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# (Optional) Bitsets

Author: Benjamin Qi

### Prerequisites

• Errichto - Bitwise Operations Pt 1

Examples of how bitsets give some unintended solutions on recent USACO problems.

## Tutorial

tl;dr some operations are around 32-64x faster compared to a boolean array. See the C++ Reference for the operations you can perform.

Resources
CPHspeeding up Hamming distance
CF

## Knapsack

Focus Problem – read through this problem before continuing!

Of course, the first step is to generate the sizes of each connected component.

1#include <bits/stdc++.h>2using namespace std;3
4struct DSU {5    vector<int> e; void init(int N) { e = vector<int>(N,-1); }6    int get(int x) { return e[x] < 0 ? x : e[x] = get(e[x]); }7    bool sameSet(int a, int b) { return get(a) == get(b); }8    int size(int x) { return -e[get(x)]; }9    bool unite(int x, int y) { // union by size10        x = get(x), y = get(y); if (x == y) return 0;

A naive knapsack solution would be as follows. For each $0\le i\le \texttt{comps.size()}$, let $\texttt{dp}[i][j]=1$ if there exists a subset of the first $i$ components whose sizes sum to $j$. Then the answer will be stored in $\texttt{dp}[i]$. This runs in $O(N^2)$ and is too slow if implemented naively, but we can use bitset to speed it up!

Note: you can't store all $N$ bitsets in memory at the same time (more on that below).

Full Solution

Challenge: This solution runs in $\approx 0.3\text{s}$ when $N=10^5$ and there are no edges. Find a faster solution which can also be sped up with bitset (my solution runs in 0.03s).

## Cowpatibility (Gold)

Focus Problem – read through this problem before continuing!

Label the cows from $0\ldots N-1$. For two cows $x$ and $y$ set adj[x][y]=1 if they share a common flavor. Then the number of pairs of cows that are compatible (counting each pair where $x$ and $y$ are distinct twice) is equal to the sum of adj[x].count() over all $x$. It remains to compute adj[x] for all $x$.

Unfortunately, storing $N$ bitsets each with $N$ bits takes up $\frac{50000^2}{32}\cdot 4=312.5\cdot 10^6$ bytes of memory, which is greater than USACO's $256$ megabyte limit. We can reduce the memory usage by half in exchange for a slight increase in time by first computing the adjacency bitsets for all $x\in [0,N/2)$, and then for all $x\in [N/2,N)$ afterwards.

First, we read in all of the flavors.

1#include <bits/stdc++.h>2using namespace std;3
4typedef long long ll;5typedef bitset<50000> B;6const int HALF = 25000;7
8int N;9B adj[HALF];10vector<int> flav;

Then for each flavor, we can look at all pairs of cows that share that flavor and update the adjacency lists for those $x\in [0,HALF)$.

1int main() {2    input();3    for (int i = 1; i <= 1000000; ++i)4        for (int x: flav[i]) if (x < HALF) 5            for (int y: flav[i]) adj[x][y] = 1;6    for (int i = 0; i < HALF; ++i) ans += adj[i].count();7}

adj[i].count() runs quickly enough since its runtime is divided by the bitset constant. However, looping over all cows in flav[i] is too slow if say, flav[i] contains all cows. Then the nested loop could take $\Theta(N^2)$ time! Of course, we can instead write the nested loop in a way that takes advantage of fast bitset operations once again.

1for (int i = 1; i <= 1000000; ++i) if (flav[i].size() > 0) {2    B b; for (int x: flav[i]) b[x] = 1;3    for (int x: flav[i]) if (x < HALF) adj[x] |= b;4}

The full main function is as follows:

Full Solution

Apparently no test case contains more than $25000$ distinct colors, so we don't actually need to split the calculation into two halves.

## Lots of Triangles

Focus Problem – read through this problem before continuing!

First, we read in the input data. cross(a,b,c) is positive iff c lies to the left of the line from a to b.

