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
28 #error EliasFanoCoding.h requires GCC
32 #error EliasFanoCoding.h requires x86_64
37 #include <type_traits>
38 #include <boost/noncopyable.hpp>
39 #include <glog/logging.h>
41 #include <folly/Bits.h>
42 #include <folly/CpuId.h>
43 #include <folly/Likely.h>
44 #include <folly/Range.h>
45 #include <folly/experimental/Select64.h>
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 lower, upper, skipPointers
122 // and forwardPointers. As EliasFanoCompressedList has
123 // no ownership of them, you need to call 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)),
147 EliasFanoEncoderV2(size_t size, ValueType upperBound)
148 : EliasFanoEncoderV2(
149 Layout::fromUpperBoundAndSize(upperBound, size).allocList()) {
152 void add(ValueType value) {
153 CHECK_GE(value, lastValue_);
155 const auto numLowerBits = result_.numLowerBits;
156 const ValueType upperBits = value >> numLowerBits;
158 // Upper sequence consists of upperBits 0-bits and (size_ + 1) 1-bits.
159 const size_t pos = upperBits + size_;
160 upper_[pos / 8] |= 1U << (pos % 8);
161 // Append numLowerBits bits to lower sequence.
162 if (numLowerBits != 0) {
163 const ValueType lowerBits = value & ((ValueType(1) << numLowerBits) - 1);
164 writeBits56(lower_, size_ * numLowerBits, numLowerBits, lowerBits);
167 /* static */ if (skipQuantum != 0) {
168 while ((skipPointersSize_ + 1) * skipQuantum <= upperBits) {
169 // Store the number of preceding 1-bits.
170 skipPointers_[skipPointersSize_++] = size_;
174 /* static */ if (forwardQuantum != 0) {
175 if ((size_ + 1) % forwardQuantum == 0) {
176 const auto pos = size_ / forwardQuantum;
177 // Store the number of preceding 0-bits.
178 forwardPointers_[pos] = upperBits;
186 const EliasFanoCompressedList& finish() const {
187 CHECK_EQ(size_, result_.size);
192 // Writes value (with len up to 56 bits) to data starting at pos-th bit.
193 static void writeBits56(unsigned char* data, size_t pos,
194 uint8_t len, uint64_t value) {
195 DCHECK_LE(uint32_t(len), 56);
196 DCHECK_EQ(0, value & ~((uint64_t(1) << len) - 1));
197 unsigned char* const ptr = data + (pos / 8);
198 uint64_t ptrv = folly::loadUnaligned<uint64_t>(ptr);
199 ptrv |= value << (pos % 8);
200 folly::storeUnaligned<uint64_t>(ptr, ptrv);
203 unsigned char* lower_ = nullptr;
204 unsigned char* upper_ = nullptr;
205 SkipValueType* skipPointers_ = nullptr;
206 SkipValueType* forwardPointers_ = nullptr;
208 ValueType lastValue_ = 0;
210 size_t skipPointersSize_ = 0;
212 EliasFanoCompressedList result_;
215 template <class Value,
218 size_t kForwardQuantum>
219 struct EliasFanoEncoderV2<Value,
222 kForwardQuantum>::Layout {
223 static Layout fromUpperBoundAndSize(size_t upperBound, size_t size) {
224 // numLowerBits can be at most 56 because of detail::writeBits56.
225 const uint8_t numLowerBits = std::min(defaultNumLowerBits(upperBound,
229 // Upper bits are stored using unary delta encoding.
230 // For example, (3 5 5 9) will be encoded as 1000011001000_2.
231 const size_t upperSizeBits =
232 (upperBound >> numLowerBits) + // Number of 0-bits to be stored.
234 const size_t upper = (upperSizeBits + 7) / 8;
236 // *** Validity checks.
237 // Shift by numLowerBits must be valid.
