Problem p02782 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <cassert>
#include <chrono>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <functional>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>

#ifdef LOCAL
#define eprintf(...) fprintf(stderr, __VA_ARGS__)
#else
#define eprintf(...) 42
#endif

#define rep(i, n) for (int i = 0, i##_len = (n); i < i##_len; ++i)
#define repp(i, m, n) for (int i = m, i##_len = (n); i < i##_len; ++i)
#define reprev(i, n) for (int i = (n - 1LL); i >= 0; --i)
#define all(x) (x).begin(), (x).end()
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> T gcd(T a, T b) { return b ? gcd(b, a % b) : a; }
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> P;
typedef long double ld;

constexpr int MOD = 1000000007;

// + - * / ^
int mod(int a) {
  int res = a % MOD;
  if (res < 0) {
    return res + MOD;
  }
  return res;
}
int mul_mod(int a, int b) {
  ll res = ((ll)a * (ll)b) % MOD;
  return mod((int)res);
}
int pow_mod(int a, int b) {
  ll res = 1;
  while (b > 0) {
    if (b & 1) {
      res = res * (ll)a % (ll)MOD;
    }
    a = mul_mod(a, a);
    b >>= 1;
  }
  return (int)res;
}
int inv_mod(int a) { return pow_mod(a, MOD - 2); }
int div_mod(int a, int b) { return mul_mod(a, inv_mod(b)); }

// ! C
constexpr int FAC_MAX = 300000;
ll fac[FAC_MAX], finv[FAC_MAX], inv[FAC_MAX];
void com_init() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < FAC_MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}
ll com_mod(int a, int b) {
  if (a < b)
    return 0;
  if (a < 0 || b < 0)
    return 0;
  return fac[a] * (finv[b] * finv[a - b] % MOD) % MOD;
}

int main(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);
  com_init();
  int r1, r2, c1, c2;
  cin >> r1 >> c1 >> r2 >> c2;
  int ans = 0;
  repp(i, c1, c2 + 1) {
    ans = mod(ans + com_mod(r2 + i + 1, i + 1));
    ans = mod(ans - com_mod(r1 + i, i + 1));
  }
  cout << ans << '\n';
  return 0;
}

Fixed submission:
#include <algorithm>
#include <bitset>
#include <cassert>
#include <chrono>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <functional>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>

#ifdef LOCAL
#define eprintf(...) fprintf(stderr, __VA_ARGS__)
#else
#define eprintf(...) 42
#endif

#define rep(i, n) for (int i = 0, i##_len = (n); i < i##_len; ++i)
#define repp(i, m, n) for (int i = m, i##_len = (n); i < i##_len; ++i)
#define reprev(i, n) for (int i = (n - 1LL); i >= 0; --i)
#define all(x) (x).begin(), (x).end()
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> T gcd(T a, T b) { return b ? gcd(b, a % b) : a; }
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> P;
typedef long double ld;

constexpr int MOD = 1000000007;

// + - * / ^
int mod(int a) {
  int res = a % MOD;
  if (res < 0) {
    return res + MOD;
  }
  return res;
}
int mul_mod(int a, int b) {
  ll res = ((ll)a * (ll)b) % MOD;
  return mod((int)res);
}
int pow_mod(int a, int b) {
  ll res = 1;
  while (b > 0) {
    if (b & 1) {
      res = res * (ll)a % (ll)MOD;
    }
    a = mul_mod(a, a);
    b >>= 1;
  }
  return (int)res;
}
int inv_mod(int a) { return pow_mod(a, MOD - 2); }
int div_mod(int a, int b) { return mul_mod(a, inv_mod(b)); }

// ! C
constexpr int FAC_MAX = 3000000;
ll fac[FAC_MAX], finv[FAC_MAX], inv[FAC_MAX];
void com_init() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < FAC_MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}
ll com_mod(int a, int b) {
  if (a < b)
    return 0;
  if (a < 0 || b < 0)
    return 0;
  return fac[a] * (finv[b] * finv[a - b] % MOD) % MOD;
}

int main(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);
  com_init();
  int r1, r2, c1, c2;
  cin >> r1 >> c1 >> r2 >> c2;
  int ans = 0;
  repp(i, c1, c2 + 1) {
    ans = mod(ans + com_mod(r2 + i + 1, i + 1));
    ans = mod(ans - com_mod(r1 + i, i + 1));
  }
  cout << ans << '\n';
  return 0;
}