetHeader())
		return c, app.(*HistogramAppender), ts, h
	}

	{ // Schema change.
		c, hApp, ts, h1 := setup(eh)
		h2 := h1.Copy()
		h2.Schema++
		_, _, _, _, ok, _ := hApp.appendable(h2)
		require.False(t, ok)

		assertNewHistogramChunkOnAppend(t, c, hApp, ts+1, h2, UnknownCounterReset)
	}

	{ // Zero threshold change.
		c, hApp, ts, h1 := setup(eh)
		h2 := h1.Copy()
		h2.ZeroThreshold += 0.1
		_, _, _, _, ok, _ := hApp.appendable(h2)
		require.False(t, ok)

		assertNewHistogramChunkOnAppend(t, c, hApp, ts+1, h2, UnknownCounterReset)
	}

	{ // New histogram that has more buckets.
		c, hApp, ts, h1 := setup(eh)
		h2 := h1.Copy()
		h2.PositiveSpans = []histogram.Span{
			{Offset: 0, Length: 3},
			{Offset: 1, Length: 1},
			{Offset: 1, Length: 4},
			{Offset: 3, Length: 3},
		}
		h2.Count += 9
		h2.ZeroCount++
		h2.Sum = 30
		// Existing histogram should get values converted from the above to:
		//   6 3 0 3 0 0 2 4 5 0 1 (previous values with some new empty buckets in between)
		// so the new histogram should have new counts >= these per-bucket counts, e.g.:
		h2.PositiveBuckets = []int64{7, -2, -4, 2, -2, -1, 2, 3, 0, -5, 1} // 7 5 1 3 1 0 2 5 5 0 1 (total 30)

		posInterjections, negInterjections, backwardPositiveInserts, backwardNegativeInserts, ok, cr := hApp.appendable(h2)
		require.NotEmpty(t, posInterjections)
		require.Empty(t, negInterjections)
		require.Empty(t, backwardPositiveInserts)
		require.Empty(t, backwardNegativeInserts)
		require.True(t, ok) // Only new buckets came in.
		require.False(t, cr)

		assertRecodedHistogramChunkOnAppend(t, c, hApp, ts+1, h2, UnknownCounterReset)
	}

	{ // New histogram that has a bucket missing.
		c, hApp, ts, h1 := setup(eh)
		h2 := h1.Copy()
		h2.PositiveSpans = []histogram.Span{
			{Offset: 0, Length: 2},
			{Offset: 5, Length: 2},
			{Offset: 3, Length: 1},
			{Offset: 1, Length: 1},
		}
		h2.Sum = 21
		h2.PositiveBuckets = []int64{6, -3, -1, 2, 1, -4} // counts: 6, 3, 2, 4, 5, 1 (total 21)

		posInterjections, negInterjections, backwardPositiveInserts, backwardNegativeInserts, ok, cr := hApp.appendable(h2)
		require.Empty(t, posInterjections)
		require.Empty(t, negInterjections)
		require.Empty(t, backwardPositiveInserts)
		require.Empty(t, backwardNegativeInserts)
		require.False(t, ok) // Need to cut a new chunk.
		require.True(t, cr)

		assertNewHistogramChunkOnAppend(t, c, hApp, ts+1, h2, CounterReset)
	}

	{ // New histogram that has buckets missing but the buckets missing were empty.
		emptyBucketH := eh.Copy()
		emptyBucketH.PositiveBuckets = []int64{6, -6, 1, 1, -2, 1, 1} // counts: 6, 0, 1, 2, 0, 1, 2 (total 12)
		c, hApp, ts, h1 := setup(emptyBucketH)
		h2 := h1.Copy()
		h2.PositiveSpans = []histogram.Span{ // Missing buckets at offset 1 and 9.
			{Offset: 0, Length: 1},
			{Offset: 3, Length: 1},
			{Offset: 3, Length: 1},
			{Offset: 4, Length: 1},
			{Offset: 1, Length: 1},
		}
		savedH2Spans := h2.PositiveSpans
		h2.PositiveBuckets = []int64{7, -5, 1, 0, 1} // counts: 7, 2, 3, 3, 4 (total 18)

		posInterjections, negInterjections, backwardPositiveInserts, backwardNegativeInserts, ok, cr := hApp.appendable(h2)
		require.Empty(t, posInterjections)
		require.Empty(t, negInterjections)
		require.NotEmpty(t, backwardPositiveInserts)
		require.Empty(t, backwardNegativeInserts)
		require.True(t, ok)
		require.False(t, cr)

		assertNoNewHistogramChunkOnAppend(t, c, hApp, ts+1, h2, UnknownCounterReset)

		// Check that h2 was recoded.
		require.Equal(t, []int64{7, -7, 2, 1, -3, 3, 1}, h2.PositiveBuckets) // counts: 7, 0, 2, 3 , 0, 3, 4 (total 18)
		require.Equal(t, emptyBucketH.PositiveSpans, h2.PositiveSpans)
		require.NotEqual(t, savedH2Spans, h2.PositiveSpans, "recoding must make a copy")
	}

	{ // New histogram that has new buckets AND buckets missing but the buckets missing were empty.
		emptyBucketH := eh.Copy()
		emptyBucketH.PositiveBuckets = []int64{6, -6, 1, 1, -2, 1, 1} // counts: 6, 0, 1, 2, 0, 1, 2 (total 12)
		c, hApp, ts, h1 := setup(emptyBucketH)
		h2 := h1.Copy()
		h2.PositiveSpans = []histogram.Span{ // Missing buckets at offset 1 and 9.
			{Offset: 0, Length: 1},
			{Offset: 3, Length: 1},
			{Offset: 3, Length: 1},
			{Offset: 4, Length: 1},
			{Offset: 1, Length: 2},
		}
		savedH2Spans := h2.PositiveSpans
		h2.PositiveBuckets = []int64{7, -5, 1, 0, 1, 1} // counts: 7, 2, 3, 3, 4, 5 (total 23)

		posInterjections, negInterjections, backwardPositiveInserts, backwardNegativeInserts, ok, cr := hApp.appendable(h2)
		require.NotEmpty(t, posInterjections)
		require.Empty(t, negInterjections)
		require.NotEmpty(t, backwardPositiveInserts)
		require.Empty(t, backwardNegativeInserts)
		require.True(t, ok)
		require.False(t, cr)

		assertRecodedHistogramChunkOnAppend(t, c, hApp, ts+1, h2, UnknownCounterReset)

