1/*
2 * Copyright 2011-present Facebook, Inc.
3 *
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
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
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.
15 */
16
17/**
18 * BitIterator
19 * Wrapper around an iterator over an integral type that iterates
20 * over its underlying bits in MSb to LSb order
21 *
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
25 *
26 * @author Tudor Bosman (tudorb@fb.com)
27 */
28
29#pragma once
30
31#include <cassert>
32#include <cinttypes>
33#include <cstdint>
34#include <cstring>
35#include <iterator>
36#include <limits>
37#include <type_traits>
38
39#include <boost/iterator/iterator_adaptor.hpp>
40
41#include <folly/Portability.h>
42#include <folly/container/detail/BitIteratorDetail.h>
43#include <folly/lang/Bits.h>
44
45namespace folly {
46
47/**
48 * Fast bit iteration facility.
49 */
50
51template <class BaseIter>
52class BitIterator;
53template <class BaseIter>
54BitIterator<BaseIter> findFirstSet(
55 BitIterator<BaseIter>,
56 BitIterator<BaseIter>);
57/**
58 * Wrapper around an iterator over an integer type that iterates
59 * over its underlying bits in LSb to MSb order.
60 *
61 * BitIterator models the same iterator concepts as the base iterator.
62 */
63template <class BaseIter>
64class BitIterator : public bititerator_detail::BitIteratorBase<BaseIter>::type {
65 public:
66 /**
67 * Return the number of bits in an element of the underlying iterator.
68 */
69 static unsigned int bitsPerBlock() {
70 return std::numeric_limits<typename std::make_unsigned<
71 typename std::iterator_traits<BaseIter>::value_type>::type>::digits;
72 }
73
74 /**
75 * Construct a BitIterator that points at a given bit offset (default 0)
76 * in iter.
77 */
78 explicit BitIterator(const BaseIter& iter, size_t bitOff = 0)
79 : bititerator_detail::BitIteratorBase<BaseIter>::type(iter),
80 bitOffset_(bitOff) {
81 assert(bitOffset_ < bitsPerBlock());
82 }
83
84 size_t bitOffset() const {
85 return bitOffset_;
86 }
87
88 void advanceToNextBlock() {
89 bitOffset_ = 0;
90 ++this->base_reference();
91 }
92
93 BitIterator& operator=(const BaseIter& other) {
94 this->~BitIterator();
95 new (this) BitIterator(other);
96 return *this;
97 }
98
99 private:
100 friend class boost::iterator_core_access;
101 friend BitIterator findFirstSet<>(BitIterator, BitIterator);
102
103 typedef bititerator_detail::BitReference<
104 typename std::iterator_traits<BaseIter>::reference,
105 typename std::iterator_traits<BaseIter>::value_type>
106 BitRef;
107
108 void advanceInBlock(size_t n) {
109 bitOffset_ += n;
110 assert(bitOffset_ < bitsPerBlock());
111 }
112
113 BitRef dereference() const {
114 return BitRef(*this->base_reference(), bitOffset_);
115 }
116
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) {
122 bitOffset_ -= bpb;
123 ++blocks;
124 }
125 this->base_reference() += blocks;
126 }
127
128 void increment() {
129 if (++bitOffset_ == bitsPerBlock()) {
130 advanceToNextBlock();
131 }
132 }
133
134 void decrement() {
135 if (bitOffset_-- == 0) {
136 bitOffset_ = bitsPerBlock() - 1;
137 --this->base_reference();
138 }
139 }
140
141 bool equal(const BitIterator& other) const {
142 return (
143 bitOffset_ == other.bitOffset_ &&
144 this->base_reference() == other.base_reference());
145 }
146
147 ssize_t distance_to(const BitIterator& other) const {
148 return ssize_t(
149 (other.base_reference() - this->base_reference()) * bitsPerBlock() +
150 other.bitOffset_ - bitOffset_);
151 }
152
153 size_t bitOffset_;
154};
155
156/**
157 * Helper function, so you can write
158 * auto bi = makeBitIterator(container.begin());
159 */
160template <class BaseIter>
161BitIterator<BaseIter> makeBitIterator(const BaseIter& iter) {
162 return BitIterator<BaseIter>(iter);
163}
164
165/**
166 * Find first bit set in a range of bit iterators.
167 * 4.5x faster than the obvious std::find(begin, end, true);
168 */
169template <class BaseIter>
170BitIterator<BaseIter> findFirstSet(
171 BitIterator<BaseIter> begin,
172 BitIterator<BaseIter> end) {
173 // shortcut to avoid ugly static_cast<>
174 static const typename std::iterator_traits<BaseIter>::value_type one = 1;
175
176 while (begin.base() != end.base()) {
177 typename std::iterator_traits<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);
181 if (firstSet) {
182 --firstSet; // now it's 0-based
183 assert(firstSet >= begin.bitOffset());
184 begin.advanceInBlock(firstSet - begin.bitOffset());
185 return begin;
186 }
187 begin.advanceToNextBlock();
188 }
189
190 // now begin points to the same block as end
191 if (end.bitOffset() != 0) { // assume end is dereferenceable
192 typename std::iterator_traits<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);
198 if (firstSet) {
199 --firstSet; // now it's 0-based
200 assert(firstSet >= begin.bitOffset());
201 begin.advanceInBlock(firstSet - begin.bitOffset());
202 return begin;
203 }
204 }
205
206 return end;
207}
208
209} // namespace folly
210