Problem p01235 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <cassert>
#include <cmath>
#include <complex>
#include <iomanip>
#include <iostream>
#include <queue>
#include <vector>

#define REP(i, s, n) for (int i = s; i < n; i++)
#define rep(i, n) REP(i, 0, n)
#define inf (1 << 29)
#define EPS (1e-11)
#define all(n) (n).begin(), (n).end()
#define COUNTER_CLOCKWISE 1
#define CLOCKWISE -1
#define ONLINE_BACK 2
#define ONLINE_FRONT -2
#define ON_SEGMENT 0
#define equals(a, b) (fabs((a) - (b)) < EPS)

using namespace std;

//*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-

class Point {
public:
  double x, y;

  Point(double x = -inf, double y = -inf) : x(x), y(y) {}

  Point operator+(Point p) { return Point(x + p.x, y + p.y); }
  Point operator-(Point p) { return Point(x - p.x, y - p.y); }
  Point operator*(double a) { return Point(a * x, a * y); }
  Point operator/(double a) { return Point(x / a, y / a); }

  bool operator<(const Point &p) const {
    return !equals(x, p.x) ? x < p.x : y < p.y;
  }

  bool operator==(const Point &p) const {
    return fabs(x - p.x) < EPS && fabs(y - p.y) < EPS;
  }
};

struct Segment {
  Point p1, p2;
  Segment(Point p1 = Point(-inf, -inf), Point p2 = Point(-inf, -inf))
      : p1(p1), p2(p2) {}
  bool operator==(const Segment &p) const { return p.p1 == p1 && p.p2 == p2; }
};

typedef Point Vector;
typedef Segment Line;
typedef vector<Point> Polygon;

ostream &operator<<(ostream &os, const Point &a) {
  os << "(" << a.x << "," << a.y << ")";
}

ostream &operator<<(ostream &os, const Segment &a) {
  os << "( " << a.p1 << " , " << a.p2 << " )";
}

double dot(Point a, Point b) { return a.x * b.x + a.y * b.y; }

double cross(Point a, Point b) { return a.x * b.y - a.y * b.x; }

double norm(Point a) { return a.x * a.x + a.y * a.y; }

double abs(Point a) { return sqrt(norm(a)); }

// rad は角度をラジアンで持たせること
Point rotate(Point a, double rad) {
  return Point(cos(rad) * a.x - sin(rad) * a.y,
               sin(rad) * a.x + cos(rad) * a.y);
}

// 度をラジアンに変換
double toRad(double agl) { return agl * M_PI / 180.0; }

int ccw(Point p0, Point p1, Point p2) {
  Point a = p1 - p0;
  Point b = p2 - p0;
  if (cross(a, b) > EPS)
    return COUNTER_CLOCKWISE;
  if (cross(a, b) < -EPS)
    return CLOCKWISE;
  if (dot(a, b) < -EPS)
    return ONLINE_BACK;
  if (norm(a) < norm(b))
    return ONLINE_FRONT;
  return ON_SEGMENT;
}

bool intersectLL(Line l, Line m) {
  return abs(cross(l.p2 - l.p1, m.p2 - m.p1)) > EPS || // non-parallel
         abs(cross(l.p2 - l.p1, m.p1 - l.p1)) < EPS;   // same line
}
bool intersectLS(Line l, Line s) {
  return cross(l.p2 - l.p1, s.p1 - l.p1) * // s[0] is left of l
             cross(l.p2 - l.p1, s.p2 - l.p1) <
         EPS; // s[1] is right of l
}
bool intersectLP(Line l, Point p) {
  return abs(cross(l.p2 - p, l.p1 - p)) < EPS;
}
bool intersectSS(Line s, Line t) {
  return ccw(s.p1, s.p2, t.p1) * ccw(s.p1, s.p2, t.p2) <= 0 &&
         ccw(t.p1, t.p2, s.p1) * ccw(t.p1, t.p2, s.p2) <= 0;
}
bool intersectSP(Line s, Point p) {
  return abs(s.p1 - p) + abs(s.p2 - p) - abs(s.p2 - s.p1) <
         EPS; // triangle inequality
}

