prometheus/tsdb/chunkenc/float_histogram_test.go

// 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 floatResult struct {
	t int64
	h *histogram.FloatHistogram
}

func TestFirstFloatHistogramExplicitCounterReset(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.FloatHistogram{
				CounterResetHint: test.hint,
			}
			chk := NewFloatHistogramChunk()
			app, err := chk.Appender()
			require.NoError(t, err)
			newChk, recoded, newApp, err := app.AppendFloatHistogram(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 TestFloatHistogramChunkSameBuckets(t *testing.T) {
	c := NewFloatHistogramChunk()
	var exp []floatResult

	// 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.AppendFloatHistogram(nil, ts, h.ToFloat(nil), false)
	require.NoError(t, err)
	require.Nil(t, chk)
	exp = append(exp, floatResult{t: ts, h: 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.AppendFloatHistogram(nil, ts, h.ToFloat(nil), false)
	require.NoError(t, err)
	require.Nil(t, chk)
	expH := h.ToFloat(nil)
	expH.CounterResetHint = histogram.NotCounterReset
	exp = append(exp, floatResult{t: ts, h: expH})
	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.AppendFloatHistogram(nil, ts, h.ToFloat(nil), false)
	require.NoError(t, err)
	require.Nil(t, chk)
	expH = h.ToFloat(nil)
	expH.CounterResetHint = histogram.NotCounterReset
	exp = append(exp, floatResult{t: ts, h: expH})
	require.Equal(t, 3, c.NumSamples())

	// 1. Expand iterator in simple case.
	it := c.Iterator(nil)
	require.NoError(t, it.Err())
	var act []floatResult
	for it.Next() == ValFloatHistogram {
		fts, fh := it.AtFloatHistogram(nil)
		act = append(act, floatResult{t: fts, h: 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 []floatResult
	for it2.Next() == ValFloatHistogram {
		fts, fh := it2.AtFloatHistogram(nil)
		act2 = append(act2, floatResult{t: fts, h: 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() == ValFloatHistogram {
		// Just iterate through without accessing anything.
	}
	it3 := c.iterator(itX)
	var act3 []floatResult
	for it3.Next() == ValFloatHistogram {
		fts, fh := it3.AtFloatHistogram(nil)
		act3 = append(act3, floatResult{t: fts, h: 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 []floatResult
	require.Equal(t, ValFloatHistogram, it4.Seek(exp[mid].t))
	// Below ones should not matter.
	require.Equal(t, ValFloatHistogram, it4.Seek(exp[mid].t))
	require.Equal(t, ValFloatHistogram, it4.Seek(exp[mid].t))
	fts, fh := it4.AtFloatHistogram(nil)
	act4 = append(act4, floatResult{t: fts, h: fh})
	for it4.Next() == ValFloatHistogram {
		fts, fh := it4.AtFloatHistogram(nil)
		act4 = append(act4, floatResult{t: fts, h: 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 TestFloatHistogramChunkBucketChanges(t *testing.T) {
	c := Chunk(NewFloatHistogramChunk())

	// 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.AppendFloatHistogram(nil, ts1, h1.ToFloat(nil), 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.(*FloatHistogramAppender)
	posInterjections, negInterjections, backwardPositiveInserts, backwardNegativeInserts, ok, cr := hApp.appendable(h2.ToFloat(nil))
	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.AppendFloatHistogram(nil, ts2, h2.ToFloat(nil), 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}
	expH2 := h2.ToFloat(nil)
	expH2.CounterResetHint = histogram.NotCounterReset
	exp := []floatResult{
		{t: ts1, h: h1.ToFloat(nil)},
		{t: ts2, h: expH2},
	}
	it := c.Iterator(nil)
	var act []floatResult
	for it.Next() == ValFloatHistogram {
		fts, fh := it.AtFloatHistogram(nil)
		act = append(act, floatResult{t: fts, h: fh})
	}
	require.NoError(t, it.Err())
	require.Equal(t, exp, act)
}

func TestFloatHistogramChunkAppendable(t *testing.T) {
	eh := &histogram.FloatHistogram{
		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: []float64{6, 3, 3, 2, 4, 5, 1},
	}

	cbh := &histogram.FloatHistogram{
		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: []float64{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.FloatHistogram) (Chunk, *FloatHistogramAppender, int64, *histogram.FloatHistogram) {
		c := Chunk(NewFloatHistogramChunk())

