EliasFanoCoding.h

/*
 * Copyright 2013-present Facebook, Inc.
 *
 * 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.
 */

/**
 * @author Philip Pronin (philipp@fb.com)
 *
 * Based on the paper by Sebastiano Vigna,
 * "Quasi-succinct indices" (arxiv:1206.4300).
 */

#pragma once

#include <algorithm>
#include <cstdlib>
#include <limits>
#include <type_traits>

#include <folly/Likely.h>
#include <folly/Portability.h>
#include <folly/Range.h>
#include <folly/experimental/CodingDetail.h>
#include <folly/experimental/Instructions.h>
#include <folly/experimental/Select64.h>
#include <folly/lang/Assume.h>
#include <folly/lang/Bits.h>
#include <glog/logging.h>

#if !FOLLY_X64
#error EliasFanoCoding.h requires x86_64
#endif

namespace folly {
namespace compression {

static_assert(kIsLittleEndian, "EliasFanoCoding.h requires little endianness");

constexpr size_t kCacheLineSize = 64;

template <class Pointer>
struct EliasFanoCompressedListBase {
  EliasFanoCompressedListBase() = default;

  template <class OtherPointer>
  EliasFanoCompressedListBase(
      const EliasFanoCompressedListBase<OtherPointer>& other)
      : size(other.size),
        numLowerBits(other.numLowerBits),
        data(other.data),
        skipPointers(reinterpret_cast<Pointer>(other.skipPointers)),
        forwardPointers(reinterpret_cast<Pointer>(other.forwardPointers)),
        lower(reinterpret_cast<Pointer>(other.lower)),
        upper(reinterpret_cast<Pointer>(other.upper)) {}

  template <class T = Pointer>
  auto free() -> decltype(::free(T(nullptr))) {
    return ::free(data.data());
  }

  size_t upperSize() const {
    return size_t(data.end() - upper);
  }

  size_t size = 0;
  uint8_t numLowerBits = 0;

  // WARNING: EliasFanoCompressedList has no ownership of data. The 7
  // bytes following the last byte should be readable.
  folly::Range<Pointer> data;

  Pointer skipPointers = nullptr;
  Pointer forwardPointers = nullptr;
  Pointer lower = nullptr;
  Pointer upper = nullptr;
};

typedef EliasFanoCompressedListBase<const uint8_t*> EliasFanoCompressedList;
typedef EliasFanoCompressedListBase<uint8_t*> MutableEliasFanoCompressedList;

template <
    class Value,
    class SkipValue = size_t,
    size_t kSkipQuantum = 0, // 0 = disabled
    size_t kForwardQuantum = 0> // 0 = disabled
struct EliasFanoEncoderV2 {
  static_assert(
      std::is_integral<Value>::value && std::is_unsigned<Value>::value,
      "Value should be unsigned integral");

  typedef EliasFanoCompressedList CompressedList;
  typedef MutableEliasFanoCompressedList MutableCompressedList;

  typedef Value ValueType;
  typedef SkipValue SkipValueType;
  struct Layout;

  static constexpr size_t skipQuantum = kSkipQuantum;
  static constexpr size_t forwardQuantum = kForwardQuantum;

  static uint8_t defaultNumLowerBits(size_t upperBound, size_t size) {
    if (UNLIKELY(size == 0 || upperBound < size)) {
      return 0;
    }
    // Result that should be returned is "floor(log(upperBound / size))".
    // In order to avoid expensive division, we rely on
    // "floor(a) - floor(b) - 1 <= floor(a - b) <= floor(a) - floor(b)".
    // Assuming "candidate = floor(log(upperBound)) - floor(log(upperBound))",
    // then result is either "candidate - 1" or "candidate".
    auto candidate = folly::findLastSet(upperBound) - folly::findLastSet(size);
    // NOTE: As size != 0, "candidate" is always < 64.
    return (size > (upperBound >> candidate)) ? candidate - 1 : candidate;
  }

