/* * Copyright (c) Meta Platforms, Inc. and affiliates. * * 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 <glog/logging.h> #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> #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), upperSizeBytes(other.upperSizeBytes), 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 size = 0; uint8_t numLowerBits = 0; size_t upperSizeBytes = 0; // WARNING: EliasFanoCompressedList has no ownership of data. The 7 bytes // following the last byte should be readable if kUpperFirst = false, 8 bytes // otherwise. Range<Pointer> data; Pointer skipPointers = nullptr; Pointer forwardPointers = nullptr; Pointer lower = nullptr; Pointer upper = nullptr; }; using EliasFanoCompressedList = EliasFanoCompressedListBase<const uint8_t*>; using MutableEliasFanoCompressedList = EliasFanoCompressedListBase<uint8_t*>; template < class Value, // SkipValue must be wide enough to be able to represent the list length. class SkipValue = uint64_t, size_t kSkipQuantum = 0, // 0 = disabled size_t kForwardQuantum = 0, // 0 = disabled bool kUpperFirst = false> struct EliasFanoEncoder { static_assert( std::is_integral_v<Value> && std::is_unsigned_v<Value>, "Value should be unsigned integral"); using CompressedList = EliasFanoCompressedList; using MutableCompressedList = MutableEliasFanoCompressedList; using ValueType = Value; using SkipValueType = SkipValue; 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 = findLastSet(upperBound) - 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(); } EliasFanoEncoder encoder(size_t(end - begin), *(end - 1)); for (; begin != end; ++begin) { encoder.add(*begin); } return encoder.finish(); } explicit EliasFanoEncoder(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'); } EliasFanoEncoder(size_t size, ValueType upperBound) : EliasFanoEncoder( Layout::fromUpperBoundAndSize(upperBound, size).allocList()) {} void add(ValueType value) { 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); } fillSkipPointersUpTo(upperBits); if constexpr (forwardQuantum != 0) { if ((size_ + 1) % forwardQuantum == 0) { DCHECK_LE(upperBits, std::numeric_limits<SkipValueType>::max()); const auto k = size_ / forwardQuantum; // Store the number of preceding 0-bits. forwardPointers_[k] = upperBits; } } lastValue_ = value; ++size_; } const MutableCompressedList& finish() { CHECK_EQ(size_, result_.size); const ValueType upperBitsUniverse = (8 * result_.upperSizeBytes - result_.size); // Populate skip pointers up to the universe upper bound (inclusive). fillSkipPointersUpTo(upperBitsUniverse); return result_; } private: void fillSkipPointersUpTo(ValueType fillBoundary) { if constexpr (skipQuantum != 0) { DCHECK_LE(size_, std::numeric_limits<SkipValueType>::max()); // The first skip pointer is omitted (it would always be 0), so the // calculation is shifted by 1. while ((skipPointersSize_ + 1) * skipQuantum <= fillBoundary) { // Store the number of preceding 1-bits. skipPointers_[skipPointersSize_++] = static_cast<SkipValueType>(size_); } } } // 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 = loadUnaligned<uint64_t>(ptr); ptrv |= value << (pos % 8); 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, bool kUpperFirst> struct EliasFanoEncoder< Value, SkipValue, kSkipQuantum, kForwardQuantum, kUpperFirst>::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(static_cast<int>(numLowerBits), 8 * sizeof(Value)); CHECK_LE( 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.upperSizeBytes = upper; 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); if constexpr (kUpperFirst) { result.upper = advance(upper); result.lower = advance(lower); } else { 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 upper bytes and the 8 bytes following the lower bytes // sequences are readable (stored value doesn't matter and won't be // changed), so we allocate additional 8 bytes, but do not include them in // size of returned value. if (size > 0) { buf = static_cast<uint8_t*>(malloc(bytes() + 8)); } 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 { // Add a and b in the domain of T. This guarantees that if T is a sub-int type, // we cast away the promotion to int, so that unsigned overflow and underflow // work as expected. template <class T, class U> FOLLY_ALWAYS_INLINE T addT(T a, U b) { static_assert(std::is_unsigned_v<T>); return static_cast<T>(a + static_cast<T>(b)); } template < class Encoder, class Instructions, class SizeType, bool kUnchecked = false> class UpperBitsReader : ForwardPointers<Encoder::forwardQuantum>, SkipPointers<Encoder::skipQuantum> { using SkipValueType = typename Encoder::SkipValueType; public: using ValueType = typename Encoder::ValueType; static constexpr SizeType kBeforeFirstPos = -1; explicit UpperBitsReader(const typename Encoder::CompressedList& list) : ForwardPointers<Encoder::forwardQuantum>(list.forwardPointers), SkipPointers<Encoder::skipQuantum>(list.skipPointers), start_(list.upper), size_(list.size), upperBound_(estimateUpperBound(list)) { reset(); } void reset() { // Pretend the bitvector is prefixed by a block of zeroes. block_ = 0; position_ = kBeforeFirstPos; outer_ = static_cast<OuterType>(-sizeof(block_t)); value_ = 0; } FOLLY_ALWAYS_INLINE SizeType position() const { return position_; } FOLLY_ALWAYS_INLINE ValueType value() const { return value_; } FOLLY_ALWAYS_INLINE bool valid() const { // Also checks that position() != kBeforeFirstPos. return position() < size(); } FOLLY_ALWAYS_INLINE SizeType size() const { return size_; } FOLLY_ALWAYS_INLINE bool previous() { if (!kUnchecked && UNLIKELY(position() == 0)) { return false; } size_t inner; block_t block; DCHECK_GE(outer_, 0); getPreviousInfo(block, inner, outer_); // Updates outer_. block_ = loadUnaligned<block_t>(start_ + outer_); block_ ^= block; --position_; return setValue(inner); } FOLLY_ALWAYS_INLINE bool next() { if (!kUnchecked && UNLIKELY(addT(position(), 1) >= size())) { return setDone(); } // Skip to the first non-zero block. while (UNLIKELY(block_ == 0)) { outer_ += sizeof(block_t); block_ = loadUnaligned<block_t>(start_ + outer_); } ++position_; size_t inner = Instructions::ctz(block_); block_ = Instructions::blsr(block_); return setValue(inner); } FOLLY_ALWAYS_INLINE bool skip(SizeType n) { DCHECK_GT(n, 0); if (!kUnchecked && UNLIKELY(addT(position_, n) >= size())) { return setDone(); } position_ += n; // n 1-bits will be read. // Use forward pointer. if constexpr (Encoder::forwardQuantum > 0) { if (UNLIKELY(n > Encoder::forwardQuantum)) { const size_t steps = position_ / Encoder::forwardQuantum; const size_t dest = 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_ = 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). FOLLY_ALWAYS_INLINE bool skipToNext(ValueType v) { DCHECK_GE(v, value_); if (!kUnchecked && UNLIKELY(v > upperBound_)) { return setDone(); } // Use skip pointer. if constexpr (Encoder::skipQuantum > 0) { // NOTE: The addition can overflow here, but that means value_ is within // skipQuantum_ distance from the maximum representable value, and thus // the last value, so the comparison is still correct. if (UNLIKELY(v >= addT(value_, Encoder::skipQuantum))) { const size_t steps = v / Encoder::skipQuantum; const size_t dest = loadUnaligned<SkipValueType>( this->skipPointers_ + (steps - 1) * sizeof(SkipValueType)); DCHECK_LE(dest, size()); if (!kUnchecked && UNLIKELY(dest == size())) { return setDone(); } 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; // outer_ and position_ rely on negative sentinel values. We enforce the // overflown bits are dropped by explicitly casting the final value to // SizeType first, followed by a potential implicit cast to size_t. size_t skip = static_cast<SizeType>(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); DCHECK_LT(outer_, (static_cast<size_t>(upperBound_) + size() + 7) / 8); block_ = loadUnaligned<block_t>(start_ + outer_); } if (LIKELY(skip)) { auto inner = select64<Instructions>(~block_, skip - 1); position_ += inner - skip + 1; block_ &= block_t(-1) << inner; } DCHECK_LT(addT(position(), 1), addT(size(), 1)); return