		// 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.AppendFloatHistogram(nil, ts, h.Copy(), false)
		require.NoError(t, err)
		require.Nil(t, chk)
		require.Equal(t, 1, c.NumSamples())
		require.Equal(t, UnknownCounterReset, c.(*FloatHistogramChunk).GetCounterResetHeader())
		return c, app.(*FloatHistogramAppender), ts, h
	}

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

		assertNewFloatHistogramChunkOnAppend(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)

		assertNewFloatHistogramChunkOnAppend(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
		h2.PositiveBuckets = []float64{7, 5, 1, 3, 1, 0, 2, 5, 5, 0, 1}

		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)

		assertRecodedFloatHistogramChunkOnAppend(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 = []float64{6, 3, 2, 4, 5, 1}

		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)

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

	{ // New histogram that has buckets missing but the buckets missing were empty.
		emptyBucketH := eh.Copy()
		emptyBucketH.PositiveBuckets = []float64{6, 0, 3, 2, 4, 0, 1}
		c, hApp, ts, h1 := setup(emptyBucketH)
		h2 := h1.Copy()
		h2.PositiveSpans = []histogram.Span{
			{Offset: 0, Length: 1},
			{Offset: 3, Length: 1},
			{Offset: 3, Length: 2},
			{Offset: 5, Length: 1},
		}
		savedH2Spans := h2.PositiveSpans
		h2.PositiveBuckets = []float64{7, 4, 3, 5, 2}

		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)

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

		// Check that h2 was recoded.
		require.Equal(t, []float64{7, 0, 4, 3, 5, 0, 2}, h2.PositiveBuckets)
		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 = []float64{6, 0, 3, 2, 4, 0, 1}
		c, hApp, ts, h1 := setup(emptyBucketH)
		h2 := h1.Copy()
		h2.PositiveSpans = []histogram.Span{
			{Offset: 0, Length: 1},
			{Offset: 3, Length: 1},
			{Offset: 3, Length: 2},
			{Offset: 5, Length: 2},
		}
		savedH2Spans := h2.PositiveSpans
		h2.PositiveBuckets = []float64{7, 4, 3, 5, 2, 3}

		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)

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

		// Check that h2 was recoded.
		require.Equal(t, []float64{7, 0, 4, 3, 5, 0, 2, 3}, h2.PositiveBuckets)
		require.Equal(t, []histogram.Span{
			{Offset: 0, Length: 2}, // Added empty bucket.
			{Offset: 2, Length: 1}, // Existing - offset adjusted.
			{Offset: 3, Length: 2}, // Existing.
			{Offset: 3, Length: 1}, // Added empty bucket.
			{Offset: 1, Length: 2}, // Existing + the extra bucket.
		}, 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 = []float64{6, 2, 3, 2, 4, 5, 1}

		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)

		assertNewFloatHistogramChunkOnAppend(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
		h2.PositiveBuckets = []float64{7, 5, 1, 3, 1, 0, 2, 5, 5, 0, 0}

		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)

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

	{
		c, hApp, ts, h1 := setup(eh)
		// 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)
		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 = []float64{1, 2, 5, 3, 3, 2, 4, 5, 1}

		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)

		assertNewFloatHistogramChunkOnAppend(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

		assertNewFloatHistogramChunkOnAppend(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 := NewFloatHistogramChunk()
		app, err := nextChunk.Appender()
		require.NoError(t, err)
		newChunk, recoded, newApp, err := app.AppendFloatHistogram(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, ValFloatHistogram)
		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 := NewFloatHistogramChunk()
		app, err := nextChunk.Appender()
		require.NoError(t, err)
		newChunk, recoded, newApp, err := app.AppendFloatHistogram(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, ValFloatHistogram)
		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
		h2.PositiveBuckets = []float64{7, 5, 1, 3, 1, 0, 2, 5, 5, 0, 1}

		nextChunk := NewFloatHistogramChunk()
		app, err := nextChunk.Appender()
		require.NoError(t, err)
		newChunk, recoded, newApp, err := app.AppendFloatHistogram(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, ValFloatHistogram)
		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([]float64, 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)

