// Copyright 2016 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 local

import (
	"encoding/binary"
	"fmt"
	"io"
	"math"

	"github.com/prometheus/common/model"
)

// Gorilla chunk encoding is inspired by the following paper:
//   Gorilla: A Fast, Scalable, In-Memory Time Series Database
//   T. Pelkonen et al., Facebook Inc.
//   http://www.vldb.org/pvldb/vol8/p1816-teller.pdf
// Note that there are significant differences in detail, some due to the way
// Prometheus chunks work, others to optimize for the Prometheus use-case.
//
// Layout of a 1024 byte gorilla chunk (big endian, wherever it matters):
// - first time (int64):                  8 bytes  bit 0000-0063
// - first value (float64):               8 bytes  bit 0064-0127
// - last time (int64):                   8 bytes  bit 0128-0191
// - last value (float64):                8 bytes  bit 0192-0255
// - first Δt (t1-t0, unsigned):          3 bytes  bit 0256-0279
// - flags (so far just encoding, byte)   1 byte   bit 0280-0287
// - bit offset for next sample           2 bytes  bit 0288-0303
// - first Δv for value encoding 1, otherwise payload
//                                        4 bytes  bit 0304-0335
// - payload                            973 bytes  bit 0336-8119
// The following only exists if the chunk is still open. Otherwise, it might be
// used by payload.
// - bit offset for current ΔΔt=0 count   2 bytes  bit 8120-8135
// - last Δt                              3 bytes  bit 8136-8159
// - special bytes for value encoding     4 bytes  bit 8160-8191
//   - for encoding 1: last Δv            4 bytes  bit 8160-8191
//   - for encoding 2: count of
//     - last leading zeros (1 byte)      1 byte   bit 8160-8167
//     - last significant bits (1 byte)   1 byte   bit 8168-8175
//
// TIMESTAMP ENCODING
//
// The 1st timestamp is saved directly.
//
// The difference to the 2nd timestamp is saved as first Δt. 3 bytes is enough
// for about 4.5h. Since we close a chunk after sitting idle for 1h, this
// limitation has no practical consequences. Should, for whatever reason, a
// larger delta be required, the chunk would be closed and the new sample added
// to a new chunk.
//
// From the 3rd timestamp on, a double-delta (ΔΔt) is saved:
//   (t_{n} - t_{n-1}) - (t_{n-2} - t_{n-1})
// To perform that operation, the last Δt is saved at the end of the chunk for
// as long the chunk is not closed yet (see above).
//
// Most of the times, ΔΔt is zero, even with the ms-precision of
// Prometheus. Therefore, we save a ΔΔt of zero as a leading '0' bit followed by
// 7 bits counting the number of consecutive ΔΔt==0 (the count is offset by -1,
// so the range of 0 to 127 represents 1 to 128 repetitions).
//
// If ΔΔt != 0, we essentially apply the Gorilla scheme verbatim (cf. section
// 4.1.1 in the paper), but with different bit buckets as Prometheus uses ms
// rather than s, and the default scrape interval is 1m rather than 4m). In
// particular:
//
// - If ΔΔt is between [-32,31], store '10' followed by a 6 bit value. This is
//   for minor irregularities in the scrape interval.
//
// - If ΔΔt is between [-65536,65535], store '110' followed by a 17 bit
//   value. This will typically happen if a scrape is missed completely.
//
// - If ΔΔt is betwees [-4194304,4194303], store '111' followed by a 23 bit
//   value.  This spans more than 1h, which is usually enough as we close a
//   chunk anyway if it doesn't receive any sample in 1h.
//
// - Should we nevertheless encounter a larger ΔΔt, we simply close the chunk
//   and overflow into a new chunk.
//
// VALUE ENCODING
//
// Value encoding can change and is determined by the two least significant bits
// of the 'flags' byte at bit position 280. (The remaining bits could be used
// for other flags in the future.) The encoding can be changed without
// transcoding upon adding the 3rd sample. After that, an encoding change
// results either in transcoding or in closing the chunk and overflowing into a
// new chunk.
//
// The 1st sample value is always saved directly. The 2nd sample value is saved
// as the last value. Upon saving the 3rd value, an encoding is chosen, and the
// chunk is prepared accordingly.
//
// The following value encodings exist (with their value in the flags byte):
//
// 0: "Zero encoding".
//
// In many time series, the value simply stays constant over a long time
// (e.g. the "up" time series). In that case, all sample values are determined
// by the 1st value, and no further value encoding is happening at all. The
// payload consists entirely of timestamps.
//
// 1: Integer double-delta encoding.
//
// Many Prometheus metrics are integer counters and change in a quite regular
// fashion, similar to timestamps. Thus, the same double-delta encoding can be
// applied. This encoding works like the timestamp encoding described above, but
// with different bit buckets and without counting of repeated ΔΔv=0. The case
// of ΔΔv=0 is represented by a single '0' bit for each occurrence. The first Δv
// is saved as an int32 at bit position 288. The most recent Δv is saved as an
// int32 at the end of the chunk (see above). If Δv cannot be represented as a
// 32 bit signed integer, no integer double-delta encoding can be applied.
//
// Bit buckets (lead-in bytes followed by (signed) value bits):
// - '0': 0 bit
// - '10': 6 bit
// - '110': 13 bit
// - '1110': 20 bit
// - '1111': 33 bit
// Since Δv is restricted to 32 bit, 33 bit are always enough for ΔΔv.
//
// 2: XOR encoding.
//
// This follows verbatim the Gorilla value encoding (cf. section 4.1.2 of the
// paper). The last count of leading zeros and the last count of meaningful bits
// in the XOR value is saved at the end of the chunk for as long as the chunk is
// not closed yet (see above). Note, though, that the number of significant bits
// is saved as (count-1), i.e. a saved value of 0 means 1 significant bit, a
// saved value of 1 means 2, and so on. Also, we save the numbers of leading
// zeros and significant bits anew if they drop a lot. Otherwise, you can easily
// be locked in with a high number of significant bits.
//
// 3: Direct encoding.
//
// If the sample values are just random, it is most efficient to save sample
// values directly as float64.
//
// ZIPPING TIMESTAMPS AND VALUES TOGETHER
//
// Usually, encoded timestamps and encoded values simply alternate. There are
// two exceptions:
//
// (1) With the "zero encoding" for values, the payload only contains
//     timestamps.
//
// (2) In a consecutive row of up to 128 ΔΔt=0 repeats, the count of timestamps
//     determines how many sample values will follow directly after another.

