2 * Copyright 2015 Facebook, Inc.
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
8 * http://www.apache.org/licenses/LICENSE-2.0
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
18 * @author Philip Pronin (philipp@fb.com)
20 * Based on the paper by Sebastiano Vigna,
21 * "Quasi-succinct indices" (arxiv:1206.4300).
24 #ifndef FOLLY_EXPERIMENTAL_ELIAS_FANO_CODING_H
25 #define FOLLY_EXPERIMENTAL_ELIAS_FANO_CODING_H
29 #include <type_traits>
30 #include <glog/logging.h>
32 #include <folly/Bits.h>
33 #include <folly/CpuId.h>
34 #include <folly/Likely.h>
35 #include <folly/Portability.h>
36 #include <folly/Range.h>
37 #include <folly/experimental/Select64.h>
40 #error EliasFanoCoding.h requires GCC
44 #error EliasFanoCoding.h requires x86_64
47 #if __BYTE_ORDER__ != __ORDER_LITTLE_ENDIAN__
48 #error EliasFanoCoding.h requires little endianness
51 namespace folly { namespace compression {
53 template <class Pointer>
54 struct EliasFanoCompressedListBase {
55 EliasFanoCompressedListBase() = default;
57 template <class OtherPointer>
58 EliasFanoCompressedListBase(
59 const EliasFanoCompressedListBase<OtherPointer>& other)
61 numLowerBits(other.numLowerBits),
63 skipPointers(reinterpret_cast<Pointer>(other.skipPointers)),
64 forwardPointers(reinterpret_cast<Pointer>(other.forwardPointers)),
65 lower(reinterpret_cast<Pointer>(other.lower)),
66 upper(reinterpret_cast<Pointer>(other.upper)) {
70 ::free(const_cast<unsigned char*>(data.data()));
73 size_t upperSize() const {
74 return data.end() - upper;
78 uint8_t numLowerBits = 0;
80 // WARNING: EliasFanoCompressedList has no ownership of data. The 7
81 // bytes following the last byte should be readable.
82 folly::Range<Pointer> data;
84 Pointer skipPointers = nullptr;
85 Pointer forwardPointers = nullptr;
86 Pointer lower = nullptr;
87 Pointer upper = nullptr;
90 typedef EliasFanoCompressedListBase<const uint8_t*> EliasFanoCompressedList;
91 typedef EliasFanoCompressedListBase<uint8_t*> MutableEliasFanoCompressedList;
93 template <class Value,
94 class SkipValue = size_t,
95 size_t kSkipQuantum = 0, // 0 = disabled
96 size_t kForwardQuantum = 0> // 0 = disabled
97 struct EliasFanoEncoderV2 {
98 static_assert(std::is_integral<Value>::value &&
99 std::is_unsigned<Value>::value,
100 "Value should be unsigned integral");
102 typedef EliasFanoCompressedList CompressedList;
104 typedef Value ValueType;
105 typedef SkipValue SkipValueType;
108 static constexpr size_t skipQuantum = kSkipQuantum;
109 static constexpr size_t forwardQuantum = kForwardQuantum;
111 static uint8_t defaultNumLowerBits(size_t upperBound, size_t size) {
112 if (size == 0 || upperBound < size) {
115 // floor(log(upperBound / size));
116 return folly::findLastSet(upperBound / size) - 1;
119 // Requires: input range (begin, end) is sorted (encoding
120 // crashes if it's not).
121 // WARNING: encode() mallocates EliasFanoCompressedList::data. As
122 // EliasFanoCompressedList has no ownership of it, you need to call
123 // free() explicitly.
124 template <class RandomAccessIterator>
125 static EliasFanoCompressedList encode(RandomAccessIterator begin,
126 RandomAccessIterator end) {
128 return EliasFanoCompressedList();
130 EliasFanoEncoderV2 encoder(end - begin, *(end - 1));
131 for (; begin != end; ++begin) {
134 return encoder.finish();
137 explicit EliasFanoEncoderV2(const MutableEliasFanoCompressedList& result)
138 : lower_(result.lower),
139 upper_(result.upper),
140 skipPointers_(reinterpret_cast<SkipValueType*>(
141 result.skipPointers)),
142 forwardPointers_(reinterpret_cast<SkipValueType*>(
143 result.forwardPointers)),
145 memset(result.data.data(), 0, result.data.size());
148 EliasFanoEncoderV2(size_t size, ValueType upperBound)
149 : EliasFanoEncoderV2(
150 Layout::fromUpperBoundAndSize(upperBound, size).allocList()) {
153 void add(ValueType value) {
154 CHECK_GE(value, lastValue_);
156 const auto numLowerBits = result_.numLowerBits;
157 const ValueType upperBits = value >> numLowerBits;
159 // Upper sequence consists of upperBits 0-bits and (size_ + 1) 1-bits.