next(); } /** * Try to prepare to skip to value. This is a constant-time operation that * will attempt to prefetch memory required for a subsequent skipTo(value) * call if the value to skip to is within this list. * * Returns: * {true, position of the reader} if the skip is valid, * {false, size()} otherwise. */ FOLLY_ALWAYS_INLINE std::pair<bool, SizeType> prepareSkipTo( ValueType v) const { if (!kUnchecked && UNLIKELY(v > upperBound_)) { return std::make_pair(false, size()); } auto position = position_; if constexpr (Encoder::skipQuantum > 0) { if (v >= addT(value_, Encoder::skipQuantum)) { auto outer = outer_; const size_t steps = v / Encoder::skipQuantum; const size_t dest = loadUnaligned<SkipValueType>( this->skipPointers_ + (steps - 1) * sizeof(SkipValueType)); DCHECK_LE(dest, size()); if (!kUnchecked && UNLIKELY(dest == size())) { return std::make_pair(false, size()); } 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 std::make_pair(true, position); } FOLLY_ALWAYS_INLINE ValueType previousValue() const { block_t block; size_t inner; OuterType outer; getPreviousInfo(block, inner, outer); return static_cast<ValueType>(8 * outer + inner - (position_ - 1)); } // Returns true if we're at the beginning of the list, or previousValue() != // value(). FOLLY_ALWAYS_INLINE bool isAtBeginningOfRun() const { DCHECK_NE(position(), kBeforeFirstPos); if (position_ == 0) { return true; } size_t bitPos = size_t(value_) + position_ - 1; return (start_[bitPos / 8] & (1 << (bitPos % 8))) == 0; } private: 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_t<ValueType, SizeType>; static ValueType estimateUpperBound( const typename Encoder::CompressedList& list) { size_t upperBound = 8 * list.upperSizeBytes - list.size; // The bitvector is byte-aligned, so we may be overestimating the universe // size. Make sure it fits in ValueType. return static_cast<ValueType>(std::min<size_t>( upperBound, std::numeric_limits<ValueType>::max() >> list.numLowerBits)); } FOLLY_ALWAYS_INLINE bool setValue(size_t inner) { value_ = static_cast<ValueType>(8 * outer_ + inner - position_); return true; } FOLLY_ALWAYS_INLINE bool setDone() { position_ = size_; return false; } // NOTE: dest is a position in the bit vector, use size_t as SizeType may // not be sufficient here. FOLLY_ALWAYS_INLINE void reposition(size_t dest) { outer_ = dest / 8; DCHECK_LT(outer_, (static_cast<size_t>(upperBound_) + size() + 7) / 8); block_ = loadUnaligned<block_t>(start_ + outer_); block_ &= ~((block_t(1) << (dest % 8)) - 1); } FOLLY_ALWAYS_INLINE void getPreviousInfo( block_t& block, size_t& inner, OuterType& outer) const { DCHECK_GT(position(), 0); DCHECK_LT(position(), size()); outer = outer_; block = 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 = loadUnaligned<block_t>(start_ + outer); } inner = 8 * sizeof(block_t) - 1 - Instructions::clz(block); } const unsigned char* const start_; const SizeType size_; // Size of the list. const ValueType upperBound_; // Upper bound of values in this list. 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. // // If the list length is known to be representable with a type narrower than the // SkipValueType used in the format, the reader footprint can be reduced by // passing the type as SizeType. template < class Encoder, class Instructions = instructions::Default, bool kUnchecked = false, class SizeT = typename Encoder::SkipValueType> class EliasFanoReader { using UpperBitsReader = detail::UpperBitsReader<Encoder, Instructions, SizeT, kUnchecked>; public: using EncoderType = Encoder; using ValueType = typename Encoder::ValueType; using SizeType = SizeT; explicit EliasFanoReader(const typename Encoder::CompressedList& list) : upper_(list), lower_(list.lower), value_(), numLowerBits_(list.numLowerBits) { DCHECK_LE(list.size, std::numeric_limits<SizeType>::max()); DCHECK(Instructions::supported()); } void reset() { upper_.reset(); } bool previous() { if (LIKELY(upper_.previous())) { return setValue(readCurrentValue()); } reset(); return false; } bool next() { if (LIKELY(upper_.next())) { return