  // Requires: input range (begin, end) is sorted (encoding
  // crashes if it's not).
  // WARNING: encode() mallocates EliasFanoCompressedList::data. As
  // EliasFanoCompressedList has no ownership of it, you need to call
  // free() explicitly.
  template <class RandomAccessIterator>
  static MutableCompressedList encode(
      RandomAccessIterator begin,
      RandomAccessIterator end) {
    if (begin == end) {
      return MutableCompressedList();
    }
    EliasFanoEncoderV2 encoder(size_t(end - begin), *(end - 1));
    for (; begin != end; ++begin) {
      encoder.add(*begin);
    }
    return encoder.finish();
  }

  explicit EliasFanoEncoderV2(const MutableCompressedList& result)
      : lower_(result.lower),
        upper_(result.upper),
        skipPointers_(reinterpret_cast<SkipValueType*>(result.skipPointers)),
        forwardPointers_(
            reinterpret_cast<SkipValueType*>(result.forwardPointers)),
        result_(result) {
    std::fill(result.data.begin(), result.data.end(), '\0');
  }

  EliasFanoEncoderV2(size_t size, ValueType upperBound)
      : EliasFanoEncoderV2(
            Layout::fromUpperBoundAndSize(upperBound, size).allocList()) {}

  void add(ValueType value) {
    CHECK_LT(value, std::numeric_limits<ValueType>::max());
    CHECK_GE(value, lastValue_);

    const auto numLowerBits = result_.numLowerBits;
    const ValueType upperBits = value >> numLowerBits;

    // Upper sequence consists of upperBits 0-bits and (size_ + 1) 1-bits.
    const size_t pos = upperBits + size_;
    upper_[pos / 8] |= 1U << (pos % 8);
    // Append numLowerBits bits to lower sequence.
    if (numLowerBits != 0) {
      const ValueType lowerBits = value & ((ValueType(1) << numLowerBits) - 1);
      writeBits56(lower_, size_ * numLowerBits, numLowerBits, lowerBits);
    }

    if /* constexpr */ (skipQuantum != 0) {
      while ((skipPointersSize_ + 1) * skipQuantum <= upperBits) {
        // Store the number of preceding 1-bits.
        skipPointers_[skipPointersSize_++] = SkipValue(size_);
      }
    }

    if /* constexpr */ (forwardQuantum != 0) {
      if ((size_ + 1) % forwardQuantum == 0) {
        const auto k = size_ / forwardQuantum;
        // Store the number of preceding 0-bits.
        forwardPointers_[k] = upperBits;
      }
    }

    lastValue_ = value;
    ++size_;
  }

  const MutableCompressedList& finish() const {
    CHECK_EQ(size_, result_.size);
    return result_;
  }

 private:
  // Writes value (with len up to 56 bits) to data starting at pos-th bit.
  static void
  writeBits56(unsigned char* data, size_t pos, uint8_t len, uint64_t value) {
    DCHECK_LE(uint32_t(len), 56);
    DCHECK_EQ(0, value & ~((uint64_t(1) << len) - 1));
    unsigned char* const ptr = data + (pos / 8);
    uint64_t ptrv = folly::loadUnaligned<uint64_t>(ptr);
    ptrv |= value << (pos % 8);
    folly::storeUnaligned<uint64_t>(ptr, ptrv);
  }

  unsigned char* lower_ = nullptr;
  unsigned char* upper_ = nullptr;
  SkipValueType* skipPointers_ = nullptr;
  SkipValueType* forwardPointers_ = nullptr;

  ValueType lastValue_ = 0;
  size_t size_ = 0;
  size_t skipPointersSize_ = 0;

  MutableCompressedList result_;
};

template <
    class Value,
    class SkipValue,
    size_t kSkipQuantum,
    size_t kForwardQuantum>
struct EliasFanoEncoderV2<Value, SkipValue, kSkipQuantum, kForwardQuantum>::
    Layout {
  static Layout fromUpperBoundAndSize(size_t upperBound, size_t size) {
    // numLowerBits can be at most 56 because of detail::writeBits56.
    const uint8_t numLowerBits =
        std::min(defaultNumLowerBits(upperBound, size), uint8_t(56));
    // *** Upper bits.
    // Upper bits are stored using unary delta encoding.
    // For example, (3 5 5 9) will be encoded as 1000011001000_2.
    const size_t upperSizeBits =
        (upperBound >> numLowerBits) + // Number of 0-bits to be stored.
        size; // 1-bits.
    const size_t upper = (upperSizeBits + 7) / 8;

