Problem p02550 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;

/// ----------------------------- (Debug) ------------------------------
#define sim template <class c
#define ris return *this
#define dor > debug &operator<<
#define eni(x)                                                                 \
  sim > typename enable_if<sizeof dud<c>(0) x 1, debug &>::type operator<<(    \
            c i) {
sim > struct rge {
  c b, e;
};
sim > rge<c> range(c i, c j) { return rge<c>{i, j}; }
sim > auto dud(c *x) -> decltype(cerr << *x, 0);
sim > char dud(...);
struct debug {
#ifndef ONLINE_JUDGE
  eni(!=) cerr << boolalpha << i;
  ris;
} eni(==) ris << range(begin(i), end(i));
}
sim, class b dor(pair<b, c> d) {
  ris << "(" << d.first << ", " << d.second << ")";
}
sim dor(rge<c> d) {
  *this << "[";
  for (auto it = d.b; it != d.e; ++it)
    *this << ", " + 2 * (it == d.b) << *it;
  ris << "]";
}
#else
  sim dor(const c &) { ris; }
#endif
}
;
vector<char *> tokenizer(const char *args) {
  char *token = new char[111];
  strcpy(token, args);
  token = strtok(token, ", ");
  vector<char *> v({token});
  while (token = strtok(NULL, ", "))
    v.push_back(token);
  return reverse(v.begin(), v.end()), v;
}
void debugg(vector<char *> args) { cerr << "\b\b "; }
template <typename Head, typename... Tail>
void debugg(vector<char *> args, Head H, Tail... T) {
  debug() << " [" << args.back() << ": " << H << "] ";
  args.pop_back();
  debugg(args, T...);
}
#define harg(...) #__VA_ARGS__
#ifndef ONLINE_JUDGE
#define dbg(...)                                                               \
  {                                                                            \
    debugg(tokenizer(harg(__VA_ARGS__, \b\t-- > Line)), __VA_ARGS__,           \
           __LINE__);                                                          \
    cerr << endl;                                                              \
  }
#else
#define dbg(...)                                                               \
  {}
#endif
/// -----------------------------------------------------------------------

typedef long long ll;
typedef vector<int> vi;
typedef pair<int, int> pi;
typedef vector<pi> vpi;
#define IOS                                                                    \
  ios::sync_with_stdio(0);                                                     \
  cin.tie(0);                                                                  \
  cout.tie(0)
#define F first
#define S second
#define PB push_back
#define EB emplace_back
#define MP make_pair
#define REP(i, a, b) for (i = a; i <= b; i++)
#define RAP(i, a, b) for (i = a; i >= b; i--)
#define spa << " " <<
#define all(x) (x).begin(), (x).end()
#define sz(x) (int)x.size()
ll mod = 998244353;
const int MX = 0x3f3f3f3f;

int main() {
  IOS;
  ll n, x, cnt = 1, ans;
  int m, i;
  cin >> n >> x >> m;
  ans = x;
  vi v;
  set<int> s;
  s.insert(x);
  dbg(x, s);
  while (cnt < n && s.find((x * x) % m) == s.end()) {
    x = (x * x) % m;
    // dbg(x,s,ans);
    s.insert(x);
    v.EB(x);
    ans += x;
    cnt++;
  }
  if (cnt < n) {
    ll sum = 0;
    int num = 0;
    x = (x * x) % m;
    // dbg(v);
    RAP(i, sz(v) - 1, 0) {
      sum += v[i];
      num++;
      if (v[i] == x)
        break;
    }
    // dbg(i,num,cnt);
    ans += ((n - cnt) / num) * sum;
    num = (n - cnt) % num;
    sum = 0;
    while (num--) {
      sum += v[i];
      i++;
    };
    ans += sum;
  }
  cout << ans;
  return 0;
}

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

/// ----------------------------- (Debug) ------------------------------
#define sim template <class c
#define ris return *this
#define dor > debug &operator<<
#define eni(x)                                                                 \
  sim > typename enable_if<sizeof dud<c>(0) x 1, debug &>::type operator<<(    \
            c i) {
sim > struct rge {
  c b, e;
};
sim > rge<c> range(c i, c j) { return rge<c>{i, j}; }
sim > auto dud(c *x) -> decltype(cerr << *x, 0);
sim > char dud(...);
struct debug {
#ifndef ONLINE_JUDGE
  eni(!=) cerr << boolalpha << i;
  ris;
} eni(==) ris << range(begin(i), end(i));
}
sim, class b dor(pair<b, c> d) {
  ris << "(" << d.first << ", " << d.second << ")";
}
sim dor(rge<c> d) {
  *this << "[";
  for (auto it = d.b; it != d.e; ++it)
    *this << ", " + 2 * (it == d.b) << *it;
  ris << "]";
}
#else
  sim dor(const c &) { ris; }
#endif
}
;
vector<char *> tokenizer(const char *args) {
  char *token = new char[111];
  strcpy(token, args);
  token = strtok(token, ", ");
  vector<char *> v({token});
  while (token = strtok(NULL, ", "))
    v.push_back(token);
  return reverse(v.begin(), v.end()), v;
}
void debugg(vector<char *> args) { cerr << "\b\b "; }
template <typename Head, typename... Tail>
void debugg(vector<char *> args, Head H, Tail... T) {
  debug() << " [" << args.back() << ": " << H << "] ";
  args.pop_back();
  debugg(args, T...);
}
#define harg(...) #__VA_ARGS__
#ifndef ONLINE_JUDGE
#define dbg(...)                                                               \
  {                                                                            \
    debugg(tokenizer(harg(__VA_ARGS__, \b\t-- > Line)), __VA_ARGS__,           \
           __LINE__);                                                          \
    cerr << endl;                                                              \
  }
#else
#define dbg(...)                                                               \
  {}
#endif
/// -----------------------------------------------------------------------

typedef long long ll;
typedef vector<int> vi;
typedef pair<int, int> pi;
typedef vector<pi> vpi;
#define IOS                                                                    \
  ios::sync_with_stdio(0);                                                     \
  cin.tie(0);                                                                  \
  cout.tie(0)
#define F first
#define S second
#define PB push_back
#define EB emplace_back
#define MP make_pair
#define REP(i, a, b) for (i = a; i <= b; i++)
#define RAP(i, a, b) for (i = a; i >= b; i--)
#define spa << " " <<
#define all(x) (x).begin(), (x).end()
#define sz(x) (int)x.size()
ll mod = 998244353;
const int MX = 0x3f3f3f3f;

int main() {
  IOS;
  ll n, x, cnt = 1, ans;
  int m, i;
  cin >> n >> x >> m;
  ans = x;
  vi v;
  set<int> s;
  s.insert(x);
  v.EB(x);
  while (cnt < n && s.find((x * x) % m) == s.end()) {
    x = (x * x) % m;
    // dbg(x,s,ans);
    s.insert(x);
    v.EB(x);
    ans += x;
    cnt++;
  }
  if (cnt < n) {
    ll sum = 0;
    int num = 0;
    x = (x * x) % m;
    // dbg(v);
    RAP(i, sz(v) - 1, 0) {
      sum += v[i];
      num++;
      if (v[i] == x)
        break;
    }
    // dbg(i,num,cnt);
    ans += ((n - cnt) / num) * sum;
    num = (n - cnt) % num;
    sum = 0;
    while (num--) {
      sum += v[i];
      i++;
    };
    ans += sum;
  }
  cout << ans;
  return 0;
}