238 CHECK_LT(numLowerBits, 8 * sizeof(Value));
239 CHECK_LT(size, std::numeric_limits<SkipValueType>::max());
240 CHECK_LT(upperBound >> numLowerBits,
241 std::numeric_limits<SkipValueType>::max());
243 return fromInternalSizes(numLowerBits, upper, size);
246 static Layout fromInternalSizes(uint8_t numLowerBits,
251 layout.numLowerBits = numLowerBits;
253 layout.lower = (numLowerBits * size + 7) / 8;
254 layout.upper = upper;
256 // *** Skip pointers.
257 // Store (1-indexed) position of every skipQuantum-th
258 // 0-bit in upper bits sequence.
259 /* static */ if (skipQuantum != 0) {
260 // 8 * upper is used here instead of upperSizeBits, as that is
261 // more serialization-friendly way (upperSizeBits doesn't need
262 // to be known by this function, unlike upper).
264 // '?: 1' is a workaround for false 'division by zero'
265 // compile-time error.
266 size_t numSkipPointers = (8 * upper - size) / (skipQuantum ?: 1);
267 layout.skipPointers = numSkipPointers * sizeof(SkipValueType);
270 // *** Forward pointers.
271 // Store (1-indexed) position of every forwardQuantum-th
272 // 1-bit in upper bits sequence.
273 /* static */ if (forwardQuantum != 0) {
274 size_t numForwardPointers = size / (forwardQuantum ?: 1);
275 layout.forwardPointers = numForwardPointers * sizeof(SkipValueType);
281 size_t bytes() const {
282 return lower + upper + skipPointers + forwardPointers;
285 template <typename Range>
286 EliasFanoCompressedListBase<typename Range::iterator>
287 openList(Range& buf) const {
288 EliasFanoCompressedListBase<typename Range::iterator> result;
290 result.numLowerBits = numLowerBits;
291 result.data = buf.subpiece(0, bytes());
293 auto advance = [&] (size_t n) {
294 auto begin = buf.data();
299 result.skipPointers = advance(skipPointers);
300 result.forwardPointers = advance(forwardPointers);
301 result.lower = advance(lower);
302 result.upper = advance(upper);
307 MutableEliasFanoCompressedList allocList() const {
308 uint8_t* buf = nullptr;
309 // WARNING: Current read/write logic assumes that the 7 bytes
310 // following the last byte of lower and upper sequences are
311 // readable (stored value doesn't matter and won't be changed),
312 // so we allocate additional 7B, but do not include them in size
313 // of returned value.
315 buf = static_cast<uint8_t*>(calloc(bytes() + 7, 1));
317 folly::MutableByteRange bufRange(buf, bytes());
318 return openList(bufRange);
322 uint8_t numLowerBits = 0;
327 size_t skipPointers = 0;
328 size_t forwardPointers = 0;
331 // NOTE: It's recommended to compile EF coding with -msse4.2, starting
332 // with Nehalem, Intel CPUs support POPCNT instruction and gcc will emit
333 // it for __builtin_popcountll intrinsic.
334 // But we provide an alternative way for the client code: it can switch to
335 // the appropriate version of EliasFanoReader<> in realtime (client should
336 // implement this switching logic itself) by specifying instruction set to
338 namespace instructions {
341 static bool supported(const folly::CpuId& cpuId = {}) {
344 static inline uint64_t popcount(uint64_t value) {
345 return __builtin_popcountll(value);
347 static inline int ctz(uint64_t value) {
349 return __builtin_ctzll(value);
351 static inline uint64_t blsr(uint64_t value) {
352 return value & (value - 1);
356 struct Nehalem : public Default {
357 static bool supported(const folly::CpuId& cpuId = {}) {
358 return cpuId.popcnt();
360 static inline uint64_t popcount(uint64_t value) {
361 // POPCNT is supported starting with Intel Nehalem, AMD K10.
363 asm ("popcntq %1, %0" : "=r" (result) : "r" (value));
368 struct Haswell : public Nehalem {
369 static bool supported(const folly::CpuId& cpuId = {}) {
370 return Nehalem::supported(cpuId) && cpuId.bmi1();
372 static inline uint64_t blsr(uint64_t value) {
373 // BMI1 is supported starting with Intel Haswell, AMD Piledriver.
374 // BLSR combines two instuctions into one and reduces register pressure.