    // *** Validity checks.
    // Shift by numLowerBits must be valid.
    CHECK_LT(numLowerBits, 8 * sizeof(Value));
    CHECK_LT(size, std::numeric_limits<SkipValueType>::max());
    CHECK_LT(
        upperBound >> numLowerBits, std::numeric_limits<SkipValueType>::max());

    return fromInternalSizes(numLowerBits, upper, size);
  }

  static Layout
  fromInternalSizes(uint8_t numLowerBits, size_t upper, size_t size) {
    Layout layout;
    layout.size = size;
    layout.numLowerBits = numLowerBits;

    layout.lower = (numLowerBits * size + 7) / 8;
    layout.upper = upper;

    // *** Skip pointers.
    // Store (1-indexed) position of every skipQuantum-th
    // 0-bit in upper bits sequence.
    if /* constexpr */ (skipQuantum != 0) {
      // 8 * upper is used here instead of upperSizeBits, as that is
      // more serialization-friendly way (upperSizeBits doesn't need
      // to be known by this function, unlike upper).

      size_t numSkipPointers = (8 * upper - size) / skipQuantum;
      layout.skipPointers = numSkipPointers * sizeof(SkipValueType);
    }

    // *** Forward pointers.
    // Store (1-indexed) position of every forwardQuantum-th
    // 1-bit in upper bits sequence.
    if /* constexpr */ (forwardQuantum != 0) {
      size_t numForwardPointers = size / forwardQuantum;
      layout.forwardPointers = numForwardPointers * sizeof(SkipValueType);
    }

    return layout;
  }

  size_t bytes() const {
    return lower + upper + skipPointers + forwardPointers;
  }

  template <class Range>
  EliasFanoCompressedListBase<typename Range::iterator> openList(
      Range& buf) const {
    EliasFanoCompressedListBase<typename Range::iterator> result;
    result.size = size;
    result.numLowerBits = numLowerBits;
    result.data = buf.subpiece(0, bytes());

    auto advance = [&](size_t n) {
      auto begin = buf.data();
      buf.advance(n);
      return begin;
    };

    result.skipPointers = advance(skipPointers);
    result.forwardPointers = advance(forwardPointers);
    result.lower = advance(lower);
    result.upper = advance(upper);

    return result;
  }

  MutableCompressedList allocList() const {
    uint8_t* buf = nullptr;
    // WARNING: Current read/write logic assumes that the 7 bytes
    // following the last byte of lower and upper sequences are
    // readable (stored value doesn't matter and won't be changed), so
    // we allocate additional 7 bytes, but do not include them in size
    // of returned value.
    if (size > 0) {
      buf = static_cast<uint8_t*>(malloc(bytes() + 7));
    }
    folly::MutableByteRange bufRange(buf, bytes());
    return openList(bufRange);
  }

  size_t size = 0;
  uint8_t numLowerBits = 0;

  // Sizes in bytes.
  size_t lower = 0;
  size_t upper = 0;
  size_t skipPointers = 0;
  size_t forwardPointers = 0;
};

namespace detail {

template <class Encoder, class Instructions, class SizeType>
class UpperBitsReader : ForwardPointers<Encoder::forwardQuantum>,
                        SkipPointers<Encoder::skipQuantum> {
  typedef typename Encoder::SkipValueType SkipValueType;

 public:
  typedef typename Encoder::ValueType ValueType;

  explicit UpperBitsReader(const typename Encoder::CompressedList& list)
      : ForwardPointers<Encoder::forwardQuantum>(list.forwardPointers),
        SkipPointers<Encoder::skipQuantum>(list.skipPointers),
        start_(list.upper) {
    reset();
  }

  void reset() {
    block_ = start_ != nullptr ? folly::loadUnaligned<block_t>(start_) : 0;
    position_ = std::numeric_limits<SizeType>::max();
    outer_ = 0;
    value_ = 0;
  }

  SizeType position() const {
    return position_;
  }
  ValueType value() const {
    return value_;
  }

  ValueType previous() {
    size_t inner;
    block_t block;
    getPreviousInfo(block, inner, outer_);
    block_ = folly::loadUnaligned<block_t>(start_ + outer_);
    block_ ^= block;
    --position_;
    return setValue(inner);
  }

  ValueType next() {
    // Skip to the first non-zero block.
    while (block_ == 0) {
      outer_ += sizeof(block_t);
      block_ = folly::loadUnaligned<block_t>(start_ + outer_);
    }

    ++position_;
    size_t inner = Instructions::ctz(block_);
    block_ = Instructions::blsr(block_);

    return setValue(inner);
  }

  ValueType skip(SizeType n) {
    DCHECK_GT(n, 0);

    position_ += n; // n 1-bits will be read.