const (
	gorillaMinLength = 128
	gorillaMaxLength = 8192

	// Useful byte offsets.
	gorillaFirstTimeOffset           = 0
	gorillaFirstValueOffset          = 8
	gorillaLastTimeOffset            = 16
	gorillaLastValueOffset           = 24
	gorillaFirstTimeDeltaOffset      = 32
	gorillaFlagOffset                = 35
	gorillaNextSampleBitOffsetOffset = 36
	gorillaFirstValueDeltaOffset     = 38
	// The following are in the "footer" and only usable if the chunk is
	// still open.
	gorillaCountOffsetBitOffset           = chunkLen - 9
	gorillaLastTimeDeltaOffset            = chunkLen - 7
	gorillaLastValueDeltaOffset           = chunkLen - 4
	gorillaLastLeadingZerosCountOffset    = chunkLen - 4
	gorillaLastSignificantBitsCountOffset = chunkLen - 3

	gorillaFirstSampleBitOffset  uint16 = 0 // Symbolic, don't really read or write here.
	gorillaSecondSampleBitOffset uint16 = 1 // Symbolic, don't really read or write here.
	// gorillaThirdSampleBitOffset is a bit special. Depending on the encoding, there can
	// be various things at this offset. It's most of the time symbolic, but in the best
	// case (zero encoding for values), it will be the real offset for the 3rd sample.
	gorillaThirdSampleBitOffset uint16 = gorillaFirstValueDeltaOffset * 8

	// If the bit offset for the next sample is above this threshold, no new
	// samples can be added to the chunk (because the payload has already
	// reached the footer). The chunk is considered closed.
	gorillaNextSampleBitOffsetThreshold = 8 * gorillaCountOffsetBitOffset

	gorillaMaxTimeDelta = 1 << 24 // What fits into a 3-byte timestamp.
)

type gorillaValueEncoding byte

const (
	gorillaZeroEncoding gorillaValueEncoding = iota
	gorillaIntDoubleDeltaEncoding
	gorillaXOREncoding
	gorillaDirectEncoding
)

// gorillaWorstCaseBitsPerSample provides the worst-case number of bits needed
// per sample with the various value encodings. The counts already include the
// up to 27 bits taken by a timestamp.
var gorillaWorstCaseBitsPerSample = map[gorillaValueEncoding]int{
	gorillaZeroEncoding:           27 + 0,
	gorillaIntDoubleDeltaEncoding: 27 + 38,
	gorillaXOREncoding:            27 + 13 + 64,
	gorillaDirectEncoding:         27 + 64,
}

// gorillaChunk implements the chunk interface.
type gorillaChunk []byte

// newGorillaChunk returns a newly allocated gorillaChunk.  For simplicity, all
// Gorilla chunks must have the length as determined by the chunkLen constant.
func newGorillaChunk(enc gorillaValueEncoding) *gorillaChunk {
	if chunkLen < gorillaMinLength || chunkLen > gorillaMaxLength {
		panic(fmt.Errorf(
			"invalid chunk length of %d bytes , need at least %d bytes and at most %d bytes",
			chunkLen, gorillaMinLength, gorillaMaxLength,
		))
	}
	if enc > gorillaDirectEncoding {
		panic(fmt.Errorf("unknown Gorilla value encoding: %v", enc))
	}
	c := make(gorillaChunk, chunkLen)
	c[gorillaFlagOffset] = byte(enc)
	return &c
}

// add implements chunk.
func (c *gorillaChunk) add(s model.SamplePair) ([]chunk, error) {
	offset := c.nextSampleOffset()
	switch {
	case offset > gorillaNextSampleBitOffsetThreshold:
		return addToOverflowChunk(c, s)
	case offset == gorillaFirstSampleBitOffset:
		return c.addFirstSample(s), nil
	case offset == gorillaSecondSampleBitOffset:
		return c.addSecondSample(s)
	}
	return c.addLaterSample(s, offset)
}