376 asm ("blsrq %1, %0" : "=r" (result) : "r" (value));
381 } // namespace instructions
385 template <class Encoder, class Instructions>
386 class UpperBitsReader {
387 typedef typename Encoder::SkipValueType SkipValueType;
389 typedef typename Encoder::ValueType ValueType;
391 explicit UpperBitsReader(const EliasFanoCompressedList& list)
392 : forwardPointers_(list.forwardPointers),
393 skipPointers_(list.skipPointers),
399 block_ = start_ != nullptr ? folly::loadUnaligned<block_t>(start_) : 0;
406 size_t position() const { return position_; }
407 ValueType value() const { return value_; }
410 // Skip to the first non-zero block.
411 while (block_ == 0) {
412 outer_ += sizeof(block_t);
413 block_ = folly::loadUnaligned<block_t>(start_ + outer_);
417 inner_ = Instructions::ctz(block_);
418 block_ = Instructions::blsr(block_);
423 ValueType skip(size_t n) {
426 position_ += n; // n 1-bits will be read.
428 // Use forward pointer.
429 if (Encoder::forwardQuantum > 0 && n > Encoder::forwardQuantum) {
430 // Workaround to avoid 'division by zero' compile-time error.
431 constexpr size_t q = Encoder::forwardQuantum ?: 1;
433 const size_t steps = position_ / q;
435 folly::loadUnaligned<SkipValueType>(
436 forwardPointers_ + (steps - 1) * sizeof(SkipValueType));
438 reposition(dest + steps * q);
439 n = position_ + 1 - steps * q; // n is > 0.
440 // Correct inner_ will be set at the end.
444 // Find necessary block.
445 while ((cnt = Instructions::popcount(block_)) < n) {
447 outer_ += sizeof(block_t);
448 block_ = folly::loadUnaligned<block_t>(start_ + outer_);
451 // Skip to the n-th one in the block.
453 inner_ = select64<Instructions>(block_, n - 1);
454 block_ &= (block_t(-1) << inner_) << 1;
459 // Skip to the first element that is >= v and located *after* the current
460 // one (so even if current value equals v, position will be increased by 1).
461 ValueType skipToNext(ValueType v) {
462 DCHECK_GE(v, value_);
465 if (Encoder::skipQuantum > 0 && v >= value_ + Encoder::skipQuantum) {
466 // Workaround to avoid 'division by zero' compile-time error.
467 constexpr size_t q = Encoder::skipQuantum ?: 1;
469 const size_t steps = v / q;
471 folly::loadUnaligned<SkipValueType>(
472 skipPointers_ + (steps - 1) * sizeof(SkipValueType));
474 reposition(dest + q * steps);
475 position_ = dest - 1;
477 // Correct inner_ and value_ will be set during the next()
480 // NOTE: Corresponding block of lower bits sequence may be
481 // prefetched here (via __builtin_prefetch), but experiments
482 // didn't show any significant improvements.
487 size_t skip = v - (8 * outer_ - position_ - 1);
489 constexpr size_t kBitsPerBlock = 8 * sizeof(block_t);
490 while ((cnt = Instructions::popcount(~block_)) < skip) {
492 position_ += kBitsPerBlock - cnt;
493 outer_ += sizeof(block_t);
494 block_ = folly::loadUnaligned<block_t>(start_ + outer_);
498 auto inner = select64<Instructions>(~block_, skip - 1);
499 position_ += inner - skip + 1;
500 block_ &= block_t(-1) << inner;
507 ValueType jump(size_t n) {
508 if (Encoder::forwardQuantum == 0 || n <= Encoder::forwardQuantum) {
511 position_ = -1; // Avoid reading the head, skip() will reposition.
516 ValueType jumpToNext(ValueType v) {
517 if (Encoder::skipQuantum == 0 || v < Encoder::skipQuantum) {
520 value_ = 0; // Avoid reading the head, skipToNext() will reposition.