		// Check that h2 was recoded.
		require.Equal(t, []int64{7, -7, 2, 1, -3, 3, 1, 1}, h2.PositiveBuckets) // counts: 7, 0, 2, 3 , 0, 3, 5 (total 23)
		require.Equal(t, []histogram.Span{
			{Offset: 0, Length: 2}, // Added empty bucket.
			{Offset: 2, Length: 1}, // Existing - offset adjusted.
			{Offset: 3, Length: 2}, // Added empty bucket.
			{Offset: 3, Length: 1}, // Existing - offset adjusted.
			{Offset: 1, Length: 2}, // Existing.
		}, h2.PositiveSpans)
		require.NotEqual(t, savedH2Spans, h2.PositiveSpans, "recoding must make a copy")
	}

	{ // New histogram that has a counter reset while buckets are same.
		c, hApp, ts, h1 := setup(eh)
		h2 := h1.Copy()
		h2.Sum = 23
		h2.PositiveBuckets = []int64{6, -4, 1, -1, 2, 1, -4} // counts: 6, 2, 3, 2, 4, 5, 1 (total 23)

		posInterjections, negInterjections, backwardPositiveInserts, backwardNegativeInserts, ok, cr := hApp.appendable(h2)
		require.Empty(t, posInterjections)
		require.Empty(t, negInterjections)
		require.Empty(t, backwardPositiveInserts)
		require.Empty(t, backwardNegativeInserts)
		require.False(t, ok) // Need to cut a new chunk.
		require.True(t, cr)

		assertNewHistogramChunkOnAppend(t, c, hApp, ts+1, h2, CounterReset)
	}

	{ // New histogram that has a counter reset while new buckets were added.
		c, hApp, ts, h1 := setup(eh)
		h2 := h1.Copy()
		h2.PositiveSpans = []histogram.Span{
			{Offset: 0, Length: 3},
			{Offset: 1, Length: 1},
			{Offset: 1, Length: 4},
			{Offset: 3, Length: 3},
		}
		h2.Sum = 29
		// Existing histogram should get values converted from the above to:
		//   6 3 0 3 0 0 2 4 5 0 1 (previous values with some new empty buckets in between)
		// so the new histogram should have new counts >= these per-bucket counts, e.g.:
		h2.PositiveBuckets = []int64{7, -2, -4, 2, -2, -1, 2, 3, 0, -5, 0} // 7 5 1 3 1 0 2 5 5 0 0 (total 29)

		posInterjections, negInterjections, backwardPositiveInserts, backwardNegativeInserts, ok, cr := hApp.appendable(h2)
		require.Empty(t, posInterjections)
		require.Empty(t, negInterjections)
		require.Empty(t, backwardPositiveInserts)
		require.Empty(t, backwardNegativeInserts)
		require.False(t, ok) // Need to cut a new chunk.
		require.True(t, cr)

		assertNewHistogramChunkOnAppend(t, c, hApp, ts+1, h2, CounterReset)
	}

	{
		// New histogram that has a counter reset while new buckets were
		// added before the first bucket and reset on first bucket.  (to
		// catch the edge case where the new bucket should be forwarded
		// ahead until first old bucket at start)
		c, hApp, ts, h1 := setup(eh)
		h2 := h1.Copy()
		h2.PositiveSpans = []histogram.Span{
			{Offset: -3, Length: 2},
			{Offset: 1, Length: 2},
			{Offset: 2, Length: 1},
			{Offset: 3, Length: 2},
			{Offset: 3, Length: 1},
			{Offset: 1, Length: 1},
		}
		h2.Sum = 26
		// Existing histogram should get values converted from the above to:
		//   0, 0, 6, 3, 3, 2, 4, 5, 1
		// so the new histogram should have new counts >= these per-bucket counts, e.g.:
		h2.PositiveBuckets = []int64{1, 1, 3, -2, 0, -1, 2, 1, -4} // counts: 1, 2, 5, 3, 3, 2, 4, 5, 1 (total 26)

		posInterjections, negInterjections, backwardPositiveInserts, backwardNegativeInserts, ok, cr := hApp.appendable(h2)
		require.Empty(t, posInterjections)
		require.Empty(t, negInterjections)
		require.Empty(t, backwardPositiveInserts)
		require.Empty(t, backwardNegativeInserts)
		require.False(t, ok) // Need to cut a new chunk.
		require.True(t, cr)

		assertNewHistogramChunkOnAppend(t, c, hApp, ts+1, h2, CounterReset)
	}

	{ // New histogram that has an explicit counter reset.
		c, hApp, ts, h1 := setup(eh)
		h2 := h1.Copy()
		h2.CounterResetHint = histogram.CounterReset

		assertNewHistogramChunkOnAppend(t, c, hApp, ts+1, h2, CounterReset)
	}

	{ // Start new chunk explicitly, and append a new histogram that is considered appendable to the previous chunk.
		_, hApp, ts, h1 := setup(eh)
		h2 := h1.Copy() // Identity is appendable.

		nextChunk := NewHistogramChunk()
		app, err := nextChunk.Appender()
		require.NoError(t, err)
		newChunk, recoded, newApp, err := app.AppendHistogram(hApp, ts+1, h2, false)
		require.NoError(t, err)
		require.Nil(t, newChunk)
		require.False(t, recoded)
		require.Equal(t, app, newApp)
		assertSampleCount(t, nextChunk, 1, ValHistogram)
		require.Equal(t, NotCounterReset, nextChunk.GetCounterResetHeader())
	}

	{ // Start new chunk explicitly, and append a new histogram that is not considered appendable to the previous chunk.
		_, hApp, ts, h1 := setup(eh)
		h2 := h1.Copy()
		h2.Count-- // Make this not appendable due to counter reset.

		nextChunk := NewHistogramChunk()
		app, err := nextChunk.Appender()
		require.NoError(t, err)
		newChunk, recoded, newApp, err := app.AppendHistogram(hApp, ts+1, h2, false)
		require.NoError(t, err)
		require.Nil(t, newChunk)
		require.False(t, recoded)
		require.Equal(t, app, newApp)
		assertSampleCount(t, nextChunk, 1, ValHistogram)
		require.Equal(t, CounterReset, nextChunk.GetCounterResetHeader())
	}