setValue(readCurrentValue()); } return false; } /** * Advances by n elements. n = 0 is allowed and has no effect. Returns false * if the end of the list is reached. position() + n must be representable by * SizeType. */ bool skip(SizeType n) { if (n == 0) { return valid(); } if (!upper_.skip(n)) { return false; } return setValue(readCurrentValue()); } /** * Skips to the first element >= value whose position is greater or equal to * the current position. * Requires that value >= value() (or that the reader is positioned before the * first element). Returns false if no such element exists. * If kCanBeAtValue is false, the requirement above becomes value > value(). */ template <bool kCanBeAtValue = true> bool skipTo(ValueType value) { if (valid()) { if constexpr (kCanBeAtValue) { DCHECK_GE(value, value_); if (UNLIKELY(value == value_)) { return true; } } else { DCHECK_GT(value, value_); } } ValueType upperValue = value >> numLowerBits_; if (UNLIKELY(!upper_.skipToNext(upperValue))) { return false; } do { if (auto cur = readCurrentValue(); LIKELY(cur >= value)) { return setValue(cur); } } while (LIKELY(upper_.next())); return false; } /** * Prepare to skip to `value` by prefetching appropriate memory in both the * upper and lower bits. */ template <bool kCanBeAtValue = true> void prepareSkipTo(ValueType value) const { if (valid()) { if constexpr (kCanBeAtValue) { DCHECK_GE(value, value_); if (UNLIKELY(value == value_)) { return; } } else { DCHECK_GT(value, value_); } } // Do minimal computation required to prefetch address used in // `readLowerPart()`. ValueType upperValue = value >> numLowerBits_; const auto [valid, upperPosition] = upper_.prepareSkipTo(upperValue); if (!valid) { return; } const auto addr = lower_ + (upperPosition * numLowerBits_ / 8); __builtin_prefetch(addr); __builtin_prefetch(addr + kCacheLineSize); } /** * Jumps to the element at position n. The reader can be in any state. Returns * false if n >= size(). */ bool jump(SizeType n) { // Also works if position() == -1. if (detail::addT(n, 1) < detail::addT(position(), 1)) { reset(); n += 1; // Initial position is -1. } else { n -= position(); } return skip(n); } /** * Jumps to the first element >= value. The reader can be in any * state. Returns false if no such element exists. * * If all the values in the list can be assumed distinct, setting * assumeDistinct = true can enable some optimizations. */ bool jumpTo(ValueType value, bool assumeDistinct = false) { if (valid() && value == value_) { if (assumeDistinct == true) { return true; } // We might be in the middle of a run of equal values, reposition by // iterating backwards to its first element. auto valueLower = Instructions::bzhi(value_, numLowerBits_); while (!upper_.isAtBeginningOfRun() && readLowerPart(position() - 1) == valueLower) { upper_.previous(); } return true; } // We need to reset if we're not in the initial state and the jump is // backwards. if (position() != UpperBitsReader::kBeforeFirstPos && (position() == size() || value < value_)) { reset(); } return skipTo(value); } ValueType previousValue() const { DCHECK_GT(position(), 0); DCHECK_LT(position(), size()); return readLowerPart(position() - 1) | (upper_.previousValue() << numLowerBits_); } SizeType size() const { return upper_.size(); } bool valid() const { return upper_.valid(); } SizeType position() const { return upper_.position(); } ValueType value() const { DCHECK(valid()); return value_; } private: FOLLY_ALWAYS_INLINE bool setValue(ValueType value) { DCHECK(valid()); value_ = value; return true; } FOLLY_ALWAYS_INLINE 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 = 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_); } FOLLY_ALWAYS_INLINE ValueType readCurrentValue() { return readLowerPart(position()) | (upper_.value() << numLowerBits_); } // Ordering of fields is counter-intutive but it optimizes the layout. UpperBitsReader upper_; const uint8_t* const lower_; ValueType value_; const uint8_t numLowerBits_; }; } // namespace compression } // namespace folly