Hello sirs, you might know that Boost has a dynamically sized bitset class, but did you know that GCC also has one? Turns out it's included in the tr2 folder. You can simply include the <tr2/dynamic_bitset> header, and use it as std::tr2::dynamic_bitset. Yes, it works on Codeforces.

Here's a code example to count triangles in a graph (abc258_g):

#include <bits/stdc++.h>
using namespace std;

#include <tr2/dynamic_bitset>
using namespace tr2;
signed main() {
    cin.tie(0)->sync_with_stdio(0); cin.exceptions(cin.failbit);
    int n; cin >> n;
    vector<dynamic_bitset<>> adj(n);
    for(int i = 0; i < n; i++){
        adj[i].resize(n);
        for(int j = 0; j < n; j++){
            char c; cin >> c;
            adj[i][j] = c-'0';
        }
    }
    int64_t ans = 0;
    for(int i = 0; i < n; i++) for(int j = i+1; j < n; j++) if(adj[i][j]){
        ans += (adj[i]&adj[j]).count();
    }
    cout << ans/3 << "\n";
}

In some problems, we might not be able to use a constant sized bitset. For example, on 1856E2 - PermuTree (hard version). Here's a submission where we replaced neal's custom_bitset template with using custom_bitset = tr2::dynamic_bitset<>;, and got accepted with little performance difference (260853192, original 217308610).

The implementation of the bitset seems identical to a version of Boost's dynamic bitset, so you can read the docs here. You can view the source here.

Comparing to std::bitset, here are some notable things I saw:

• They include more set operations, such as set difference, and checking if a bitset is a subset of another bitset.
• They include lexicographical comparisons between bitsets.
• You can append single bits, as well as whole blocks (i.e. integers)
• You can also resize it like a vector. (For some reason there's no pop_back though...)
• Find first and find next are also supported, without weird function names like in std::bitset. Unfortunately, there's still no find previous :(
• You can specify the word type and allocator as template arguments.

Of course, it also has all the normal std::bitset functionality. However, I'm not sure how fast this is compared to std::bitset; you can let me know in the comments. Interestingly, it seems to be using 64 bit integers on Codeforces.

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Thanks qmk for helping with the blog.