	{ // Start new chunk explicitly, and append a new histogram that would need recoding if we added it to the chunk.
		_, hApp, ts, h1 := setup(eh)
		h2 := h1.Copy()
		h2.PositiveSpans = []histogram.Span{
			{Offset: 0, Length: 3},
			{Offset: 1, Length: 1},
			{Offset: 1, Length: 4},
			{Offset: 3, Length: 3},
		}
		h2.Count += 9
		h2.ZeroCount++
		h2.Sum = 30
		// Existing histogram should get values converted from the above to:
		//   6 3 0 3 0 0 2 4 5 0 1 (previous values with some new empty buckets in between)
		// so the new histogram should have new counts >= these per-bucket counts, e.g.:
		h2.PositiveBuckets = []int64{7, -2, -4, 2, -2, -1, 2, 3, 0, -5, 1} // 7 5 1 3 1 0 2 5 5 0 1 (total 30)

		nextChunk := NewHistogramChunk()
		app, err := nextChunk.Appender()
		require.NoError(t, err)
		newChunk, recoded, newApp, err := app.AppendHistogram(hApp, ts+1, h2, false)
		require.NoError(t, err)
		require.Nil(t, newChunk)
		require.False(t, recoded)
		require.Equal(t, app, newApp)
		assertSampleCount(t, nextChunk, 1, ValHistogram)
		require.Equal(t, NotCounterReset, nextChunk.GetCounterResetHeader())
	}

	{
		// Start a new chunk with a histogram that has an empty bucket.
		// Add a histogram that has the same bucket missing.
		// This should be appendable and can happen if we are merging from chunks
		// where the first sample came from a recoded chunk that added the
		// empty bucket.
		h1 := eh.Copy()
		// Add a bucket that is empty -10 offsets from the first bucket.
		h1.PositiveSpans = make([]histogram.Span, len(eh.PositiveSpans)+1)
		h1.PositiveSpans[0] = histogram.Span{Offset: eh.PositiveSpans[0].Offset - 10, Length: 1}
		h1.PositiveSpans[1] = histogram.Span{Offset: eh.PositiveSpans[0].Offset + 9, Length: eh.PositiveSpans[0].Length}
		for i, v := range eh.PositiveSpans[1:] {
			h1.PositiveSpans[i+2] = v
		}
		h1.PositiveBuckets = make([]int64, len(eh.PositiveBuckets)+1)
		h1.PositiveBuckets[0] = 0
		for i, v := range eh.PositiveBuckets {
			h1.PositiveBuckets[i+1] = v
		}

		c, hApp, ts, _ := setup(h1)
		h2 := eh.Copy()

		posInterjections, negInterjections, backwardPositiveInserts, backwardNegativeInserts, ok, cr := hApp.appendable(h2)
		require.Empty(t, posInterjections)
		require.Empty(t, negInterjections)
		require.NotEmpty(t, backwardPositiveInserts)
		require.Empty(t, backwardNegativeInserts)
		require.True(t, ok)
		require.False(t, cr)

		assertNoNewHistogramChunkOnAppend(t, c, hApp, ts+1, h2, UnknownCounterReset)
	}

	{ // Custom buckets, no change.
		c, hApp, ts, h1 := setup(cbh)
		h2 := h1.Copy()
		_, _, _, _, ok, _ := hApp.appendable(h2)
		require.True(t, ok)

		assertNoNewHistogramChunkOnAppend(t, c, hApp, ts+1, h2, UnknownCounterReset)
	}

	{ // Custom buckets, increase in bucket counts but no change in layout.
		c, hApp, ts, h1 := setup(cbh)
		h2 := h1.Copy()
		h2.Count++
		h2.PositiveBuckets = []int64{6, -3, 0, -1, 2, 1, -3}
		_, _, _, _, ok, _ := hApp.appendable(h2)
		require.True(t, ok)

		assertNoNewHistogramChunkOnAppend(t, c, hApp, ts+1, h2, UnknownCounterReset)
	}

	{ // Custom buckets, decrease in bucket counts but no change in layout.
		c, hApp, ts, h1 := setup(cbh)
		h2 := h1.Copy()
		h2.Count--
		h2.PositiveBuckets = []int64{6, -3, 0, -1, 2, 1, -5}
		_, _, _, _, ok, _ := hApp.appendable(h2)
		require.False(t, ok)

		assertNewHistogramChunkOnAppend(t, c, hApp, ts+1, h2, CounterReset)
	}

	{ // Custom buckets, change only in custom bounds.
		c, hApp, ts, h1 := setup(cbh)
		h2 := h1.Copy()
		h2.CustomValues = []float64{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21}
		_, _, _, _, ok, _ := hApp.appendable(h2)
		require.False(t, ok)

		assertNewHistogramChunkOnAppend(t, c, hApp, ts+1, h2, CounterReset)
	}

	{ // Custom buckets, with more buckets.
		c, hApp, ts, h1 := setup(cbh)
		h2 := h1.Copy()
		h2.PositiveSpans = []histogram.Span{
			{Offset: 0, Length: 3},
			{Offset: 1, Length: 1},
			{Offset: 1, Length: 4},
			{Offset: 3, Length: 3},
		}
		h2.Count += 6
		h2.Sum = 30
		// Existing histogram should get values converted from the above to:
		//   6 3 0 3 0 0 2 4 5 0 1 (previous values with some new empty buckets in between)
		// so the new histogram should have new counts >= these per-bucket counts, e.g.:
		h2.PositiveBuckets = []int64{7, -2, -4, 2, -2, -1, 2, 3, 0, -5, 1} // 7 5 1 3 1 0 2 5 5 0 1 (total 30)

		posInterjections, negInterjections, backwardPositiveInserts, backwardNegativeInserts, ok, cr := hApp.appendable(h2)
		require.NotEmpty(t, posInterjections)
		require.Empty(t, negInterjections)
		require.Empty(t, backwardPositiveInserts)
		require.Empty(t, backwardNegativeInserts)
		require.True(t, ok) // Only new buckets came in.
		require.False(t, cr)

		assertRecodedHistogramChunkOnAppend(t, c, hApp, ts+1, h2, UnknownCounterReset)
	}
}