    // Use forward pointer.
    if (Encoder::forwardQuantum > 0 && n > Encoder::forwardQuantum) {
      const size_t steps = position_ / Encoder::forwardQuantum;
      const size_t dest = folly::loadUnaligned<SkipValueType>(
          this->forwardPointers_ + (steps - 1) * sizeof(SkipValueType));

      reposition(dest + steps * Encoder::forwardQuantum);
      n = position_ + 1 - steps * Encoder::forwardQuantum; // n is > 0.
    }

    size_t cnt;
    // Find necessary block.
    while ((cnt = Instructions::popcount(block_)) < n) {
      n -= cnt;
      outer_ += sizeof(block_t);
      block_ = folly::loadUnaligned<block_t>(start_ + outer_);
    }

    // Skip to the n-th one in the block.
    DCHECK_GT(n, 0);
    size_t inner = select64<Instructions>(block_, n - 1);
    block_ &= (block_t(-1) << inner) << 1;

    return setValue(inner);
  }

  // Skip to the first element that is >= v and located *after* the current
  // one (so even if current value equals v, position will be increased by 1).
  ValueType skipToNext(ValueType v) {
    DCHECK_GE(v, value_);

    // Use skip pointer.
    if (Encoder::skipQuantum > 0 && v >= value_ + Encoder::skipQuantum) {
      const size_t steps = v / Encoder::skipQuantum;
      const size_t dest = folly::loadUnaligned<SkipValueType>(
          this->skipPointers_ + (steps - 1) * sizeof(SkipValueType));

      reposition(dest + Encoder::skipQuantum * steps);
      position_ = dest - 1;

      // Correct value_ will be set during the next() call at the end.

      // NOTE: Corresponding block of lower bits sequence may be
      // prefetched here (via __builtin_prefetch), but experiments
      // didn't show any significant improvements.
    }

    // Skip by blocks.
    size_t cnt;
    size_t skip = v - (8 * outer_ - position_ - 1);

    constexpr size_t kBitsPerBlock = 8 * sizeof(block_t);
    while ((cnt = Instructions::popcount(~block_)) < skip) {
      skip -= cnt;
      position_ += kBitsPerBlock - cnt;
      outer_ += sizeof(block_t);
      block_ = folly::loadUnaligned<block_t>(start_ + outer_);
    }

    if (LIKELY(skip)) {
      auto inner = select64<Instructions>(~block_, skip - 1);
      position_ += inner - skip + 1;
      block_ &= block_t(-1) << inner;
    }

    next();
    return value_;
  }

  /**
   * Prepare to skip to `value`. This is a constant-time operation that will
   * prefetch memory required for a `skipTo(value)` call.
   *
   * @return position of reader
   */
  SizeType prepareSkipTo(ValueType v) const {
    auto position = position_;

    if (Encoder::skipQuantum > 0 && v >= value_ + Encoder::skipQuantum) {
      auto outer = outer_;
      const size_t steps = v / Encoder::skipQuantum;
      const size_t dest = folly::loadUnaligned<SkipValueType>(
          this->skipPointers_ + (steps - 1) * sizeof(SkipValueType));

      position = dest - 1;
      outer = (dest + Encoder::skipQuantum * steps) / 8;

      // Prefetch up to the beginning of where we linear search. After that,
      // hardware prefetching will outperform our own. In addition, this
      // simplifies calculating what to prefetch as we don't have to calculate
      // the entire destination address. Two cache lines are prefetched because
      // this results in fewer cycles used (based on practical results) than
      // one. However, three cache lines does not have any additional effect.
      const auto addr = start_ + outer;
      __builtin_prefetch(addr);
      __builtin_prefetch(addr + kCacheLineSize);
    }

    return position;
  }

  ValueType jump(size_t n) {
    if (Encoder::forwardQuantum == 0 || n <= Encoder::forwardQuantum) {
      reset();
    } else {
      // Avoid reading the head, skip() will reposition.
      position_ = std::numeric_limits<SizeType>::max();
    }
    return skip(n);
  }