160 const size_t pos = upperBits + size_;
161 upper_[pos / 8] |= 1U << (pos % 8);
162 // Append numLowerBits bits to lower sequence.
163 if (numLowerBits != 0) {
164 const ValueType lowerBits = value & ((ValueType(1) << numLowerBits) - 1);
165 writeBits56(lower_, size_ * numLowerBits, numLowerBits, lowerBits);
168 /* static */ if (skipQuantum != 0) {
169 while ((skipPointersSize_ + 1) * skipQuantum <= upperBits) {
170 // Store the number of preceding 1-bits.
171 skipPointers_[skipPointersSize_++] = size_;
175 /* static */ if (forwardQuantum != 0) {
176 if ((size_ + 1) % forwardQuantum == 0) {
177 const auto pos = size_ / forwardQuantum;
178 // Store the number of preceding 0-bits.
179 forwardPointers_[pos] = upperBits;
187 const EliasFanoCompressedList& finish() const {
188 CHECK_EQ(size_, result_.size);
193 // Writes value (with len up to 56 bits) to data starting at pos-th bit.
194 static void writeBits56(unsigned char* data, size_t pos,
195 uint8_t len, uint64_t value) {
196 DCHECK_LE(uint32_t(len), 56);
197 DCHECK_EQ(0, value & ~((uint64_t(1) << len) - 1));
198 unsigned char* const ptr = data + (pos / 8);
199 uint64_t ptrv = folly::loadUnaligned<uint64_t>(ptr);
200 ptrv |= value << (pos % 8);
201 folly::storeUnaligned<uint64_t>(ptr, ptrv);
204 unsigned char* lower_ = nullptr;
205 unsigned char* upper_ = nullptr;
206 SkipValueType* skipPointers_ = nullptr;
207 SkipValueType* forwardPointers_ = nullptr;
209 ValueType lastValue_ = 0;
211 size_t skipPointersSize_ = 0;
213 EliasFanoCompressedList result_;
216 template <class Value,
219 size_t kForwardQuantum>
220 struct EliasFanoEncoderV2<Value,
223 kForwardQuantum>::Layout {
224 static Layout fromUpperBoundAndSize(size_t upperBound, size_t size) {
225 // numLowerBits can be at most 56 because of detail::writeBits56.
226 const uint8_t numLowerBits = std::min(defaultNumLowerBits(upperBound,
230 // Upper bits are stored using unary delta encoding.
231 // For example, (3 5 5 9) will be encoded as 1000011001000_2.
232 const size_t upperSizeBits =
233 (upperBound >> numLowerBits) + // Number of 0-bits to be stored.
235 const size_t upper = (upperSizeBits + 7) / 8;
237 // *** Validity checks.
238 // Shift by numLowerBits must be valid.
239 CHECK_LT(numLowerBits, 8 * sizeof(Value));
240 CHECK_LT(size, std::numeric_limits<SkipValueType>::max());
241 CHECK_LT(upperBound >> numLowerBits,
242 std::numeric_limits<SkipValueType>::max());
244 return fromInternalSizes(numLowerBits, upper, size);
247 static Layout fromInternalSizes(uint8_t numLowerBits,
252 layout.numLowerBits = numLowerBits;
254 layout.lower = (numLowerBits * size + 7) / 8;
255 layout.upper = upper;
257 // *** Skip pointers.
258 // Store (1-indexed) position of every skipQuantum-th
259 // 0-bit in upper bits sequence.
260 /* static */ if (skipQuantum != 0) {
261 // 8 * upper is used here instead of upperSizeBits, as that is
262 // more serialization-friendly way (upperSizeBits doesn't need
263 // to be known by this function, unlike upper).
265 // '?: 1' is a workaround for false 'division by zero'
266 // compile-time error.
