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iterative_peeling.cpp
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iterative_peeling.cpp
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#include <iostream>
#include <fstream>
#include <string>
#include <stdio.h>
#include <stdlib.h>
#include<cassert>
#include <vector>
#include <queue>
#include<list>
#include <set>
#include<cstring>
#include<ctime>
#include <unordered_set>
#include <algorithm>
#include <numeric>
#include <chrono>
using namespace std;
////////////////////////////////////////////////////////////////////////////////////////
// Helper for fast input
inline char GET_CHAR(){
const int maxn = 131072;
static char buf[maxn],*p1=buf,*p2=buf;
return p1==p2&&(p2=(p1=buf)+fread(buf,1,maxn,stdin),p1==p2)?EOF:*p1++;
}
inline int getInt() {
int res(0);
char c = GET_CHAR();
while(c < '0') c = GET_CHAR();
while(c >= '0') {
res = res * 10 + (c - '0');
c = GET_CHAR();
}
return res;
}
////////////////////////////////////////////////////////////////////////////////////////
int n, m;
struct Edge {
int y, next;
};
struct Node {
int deg, next, prev, idx;
inline void clear() {
deg = next = prev = 0;
idx = -1;
}
};
Node * lists;
__inline void linklists (int x, int y) {
if(y == 0) return;
lists[x].next = y;
lists[y].prev = x;
};
int * nxt, * prv, *itr;
__inline void linknodes (int x, int y) {
if(y == -1) return;
nxt[x] = y;
prv[y] = x;
};
__inline void eraselist(int x) {
lists[lists[x].prev].next = lists[x].next;
if(lists[x].next != 0) lists[lists[x].next].prev = lists[x].prev;
};
__inline void erasenode (int x) {
if(prv[x] == -1) {
lists[itr[x]].idx = nxt[x];
}
if(prv[x] != -1) nxt[prv[x]] = nxt[x];
if(nxt[x] != -1) prv[nxt[x]] = prv[x];
};
int l = 0;
Edge * edges;
int * idx;
__inline void build(int x, int y) {
edges[++l].next = idx[x];
edges[l].y = y;
idx[x] = l;
};
//////////////////////////////////////////////////////////////////////////////////////////////////////
// Main
int main(int argc, char** argv) {
auto startio = chrono::steady_clock::now();
int iters = atoi(argv[1]);
n = getInt(); m = getInt();
edges = new Edge[m * 2 + 10];
idx = new int[n];
memset(idx, 0, sizeof(int) * n);
int * init_deg = new int[n];
memset(init_deg, 0, sizeof(int) * n);
l = 0;
lists = new Node[n + 2 * m + 10];
int n_list = 0;
itr = new int[n];
int * deg = new int[n], * w = new int[n], * pos = new int [n];
memset(deg, 0, sizeof(int) * n);
memset(w, 0, sizeof(int) * n); //initial vertex weights=0, i.e., no self loops at the start
memset(pos, 0, sizeof(int) * n);
prv = new int[n]; nxt = new int[n];
for (int i = 0; i < m; i++) {
int p, q;
p = getInt(); q = getInt();
p -= 1;
q -= 1;
build(p, q);
build(q, p);
init_deg[p]++;
init_deg[q]++;
}
pair<int, int> * deg_sorted = new pair<int, int>[n];
vector<int> m_ans;
double mm_density = 0;
auto endio = chrono::steady_clock::now();
int sum_iter_times = 0;
int init_time = chrono::duration_cast<chrono::milliseconds>(endio - startio).count();
cout << "Time for reading input and initialization: " << init_time << " ms" << endl;
for (int tt = 0; tt < iters; tt++) {
auto startiter = chrono::steady_clock::now();
for (int i = 0; i < n; i++) {
nxt[i] = prv[i] = -1;
pos[i] = 0;
deg[i] = w[i] + init_deg[i]; //degree for this iteration is "vertex weight" + actual degree
deg_sorted[i] = make_pair(deg[i], i);
}
sort(deg_sorted, deg_sorted + n);
n_list = 0;
for(int i = 0; i < n; i++) {
int v = deg_sorted[i].second;
if(n_list == 0 || lists[n_list].deg != deg_sorted[i].first) {
++n_list;
lists[n_list].clear();
linklists(n_list - 1, n_list);
lists[n_list].deg = deg_sorted[i].first;
}
linknodes(v, lists[n_list].idx);
lists[n_list].idx = v;
itr[v] = n_list;
}
double max_density = (double)m / n;
int cur_m = m, cur_n = n;
vector<int> ans;
int max_size = 0;
while(lists[0].next) {
int i = lists[0].next;
int k = lists[i].idx;
if(nxt[k] == -1) {
eraselist(i);
}else {
erasenode(k);
}
pos[k] = -1;
w[k] = deg[k]; //increment vertex weight for the next iteration (self loops)
cur_n -= 1;
ans.push_back(k);
for (int p = idx[k]; p; p = edges[p].next) { //decrement degrees of k's neighbors
int j = edges[p].y;
if(pos[j] == -1) continue;
cur_m -= 1;
int i = itr[j];
erasenode(j);
int i1 = lists[i].prev;
if(lists[i].idx == -1) eraselist(i);
deg[j]--;
prv[j] = nxt[j] = -1;
if(i1 == 0 || lists[i1].deg != deg[j]) {
++n_list;
lists[n_list].clear();
itr[j] = n_list;
int i2 = lists[i1].next;
lists[n_list].deg = deg[j];
lists[n_list].idx = j;
linklists(i1, n_list);
if(i2) linklists(n_list, i2);
}
else {
linknodes(j, lists[i1].idx);
lists[i1].idx = j;
itr[j] = i1;
}
}
if(cur_n == 0) continue;
if(max_density < (double)cur_m / cur_n) {
max_size = ans.size();
}
max_density = max(max_density, (double)cur_m / cur_n);
}
reverse(ans.begin(), ans.end());
ans.resize(n - max_size);
if(max_density > mm_density) {
m_ans = ans;
mm_density = max_density;
}
auto enditer = chrono::steady_clock::now();
int elapsed = chrono::duration_cast<chrono::milliseconds>(enditer - startiter).count();
sum_iter_times += elapsed;
cout << "Max density until iteration " << tt+1 <<": " << mm_density << endl;
cout << "Avg time per iteration: " << sum_iter_times/(tt+1) << " ms" << endl;
cout << "Total time: " << sum_iter_times + init_time << " ms" << endl;
}
string output_file;
ofstream outfile;
if (argc >= 3)
{
output_file = argv[2];
}
else
{
output_file = "soln.tmp";
}
outfile.open(output_file.c_str());
for (int i : m_ans)
{
outfile << i+1 << endl;
}
return 0;
}