func assertNewHistogramChunkOnAppend(t *testing.T, oldChunk Chunk, hApp *HistogramAppender, ts int64, h *histogram.Histogram, expectHeader CounterResetHeader) {
	oldChunkBytes := oldChunk.Bytes()
	newChunk, recoded, newAppender, err := hApp.AppendHistogram(nil, ts, h, false)
	require.Equal(t, oldChunkBytes, oldChunk.Bytes()) // Sanity check that previous chunk is untouched.
	require.NoError(t, err)
	require.NotNil(t, newChunk)
	require.False(t, recoded)
	require.NotEqual(t, oldChunk, newChunk)
	require.Equal(t, expectHeader, newChunk.(*HistogramChunk).GetCounterResetHeader())
	require.NotNil(t, newAppender)
	require.NotEqual(t, hApp, newAppender)
	assertSampleCount(t, newChunk, 1, ValHistogram)
}

func assertNoNewHistogramChunkOnAppend(t *testing.T, currChunk Chunk, hApp *HistogramAppender, ts int64, h *histogram.Histogram, expectHeader CounterResetHeader) {
	prevChunkBytes := currChunk.Bytes()
	newChunk, recoded, newAppender, err := hApp.AppendHistogram(nil, ts, h, false)
	require.Greater(t, len(currChunk.Bytes()), len(prevChunkBytes)) // Check that current chunk is bigger than previously.
	require.NoError(t, err)
	require.Nil(t, newChunk)
	require.False(t, recoded)
	require.Equal(t, expectHeader, currChunk.(*HistogramChunk).GetCounterResetHeader())
	require.NotNil(t, newAppender)
	require.Equal(t, hApp, newAppender)
	assertSampleCount(t, currChunk, 2, ValHistogram)
}

func assertRecodedHistogramChunkOnAppend(t *testing.T, prevChunk Chunk, hApp *HistogramAppender, ts int64, h *histogram.Histogram, expectHeader CounterResetHeader) {
	prevChunkBytes := prevChunk.Bytes()
	newChunk, recoded, newAppender, err := hApp.AppendHistogram(nil, ts, h, false)
	require.Equal(t, prevChunkBytes, prevChunk.Bytes()) // Sanity check that previous chunk is untouched. This may change in the future if we implement in-place recoding.
	require.NoError(t, err)
	require.NotNil(t, newChunk)
	require.True(t, recoded)
	require.NotEqual(t, prevChunk, newChunk)
	require.Equal(t, expectHeader, newChunk.(*HistogramChunk).GetCounterResetHeader())
	require.NotNil(t, newAppender)
	require.NotEqual(t, hApp, newAppender)
	assertSampleCount(t, newChunk, 2, ValHistogram)
}

func assertSampleCount(t *testing.T, c Chunk, exp int64, vtype ValueType) {
	count := int64(0)
	it := c.Iterator(nil)
	require.NoError(t, it.Err())
	for it.Next() == vtype {
		count++
	}
	require.NoError(t, it.Err())
	require.Equal(t, exp, count)
}