  ValueType jumpToNext(ValueType v) {
    if (Encoder::skipQuantum == 0 || v < Encoder::skipQuantum) {
      reset();
    } else {
      value_ = 0; // Avoid reading the head, skipToNext() will reposition.
    }
    return skipToNext(v);
  }

  ValueType previousValue() const {
    block_t block;
    size_t inner;
    OuterType outer;
    getPreviousInfo(block, inner, outer);
    return static_cast<ValueType>(8 * outer + inner - (position_ - 1));
  }

  void setDone(SizeType endPos) {
    position_ = endPos;
  }

 private:
  ValueType setValue(size_t inner) {
    value_ = static_cast<ValueType>(8 * outer_ + inner - position_);
    return value_;
  }

  void reposition(SizeType dest) {
    outer_ = dest / 8;
    block_ = folly::loadUnaligned<block_t>(start_ + outer_);
    block_ &= ~((block_t(1) << (dest % 8)) - 1);
  }

  using block_t = uint64_t;
  // The size in bytes of the upper bits is limited by n + universe / 8,
  // so a type that can hold either sizes or values is sufficient.
  using OuterType = typename std::common_type<ValueType, SizeType>::type;

  void getPreviousInfo(block_t& block, size_t& inner, OuterType& outer) const {
    DCHECK_NE(position(), std::numeric_limits<SizeType>::max());
    DCHECK_GT(position(), 0);

    outer = outer_;
    block = folly::loadUnaligned<block_t>(start_ + outer);
    inner = size_t(value_) - 8 * outer_ + position_;
    block &= (block_t(1) << inner) - 1;
    while (UNLIKELY(block == 0)) {
      DCHECK_GT(outer, 0);
      outer -= std::min<OuterType>(sizeof(block_t), outer);
      block = folly::loadUnaligned<block_t>(start_ + outer);
    }
    inner = 8 * sizeof(block_t) - 1 - Instructions::clz(block);
  }

  const unsigned char* const start_;
  block_t block_;
  SizeType position_; // Index of current value (= #reads - 1).
  OuterType outer_; // Outer offset: number of consumed bytes in upper.
  ValueType value_;
};

} // namespace detail

// If kUnchecked = true the caller must guarantee that all the
// operations return valid elements, i.e., they would never return
// false if checked.
template <
    class Encoder,
    class Instructions = instructions::Default,
    bool kUnchecked = false,
    class SizeType = size_t>
class EliasFanoReader {
 public:
  typedef Encoder EncoderType;
  typedef typename Encoder::ValueType ValueType;

  explicit EliasFanoReader(const typename Encoder::CompressedList& list)
      : upper_(list),
        lower_(list.lower),
        size_(list.size),
        numLowerBits_(list.numLowerBits) {
    DCHECK(Instructions::supported());
    // To avoid extra branching during skipTo() while reading
    // upper sequence we need to know the last element.
    // If kUnchecked == true, we do not check that skipTo() is called
    // within the bounds, so we can avoid initializing lastValue_.
    if (kUnchecked || UNLIKELY(list.size == 0)) {
      lastValue_ = 0;
      return;
    }
    ValueType lastUpperValue = ValueType(8 * list.upperSize() - size_);
    auto it = list.upper + list.upperSize() - 1;
    DCHECK_NE(*it, 0);
    lastUpperValue -= 8 - folly::findLastSet(*it);
    lastValue_ = readLowerPart(size_ - 1) | (lastUpperValue << numLowerBits_);
  }

  void reset() {
    upper_.reset();
    value_ = kInvalidValue;
  }

  bool previous() {
    if (!kUnchecked && UNLIKELY(position() == 0)) {
      reset();
      return false;
    }
    upper_.previous();
    value_ =
        readLowerPart(upper_.position()) | (upper_.value() << numLowerBits_);
    return true;
  }

  bool next() {
    if (!kUnchecked && UNLIKELY(position() + 1 >= size_)) {
      return setDone();
    }
    upper_.next();
    value_ =
        readLowerPart(upper_.position()) | (upper_.value() << numLowerBits_);
    return true;
  }

  bool skip(SizeType n) {
    CHECK_GT(n, 0);

    if (kUnchecked || LIKELY(position() + n < size_)) {
      if (LIKELY(n < kLinearScanThreshold)) {
        for (SizeType i = 0; i < n; ++i) {
          upper_.next();
        }
      } else {
        upper_.skip(n);
      }
      value_ =
          readLowerPart(upper_.position()) | (upper_.value() << numLowerBits_);
      return true;
    }

    return setDone();
  }

  bool skipTo(ValueType value) {
    // Also works when value_ == kInvalidValue.
    if (value != kInvalidValue) {
      DCHECK_GE(value + 1, value_ + 1);
    }

    if (!kUnchecked && value > lastValue_) {
      return setDone();
    } else if (value == value_) {
      return true;
    }