522 return skipToNext(v);
525 void setDone(size_t endPos) {
530 ValueType setValue() {
531 value_ = static_cast<ValueType>(8 * outer_ + inner_ - position_);
535 void reposition(size_t dest) {
537 block_ = folly::loadUnaligned<block_t>(start_ + outer_);
538 block_ &= ~((block_t(1) << (dest % 8)) - 1);
541 typedef unsigned long long block_t;
542 const unsigned char* const forwardPointers_;
543 const unsigned char* const skipPointers_;
544 const unsigned char* const start_;
546 size_t outer_; // Outer offset: number of consumed bytes in upper.
547 size_t inner_; // Inner offset: (bit) position in current block.
548 size_t position_; // Index of current value (= #reads - 1).
552 } // namespace detail
554 template <class Encoder,
555 class Instructions = instructions::Default>
556 class EliasFanoReader : private boost::noncopyable {
558 typedef Encoder EncoderType;
559 typedef typename Encoder::ValueType ValueType;
561 explicit EliasFanoReader(const EliasFanoCompressedList& list)
565 lowerMask_((ValueType(1) << list.numLowerBits) - 1),
566 numLowerBits_(list.numLowerBits) {
567 DCHECK(Instructions::supported());
568 // To avoid extra branching during skipTo() while reading
569 // upper sequence we need to know the last element.
570 if (UNLIKELY(list.size == 0)) {
574 ValueType lastUpperValue = 8 * list.upperSize() - size_;
575 auto it = list.upper + list.upperSize() - 1;
577 lastUpperValue -= 8 - folly::findLastSet(*it);
578 lastValue_ = readLowerPart(size_ - 1) | (lastUpperValue << numLowerBits_);
587 if (UNLIKELY(position() + 1 >= size_)) {
591 value_ = readLowerPart(upper_.position()) |
592 (upper_.value() << numLowerBits_);
596 bool skip(size_t n) {
599 if (LIKELY(position() + n < size_)) {
600 if (LIKELY(n < kLinearScanThreshold)) {
601 for (size_t i = 0; i < n; ++i) upper_.next();
605 value_ = readLowerPart(upper_.position()) |
606 (upper_.value() << numLowerBits_);
613 bool skipTo(ValueType value) {
614 DCHECK_GE(value, value_);
615 if (value <= value_) {
617 } else if (value > lastValue_) {
621 size_t upperValue = (value >> numLowerBits_);
622 size_t upperSkip = upperValue - upper_.value();
623 // The average density of ones in upper bits is 1/2.
624 // LIKELY here seems to make things worse, even for small skips.
625 if (upperSkip < 2 * kLinearScanThreshold) {
628 } while (UNLIKELY(upper_.value() < upperValue));
630 upper_.skipToNext(upperValue);
637 bool jump(size_t n) {
638 if (LIKELY(n - 1 < size_)) { // n > 0 && n <= size_
639 value_ = readLowerPart(n - 1) | (upper_.jump(n) << numLowerBits_);
648 bool jumpTo(ValueType value) {
652 } else if (value > lastValue_) {
656 upper_.jumpToNext(value >> numLowerBits_);
661 size_t size() const { return size_; }
663 size_t position() const { return upper_.position(); }
664 ValueType value() const { return value_; }
668 value_ = std::numeric_limits<ValueType>::max();
669 upper_.setDone(size_);
673 ValueType readLowerPart(size_t i) const {
675 const size_t pos = i * numLowerBits_;
676 const unsigned char* ptr = lower_ + (pos / 8);
677 const uint64_t ptrv = folly::loadUnaligned<uint64_t>(ptr);
678 return lowerMask_ & (ptrv >> (pos % 8));
681 void iterateTo(ValueType value) {
683 value_ = readLowerPart(upper_.position()) |
684 (upper_.value() << numLowerBits_);
685 if (LIKELY(value_ >= value)) break;
690 constexpr static size_t kLinearScanThreshold = 8;
693 const uint8_t* lower_;
694 detail::UpperBitsReader<Encoder, Instructions> upper_;
695 const ValueType lowerMask_;
696 ValueType value_ = 0;
697 ValueType lastValue_;
698 uint8_t numLowerBits_;
703 #endif // FOLLY_EXPERIMENTAL_ELIAS_FANO_CODING_H