func TestHistogramChunkAppendableWithEmptySpan(t *testing.T) {
	tests := map[string]struct {
		h1 *histogram.Histogram
		h2 *histogram.Histogram
	}{
		"empty span in old and new histogram": {
			h1: &histogram.Histogram{
				Schema:        0,
				Count:         21,
				Sum:           1234.5,
				ZeroThreshold: 0.001,
				ZeroCount:     4,
				PositiveSpans: []histogram.Span{
					{Offset: 0, Length: 4},
					{Offset: 0, Length: 0},
					{Offset: 0, Length: 3},
				},
				PositiveBuckets: []int64{1, 1, -1, 0, 0, 0, 0},
				NegativeSpans: []histogram.Span{
					{Offset: 1, Length: 4},
					{Offset: 2, Length: 0},
					{Offset: 2, Length: 3},
				},
				NegativeBuckets: []int64{1, 1, -1, 1, 0, 0, 0},
			},
			h2: &histogram.Histogram{
				Schema:        0,
				Count:         37,
				Sum:           2345.6,
				ZeroThreshold: 0.001,
				ZeroCount:     5,
				PositiveSpans: []histogram.Span{
					{Offset: 0, Length: 4},
					{Offset: 0, Length: 0},
					{Offset: 0, Length: 3},
				},
				PositiveBuckets: []int64{1, 2, -2, 1, -1, 0, 0},
				NegativeSpans: []histogram.Span{
					{Offset: 1, Length: 4},
					{Offset: 2, Length: 0},
					{Offset: 2, Length: 3},
				},
				NegativeBuckets: []int64{1, 3, -2, 5, -2, 0, -3},
			},
		},
		"empty span in old histogram": {
			h1: &histogram.Histogram{
				Schema:        0,
				Count:         21,
				Sum:           1234.5,
				ZeroThreshold: 0.001,
				ZeroCount:     4,
				PositiveSpans: []histogram.Span{
					{Offset: 1, Length: 0}, // This span will disappear.
					{Offset: 2, Length: 4},
					{Offset: 0, Length: 3},
				},
				PositiveBuckets: []int64{1, 1, -1, 0, 0, 0, 0},
				NegativeSpans: []histogram.Span{
					{Offset: 1, Length: 4},
					{Offset: 2, Length: 0},
					{Offset: 2, Length: 3},
				},
				NegativeBuckets: []int64{1, 1, -1, 1, 0, 0, 0},
			},
			h2: &histogram.Histogram{
				Schema:        0,
				Count:         37,
				Sum:           2345.6,
				ZeroThreshold: 0.001,
				ZeroCount:     5,
				PositiveSpans: []histogram.Span{
					{Offset: 3, Length: 4},
					{Offset: 0, Length: 3},
				},
				PositiveBuckets: []int64{1, 2, -2, 1, -1, 0, 0},
				NegativeSpans: []histogram.Span{
					{Offset: 1, Length: 4},
					{Offset: 2, Length: 0},
					{Offset: 2, Length: 3},
				},
				NegativeBuckets: []int64{1, 3, -2, 5, -2, 0, -3},
			},
		},
		"empty span in new histogram": {
			h1: &histogram.Histogram{
				Schema:        0,
				Count:         21,
				Sum:           1234.5,
				ZeroThreshold: 0.001,
				ZeroCount:     4,
				PositiveSpans: []histogram.Span{
					{Offset: 0, Length: 4},
					{Offset: 3, Length: 3},
				},
				PositiveBuckets: []int64{1, 1, -1, 0, 0, 0, 0},
				NegativeSpans: []histogram.Span{
					{Offset: 1, Length: 4},
					{Offset: 2, Length: 0},
					{Offset: 2, Length: 3},
				},
				NegativeBuckets: []int64{1, 1, -1, 1, 0, 0, 0},
			},
			h2: &histogram.Histogram{
				Schema:        0,
				Count:         37,
				Sum:           2345.6,
				ZeroThreshold: 0.001,
				ZeroCount:     5,
				PositiveSpans: []histogram.Span{
					{Offset: 0, Length: 4},
					{Offset: 1, Length: 0}, // This span is new.
					{Offset: 2, Length: 3},
				},
				PositiveBuckets: []int64{1, 2, -2, 1, -1, 0, 0},
				NegativeSpans: []histogram.Span{
					{Offset: 1, Length: 4},
					{Offset: 2, Length: 0},
					{Offset: 2, Length: 3},
				},
				NegativeBuckets: []int64{1, 3, -2, 5, -2, 0, -3},
			},
		},
		"two empty spans mixing offsets": {
			h1: &histogram.Histogram{
				Schema:        0,
				Count:         21,
				Sum:           1234.5,
				ZeroThreshold: 0.001,
				ZeroCount:     4,
				PositiveSpans: []histogram.Span{
					{Offset: 0, Length: 4},
					{Offset: 1, Length: 0},
					{Offset: 3, Length: 0},
					{Offset: 4, Length: 3},
				},
				PositiveBuckets: []int64{1, 1, -1, 0, 0, 0, 0},
				NegativeSpans: []histogram.Span{
					{Offset: 1, Length: 4},
					{Offset: 2, Length: 0},
					{Offset: 2, Length: 3},
				},
				NegativeBuckets: []int64{1, 1, -1, 1, 0, 0, 0},
			},
			h2: &histogram.Histogram{
				Schema:        0,
				Count:         37,
				Sum:           2345.6,
				ZeroThreshold: 0.001,
				ZeroCount:     5,
				PositiveSpans: []histogram.Span{
					{Offset: 0, Length: 4},
					{Offset: 3, Length: 0},
					{Offset: 1, Length: 0},
					{Offset: 4, Length: 3},
				},
				PositiveBuckets: []int64{1, 2, -2, 1, -1, 0, 0},
				NegativeSpans: []histogram.Span{
					{Offset: 1, Length: 4},
					{Offset: 2, Length: 0},
					{Offset: 2, Length: 3},
				},
				NegativeBuckets: []int64{1, 3, -2, 5, -2, 0, -3},
			},
		},
		"empty span in old and new custom buckets histogram": {
			h1: &histogram.Histogram{
				Schema: histogram.CustomBucketsSchema,
				Count:  7,
				Sum:    1234.5,
				PositiveSpans: []histogram.Span{
					{Offset: 0, Length: 4},
					{Offset: 0, Length: 0},
					{Offset: 0, Length: 3},
				},
				PositiveBuckets: []int64{1, 1, -1, 0, 0, 0, 0},
				CustomValues:    []float64{1, 2, 3, 4, 5, 6, 7, 8, 9, 10},
			},
			h2: &histogram.Histogram{
				Schema: histogram.CustomBucketsSchema,
				Count:  10,
				Sum:    2345.6,
				PositiveSpans: []histogram.Span{
					{Offset: 0, Length: 4},
					{Offset: 0, Length: 0},
					{Offset: 0, Length: 3},
				},
				PositiveBuckets: []int64{1, 2, -2, 1, -1, 0, 0},
				CustomValues:    []float64{1, 2, 3, 4, 5, 6, 7, 8, 9, 10},
			},
		},
	}

	for name, tc := range tests {
		t.Run(name, func(t *testing.T) {
			c := Chunk(NewHistogramChunk())

			// Create fresh appender and add the first histogram.
			app, err := c.Appender()
			require.NoError(t, err)
			require.Equal(t, 0, c.NumSamples())

			_, _, _, err = app.AppendHistogram(nil, 1, tc.h1, true)
			require.NoError(t, err)
			require.Equal(t, 1, c.NumSamples())
			hApp, _ := app.(*HistogramAppender)

			pI, nI, bpI, bnI, okToAppend, counterReset := hApp.appendable(tc.h2)
			require.Empty(t, pI)
			require.Empty(t, nI)
			require.Empty(t, bpI)
			require.Empty(t, bnI)
			require.True(t, okToAppend)
			require.False(t, counterReset)
		})
	}
}

func TestAtFloatHistogram(t *testing.T) {
	input := []histogram.Histogram{
		{
			Schema:        0,
			Count:         21,
			Sum:           1234.5,
			ZeroThreshold: 0.001,
			ZeroCount:     4,
			PositiveSpans: []histogram.Span{
				{Offset: 0, Length: 4},
				{Offset: 0, Length: 0},
				{Offset: 0, Length: 3},
			},
			PositiveBuckets: []int64{1, 1, -1, 0, 0, 0, 0},
			NegativeSpans: []histogram.Span{
				{Offset: 1, Length: 4},
				{Offset: 2, Length: 0},
				{Offset: 2, Length: 3},
			},
			NegativeBuckets: []int64{1, 1, -1, 1, 0, 0, 0},
		},
		{
			Schema:        0,
			Count:         36,
			Sum:           2345.6,
			ZeroThreshold: 0.001,
			ZeroCount:     5,
			PositiveSpans: []histogram.Span{
				{Offset: 0, Length: 4},
				{Offset: 0, Length: 0},
				{Offset: 0, Length: 3},
			},
			PositiveBuckets: []int64{1, 2, -2, 1, -1, 0, 0},
			NegativeSpans: []histogram.Span{
				{Offset: 1, Length: 4},
				{Offset: 2, Length: 0},
				{Offset: 2, Length: 3},
			},
			NegativeBuckets: []int64{1, 3, -2, 5, -2, 0, -3},
		},
		{
			Schema:        0,
			Count:         36,
			Sum:           1111.1,
			ZeroThreshold: 0.001,
			ZeroCount:     5,
			PositiveSpans: []histogram.Span{
				{Offset: 0, Length: 4},
				{Offset: 0, Length: 0},
				{Offset: 0, Length: 3},
			},
			PositiveBuckets: []int64{1, 2, -2, 2, -1, 0, 0},
			NegativeSpans: []histogram.Span{
				{Offset: 1, Length: 4},
				{Offset: 2, Length: 0},
				{Offset: 2, Length: 3},
			},
			NegativeBuckets: []int64{1, 3, -2, 5, -1, 0, -3},
		},
	}