		assertNoNewFloatHistogramChunkOnAppend(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)

		assertNoNewFloatHistogramChunkOnAppend(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 = []float64{6, 3, 3, 2, 4, 5, 2}
		_, _, _, _, ok, _ := hApp.appendable(h2)
		require.True(t, ok)

		assertNoNewFloatHistogramChunkOnAppend(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 = []float64{6, 3, 3, 2, 4, 5, 0}
		_, _, _, _, ok, _ := hApp.appendable(h2)
		require.False(t, ok)

		assertNewFloatHistogramChunkOnAppend(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)

		assertNewFloatHistogramChunkOnAppend(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 = []float64{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)

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

func assertNewFloatHistogramChunkOnAppend(t *testing.T, oldChunk Chunk, hApp *FloatHistogramAppender, ts int64, h *histogram.FloatHistogram, expectHeader CounterResetHeader) {
	oldChunkBytes := oldChunk.Bytes()
	newChunk, recoded, newAppender, err := hApp.AppendFloatHistogram(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.(*FloatHistogramChunk).GetCounterResetHeader())
	require.NotNil(t, newAppender)
	require.NotEqual(t, hApp, newAppender)
	assertSampleCount(t, newChunk, 1, ValFloatHistogram)
}

func assertNoNewFloatHistogramChunkOnAppend(t *testing.T, oldChunk Chunk, hApp *FloatHistogramAppender, ts int64, h *histogram.FloatHistogram, expectHeader CounterResetHeader) {
	oldChunkBytes := oldChunk.Bytes()
	newChunk, recoded, newAppender, err := hApp.AppendFloatHistogram(nil, ts, h, false)
	require.Greater(t, len(oldChunk.Bytes()), len(oldChunkBytes)) // Check that current chunk is bigger than previously.
	require.NoError(t, err)
	require.Nil(t, newChunk)
	require.False(t, recoded)
	require.Equal(t, expectHeader, oldChunk.(*FloatHistogramChunk).GetCounterResetHeader())
	require.NotNil(t, newAppender)
	require.Equal(t, hApp, newAppender)
	assertSampleCount(t, oldChunk, 2, ValFloatHistogram)
}

func assertRecodedFloatHistogramChunkOnAppend(t *testing.T, prevChunk Chunk, hApp *FloatHistogramAppender, ts int64, h *histogram.FloatHistogram, expectHeader CounterResetHeader) {
	prevChunkBytes := prevChunk.Bytes()
	newChunk, recoded, newAppender, err := hApp.AppendFloatHistogram(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.(*FloatHistogramChunk).GetCounterResetHeader())
	require.NotNil(t, newAppender)
	require.NotEqual(t, hApp, newAppender)
	assertSampleCount(t, newChunk, 2, ValFloatHistogram)
}

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

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

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

			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 TestFloatHistogramChunkAppendableGauge(t *testing.T) {
	eh := &histogram.FloatHistogram{
		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: []float64{6, 3, 3, 2, 4, 5, 1},
	}

	cbh := &histogram.FloatHistogram{
		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: []float64{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.FloatHistogram) (Chunk, *FloatHistogramAppender, int64, *histogram.FloatHistogram) {
		c := Chunk(NewFloatHistogramChunk())

		// 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.AppendFloatHistogram(nil, ts, h.Copy(), false)
		require.NoError(t, err)
		require.Nil(t, chk)
		require.Equal(t, 1, c.NumSamples())
		require.Equal(t, GaugeType, c.(*FloatHistogramChunk).GetCounterResetHeader())
		return c, app.(*FloatHistogramAppender), ts, h
	}

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

		assertNewFloatHistogramChunkOnAppend(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)

		assertNewFloatHistogramChunkOnAppend(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 = []float64{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)

		assertRecodedFloatHistogramChunkOnAppend(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 = []float64{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)

		assertNoNewFloatHistogramChunkOnAppend(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 = []float64{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)

		assertRecodedFloatHistogramChunkOnAppend(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 = []float64{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)

		assertNoNewFloatHistogramChunkOnAppend(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 = []float64{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)

		assertRecodedFloatHistogramChunkOnAppend(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 = []float64{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)

		assertRecodedFloatHistogramChunkOnAppend(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)

		assertNoNewFloatHistogramChunkOnAppend(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 = []float64{6, 3, 3, 2, 4, 5, 2}
		_, _, _, _, _, _, ok := hApp.appendableGauge(h2)
		require.True(t, ok)

		assertNoNewFloatHistogramChunkOnAppend(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 = []float64{6, 3, 3, 2, 4, 5, 0}
		_, _, _, _, _, _, ok := hApp.appendableGauge(h2)
		require.True(t, ok)

		assertNoNewFloatHistogramChunkOnAppend(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)

		assertNewFloatHistogramChunkOnAppend(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 = []float64{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.