267 size_t numSkipPointers = (8 * upper - size) / (skipQuantum ?: 1);
268 layout.skipPointers = numSkipPointers * sizeof(SkipValueType);
271 // *** Forward pointers.
272 // Store (1-indexed) position of every forwardQuantum-th
273 // 1-bit in upper bits sequence.
274 /* static */ if (forwardQuantum != 0) {
275 size_t numForwardPointers = size / (forwardQuantum ?: 1);
276 layout.forwardPointers = numForwardPointers * sizeof(SkipValueType);
282 size_t bytes() const {
283 return lower + upper + skipPointers + forwardPointers;
286 template <typename Range>
287 EliasFanoCompressedListBase<typename Range::iterator>
288 openList(Range& buf) const {
289 EliasFanoCompressedListBase<typename Range::iterator> result;
291 result.numLowerBits = numLowerBits;
292 result.data = buf.subpiece(0, bytes());
294 auto advance = [&] (size_t n) {
295 auto begin = buf.data();
300 result.skipPointers = advance(skipPointers);
301 result.forwardPointers = advance(forwardPointers);
302 result.lower = advance(lower);
303 result.upper = advance(upper);
308 MutableEliasFanoCompressedList allocList() const {
309 uint8_t* buf = nullptr;
310 // WARNING: Current read/write logic assumes that the 7 bytes
311 // following the last byte of lower and upper sequences are
312 // readable (stored value doesn't matter and won't be changed), so
313 // we allocate additional 7 bytes, but do not include them in size
314 // of returned value.
316 buf = static_cast<uint8_t*>(malloc(bytes() + 7));
318 folly::MutableByteRange bufRange(buf, bytes());
319 return openList(bufRange);
323 uint8_t numLowerBits = 0;
328 size_t skipPointers = 0;
329 size_t forwardPointers = 0;
332 // NOTE: It's recommended to compile EF coding with -msse4.2, starting
333 // with Nehalem, Intel CPUs support POPCNT instruction and gcc will emit
334 // it for __builtin_popcountll intrinsic.
335 // But we provide an alternative way for the client code: it can switch to
336 // the appropriate version of EliasFanoReader<> in realtime (client should
337 // implement this switching logic itself) by specifying instruction set to
339 namespace instructions {
342 static bool supported(const folly::CpuId& cpuId = {}) {
345 static inline uint64_t popcount(uint64_t value) {
346 return __builtin_popcountll(value);
348 static inline int ctz(uint64_t value) {
350 return __builtin_ctzll(value);
352 static inline int clz(uint64_t value) {
354 return __builtin_clzll(value);
356 static inline uint64_t blsr(uint64_t value) {
357 return value & (value - 1);
361 struct Nehalem : public Default {
362 static bool supported(const folly::CpuId& cpuId = {}) {
363 return cpuId.popcnt();
365 static inline uint64_t popcount(uint64_t value) {
366 // POPCNT is supported starting with Intel Nehalem, AMD K10.
368 asm ("popcntq %1, %0" : "=r" (result) : "r" (value));
373 struct Haswell : public Nehalem {
374 static bool supported(const folly::CpuId& cpuId = {}) {
375 return Nehalem::supported(cpuId) && cpuId.bmi1();
377 static inline uint64_t blsr(uint64_t value) {
378 // BMI1 is supported starting with Intel Haswell, AMD Piledriver.
379 // BLSR combines two instuctions into one and reduces register pressure.
381 asm ("blsrq %1, %0" : "=r" (result) : "r" (value));
386 } // namespace instructions
390 template <class Encoder, class Instructions>
391 class UpperBitsReader {
392 typedef typename Encoder::SkipValueType SkipValueType;
394 typedef typename Encoder::ValueType ValueType;
396 explicit UpperBitsReader(const EliasFanoCompressedList& list)
397 : forwardPointers_(list.forwardPointers),
398 skipPointers_(list.skipPointers),
404 block_ = start_ != nullptr ? folly::loadUnaligned<block_t>(start_) : 0;
411 size_t position() const { return position_; }
412 ValueType value() const { return value_; }
415 // Skip to the first non-zero block.