// clone implements chunk.
func (c gorillaChunk) clone() chunk {
	clone := make(gorillaChunk, len(c))
	copy(clone, c)
	return &clone
}

// newIterator implements chunk.
func (c gorillaChunk) newIterator() chunkIterator {
	return newGorillaChunkIterator(c)
}

// marshal implements chunk.
func (c gorillaChunk) marshal(w io.Writer) error {
	n, err := w.Write(c)
	if err != nil {
		return err
	}
	if n != cap(c) {
		return fmt.Errorf("wanted to write %d bytes, wrote %d", cap(c), n)
	}
	return nil
}

// marshalToBuf implements chunk.
func (c gorillaChunk) marshalToBuf(buf []byte) error {
	n := copy(buf, c)
	if n != len(c) {
		return fmt.Errorf("wanted to copy %d bytes to buffer, copied %d", len(c), n)
	}
	return nil
}

// unmarshal implements chunk.
func (c gorillaChunk) unmarshal(r io.Reader) error {
	_, err := io.ReadFull(r, c)
	return err
}

// unmarshalFromBuf implements chunk.
func (c gorillaChunk) unmarshalFromBuf(buf []byte) error {
	if copied := copy(c, buf); copied != cap(c) {
		return fmt.Errorf("insufficient bytes copied from buffer during unmarshaling, want %d, got %d", cap(c), copied)
	}
	return nil
}

// encoding implements chunk.
func (c gorillaChunk) encoding() chunkEncoding { return gorilla }

// firstTime implements chunk.
func (c gorillaChunk) firstTime() model.Time {
	return model.Time(
		binary.BigEndian.Uint64(
			c[gorillaFirstTimeOffset:],
		),
	)
}

func (c gorillaChunk) firstValue() model.SampleValue {
	return model.SampleValue(
		math.Float64frombits(
			binary.BigEndian.Uint64(
				c[gorillaFirstValueOffset:],
			),
		),
	)
}

func (c gorillaChunk) lastTime() model.Time {
	return model.Time(
		binary.BigEndian.Uint64(
			c[gorillaLastTimeOffset:],
		),
	)
}

func (c gorillaChunk) lastValue() model.SampleValue {
	return model.SampleValue(
		math.Float64frombits(
			binary.BigEndian.Uint64(
				c[gorillaLastValueOffset:],
			),
		),
	)
}

func (c gorillaChunk) firstTimeDelta() model.Time {
	// Only the first 3 bytes are actually the timestamp, so get rid of the
	// last one by bitshifting.
	return model.Time(c[gorillaFirstTimeDeltaOffset+2]) |
		model.Time(c[gorillaFirstTimeDeltaOffset+1])<<8 |
		model.Time(c[gorillaFirstTimeDeltaOffset])<<16
}

// firstValueDelta returns an undefined result if the encoding type is not 1.
func (c gorillaChunk) firstValueDelta() int32 {
	return int32(binary.BigEndian.Uint32(c[gorillaFirstValueDeltaOffset:]))
}

// lastTimeDelta returns an undefined result if the chunk is closed already.
func (c gorillaChunk) lastTimeDelta() model.Time {
	return model.Time(c[gorillaLastTimeDeltaOffset+2]) |
		model.Time(c[gorillaLastTimeDeltaOffset+1])<<8 |
		model.Time(c[gorillaLastTimeDeltaOffset])<<16
}

// setLastTimeDelta must not be called if the chunk is closed already. It most
// not be called with a time that doesn't fit into 24bit, either.
func (c gorillaChunk) setLastTimeDelta(dT model.Time) {
	if dT > gorillaMaxTimeDelta {
		panic("Δt overflows 24 bit")
	}
	c[gorillaLastTimeDeltaOffset] = byte(dT >> 16)
	c[gorillaLastTimeDeltaOffset+1] = byte(dT >> 8)
	c[gorillaLastTimeDeltaOffset+2] = byte(dT)
}

// lastValueDelta returns an undefined result if the chunk is closed already.
func (c gorillaChunk) lastValueDelta() int32 {
	return int32(binary.BigEndian.Uint32(c[gorillaLastValueDeltaOffset:]))
}

// setLastValueDelta must not be called if the chunk is closed already.
func (c gorillaChunk) setLastValueDelta(dV int32) {
	binary.BigEndian.PutUint32(c[gorillaLastValueDeltaOffset:], uint32(dV))
}

func (c gorillaChunk) nextSampleOffset() uint16 {
	return binary.BigEndian.Uint16(c[gorillaNextSampleBitOffsetOffset:])
}

func (c gorillaChunk) setNextSampleOffset(offset uint16) {
	binary.BigEndian.PutUint16(c[gorillaNextSampleBitOffsetOffset:], offset)
}

func (c gorillaChunk) zeroDDTRepeats() (repeats uint64, offset uint16) {
	offset = binary.BigEndian.Uint16(c[gorillaCountOffsetBitOffset:])
	if offset == 0 {
		return 0, 0
	}
	return c.readBitPattern(offset, 7) + 1, offset
}