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

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

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

		h := tsdbutil.GenerateTestFloatHistogram(0)
		var isRecoded bool
		c, isRecoded, app, err = app.AppendFloatHistogram(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.AppendFloatHistogram(nil, 2, h2, true)
		require.Nil(t, c)
		require.False(t, isRecoded)
		require.EqualError(t, err, "float histogram schema change")
	})
	t.Run("counter reset error", func(t *testing.T) {
		c := Chunk(NewFloatHistogramChunk())

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

		h := tsdbutil.GenerateTestFloatHistogram(0)
		var isRecoded bool
		c, isRecoded, app, err = app.AppendFloatHistogram(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.AppendFloatHistogram(nil, 2, h2, true)
		require.Nil(t, c)
		require.False(t, isRecoded)
		require.EqualError(t, err, "float histogram counter reset")
	})
	t.Run("counter reset error with custom buckets", func(t *testing.T) {
		c := Chunk(NewFloatHistogramChunk())

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

		h := tsdbutil.GenerateTestCustomBucketsFloatHistogram(0)
		var isRecoded bool
		c, isRecoded, app, err = app.AppendFloatHistogram(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.AppendFloatHistogram(nil, 2, h2, true)
		require.Nil(t, c)
		require.False(t, isRecoded)
		require.EqualError(t, err, "float histogram counter reset")
	})
}

func TestFloatHistogramUniqueSpansAfterNext(t *testing.T) {
	// Create two histograms with the same schema and spans.
	h1 := &histogram.FloatHistogram{
		Schema:        1,
		ZeroThreshold: 1e-100,
		Count:         10,
		ZeroCount:     2,
		Sum:           15.0,
		PositiveSpans: []histogram.Span{
			{Offset: 0, Length: 2},
			{Offset: 1, Length: 2},
		},
		PositiveBuckets: []float64{1, 2, 3, 4},
		NegativeSpans: []histogram.Span{
			{Offset: 1, Length: 1},
		},
		NegativeBuckets: []float64{2},
	}

	h2 := h1.Copy()

	// Create a chunk and append both histograms.
	c := NewFloatHistogramChunk()
	app, err := c.Appender()
	require.NoError(t, err)

	_, _, _, err = app.AppendFloatHistogram(nil, 0, h1, false)
	require.NoError(t, err)

	_, _, _, err = app.AppendFloatHistogram(nil, 1, h2, false)
	require.NoError(t, err)

	// Create an iterator and advance to the first histogram.
	it := c.Iterator(nil)
	require.Equal(t, ValFloatHistogram, it.Next())
	_, rh1 := it.AtFloatHistogram(nil)

	// Advance to the second histogram and retrieve it.
	require.Equal(t, ValFloatHistogram, it.Next())
	_, rh2 := it.AtFloatHistogram(nil)

	require.Equal(t, rh1.PositiveSpans, h1.PositiveSpans, "Returned positive spans are as expected")
	require.Equal(t, rh1.NegativeSpans, h1.NegativeSpans, "Returned negative spans are as expected")
	require.Equal(t, rh2.PositiveSpans, h1.PositiveSpans, "Returned positive spans are as expected")
	require.Equal(t, rh2.NegativeSpans, h1.NegativeSpans, "Returned negative spans are as expected")

	// Check that the spans for h1 and h2 are unique slices.
	require.NotSame(t, &rh1.PositiveSpans[0], &rh2.PositiveSpans[0], "PositiveSpans should be unique between histograms")
	require.NotSame(t, &rh1.NegativeSpans[0], &rh2.NegativeSpans[0], "NegativeSpans should be unique between histograms")
}