在 std::bitset 中查找设置为 true 的最高索引

Question

I'm using std::bitset<9> deck in my program. By default it initializes all bits to 0.

Somewhere in my code I'm doing: deck.flip(index) where index can be any number from 0-8 (bitset indexes).

Now what I need is to find the index of the last bit set to true .

Eg:

000100010 - bitset
876543210 - index

In this example the latest bit set to true is at index 5 . That is what I would like to find using std::bitset .

I have read the documentation https://en.cppreference.com/w/cpp/utility/bitset and found no useful function to do this, and as a beginner I'm having a hard time figuring out, how can I do this?

(Sorry if question is poorly written, english is my second language)

Answer 1

It seems that only option is to iterate over bit set and find last bit set since bit set does not have support for iterators.

#include <iostream>
#include <bitset>
#include <optional>

template <size_t N>
constexpr std::optional<int> find_last_index_of_bit_set(std::bitset<N> set)
{
    for (int i = set.size()-1; i > -1; --i) {
      if (set.test(i)) {
          return std::optional(i);
      }
    }
    return std::nullopt;
}

int main() 
{
    std::bitset<10> bitSet("000100010");

    auto pos = find_last_index_of_bit_set(bitSet);

    if (pos.has_value()) {
        std::cout << "Max index is: " << pos.value() << '\n';
    } else {
        std::cout << "No bit set";
    }
}

Answer 2

Here's a gcc/clang specific way using the __builtin_clzl() intrinsic, which returns the number of leading 0 bits starting from the most significant:

#include <bitset>
#include <cassert>
#include <climits>
#include <iostream>

template<std::size_t N>
int msb(const std::bitset<N> &bs) {
  static_assert(N <= (CHAR_BIT * sizeof(unsigned long)), "bitset is too big");
  auto n = bs.to_ulong();
  // C++20 will have std::countl_zero() but in the meantime:
  assert(n != 0); // clz is undefined if n is 0. Maybe return -1 in this case?
  return (CHAR_BIT * sizeof n) - __builtin_clzl(n) - 1;
}

int main() {
  std::bitset<9> bs;
  bs[1] = 1;
  bs[5] = 1;
  std::cout << "Highest set bit is " << msb(bs) << '\n';
  return 0;
}

will print Highest set bit is 5 . This approach only works with bitsets equal or smaller than the number of bits in an unsigned long , of course (Though it can go up to unsigned long long with a few minor changes)