func (c gorillaChunk) setZeroDDTRepeats(repeats uint64, offset uint16) {
	switch repeats {
	case 0:
		// Just clear the offset.
		binary.BigEndian.PutUint16(c[gorillaCountOffsetBitOffset:], 0)
		return
	case 1:
		// First time we set a repeat here, so set the offset. But only
		// if we haven't reached the footer yet. (If that's the case, we
		// would overwrite ourselves below, and we don't need the offset
		// later anyway because no more samples will be added to this
		// chunk.)
		if offset+7 <= gorillaNextSampleBitOffsetThreshold {
			binary.BigEndian.PutUint16(c[gorillaCountOffsetBitOffset:], offset)
		}
	default:
		// For a change, we are writing somewhere where we have written
		// before. We need to clear the bits first.
		posIn1stByte := offset % 8
		c[offset/8] &^= bitMask[7][posIn1stByte]
		if posIn1stByte > 1 {
			c[offset/8+1] &^= bitMask[posIn1stByte-1][0]
		}
	}
	c.addBitPattern(offset, repeats-1, 7)
}

func (c gorillaChunk) setLastSample(s model.SamplePair) {
	binary.BigEndian.PutUint64(
		c[gorillaLastTimeOffset:],
		uint64(s.Timestamp),
	)
	binary.BigEndian.PutUint64(
		c[gorillaLastValueOffset:],
		math.Float64bits(float64(s.Value)),
	)
}

// addFirstSample is a helper method only used by c.add(). It adds timestamp and
// value as base time and value.
func (c *gorillaChunk) addFirstSample(s model.SamplePair) []chunk {
	binary.BigEndian.PutUint64(
		(*c)[gorillaFirstTimeOffset:],
		uint64(s.Timestamp),
	)
	binary.BigEndian.PutUint64(
		(*c)[gorillaFirstValueOffset:],
		math.Float64bits(float64(s.Value)),
	)
	c.setLastSample(s) // To simplify handling of single-sample chunks.
	c.setNextSampleOffset(gorillaSecondSampleBitOffset)
	return []chunk{c}
}

// addSecondSample is a helper method only used by c.add(). It calculates the
// first time delta from the provided sample and adds it to the chunk together
// with the provided sample as the last sample.
func (c *gorillaChunk) addSecondSample(s model.SamplePair) ([]chunk, error) {
	firstTimeDelta := s.Timestamp - c.firstTime()
	if firstTimeDelta < 0 {
		return nil, fmt.Errorf("first Δt is less than zero: %v", firstTimeDelta)
	}
	if firstTimeDelta > gorillaMaxTimeDelta {
		return addToOverflowChunk(c, s)
	}
	(*c)[gorillaFirstTimeDeltaOffset] = byte(firstTimeDelta >> 16)
	(*c)[gorillaFirstTimeDeltaOffset+1] = byte(firstTimeDelta >> 8)
	(*c)[gorillaFirstTimeDeltaOffset+2] = byte(firstTimeDelta)

	// Also set firstTimeDelta as the last time delta to be able to use the
	// normal methods for adding later samples.
	c.setLastTimeDelta(firstTimeDelta)

	c.setLastSample(s)
	c.setNextSampleOffset(gorillaThirdSampleBitOffset)
	return []chunk{c}, nil
}

// addLaterSample is a helper method only used by c.add(). It adds a third or
// later sample.
func (c *gorillaChunk) addLaterSample(s model.SamplePair, offset uint16) ([]chunk, error) {
	var (
		lastTime      = c.lastTime()
		lastTimeDelta = c.lastTimeDelta()
		newTimeDelta  = s.Timestamp - lastTime
		lastValue     = c.lastValue()
		encoding      = gorillaValueEncoding((*c)[gorillaFlagOffset])
	)

	if newTimeDelta < 0 {
		return nil, fmt.Errorf("Δt is less than zero: %v", newTimeDelta)
	}
	if newTimeDelta > gorillaMaxTimeDelta {
		return addToOverflowChunk(c, s)
	}

	if offset == gorillaThirdSampleBitOffset {
		offset, encoding = c.prepForThirdSample(lastValue, s.Value, encoding)
	}

	// Analyze worst case, does it fit? If not, overflow into new chunk.
	if int(offset)+gorillaWorstCaseBitsPerSample[encoding] > chunkLen*8 {
		return addToOverflowChunk(c, s)
	}