Point projection(Line l, Point p) {
  double t = dot(p - l.p1, l.p1 - l.p2) / norm(l.p1 - l.p2);
  return l.p1 + (l.p1 - l.p2) * t;
}
Point reflection(Line l, Point p) { return p + (projection(l, p) - p) * 2; }
double distanceLP(Line l, Point p) { return abs(p - projection(l, p)); }
double distanceLL(Line l, Line m) {
  return intersectLL(l, m) ? 0 : distanceLP(l, m.p1);
}

double distanceLS(Line l, Line s) {
  if (intersectLS(l, s))
    return 0;
  return min(distanceLP(l, s.p1), distanceLP(l, s.p2));
}
double distanceSP(Line s, Point p) {
  Point r = projection(s, p);
  if (intersectSP(s, r))
    return abs(r - p);
  return min(abs(s.p1 - p), abs(s.p2 - p));
}

double distanceSS(Line s, Line t) {
  if (intersectSS(s, t))
    return 0;
  return min(min(distanceSP(s, t.p1), distanceSP(s, t.p2)),
             min(distanceSP(t, s.p1), distanceSP(t, s.p2)));
}
Point crosspoint(Line l, Line m) {
  double A = cross(l.p2 - l.p1, m.p2 - m.p1);
  double B = cross(l.p2 - l.p1, l.p2 - m.p1);
  if (abs(A) < EPS && abs(B) < EPS)
    return m.p1; // same line
  if (abs(A) < EPS)
    assert(false); // !!!PRECONDITION NOT SATISFIED!!!
  return m.p1 + (m.p2 - m.p1) * (B / A);
}

// -*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-|

struct SP {
  Segment seg;
  int type; // type 0 -> Point, type 1 -> Segment;
  SP(Segment seg = Segment(), int type = inf) : seg(seg), type(type) {}
};

struct Edge {
  int from, to;
  double cost;
  Edge(int from = inf, int to = inf, double cost = inf)
      : from(from), to(to), cost(cost) {}
  bool operator<(const Edge &a) const { return cost < a.cost; }
};

typedef vector<Edge> vE;
typedef vector<vE> vvE;

int N;
vector<SP> vec;

vector<vector<Edge>> segmentArrangementVer1(vector<Segment> vs,
                                            vector<Point> &ps) {

  rep(i, vs.size()) REP(j, i + 1, vs.size()) if (intersectSS(vs[i], vs[j]))
      ps.push_back(Point(crosspoint(vs[i], vs[j])));

  //
  ps.push_back(vec[0].seg.p1);
  ps.push_back(vec[1].seg.p2);
  //

  sort(all(ps));
  ps.erase(unique(all(ps)), ps.end());

  vector<vector<Edge>> ret(ps.size());

  for (int i = 0; i < vs.size(); i++) {
    vector<pair<double, int>> list;
    rep(j, ps.size()) if (intersectSP(vs[i], ps[j]))
        list.push_back(pair<double, int>(norm(vs[i].p1 - ps[j]), j));

    sort(all(list));

    // for(int j=0;j<list.size()-1;++j) is not good
    for (int j = 0; j + 1 < list.size(); ++j) {
      int from = list[j].second, to = list[j + 1].second;
      double cost = abs(ps[from] - ps[to]);
      ret[from].push_back(Edge(from, to, cost));
      ret[to].push_back(Edge(to, from, cost));
    }
  }
  return ret;
}

void toSegOnly(vector<Segment> &vs) {
  rep(i, 2) {
    REP(j, 2, 2 + N) {
      assert(vec[i].seg.p1 == vec[i].seg.p2);
      Point prj = projection(vec[j].seg, vec[i].seg.p1);
      // cout << i << "," << j << " : " << prj << endl;
      if (intersectSP(vec[j].seg, prj)) {
        if (vec[i].seg.p1 == prj)
          continue;
        // cout << vec[i].seg.p1 << " intersect : " << i << " and " << j << " "
        // << prj << endl;
        Point e = vec[i].seg.p1 - prj;
        // cout << "fe : " << e << endl;
        e = (e / abs(e)) * 1000;
        // cout << "e : " << e << endl;
        vs.push_back(Segment(vec[i].seg.p1 - e, prj + e));
      }
    }
  }
  /*
  cout << "VS =----------------- " << vs.size() << endl;
  rep(i,vs.size())
    cout << vs[i] << endl;
  */
  REP(j, 2, 2 + N) {
    Segment seg =
        Segment(vec[j].seg.p1 - vec[j].seg.p2, vec[j].seg.p2 - vec[j].seg.p2);
    Point rig = rotate(seg.p1, toRad(90.0));
    rig = (rig / abs(rig)) * 1000;
    vs.push_back(Segment(vec[j].seg.p1 - rig, vec[j].seg.p1 + rig));
    vs.push_back(Segment(vec[j].seg.p2 - rig, vec[j].seg.p2 + rig));
  }
}