416 while (block_ == 0) {
417 outer_ += sizeof(block_t);
418 block_ = folly::loadUnaligned<block_t>(start_ + outer_);
422 inner_ = Instructions::ctz(block_);
423 block_ = Instructions::blsr(block_);
428 ValueType skip(size_t n) {
431 position_ += n; // n 1-bits will be read.
433 // Use forward pointer.
434 if (Encoder::forwardQuantum > 0 && n > Encoder::forwardQuantum) {
435 // Workaround to avoid 'division by zero' compile-time error.
436 constexpr size_t q = Encoder::forwardQuantum ?: 1;
438 const size_t steps = position_ / q;
440 folly::loadUnaligned<SkipValueType>(
441 forwardPointers_ + (steps - 1) * sizeof(SkipValueType));
443 reposition(dest + steps * q);
444 n = position_ + 1 - steps * q; // n is > 0.
445 // Correct inner_ will be set at the end.
449 // Find necessary block.
450 while ((cnt = Instructions::popcount(block_)) < n) {
452 outer_ += sizeof(block_t);
453 block_ = folly::loadUnaligned<block_t>(start_ + outer_);
456 // Skip to the n-th one in the block.
458 inner_ = select64<Instructions>(block_, n - 1);
459 block_ &= (block_t(-1) << inner_) << 1;
464 // Skip to the first element that is >= v and located *after* the current
465 // one (so even if current value equals v, position will be increased by 1).
466 ValueType skipToNext(ValueType v) {
467 DCHECK_GE(v, value_);
470 if (Encoder::skipQuantum > 0 && v >= value_ + Encoder::skipQuantum) {
471 // Workaround to avoid 'division by zero' compile-time error.
472 constexpr size_t q = Encoder::skipQuantum ?: 1;
474 const size_t steps = v / q;
476 folly::loadUnaligned<SkipValueType>(
477 skipPointers_ + (steps - 1) * sizeof(SkipValueType));
479 reposition(dest + q * steps);
480 position_ = dest - 1;
482 // Correct inner_ and value_ will be set during the next()
485 // NOTE: Corresponding block of lower bits sequence may be
486 // prefetched here (via __builtin_prefetch), but experiments
487 // didn't show any significant improvements.
492 size_t skip = v - (8 * outer_ - position_ - 1);
494 constexpr size_t kBitsPerBlock = 8 * sizeof(block_t);
495 while ((cnt = Instructions::popcount(~block_)) < skip) {
497 position_ += kBitsPerBlock - cnt;
498 outer_ += sizeof(block_t);
499 block_ = folly::loadUnaligned<block_t>(start_ + outer_);
503 auto inner = select64<Instructions>(~block_, skip - 1);
504 position_ += inner - skip + 1;
505 block_ &= block_t(-1) << inner;
512 ValueType jump(size_t n) {
513 if (Encoder::forwardQuantum == 0 || n <= Encoder::forwardQuantum) {
516 position_ = -1; // Avoid reading the head, skip() will reposition.
521 ValueType jumpToNext(ValueType v) {
522 if (Encoder::skipQuantum == 0 || v < Encoder::skipQuantum) {
525 value_ = 0; // Avoid reading the head, skipToNext() will reposition.
527 return skipToNext(v);
530 ValueType previousValue() const {
531 DCHECK_NE(position(), -1);
532 DCHECK_GT(position(), 0);
534 size_t outer = outer_;
535 block_t block = folly::loadUnaligned<block_t>(start_ + outer);
536 block &= (block_t(1) << inner_) - 1;
538 while (UNLIKELY(block == 0)) {
539 DCHECK_GE(outer, sizeof(block_t));
540 outer -= sizeof(block_t);
541 block = folly::loadUnaligned<block_t>(start_ + outer);
544 auto inner = 8 * sizeof(block_t) - 1 - Instructions::clz(block);
545 return static_cast<ValueType>(8 * outer + inner - (position_ - 1));
548 void setDone(size_t endPos) {
553 ValueType setValue() {
554 value_ = static_cast<ValueType>(8 * outer_ + inner_ - position_);
558 void reposition(size_t dest) {
560 block_ = folly::loadUnaligned<block_t>(start_ + outer_);
561 block_ &= ~((block_t(1) << (dest % 8)) - 1);
564 typedef uint64_t block_t;
565 const unsigned char* const forwardPointers_;
566 const unsigned char* const skipPointers_;
567 const unsigned char* const start_;
569 size_t outer_; // Outer offset: number of consumed bytes in upper.