	// Transcoding/overflow decisions first.
	if encoding == gorillaZeroEncoding && s.Value != lastValue {
		// Cannot go on with zero encoding.
		if offset > chunkLen*4 {
			// Chunk already half full. Don't transcode, overflow instead.
			return addToOverflowChunk(c, s)
		}
		if isInt32(s.Value - lastValue) {
			// Trying int encoding looks promising.
			return transcodeAndAdd(newGorillaChunk(gorillaIntDoubleDeltaEncoding), c, s)
		}
		return transcodeAndAdd(newGorillaChunk(gorillaXOREncoding), c, s)
	}
	if encoding == gorillaIntDoubleDeltaEncoding && !isInt32(s.Value-lastValue) {
		// Cannot go on with int encoding.
		if offset > chunkLen*4 {
			// Chunk already half full. Don't transcode, overflow instead.
			return addToOverflowChunk(c, s)
		}
		return transcodeAndAdd(newGorillaChunk(gorillaXOREncoding), c, s)
	}

	offset, overflow := c.addDDTime(offset, lastTimeDelta, newTimeDelta)
	if overflow {
		return addToOverflowChunk(c, s)
	}
	switch encoding {
	case gorillaZeroEncoding:
		// Nothing to do.
	case gorillaIntDoubleDeltaEncoding:
		offset = c.addDDValue(offset, lastValue, s.Value)
	case gorillaXOREncoding:
		offset = c.addXORValue(offset, lastValue, s.Value)
	case gorillaDirectEncoding:
		offset = c.addBitPattern(offset, math.Float64bits(float64(s.Value)), 64)
	default:
		return nil, fmt.Errorf("unknown Gorilla value encoding: %v", encoding)
	}

	c.setNextSampleOffset(offset)
	c.setLastSample(s)
	return []chunk{c}, nil
}

func (c gorillaChunk) prepForThirdSample(
	lastValue, newValue model.SampleValue, encoding gorillaValueEncoding,
) (uint16, gorillaValueEncoding) {
	var (
		offset                   = gorillaThirdSampleBitOffset
		firstValue               = c.firstValue()
		firstValueDelta          = lastValue - firstValue
		firstXOR                 = math.Float64bits(float64(firstValue)) ^ math.Float64bits(float64(lastValue))
		_, firstSignificantBits  = countBits(firstXOR)
		secondXOR                = math.Float64bits(float64(lastValue)) ^ math.Float64bits(float64(newValue))
		_, secondSignificantBits = countBits(secondXOR)
	)
	// Now pick an initial encoding and prepare things accordingly.
	// However, never pick an encoding "below" the one initially set.
	switch {
	case encoding == gorillaZeroEncoding && lastValue == firstValue && lastValue == newValue:
		// Stay at zero encoding.
		// No value to be set.
		// No offset change required.
	case encoding <= gorillaIntDoubleDeltaEncoding && isInt32(firstValueDelta):
		encoding = gorillaIntDoubleDeltaEncoding
		binary.BigEndian.PutUint32(
			c[gorillaFirstValueDeltaOffset:],
			uint32(int32(firstValueDelta)),
		)
		c.setLastValueDelta(int32(firstValueDelta))
		offset += 32
	case encoding == gorillaDirectEncoding || firstSignificantBits+secondSignificantBits > 100:
		// Heuristics based on three samples only is a bit weak,
		// but if we need 50+13 = 63 bits per sample already
		// now, we might be better off going for direct encoding.
		encoding = gorillaDirectEncoding
		// Put bit pattern directly where otherwise the delta would have gone.
		binary.BigEndian.PutUint64(
			c[gorillaFirstValueDeltaOffset:],
			math.Float64bits(float64(lastValue)),
		)
		offset += 64
	default:
		encoding = gorillaXOREncoding
		offset = c.addXORValue(offset, firstValue, lastValue)
	}
	c[gorillaFlagOffset] = byte(encoding)
	c.setNextSampleOffset(offset)
	return offset, encoding
}

// addDDTime requires that lastTimeDelta and newTimeDelta are positive and don't overflow 24bit.
func (c gorillaChunk) addDDTime(offset uint16, lastTimeDelta, newTimeDelta model.Time) (newOffset uint16, overflow bool) {
	timeDD := newTimeDelta - lastTimeDelta

	if !isSignedIntN(int64(timeDD), 23) {
		return offset, true
	}

	c.setLastTimeDelta(newTimeDelta)
	repeats, repeatsOffset := c.zeroDDTRepeats()

	if timeDD == 0 {
		if repeats == 0 || repeats == 128 {
			// First zeroDDT, or counter full, prepare new counter.
			offset = c.addZeroBit(offset)
			repeatsOffset = offset
			offset += 7
			repeats = 0
		}
		c.setZeroDDTRepeats(repeats+1, repeatsOffset)
		return offset, false
	}

	// No zero repeat. If we had any before, clear the DDT offset.
	c.setZeroDDTRepeats(0, repeatsOffset)

	switch {
	case isSignedIntN(int64(timeDD), 6):
		offset = c.addOneBitsWithTrailingZero(offset, 1)
		offset = c.addSignedInt(offset, int64(timeDD), 6)
	case isSignedIntN(int64(timeDD), 17):
		offset = c.addOneBitsWithTrailingZero(offset, 2)
		offset = c.addSignedInt(offset, int64(timeDD), 17)
	case isSignedIntN(int64(timeDD), 23):
		offset = c.addOneBits(offset, 3)
		offset = c.addSignedInt(offset, int64(timeDD), 23)
	default:
		panic("unexpected required bits for ΔΔt")
	}
	return offset, false
}