    ValueType upperValue = (value >> numLowerBits_);
    ValueType upperSkip = upperValue - upper_.value();
    // The average density of ones in upper bits is 1/2.
    // LIKELY here seems to make things worse, even for small skips.
    if (upperSkip < 2 * kLinearScanThreshold) {
      do {
        upper_.next();
      } while (UNLIKELY(upper_.value() < upperValue));
    } else {
      upper_.skipToNext(upperValue);
    }

    iterateTo(value);
    return true;
  }

  /**
   * Prepare to skip to `value` by prefetching appropriate memory in both the
   * upper and lower bits.
   */
  void prepareSkipTo(ValueType value) const {
    // Also works when value_ == kInvalidValue.
    if (value != kInvalidValue) {
      DCHECK_GE(value + 1, value_ + 1);
    }

    if ((!kUnchecked && value > lastValue_) || (value == value_)) {
      return;
    }

    // Do minimal computation required to prefetch address used in
    // `readLowerPart()`.
    ValueType upperValue = (value >> numLowerBits_);
    const auto upperPosition = upper_.prepareSkipTo(upperValue);
    const auto addr = lower_ + (upperPosition * numLowerBits_ / 8);
    __builtin_prefetch(addr);
    __builtin_prefetch(addr + kCacheLineSize);
  }

  bool jump(SizeType n) {
    if (LIKELY(n < size_)) { // Also checks that n != -1.
      value_ = readLowerPart(n) | (upper_.jump(n + 1) << numLowerBits_);
      return true;
    }
    return setDone();
  }

  bool jumpTo(ValueType value) {
    if (!kUnchecked && value > lastValue_) {
      return setDone();
    }

    upper_.jumpToNext(value >> numLowerBits_);
    iterateTo(value);
    return true;
  }

  ValueType lastValue() const {
    CHECK(!kUnchecked);
    return lastValue_;
  }

  ValueType previousValue() const {
    DCHECK_GT(position(), 0);
    DCHECK_LT(position(), size());
    return readLowerPart(upper_.position() - 1) |
        (upper_.previousValue() << numLowerBits_);
  }

  SizeType size() const {
    return size_;
  }

  bool valid() const {
    return position() < size(); // Also checks that position() != -1.
  }

  SizeType position() const {
    return upper_.position();
  }
  ValueType value() const {
    DCHECK(valid());
    return value_;
  }

 private:
  // Must hold kInvalidValue + 1 == 0.
  constexpr static ValueType kInvalidValue =
      std::numeric_limits<ValueType>::max();

  bool setDone() {
    value_ = kInvalidValue;
    upper_.setDone(size_);
    return false;
  }

  ValueType readLowerPart(SizeType i) const {
    DCHECK_LT(i, size_);
    const size_t pos = i * numLowerBits_;
    const unsigned char* ptr = lower_ + (pos / 8);
    const uint64_t ptrv = folly::loadUnaligned<uint64_t>(ptr);
    // This removes the branch in the fallback implementation of
    // bzhi. The condition is verified at encoding time.
    assume(numLowerBits_ < sizeof(ValueType) * 8);
    return Instructions::bzhi(ptrv >> (pos % 8), numLowerBits_);
  }

  void iterateTo(ValueType value) {
    while (true) {
      value_ =
          readLowerPart(upper_.position()) | (upper_.value() << numLowerBits_);
      if (LIKELY(value_ >= value)) {
        break;
      }
      upper_.next();
    }
  }

  constexpr static size_t kLinearScanThreshold = 8;

  detail::UpperBitsReader<Encoder, Instructions, SizeType> upper_;
  const uint8_t* lower_;
  SizeType size_;
  ValueType value_ = kInvalidValue;
  ValueType lastValue_;
  uint8_t numLowerBits_;
};

} // namespace compression
} // namespace folly