	expOutput := []*histogram.FloatHistogram{
		{
			Schema:        0,
			Count:         21,
			Sum:           1234.5,
			ZeroThreshold: 0.001,
			ZeroCount:     4,
			PositiveSpans: []histogram.Span{
				{Offset: 0, Length: 4},
				{Offset: 0, Length: 0},
				{Offset: 0, Length: 3},
			},
			PositiveBuckets: []float64{1, 2, 1, 1, 1, 1, 1},
			NegativeSpans: []histogram.Span{
				{Offset: 1, Length: 4},
				{Offset: 2, Length: 0},
				{Offset: 2, Length: 3},
			},
			NegativeBuckets: []float64{1, 2, 1, 2, 2, 2, 2},
		},
		{
			CounterResetHint: histogram.NotCounterReset,
			Schema:           0,
			Count:            36,
			Sum:              2345.6,
			ZeroThreshold:    0.001,
			ZeroCount:        5,
			PositiveSpans: []histogram.Span{
				{Offset: 0, Length: 4},
				{Offset: 0, Length: 0},
				{Offset: 0, Length: 3},
			},
			PositiveBuckets: []float64{1, 3, 1, 2, 1, 1, 1},
			NegativeSpans: []histogram.Span{
				{Offset: 1, Length: 4},
				{Offset: 2, Length: 0},
				{Offset: 2, Length: 3},
			},
			NegativeBuckets: []float64{1, 4, 2, 7, 5, 5, 2},
		},
		{
			CounterResetHint: histogram.NotCounterReset,
			Schema:           0,
			Count:            36,
			Sum:              1111.1,
			ZeroThreshold:    0.001,
			ZeroCount:        5,
			PositiveSpans: []histogram.Span{
				{Offset: 0, Length: 4},
				{Offset: 0, Length: 0},
				{Offset: 0, Length: 3},
			},
			PositiveBuckets: []float64{1, 3, 1, 3, 2, 2, 2},
			NegativeSpans: []histogram.Span{
				{Offset: 1, Length: 4},
				{Offset: 2, Length: 0},
				{Offset: 2, Length: 3},
			},
			NegativeBuckets: []float64{1, 4, 2, 7, 6, 6, 3},
		},
	}

	chk := NewHistogramChunk()
	app, err := chk.Appender()
	require.NoError(t, err)
	for i := range input {
		newc, _, _, err := app.AppendHistogram(nil, int64(i), &input[i], false)
		require.NoError(t, err)
		require.Nil(t, newc)
	}
	it := chk.Iterator(nil)
	i := int64(0)
	for it.Next() != ValNone {
		ts, h := it.AtFloatHistogram(nil)
		require.Equal(t, i, ts)
		require.Equal(t, expOutput[i], h, "histogram %d unequal", i)
		i++
	}
}

func TestHistogramChunkAppendableGauge(t *testing.T) {
	eh := &histogram.Histogram{
		CounterResetHint: histogram.GaugeType,
		Count:            5,
		ZeroCount:        2,
		Sum:              18.4,
		ZeroThreshold:    1e-125,
		Schema:           1,
		PositiveSpans: []histogram.Span{
			{Offset: 0, Length: 2},
			{Offset: 2, Length: 1},
			{Offset: 3, Length: 2},
			{Offset: 3, Length: 1},
			{Offset: 1, Length: 1},
		},
		PositiveBuckets: []int64{6, -3, 0, -1, 2, 1, -4}, // {6, 3, 3, 2, 4, 5, 1}
	}

	cbh := &histogram.Histogram{
		CounterResetHint: histogram.GaugeType,
		Count:            24,
		Sum:              18.4,
		Schema:           histogram.CustomBucketsSchema,
		PositiveSpans: []histogram.Span{
			{Offset: 0, Length: 2},
			{Offset: 2, Length: 1},
			{Offset: 3, Length: 2},
			{Offset: 3, Length: 1},
			{Offset: 1, Length: 1},
		},
		PositiveBuckets: []int64{6, -3, 0, -1, 2, 1, -4}, // {6, 3, 3, 2, 4, 5, 1}
		CustomValues:    []float64{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20},
	}

	setup := func(h *histogram.Histogram) (Chunk, *HistogramAppender, int64, *histogram.Histogram) {
		c := Chunk(NewHistogramChunk())

		// Create fresh appender and add the first histogram.
		app, err := c.Appender()
		require.NoError(t, err)
		require.Equal(t, 0, c.NumSamples())

		ts := int64(1234567890)

		chk, _, app, err := app.AppendHistogram(nil, ts, h.Copy(), false)
		require.NoError(t, err)
		require.Nil(t, chk)
		require.Equal(t, 1, c.NumSamples())
		require.Equal(t, GaugeType, c.(*HistogramChunk).GetCounterResetHeader())

		return c, app.(*HistogramAppender), ts, h
	}

	{ // Schema change.
		c, hApp, ts, h1 := setup(eh)
		h2 := h1.Copy()
		h2.Schema++
		_, _, _, _, _, _, ok := hApp.appendableGauge(h2)
		require.False(t, ok)

		assertNewHistogramChunkOnAppend(t, c, hApp, ts+1, h2, GaugeType)
	}

	{ // Zero threshold change.
		c, hApp, ts, h1 := setup(eh)
		h2 := h1.Copy()
		h2.ZeroThreshold += 0.1
		_, _, _, _, _, _, ok := hApp.appendableGauge(h2)
		require.False(t, ok)