// addDDValue requires that newValue-lastValue can be represented with an int32.
func (c gorillaChunk) addDDValue(offset uint16, lastValue, newValue model.SampleValue) uint16 {
	newValueDelta := int64(newValue - lastValue)
	lastValueDelta := c.lastValueDelta()
	valueDD := newValueDelta - int64(lastValueDelta)
	c.setLastValueDelta(int32(newValueDelta))

	switch {
	case valueDD == 0:
		return c.addZeroBit(offset)
	case isSignedIntN(valueDD, 6):
		offset = c.addOneBitsWithTrailingZero(offset, 1)
		return c.addSignedInt(offset, valueDD, 6)
	case isSignedIntN(valueDD, 13):
		offset = c.addOneBitsWithTrailingZero(offset, 2)
		return c.addSignedInt(offset, valueDD, 13)
	case isSignedIntN(valueDD, 20):
		offset = c.addOneBitsWithTrailingZero(offset, 3)
		return c.addSignedInt(offset, valueDD, 20)
	case isSignedIntN(valueDD, 33):
		offset = c.addOneBits(offset, 4)
		return c.addSignedInt(offset, valueDD, 33)
	default:
		panic("unexpected required bits for ΔΔv")
	}
}

func (c gorillaChunk) addXORValue(offset uint16, lastValue, newValue model.SampleValue) uint16 {
	lastPattern := math.Float64bits(float64(lastValue))
	newPattern := math.Float64bits(float64(newValue))
	xor := lastPattern ^ newPattern
	if xor == 0 {
		return c.addZeroBit(offset)
	}

	lastLeadingBits := c[gorillaLastLeadingZerosCountOffset]
	lastSignificantBits := c[gorillaLastSignificantBitsCountOffset]
	newLeadingBits, newSignificantBits := countBits(xor)

	// Short entry if the new significant bits fit into the same box as the
	// last significant bits.  However, should the new significant bits be
	// shorter by 10 or more, go for a long entry instead, as we will
	// probably save more (11 bit one-time overhead, potentially more to
	// save later).
	if newLeadingBits >= lastLeadingBits &&
		newLeadingBits+newSignificantBits <= lastLeadingBits+lastSignificantBits &&
		lastSignificantBits-newSignificantBits < 10 {
		offset = c.addOneBitsWithTrailingZero(offset, 1)
		return c.addBitPattern(
			offset,
			xor>>(64-lastLeadingBits-lastSignificantBits),
			uint16(lastSignificantBits),
		)
	}

	// Long entry.
	c[gorillaLastLeadingZerosCountOffset] = newLeadingBits
	c[gorillaLastSignificantBitsCountOffset] = newSignificantBits
	offset = c.addOneBits(offset, 2)
	offset = c.addBitPattern(offset, uint64(newLeadingBits), 5)
	offset = c.addBitPattern(offset, uint64(newSignificantBits-1), 6) // Note -1!
	return c.addBitPattern(
		offset,
		xor>>(64-newLeadingBits-newSignificantBits),
		uint16(newSignificantBits),
	)
}

func (c gorillaChunk) addZeroBit(offset uint16) uint16 {
	if offset < gorillaNextSampleBitOffsetThreshold {
		// Writing a zero to a never touched area is a no-op.
		// Just increase the offset.
		return offset + 1
	}
	c[offset/8] &^= bitMask[1][offset%8]
	return offset + 1
}

func (c gorillaChunk) addOneBits(offset uint16, n uint16) uint16 {
	if n > 7 {
		panic("unexpected number of control bits")
	}
	b := 8 - offset%8
	if b > n {
		b = n
	}
	c[offset/8] |= bitMask[b][offset%8]
	offset += b
	b = n - b
	if b > 0 {
		c[offset/8] |= bitMask[b][0]
		offset += b
	}
	return offset
}
func (c gorillaChunk) addOneBitsWithTrailingZero(offset uint16, n uint16) uint16 {
	offset = c.addOneBits(offset, n)
	return c.addZeroBit(offset)
}

// addSignedInt adds i as a signed integer with n bits. It requires i to be
// representable as such. (Check with isSignedIntN first.)
func (c gorillaChunk) addSignedInt(offset uint16, i int64, n uint16) uint16 {
	if i < 0 && n < 64 {
		i += 1 << n
	}
	return c.addBitPattern(offset, uint64(i), n)
}

// addBitPattern adds the last n bits of the given pattern. Other bits in the
// pattern must be 0.
func (c gorillaChunk) addBitPattern(offset uint16, pattern uint64, n uint16) uint16 {
	var (
		byteOffset  = offset / 8
		bitsToWrite = 8 - offset%8
		newOffset   = offset + n
	)

	// Clean up the parts of the footer we will write into. (But not more as
	// we are still using the value related part of the footer when we have
	// already overwritten timestamp related parts.)
	if newOffset > gorillaNextSampleBitOffsetThreshold {
		pos := offset
		if pos < gorillaNextSampleBitOffsetThreshold {
			pos = gorillaNextSampleBitOffsetThreshold
		}
		for pos < newOffset {
			posInByte := pos % 8
			bitsToClear := newOffset - pos
			if bitsToClear > 8-posInByte {
				bitsToClear = 8 - posInByte
			}
			c[pos/8] &^= bitMask[bitsToClear][posInByte]
			pos += bitsToClear
		}
	}

	for n > 0 {
		if n <= bitsToWrite {
			c[byteOffset] |= byte(pattern << (bitsToWrite - n))
			break
		}
		c[byteOffset] |= byte(pattern >> (n - bitsToWrite))
		n -= bitsToWrite
		bitsToWrite = 8
		byteOffset++
	}
	return newOffset
}

