Problem p01128 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <complex>
#include <cstdio>
#include <vector>

#define mp make_pair
#define pb push_back
#define rep(i, n) for (int i = 0; i < (n); i++)

using namespace std;

typedef complex<double> Point;

const double EPS = 1e-9;

class Line : public vector<Point> {
public:
  Line() {}
  Line(const Point &a, const Point &b) { pb(a), pb(b); }
};

class Segment : public Line {
public:
  Segment() {}
  Segment(const Point &a, const Point &b) : Line(a, b) {}
};

double dot(const Point &a, const Point &b) {
  return real(a) * real(b) + imag(a) * imag(b);
}

double cross(const Point &a, const Point &b) {
  return real(a) * imag(b) - imag(a) * real(b);
}

enum { CCW = 1, CW = -1, ON = 0 };
int ccw(const Point &a, Point b, Point c) {
  b -= a, c -= a;
  double rotdir = cross(b, c);
  if (rotdir > EPS)
    return CCW;
  if (rotdir < -EPS)
    return CW;
  return ON;
}

inline void calc_abc(const Line &l, double &a, double &b,
                     double &c) { // l : ax+by+c=0
  a = imag(l[0]) - imag(l[1]);
  b = real(l[1]) - real(l[0]);
  c = cross(l[0], l[1]);
}

bool intersect(const Segment &s, const Segment &t, Point *p = NULL) {
  if (max(real(s[0]), real(s[1])) < min(real(t[0]), real(t[1])) - EPS ||
      max(real(t[0]), real(t[1])) < min(real(s[0]), real(s[1])) - EPS ||
      max(imag(s[0]), imag(s[1])) < min(imag(t[0]), imag(t[1])) - EPS ||
      max(imag(t[0]), imag(t[1])) < min(imag(s[0]), imag(s[1])) - EPS)
    return false;

  if (ccw(s[0], s[1], t[0]) * ccw(s[0], s[1], t[1]) <= 0 &&
      ccw(t[0], t[1], s[0]) * ccw(t[0], t[1], s[1]) <= 0) {
    if (p) {
      double a1, b1, c1, a2, b2, c2;
      calc_abc(s, a1, b1, c1);
      calc_abc(t, a2, b2, c2);
      double det = a1 * b2 - a2 * b1;
      if (abs(det) < EPS) { // s is parallel to t
        Point q[3] = {s[0], s[1], t[0]};
        rep(i, 3) {
          if (dot(q[i] - s[0], q[i] - s[1]) < EPS &&
              dot(q[i] - t[0], q[i] - t[1]) < EPS) {
            *p = q[i];
            break;
          }
        }
      } else
        *p = Point(b1 * c2 - b2 * c1, a2 * c1 - a1 * c2) / det;
    }
    return true;
  }
  return false;
}

int n;
Point A, B;
Segment AB, S[100];
bool own[100], loc[100];

int solve() {
  vector<pair<double, int>> pts;
  rep(i, n) {
    Point Q;
    if (intersect(AB, S[i], &Q))
      pts.pb(mp(abs(A - Q), i));
  }

  sort(pts.begin(), pts.end());

  int cnt = 0, id = pts[0].second;
  bool now = (own[id] ? loc[id] : !loc[id]);
  for (int i = 1; i < pts.size(); i++) {
    id = pts[i].second;
    if (own[id]) {
      if (now != loc[id]) {
        cnt++;
        now = !now;
      }
    } else {
      if (now == loc[id]) {
        cnt++;
        now = !now;
      }
    }
  }
  return cnt;
}

int main() {
  int T;
  scanf("%d", &T);
  while (T--) {
    int xa, ya, xb, yb;
    scanf("%d%d%d%d%d", &xa, &ya, &xb, &yb, &n);
    A = Point(xa, ya);
    B = Point(xb, yb);
    AB = Segment(A, B);
    rep(i, n) {
      int x1, y1, x2, y2, o, l;
      scanf("%d%d%d%d%d%d", &x1, &y1, &x2, &y2, &o, &l);
      S[i] = Segment(Point(x1, y1), Point(x2, y2));
      own[i] = o;
      loc[i] = l;
    }
    printf("%d\n", solve());
  }

  return 0;
}

Fixed submission:
#include <algorithm>
#include <complex>
#include <cstdio>
#include <vector>