570 size_t inner_; // Inner offset: (bit) position in current block.
571 size_t position_; // Index of current value (= #reads - 1).
575 } // namespace detail
577 // If kUnchecked = true the caller must guarantee that all the
578 // operations return valid elements, i.e., they would never return
580 template <class Encoder,
581 class Instructions = instructions::Default,
582 bool kUnchecked = false>
583 class EliasFanoReader {
585 typedef Encoder EncoderType;
586 typedef typename Encoder::ValueType ValueType;
588 explicit EliasFanoReader(const EliasFanoCompressedList& list)
592 lowerMask_((ValueType(1) << list.numLowerBits) - 1),
593 numLowerBits_(list.numLowerBits) {
594 DCHECK(Instructions::supported());
595 // To avoid extra branching during skipTo() while reading
596 // upper sequence we need to know the last element.
597 // If kUnchecked == true, we do not check that skipTo() is called
598 // within the bounds, so we can avoid initializing lastValue_.
599 if (kUnchecked || UNLIKELY(list.size == 0)) {
603 ValueType lastUpperValue = 8 * list.upperSize() - size_;
604 auto it = list.upper + list.upperSize() - 1;
606 lastUpperValue -= 8 - folly::findLastSet(*it);
607 lastValue_ = readLowerPart(size_ - 1) | (lastUpperValue << numLowerBits_);
616 if (!kUnchecked && UNLIKELY(position() + 1 >= size_)) {
620 value_ = readLowerPart(upper_.position()) |
621 (upper_.value() << numLowerBits_);
625 bool skip(size_t n) {
628 if (kUnchecked || LIKELY(position() + n < size_)) {
629 if (LIKELY(n < kLinearScanThreshold)) {
630 for (size_t i = 0; i < n; ++i) upper_.next();
634 value_ = readLowerPart(upper_.position()) |
635 (upper_.value() << numLowerBits_);
642 bool skipTo(ValueType value) {
643 DCHECK_GE(value, value_);
644 if (value <= value_) {
646 } else if (!kUnchecked && value > lastValue_) {
650 size_t upperValue = (value >> numLowerBits_);
651 size_t upperSkip = upperValue - upper_.value();
652 // The average density of ones in upper bits is 1/2.
653 // LIKELY here seems to make things worse, even for small skips.
654 if (upperSkip < 2 * kLinearScanThreshold) {
657 } while (UNLIKELY(upper_.value() < upperValue));
659 upper_.skipToNext(upperValue);
666 bool jump(size_t n) {
667 if (LIKELY(n - 1 < size_)) { // n > 0 && n <= size_
668 value_ = readLowerPart(n - 1) | (upper_.jump(n) << numLowerBits_);
677 bool jumpTo(ValueType value) {
681 } else if (!kUnchecked && value > lastValue_) {
685 upper_.jumpToNext(value >> numLowerBits_);
690 ValueType previousValue() const {
691 DCHECK_GT(position(), 0);
692 DCHECK_LT(position(), size());
693 return readLowerPart(upper_.position() - 1) |
694 (upper_.previousValue() << numLowerBits_);
697 size_t size() const { return size_; }
699 size_t position() const { return upper_.position(); }
700 ValueType value() const { return value_; }
704 value_ = std::numeric_limits<ValueType>::max();
705 upper_.setDone(size_);
709 ValueType readLowerPart(size_t i) const {
711 const size_t pos = i * numLowerBits_;
712 const unsigned char* ptr = lower_ + (pos / 8);
713 const uint64_t ptrv = folly::loadUnaligned<uint64_t>(ptr);
714 return lowerMask_ & (ptrv >> (pos % 8));
717 void iterateTo(ValueType value) {
719 value_ = readLowerPart(upper_.position()) |
720 (upper_.value() << numLowerBits_);
721 if (LIKELY(value_ >= value)) break;
726 constexpr static size_t kLinearScanThreshold = 8;
729 const uint8_t* lower_;
730 detail::UpperBitsReader<Encoder, Instructions> upper_;
731 const ValueType lowerMask_;
732 ValueType value_ = 0;
733 ValueType lastValue_;
734 uint8_t numLowerBits_;
739 #endif // FOLLY_EXPERIMENTAL_ELIAS_FANO_CODING_H