		assertNewHistogramChunkOnAppend(t, c, hApp, ts+1, h2, GaugeType)
	}

	{ // New histogram that has more buckets.
		c, hApp, ts, h1 := setup(eh)
		h2 := h1.Copy()
		h2.PositiveSpans = []histogram.Span{
			{Offset: 0, Length: 3},
			{Offset: 1, Length: 1},
			{Offset: 1, Length: 4},
			{Offset: 3, Length: 3},
		}
		h2.Count += 9
		h2.ZeroCount++
		h2.Sum = 30
		h2.PositiveBuckets = []int64{7, -2, -4, 2, -2, -1, 2, 3, 0, -5, 1} // {7, 5, 1, 3, 1, 0, 2, 5, 5, 0, 1}

		pI, nI, pBackwardI, nBackwardI, _, _, ok := hApp.appendableGauge(h2)
		require.NotEmpty(t, pI)
		require.Empty(t, nI)
		require.Empty(t, pBackwardI)
		require.Empty(t, nBackwardI)
		require.True(t, ok)

		assertRecodedHistogramChunkOnAppend(t, c, hApp, ts+1, h2, GaugeType)
	}

	{ // New histogram that has buckets missing.
		c, hApp, ts, h1 := setup(eh)
		h2 := h1.Copy()
		h2.PositiveSpans = []histogram.Span{
			{Offset: 0, Length: 2},
			{Offset: 2, Length: 1},
			{Offset: 3, Length: 1},
			{Offset: 4, Length: 1},
			{Offset: 1, Length: 1},
		}
		h2.Count -= 4
		h2.Sum--
		h2.PositiveBuckets = []int64{6, -3, 0, -1, 3, -4} // {6, 3, 3, 2, 5, 1}

		pI, nI, pBackwardI, nBackwardI, _, _, ok := hApp.appendableGauge(h2)
		require.Empty(t, pI)
		require.Empty(t, nI)
		require.NotEmpty(t, pBackwardI)
		require.Empty(t, nBackwardI)
		require.True(t, ok)

		assertNoNewHistogramChunkOnAppend(t, c, hApp, ts+1, h2, GaugeType)
	}

	{ // New histogram that has a bucket missing and new buckets.
		c, hApp, ts, h1 := setup(eh)
		h2 := h1.Copy()
		h2.PositiveSpans = []histogram.Span{
			{Offset: 0, Length: 2},
			{Offset: 5, Length: 2},
			{Offset: 3, Length: 1},
			{Offset: 1, Length: 1},
		}
		h2.Sum = 21
		h2.PositiveBuckets = []int64{6, -3, -1, 2, 1, -4} // {6, 3, 2, 4, 5, 1}

		pI, nI, pBackwardI, nBackwardI, _, _, ok := hApp.appendableGauge(h2)
		require.NotEmpty(t, pI)
		require.NotEmpty(t, pBackwardI)
		require.Empty(t, nI)
		require.Empty(t, nBackwardI)
		require.True(t, ok)

		assertRecodedHistogramChunkOnAppend(t, c, hApp, ts+1, h2, GaugeType)
	}

	{ // New histogram that has a counter reset while buckets are same.
		c, hApp, ts, h1 := setup(eh)
		h2 := h1.Copy()
		h2.Sum = 23
		h2.PositiveBuckets = []int64{6, -4, 1, -1, 2, 1, -4} // {6, 2, 3, 2, 4, 5, 1}

		pI, nI, pBackwardI, nBackwardI, _, _, ok := hApp.appendableGauge(h2)
		require.Empty(t, pI)
		require.Empty(t, nI)
		require.Empty(t, pBackwardI)
		require.Empty(t, nBackwardI)
		require.True(t, ok)

		assertNoNewHistogramChunkOnAppend(t, c, hApp, ts+1, h2, GaugeType)
	}

	{ // New histogram that has a counter reset while new buckets were added.
		c, hApp, ts, h1 := setup(eh)
		h2 := h1.Copy()
		h2.PositiveSpans = []histogram.Span{
			{Offset: 0, Length: 3},
			{Offset: 1, Length: 1},
			{Offset: 1, Length: 4},
			{Offset: 3, Length: 3},
		}
		h2.Sum = 29
		h2.PositiveBuckets = []int64{7, -2, -4, 2, -2, -1, 2, 3, 0, -5, 0} // {7, 5, 1, 3, 1, 0, 2, 5, 5, 0, 0}

		pI, nI, pBackwardI, nBackwardI, _, _, ok := hApp.appendableGauge(h2)
		require.NotEmpty(t, pI)
		require.Empty(t, nI)
		require.Empty(t, pBackwardI)
		require.Empty(t, nBackwardI)
		require.True(t, ok)

		assertRecodedHistogramChunkOnAppend(t, c, hApp, ts+1, h2, GaugeType)
	}

	{
		// New histogram that has a counter reset while new buckets were
		// added before the first bucket and reset on first bucket.
		c, hApp, ts, h1 := setup(eh)
		h2 := h1.Copy()
		h2.PositiveSpans = []histogram.Span{
			{Offset: -3, Length: 2},
			{Offset: 1, Length: 2},
			{Offset: 2, Length: 1},
			{Offset: 3, Length: 2},
			{Offset: 3, Length: 1},
			{Offset: 1, Length: 1},
		}
		h2.Sum = 26
		h2.PositiveBuckets = []int64{1, 1, 3, -2, 0, -1, 2, 1, -4} // {1, 2, 5, 3, 3, 2, 4, 5, 1}

		pI, nI, pBackwardI, nBackwardI, _, _, ok := hApp.appendableGauge(h2)
		require.NotEmpty(t, pI)
		require.Empty(t, nI)
		require.Empty(t, pBackwardI)
		require.Empty(t, nBackwardI)
		require.True(t, ok)

		assertRecodedHistogramChunkOnAppend(t, c, hApp, ts+1, h2, GaugeType)
	}

	{ // Custom buckets, no change.
		c, hApp, ts, h1 := setup(cbh)
		h2 := h1.Copy()
		_, _, _, _, _, _, ok := hApp.appendableGauge(h2)
		require.True(t, ok)

		assertNoNewHistogramChunkOnAppend(t, c, hApp, ts+1, h2, GaugeType)
	}

	{ // Custom buckets, increase in bucket counts but no change in layout.
		c, hApp, ts, h1 := setup(cbh)
		h2 := h1.Copy()
		h2.Count++
		h2.PositiveBuckets = []int64{6, -3, 0, -1, 2, 1, -3}
		_, _, _, _, _, _, ok := hApp.appendableGauge(h2)
		require.True(t, ok)

