folly/folly/container/BitIterator.h

/*
 * 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.
 */

/**
 * BitIterator
 *    Wrapper around an iterator over an integral type that iterates
 *    over its underlying bits in MSb to LSb order
 *
 * findFirstSet(BitIterator begin, BitIterator end)
 *    return a BitIterator pointing to the first 1 bit in [begin, end), or
 *    end if all bits in [begin, end) are 0
 */

#pragma once

#include <cassert>
#include <cinttypes>
#include <cstdint>
#include <cstring>
#include <iterator>
#include <limits>
#include <type_traits>

#include <boost/iterator/iterator_adaptor.hpp>

#include <folly/Portability.h>
#include <folly/container/detail/BitIteratorDetail.h>
#include <folly/lang/Bits.h>

namespace folly {

/**
 * Fast bit iteration facility.
 */

template <class BaseIter>
class BitIterator;
template <class BaseIter>
BitIterator<BaseIter> findFirstSet(
    BitIterator<BaseIter>, BitIterator<BaseIter>);
/**
 * Wrapper around an iterator over an integer type that iterates
 * over its underlying bits in LSb to MSb order.
 *
 * BitIterator models the same iterator concepts as the base iterator.
 */
template <class BaseIter>
class BitIterator : public bititerator_detail::BitIteratorBase<BaseIter>::type { … };

/**
 * Helper function, so you can write
 * auto bi = makeBitIterator(container.begin());
 */
template <class BaseIter>
BitIterator<BaseIter> makeBitIterator(const BaseIter& iter) { … }

/**
 * Find first bit set in a range of bit iterators.
 * 4.5x faster than the obvious std::find(begin, end, true);
 */
template <class BaseIter>
BitIterator<BaseIter> findFirstSet(
    BitIterator<BaseIter> begin, BitIterator<BaseIter> end) { … }

} // namespace folly