// readBitPattern reads n bits at the given offset and returns them as the last
// n bits in a uint64.
func (c gorillaChunk) readBitPattern(offset, n uint16) uint64 {
	var (
		result                   uint64
		byteOffset               = offset / 8
		bitOffset                = offset % 8
		trailingBits, bitsToRead uint16
	)

	for n > 0 {
		trailingBits = 0
		bitsToRead = 8 - bitOffset
		if bitsToRead > n {
			trailingBits = bitsToRead - n
			bitsToRead = n
		}
		result <<= bitsToRead
		result |= uint64(
			(c[byteOffset] & bitMask[bitsToRead][bitOffset]) >> trailingBits,
		)
		n -= bitsToRead
		byteOffset++
		bitOffset = 0
	}
	return result
}

type gorillaChunkIterator struct {
	c                    gorillaChunk
	pos, len             uint16
	t, dT                model.Time
	repeats              byte // Repeats of ΔΔt=0.
	v                    model.SampleValue
	dV                   int64 // Only used for int value encoding.
	leading, significant uint16
	enc                  gorillaValueEncoding
	lastError            error
	rewound              bool
	nextT                model.Time        // Only for rewound state.
	nextV                model.SampleValue // Only for rewound state.
}

func newGorillaChunkIterator(c gorillaChunk) *gorillaChunkIterator {
	return &gorillaChunkIterator{
		c:           c,
		len:         c.nextSampleOffset(),
		t:           model.Earliest,
		enc:         gorillaValueEncoding(c[gorillaFlagOffset]),
		significant: 1,
	}
}

// lastTimestamp implements chunkIterator.
func (it *gorillaChunkIterator) lastTimestamp() (model.Time, error) {
	if it.len == gorillaFirstSampleBitOffset {
		// No samples in the chunk yet.
		return model.Earliest, it.lastError
	}
	return it.c.lastTime(), it.lastError
}

// contains implements chunkIterator.
func (it *gorillaChunkIterator) contains(t model.Time) (bool, error) {
	last, err := it.lastTimestamp()
	if err != nil {
		it.lastError = err
		return false, err
	}
	return !t.Before(it.c.firstTime()) &&
		!t.After(last), it.lastError
}

// scan implements chunkIterator.
func (it *gorillaChunkIterator) scan() bool {
	if it.lastError != nil {
		return false
	}
	if it.rewound {
		it.t = it.nextT
		it.v = it.nextV
		it.rewound = false
		return true
	}
	if it.pos >= it.len && it.repeats == 0 {
		return false
	}
	if it.pos == gorillaFirstSampleBitOffset {
		it.t = it.c.firstTime()
		it.v = it.c.firstValue()
		it.pos = gorillaSecondSampleBitOffset
		return it.lastError == nil
	}
	if it.pos == gorillaSecondSampleBitOffset {
		if it.len == gorillaThirdSampleBitOffset {
			// Special case: Chunk has only two samples.
			it.t = it.c.lastTime()
			it.v = it.c.lastValue()
			it.pos = it.len
			return it.lastError == nil
		}
		it.dT = it.c.firstTimeDelta()
		it.t += it.dT
		// Value depends on encoding.
		switch it.enc {
		case gorillaZeroEncoding:
			it.pos = gorillaThirdSampleBitOffset
		case gorillaIntDoubleDeltaEncoding:
			it.dV = int64(it.c.firstValueDelta())
			it.v += model.SampleValue(it.dV)
			it.pos = gorillaThirdSampleBitOffset + 32
		case gorillaXOREncoding:
			it.pos = gorillaThirdSampleBitOffset
			it.readXOR()
		case gorillaDirectEncoding:
			it.v = model.SampleValue(math.Float64frombits(
				binary.BigEndian.Uint64(it.c[gorillaThirdSampleBitOffset/8:]),
			))
			it.pos = gorillaThirdSampleBitOffset + 64
		default:
			it.lastError = fmt.Errorf("unknown Gorilla value encoding: %v", it.enc)
		}
		return it.lastError == nil
	}
	// 3rd sample or later does not have special cases anymore.
	it.readDDT()
	switch it.enc {
	case gorillaZeroEncoding:
		// Do nothing.
	case gorillaIntDoubleDeltaEncoding:
		it.readDDV()
	case gorillaXOREncoding:
		it.readXOR()
	case gorillaDirectEncoding:
		it.v = model.SampleValue(math.Float64frombits(it.readBitPattern(64)))
		return it.lastError == nil
	default:
		it.lastError = fmt.Errorf("unknown Gorilla value encoding: %v", it.enc)
		return false
	}
	return it.lastError == nil
}