		assertNoNewHistogramChunkOnAppend(t, c, hApp, ts+1, h2, GaugeType)
	}

	{ // Custom buckets, decrease in bucket counts but no change in layout.
		c, hApp, ts, h1 := setup(cbh)
		h2 := h1.Copy()
		h2.Count--
		h2.PositiveBuckets = []int64{6, -3, 0, -1, 2, 1, -5}
		_, _, _, _, _, _, ok := hApp.appendableGauge(h2)
		require.True(t, ok)

		assertNoNewHistogramChunkOnAppend(t, c, hApp, ts+1, h2, GaugeType)
	}

	{ // Custom buckets, change only in custom bounds.
		c, hApp, ts, h1 := setup(cbh)
		h2 := h1.Copy()
		h2.CustomValues = []float64{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21}
		_, _, _, _, _, _, ok := hApp.appendableGauge(h2)
		require.False(t, ok)

		assertNewHistogramChunkOnAppend(t, c, hApp, ts+1, h2, GaugeType)
	}

	{ // Custom buckets, with more buckets.
		c, hApp, ts, h1 := setup(cbh)
		h2 := h1.Copy()
		h2.PositiveSpans = []histogram.Span{
			{Offset: 0, Length: 3},
			{Offset: 1, Length: 1},
			{Offset: 1, Length: 4},
			{Offset: 3, Length: 3},
		}
		h2.Count += 6
		h2.Sum = 30
		// Existing histogram should get values converted from the above to:
		//   6 3 0 3 0 0 2 4 5 0 1 (previous values with some new empty buckets in between)
		// so the new histogram should have new counts >= these per-bucket counts, e.g.:
		h2.PositiveBuckets = []int64{7, -2, -4, 2, -2, -1, 2, 3, 0, -5, 1} // 7 5 1 3 1 0 2 5 5 0 1 (total 30)

		posInterjections, negInterjections, pBackwardI, nBackwardI, _, _, ok := hApp.appendableGauge(h2)
		require.NotEmpty(t, posInterjections)
		require.Empty(t, negInterjections)
		require.Empty(t, pBackwardI)
		require.Empty(t, nBackwardI)
		require.True(t, ok) // Only new buckets came in.

		assertRecodedHistogramChunkOnAppend(t, c, hApp, ts+1, h2, GaugeType)
	}
}

func TestHistogramAppendOnlyErrors(t *testing.T) {
	t.Run("schema change error", func(t *testing.T) {
		c := Chunk(NewHistogramChunk())

		// Create fresh appender and add the first histogram.
		app, err := c.Appender()
		require.NoError(t, err)

		h := tsdbutil.GenerateTestHistogram(0)
		var isRecoded bool
		c, isRecoded, app, err = app.AppendHistogram(nil, 1, h, true)
		require.Nil(t, c)
		require.False(t, isRecoded)
		require.NoError(t, err)

		// Add erroring histogram.
		h2 := h.Copy()
		h2.Schema++
		c, isRecoded, _, err = app.AppendHistogram(nil, 2, h2, true)
		require.Nil(t, c)
		require.False(t, isRecoded)
		require.EqualError(t, err, "histogram schema change")
	})
	t.Run("counter reset error", func(t *testing.T) {
		c := Chunk(NewHistogramChunk())

		// Create fresh appender and add the first histogram.
		app, err := c.Appender()
		require.NoError(t, err)

		h := tsdbutil.GenerateTestHistogram(0)
		var isRecoded bool
		c, isRecoded, app, err = app.AppendHistogram(nil, 1, h, true)
		require.Nil(t, c)
		require.False(t, isRecoded)
		require.NoError(t, err)

		// Add erroring histogram.
		h2 := h.Copy()
		h2.CounterResetHint = histogram.CounterReset
		c, isRecoded, _, err = app.AppendHistogram(nil, 2, h2, true)
		require.Nil(t, c)
		require.False(t, isRecoded)
		require.EqualError(t, err, "histogram counter reset")
	})
	t.Run("counter reset error with custom buckets", func(t *testing.T) {
		c := Chunk(NewHistogramChunk())

		// Create fresh appender and add the first histogram.
		app, err := c.Appender()
		require.NoError(t, err)

		h := tsdbutil.GenerateTestCustomBucketsHistogram(0)
		var isRecoded bool
		c, isRecoded, app, err = app.AppendHistogram(nil, 1, h, true)
		require.Nil(t, c)
		require.False(t, isRecoded)
		require.NoError(t, err)

		// Add erroring histogram.
		h2 := h.Copy()
		h2.CustomValues = []float64{0, 1, 2, 3, 4, 5, 6, 7}
		c, isRecoded, _, err = app.AppendHistogram(nil, 2, h2, true)
		require.Nil(t, c)
		require.False(t, isRecoded)
		require.EqualError(t, err, "histogram counter reset")
	})
}

func BenchmarkAppendable(b *testing.B) {
	// Create a histogram with a bunch of spans and buckets.
	const (
		numSpans   = 1000
		spanLength = 10
	)
	h := &histogram.Histogram{
		Schema:        0,
		Count:         100,
		Sum:           1000,
		ZeroThreshold: 0.001,
		ZeroCount:     5,
	}
	for i := 0; i < numSpans; i++ {
		h.PositiveSpans = append(h.PositiveSpans, histogram.Span{Offset: 5, Length: spanLength})
		h.NegativeSpans = append(h.NegativeSpans, histogram.Span{Offset: 5, Length: spanLength})
		for j := 0; j < spanLength; j++ {
			h.PositiveBuckets = append(h.PositiveBuckets, int64(j))
			h.NegativeBuckets = append(h.NegativeBuckets, int64(j))
		}
	}

	c := Chunk(NewHistogramChunk())

	// Create fresh appender and add the first histogram.
	app, err := c.Appender()
	if err != nil {
		b.Fatal(err)
	}

	_, _, _, err = app.AppendHistogram(nil, 1, h, true)
	if err != nil {
		b.Fatal(err)
	}

	hApp := app.(*HistogramAppender)

	isAppendable := true
	for i := 0; i < b.N; i++ {
		_, _, _, _, ok, _ := hApp.appendable(h)
		isAppendable = isAppendable && ok
	}
	if !isAppendable {
		b.Fail()
	}
}