1#include <bits/stdc++.h>2using namespace std;3
4typedef long long ll;5typedef pair<ll,ll> P;6
7#define f first8#define s second9
10ll cross(P a, P b, P c) {

There are $O(N^3)$ possible lots. Trying all possible lots and counting the number of trees that lie within each in $O(N)$ for a total time complexity of $O(N^4)$ should solve somewhere between 2 and 5 test cases. Given a triangle t, t, t with positive area, tree x lies within it iff x is to the left of each of sides (t,t),(t,t), and (t,t).

1int main() {2    input();3    vector<int> res(N-2);4    for (int i = 0; i < N; ++i) for (int j = i+1; j < N; ++j)5        for (int k = j+1; k < N; ++k) {6            vector<int> t = {i,j,k};7            if (cross(v[t],v[t],v[t]) < 0) swap(t,t);8            int cnt = 0;9            for (int x = 0; x < N; ++x) {10                if (cross(v[t],v[t],v[x]) <= 0) continue;

The analysis describes how to count the number of trees within a lot in $O(1)$, which is sufficient to solve the problem. However, $O(N)$ is actually sufficient as long as we divide by the bitset constant. Let b[i][j][k]=1 if k lies to the left of side (i,j). Then x lies within triangle (t,t,t) as long as b[t][t][x]=b[t][t][x]=b[t][t][x]=1. We can count the number of x such that this holds true by taking the bitwise AND of the bitsets for all three sides and then counting the number of bits in the result.

Fast Solution

## Knapsack Again

(GP of Bytedance 2020 F)

Given $n$ ($n\le 2\cdot 10^4$) positive integers $a_1,\ldots,a_n$ ($a_i\le 2\cdot 10^4$), find the max possible sum of a subset of $a_1,\ldots,a_n$ whose sum does not exceed $c$.

Consider the case when $\sum a_i\ge c$. The intended solution runs in $O(n\cdot \max(a_i))$; see here for more information. However, we'll solve it with bitset instead.

As with the first problem in this module, let $\texttt{dp}[i][j]=1$ if there exists a subset of the first numbers components that sums to $j$. This solution runs in $O(n\cdot \sum a_i)$ time, which is too slow even if we use bitset.

Taking inspiration from this CF blog post, we'll first shuffle the integers randomly and perform the DP with the following modification:

• If $\left|\frac{ci}{n}-j\right| \ge X$ for some $X$ that we choose, then set $\texttt{dp}[i][j]=0$.

Since we only need to keep track of $2X+1$ values for each $i$, this solution runs in $O(nX)$ time, which runs in time with $X=5\cdot 10^5$ using bitset.

Intuitively, the random shuffle reduces the optimal subset to some random walk which should have variance at most $\max a_i\cdot \sqrt N$, so it suffices to take $X\approx \max a_i\cdot \sqrt N$. (Though I'm not completely convinced that this works, does anyone know how to bound the failure probability of this algorithm precisely?)

Solution

## Other Applications

Use to speed up the following:

• Gaussian Elimination in $O(N^3)$
• Bipartite matching in $O(N^3)$ (dense graph with $N$ vertices on each side)
• Though $O(NM)$ Kuhn's algorithm is probably fast enough ...
• BFS through dense graph in $O(N^2)$
• In general, passing solutions with an additional factor of $N$.

Operations such as _Find_first() and _Find_next() mentioned in Errichto's blog can be helpful.

Resources
GFGonly resource I could find :P

A comment regarding the last two applications:

StatusSourceProblem NameDifficultyTagsSolution
CSAHard
Show Tags

DSU

Check CSA

In USACO Camp, a similar problem appeared with $N\le 10^5$ and a $6$ second time limit (presumably to allow $O(N\log ^2N)$ solutions to pass). I had already done this problem but forgot how I had solved it decided to try something new. Try to guess what I did!

## Problems

StatusSourceProblem NameDifficultyTagsSolution
PlatNormal
Show Tags

Bitset, Sliding Window

External Sol
Baltic OINormal
Show Tags

Bitset

External Sol
Baltic OINormal
Show Tags

Knapsack, Bitset

IZhOHard
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Knapsack, Bitset

View Solution
Baltic OIHard
Show Tags

Knapsack, Bitset

COCIHard
Show Tags

Bitset

View Solution