// findAtOrBefore implements chunkIterator.
func (it *gorillaChunkIterator) findAtOrBefore(t model.Time) bool {
	if it.len == 0 || t.Before(it.c.firstTime()) {
		return false
	}
	last := it.c.lastTime()
	if !t.Before(last) {
		it.t = last
		it.v = it.c.lastValue()
		it.pos = it.len
		return true
	}
	if t == it.t {
		return it.lastError == nil
	}
	if t.Before(it.t) || it.rewound {
		it.reset()
	}

	var (
		prevT = model.Earliest
		prevV model.SampleValue
	)
	for it.scan() && t.After(it.t) {
		prevT = it.t
		prevV = it.v
		// TODO(beorn7): If we are in a repeat, we could iterate forward
		// much faster.
	}
	if t == it.t {
		return it.lastError == nil
	}
	it.rewind(prevT, prevV)
	return it.lastError == nil
}

// findAtOrAfter implements chunkIterator.
func (it *gorillaChunkIterator) findAtOrAfter(t model.Time) bool {
	if it.len == 0 || t.After(it.c.lastTime()) {
		return false
	}
	first := it.c.firstTime()
	if !t.After(first) {
		it.reset()
		return it.scan()
	}
	if t == it.t {
		return it.lastError == nil
	}
	if t.Before(it.t) {
		it.reset()
	}
	for it.scan() && t.After(it.t) {
		// TODO(beorn7): If we are in a repeat, we could iterate forward
		// much faster.
	}
	return it.lastError == nil
}

// value implements chunkIterator.
func (it *gorillaChunkIterator) value() model.SamplePair {
	return model.SamplePair{
		Timestamp: it.t,
		Value:     it.v,
	}
}

// err implements chunkIterator.
func (it *gorillaChunkIterator) err() error {
	return it.lastError
}

func (it *gorillaChunkIterator) readDDT() {
	if it.repeats > 0 {
		it.repeats--
	} else {
		switch it.readControlBits(3) {
		case 0:
			it.repeats = byte(it.readBitPattern(7))
		case 1:
			it.dT += model.Time(it.readSignedInt(6))
		case 2:
			it.dT += model.Time(it.readSignedInt(17))
		case 3:
			it.dT += model.Time(it.readSignedInt(23))
		default:
			panic("unexpected number of control bits")
		}
	}
	it.t += it.dT
}

func (it *gorillaChunkIterator) readDDV() {
	switch it.readControlBits(4) {
	case 0:
		// Do nothing.
	case 1:
		it.dV += it.readSignedInt(6)
	case 2:
		it.dV += it.readSignedInt(13)
	case 3:
		it.dV += it.readSignedInt(20)
	case 4:
		it.dV += it.readSignedInt(33)
	default:
		panic("unexpected number of control bits")
	}
	it.v += model.SampleValue(it.dV)
}

func (it *gorillaChunkIterator) readXOR() {
	switch it.readControlBits(2) {
	case 0:
		return
	case 1:
		// Do nothing right now. All done below.
	case 2:
		it.leading = uint16(it.readBitPattern(5))
		it.significant = uint16(it.readBitPattern(6)) + 1
	default:
		panic("unexpected number of control bits")
	}
	pattern := math.Float64bits(float64(it.v))
	pattern ^= it.readBitPattern(it.significant) << (64 - it.significant - it.leading)
	it.v = model.SampleValue(math.Float64frombits(pattern))
}

// readControlBits reads successive 1-bits and stops after reading the first
// 0-bit. It also stops once it has read max bits. It returns the number of read
// 1-bits.
func (it *gorillaChunkIterator) readControlBits(max uint16) uint16 {
	var count uint16
	for count < max && int(it.pos/8) < len(it.c) {
		b := it.c[it.pos/8] & bitMask[1][it.pos%8]
		it.pos++
		if b == 0 {
			return count
		}
		count++
	}
	if int(it.pos/8) >= len(it.c) {
		it.lastError = errChunkBoundsExceeded
	}
	return count
}

func (it *gorillaChunkIterator) readBitPattern(n uint16) uint64 {
	if len(it.c)*8 < int(it.pos)+int(n) {
		it.lastError = errChunkBoundsExceeded
		return 0
	}
	u := it.c.readBitPattern(it.pos, n)
	it.pos += n
	return u
}

func (it *gorillaChunkIterator) readSignedInt(n uint16) int64 {
	u := it.readBitPattern(n)
	if n < 64 && u >= 1<<(n-1) {
		u -= 1 << n
	}
	return int64(u)
}

// reset puts the chunk iterator into the state it had upon creation.
func (it *gorillaChunkIterator) reset() {
	it.pos = 0
	it.t = model.Earliest
	it.dT = 0
	it.repeats = 0
	it.v = 0
	it.dV = 0
	it.leading = 0
	it.significant = 1
	it.rewound = false
}

// rewind "rewinds" the chunk iterator by one step. Since one cannot simply
// rewind a Gorilla chunk, the old values have to be provided by the
// caller. Rewinding an already rewound chunk panics. After a call of scan or
// reset, a chunk can be rewound again.
func (it *gorillaChunkIterator) rewind(t model.Time, v model.SampleValue) {
	if it.rewound {
		panic("cannot rewind Gorilla chunk twice")
	}
	it.rewound = true
	it.nextT = it.t
	it.nextV = it.v
	it.t = t
	it.v = v
}