Hey can anyone explain me following part of this question.

```
map<ll,ll>M[100011];
void join(int u,int v) {
u = f(u);
v = f(v);
if(sz[u] < sz[v]) swap(u, v);
sz[u] += sz[v];
pa[v] = u;
for(auto x:M[v]) {
for(int j=-D;j<=D;j++) {
if(M[u].find(x.f+j)!=M[u].end()) {
ans+=x.s*M[u][x.f+j];
}
}
}
for(auto x:M[v]) {
M[u][x.f]+=x.s;
}
}
```

Other part is same as general implementation as well as constraints.