#define mp make_pair
#define pb push_back
#define rep(i, n) for (int i = 0; i < (n); i++)

using namespace std;

typedef complex<double> Point;

const double EPS = 1e-9;

class Line : public vector<Point> {
public:
  Line() {}
  Line(const Point &a, const Point &b) { pb(a), pb(b); }
};

class Segment : public Line {
public:
  Segment() {}
  Segment(const Point &a, const Point &b) : Line(a, b) {}
};

double dot(const Point &a, const Point &b) {
  return real(a) * real(b) + imag(a) * imag(b);
}

double cross(const Point &a, const Point &b) {
  return real(a) * imag(b) - imag(a) * real(b);
}

enum { CCW = 1, CW = -1, ON = 0 };
int ccw(const Point &a, Point b, Point c) {
  b -= a, c -= a;
  double rotdir = cross(b, c);
  if (rotdir > EPS)
    return CCW;
  if (rotdir < -EPS)
    return CW;
  return ON;
}

inline void calc_abc(const Line &l, double &a, double &b,
                     double &c) { // l : ax+by+c=0
  a = imag(l[0]) - imag(l[1]);
  b = real(l[1]) - real(l[0]);
  c = cross(l[0], l[1]);
}

bool intersect(const Segment &s, const Segment &t, Point *p = NULL) {
  if (max(real(s[0]), real(s[1])) < min(real(t[0]), real(t[1])) - EPS ||
      max(real(t[0]), real(t[1])) < min(real(s[0]), real(s[1])) - EPS ||
      max(imag(s[0]), imag(s[1])) < min(imag(t[0]), imag(t[1])) - EPS ||
      max(imag(t[0]), imag(t[1])) < min(imag(s[0]), imag(s[1])) - EPS)
    return false;

  if (ccw(s[0], s[1], t[0]) * ccw(s[0], s[1], t[1]) <= 0 &&
      ccw(t[0], t[1], s[0]) * ccw(t[0], t[1], s[1]) <= 0) {
    if (p) {
      double a1, b1, c1, a2, b2, c2;
      calc_abc(s, a1, b1, c1);
      calc_abc(t, a2, b2, c2);
      double det = a1 * b2 - a2 * b1;
      if (abs(det) < EPS) { // s is parallel to t
        Point q[3] = {s[0], s[1], t[0]};
        rep(i, 3) {
          if (dot(q[i] - s[0], q[i] - s[1]) < EPS &&
              dot(q[i] - t[0], q[i] - t[1]) < EPS) {
            *p = q[i];
            break;
          }
        }
      } else
        *p = Point(b1 * c2 - b2 * c1, a2 * c1 - a1 * c2) / det;
    }
    return true;
  }
  return false;
}

int n;
Point A, B;
Segment AB, S[100];
bool own[100], loc[100];

int solve() {
  vector<pair<double, int>> pts;
  rep(i, n) {
    Point Q;
    if (intersect(AB, S[i], &Q))
      pts.pb(mp(abs(A - Q), i));
  }
  if (pts.size() == 0)
    return 0;

  sort(pts.begin(), pts.end());

  int cnt = 0, id = pts[0].second;
  bool now = (own[id] ? loc[id] : !loc[id]);
  for (int i = 1; i < pts.size(); i++) {
    id = pts[i].second;
    if (own[id]) {
      if (now != loc[id]) {
        cnt++;
        now = !now;
      }
    } else {
      if (now == loc[id]) {
        cnt++;
        now = !now;
      }
    }
  }
  return cnt;
}

int main() {
  int T;
  scanf("%d", &T);
  while (T--) {
    int xa, ya, xb, yb;
    scanf("%d%d%d%d%d", &xa, &ya, &xb, &yb, &n);
    A = Point(xa, ya);
    B = Point(xb, yb);
    AB = Segment(A, B);
    rep(i, n) {
      int x1, y1, x2, y2, o, l;
      scanf("%d%d%d%d%d%d", &x1, &y1, &x2, &y2, &o, &l);
      S[i] = Segment(Point(x1, y1), Point(x2, y2));
      own[i] = o;
      loc[i] = l;
    }
    printf("%d\n", solve());
  }

  return 0;
}