2 * Copyright 2017 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.
19 * Wrapper around an iterator over an integral type that iterates
20 * over its underlying bits in MSb to LSb order
22 * findFirstSet(BitIterator begin, BitIterator end)
23 * return a BitIterator pointing to the first 1 bit in [begin, end), or
24 * end if all bits in [begin, end) are 0
26 * @author Tudor Bosman (tudorb@fb.com)
37 #include <type_traits>
39 #include <boost/iterator/iterator_adaptor.hpp>
41 #include <folly/Portability.h>
42 #include <folly/detail/BitIteratorDetail.h>
43 #include <folly/lang/Bits.h>
48 * Fast bit iteration facility.
51 template <class BaseIter>
53 template <class BaseIter>
54 BitIterator<BaseIter> findFirstSet(
55 BitIterator<BaseIter>,
56 BitIterator<BaseIter>);
58 * Wrapper around an iterator over an integer type that iterates
59 * over its underlying bits in LSb to MSb order.
61 * BitIterator models the same iterator concepts as the base iterator.
63 template <class BaseIter>
64 class BitIterator : public bititerator_detail::BitIteratorBase<BaseIter>::type {
67 * Return the number of bits in an element of the underlying iterator.
69 static unsigned int bitsPerBlock() {
70 return std::numeric_limits<typename std::make_unsigned<
71 typename std::iterator_traits<BaseIter>::value_type>::type>::digits;
75 * Construct a BitIterator that points at a given bit offset (default 0)
78 explicit BitIterator(const BaseIter& iter, size_t bitOff = 0)
79 : bititerator_detail::BitIteratorBase<BaseIter>::type(iter),
81 assert(bitOffset_ < bitsPerBlock());
84 size_t bitOffset() const {
88 void advanceToNextBlock() {
90 ++this->base_reference();
93 BitIterator& operator=(const BaseIter& other) {
95 new (this) BitIterator(other);
100 friend class boost::iterator_core_access;
101 friend BitIterator findFirstSet<>(BitIterator, BitIterator);
103 typedef bititerator_detail::BitReference<
104 typename std::iterator_traits<BaseIter>::reference,
105 typename std::iterator_traits<BaseIter>::value_type>
108 void advanceInBlock(size_t n) {
110 assert(bitOffset_ < bitsPerBlock());
113 BitRef dereference() const {
114 return BitRef(*this->base_reference(), bitOffset_);
117 void advance(ssize_t n) {
118 size_t bpb = bitsPerBlock();
119 ssize_t blocks = n / ssize_t(bpb);
120 bitOffset_ += n % bpb;
121 if (bitOffset_ >= bpb) {
125 this->base_reference() += blocks;
129 if (++bitOffset_ == bitsPerBlock()) {
130 advanceToNextBlock();
135 if (bitOffset_-- == 0) {
136 bitOffset_ = bitsPerBlock() - 1;
137 --this->base_reference();
141 bool equal(const BitIterator& other) const {
143 bitOffset_ == other.bitOffset_ &&
144 this->base_reference() == other.base_reference());
147 ssize_t distance_to(const BitIterator& other) const {
149 (other.base_reference() - this->base_reference()) * bitsPerBlock() +
150 other.bitOffset_ - bitOffset_);
157 * Helper function, so you can write
158 * auto bi = makeBitIterator(container.begin());
160 template <class BaseIter>
161 BitIterator<BaseIter> makeBitIterator(const BaseIter& iter) {
162 return BitIterator<BaseIter>(iter);
166 * Find first bit set in a range of bit iterators.
167 * 4.5x faster than the obvious std::find(begin, end, true);
169 template <class BaseIter>
170 BitIterator<BaseIter> findFirstSet(
171 BitIterator<BaseIter> begin,
172 BitIterator<BaseIter> end) {
173 // shortcut to avoid ugly static_cast<>
174 static const typename BaseIter::value_type one = 1;
176 while (begin.base() != end.base()) {
177 typename BaseIter::value_type v = *begin.base();
178 // mask out the bits that don't matter (< begin.bitOffset)
179 v &= ~((one << begin.bitOffset()) - 1);
180 size_t firstSet = findFirstSet(v);
182 --firstSet; // now it's 0-based
183 assert(firstSet >= begin.bitOffset());
184 begin.advanceInBlock(firstSet - begin.bitOffset());
187 begin.advanceToNextBlock();
190 // now begin points to the same block as end
191 if (end.bitOffset() != 0) { // assume end is dereferenceable
192 typename BaseIter::value_type v = *begin.base();
193 // mask out the bits that don't matter (< begin.bitOffset)
194 v &= ~((one << begin.bitOffset()) - 1);
195 // mask out the bits that don't matter (>= end.bitOffset)
196 v &= (one << end.bitOffset()) - 1;
197 size_t firstSet = findFirstSet(v);
199 --firstSet; // now it's 0-based
200 assert(firstSet >= begin.bitOffset());
201 begin.advanceInBlock(firstSet - begin.bitOffset());