/*
* 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.
*/
/**
* 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/compression/Instructions.h>
#include <folly/compression/Select64.h>
#include <folly/compression/elias_fano/CodingDetail.h>
#include <folly/lang/Assume.h>
#include <folly/lang/Bits.h>
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 (FOLLY_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_);
CHECK_LT(size_, result_.size)
<< "add() called more times than the size specified in construction";
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)
<< "Number of add()s must be equal to the size specified in construction";
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_assert(
std::is_integral_v<SizeType> && std::is_unsigned_v<SizeType>,
"SizeType should be unsigned integral");
// Functions like `jump()` and `next()` rely on this being the predecessor
// of 0. `valid()` also needs it to be the largest possible `SizeType`.
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 {
// SizeType is unsigned, so this also ensures position() != kBeforeFirstPos
return position() < size();
}
FOLLY_ALWAYS_INLINE SizeType size() const { return size_; }
FOLLY_ALWAYS_INLINE bool previous() {
if (!kUnchecked && FOLLY_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 && FOLLY_UNLIKELY(addT(position(), 1) >= size())) {
return setDone();
}
// Skip to the first non-zero block.
while (FOLLY_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 && FOLLY_UNLIKELY(addT(position_, n) >= size())) {
return setDone();
}
position_ += n; // n 1-bits will be read.
// Use forward pointer.
if constexpr (Encoder::forwardQuantum > 0) {
if (FOLLY_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 && FOLLY_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 (FOLLY_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 && FOLLY_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 (FOLLY_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 && FOLLY_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 && FOLLY_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 (FOLLY_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 (FOLLY_LIKELY(upper_.previous())) {
return setValue(readCurrentValue());
}
reset();
return false;
}
bool next() {
if (FOLLY_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 (FOLLY_UNLIKELY(value == value_)) {
return true;
}
} else {
DCHECK_GT(value, value_);
}
}
ValueType upperValue = value >> numLowerBits_;
if (FOLLY_UNLIKELY(!upper_.skipToNext(upperValue))) {
return false;
}
do {
if (auto cur = readCurrentValue(); FOLLY_LIKELY(cur >= value)) {
return setValue(cur);
}
} while (FOLLY_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 (FOLLY_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, since `kBeforeFirstPos + 1 == 0`.
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
// bextr. The condition is verified at encoding time.
assume(numLowerBits_ < sizeof(ValueType) * 8);
assume((pos % 8) + numLowerBits_ < 64);
return Instructions::bextr(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
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