// segment merge--------
bool merge_if_able(Segment &s, Segment t) {
  if (abs(cross(s.p2 - s.p1, t.p2 - t.p1)) > EPS)
    return false;
  if (ccw(s.p1, t.p1, s.p2) == +1 || ccw(s.p1, t.p1, s.p2) == -1)
    return false; // not on the same line
  if (ccw(s.p1, s.p2, t.p1) == -2 || ccw(t.p1, t.p2, s.p1) == -2)
    return false; // separated
  s = Segment(min(s.p1, t.p1), max(s.p2, t.p2));
  return true;
}
void merge_segments(vector<Segment> &segs) {
  for (int i = 0; i < segs.size(); ++i)
    if (segs[i].p2 < segs[i].p1)
      swap(segs[i].p2, segs[i].p1);
  for (int i = 0; i < segs.size(); ++i)
    for (int j = i + 1; j < segs.size(); ++j)
      if (merge_if_able(segs[i], segs[j]))
        segs[j--] = segs.back(), segs.pop_back();
}
// -------------------

struct Pox {
  int cur;
  double cost;
  Pox(int cur = inf, double cost = inf) : cur(cur), cost(cost) {}
  bool operator<(const Pox &a) const { return cost > a.cost; }
};

void dijkstra(vvE &G, int st, int ed) {
  int n = G.size();
  double mincost[n];
  rep(i, n) mincost[i] = inf;
  mincost[st] = 0;
  priority_queue<Pox> Q;
  Q.push(Pox(st, 0));

  while (!Q.empty()) {
    Pox pox = Q.top();
    Q.pop();

    if (pox.cur == ed) {
      cout << setiosflags(ios::fixed) << setprecision(5) << pox.cost << endl;
      return;
    }

    rep(i, G[pox.cur].size()) {
      int next = G[pox.cur][i].to;
      double cost = G[pox.cur][i].cost;
      if (!equals(mincost[next], pox.cost + cost) &&
          mincost[next] > pox.cost + cost) {
        mincost[next] = pox.cost + cost;
        Q.push(Pox(next, pox.cost + cost));
      }
    }
  }
  cout << "Impossible" << endl;
}

void compute() {
  vector<Segment> vs;
  toSegOnly(vs);

  /*
  cout << "vs------bef" << endl;
  rep(i,vs.size())
    {
      cout << vs[i] << endl;
    }


  rep(i,vs.size())
    if(!equals(vs[i].p1.x,vs[i].p2.x) && vs[i].p1.x > vs[i].p2.x)
      swap(vs[i].p1,vs[i].p2);
  */
  merge_segments(vs);
  /*
  cout << "vs------aft" << endl;
  rep(i,vs.size())
    {
      cout << vs[i] << endl;
    }
  */

  vector<Point> ps;
  vvE G = segmentArrangementVer1(vs, ps);

  int st_index, ed_index;
  rep(i, ps.size()) {
    // cout << "ps[" << i << "] = "<< ps[i] << endl;
    if (ps[i] == vec[0].seg.p1)
      st_index = i;
    if (ps[i] == vec[1].seg.p1)
      ed_index = i;
  }

  // cout << "st = " << st_index << endl;
  // cout << "ed = " << ed_index << endl;

  /*
  cout << "G-----" << endl;
  rep(i,G.size())
    {
      cout << "node " << i << endl;
      rep(j,G[i].size())
        {
          cout << "(" << G[i][j].from << "," << G[i][j].to << "," <<
  G[i][j].cost << ") ";
        }
      cout << endl;
    }
  */
  dijkstra(G, st_index, ed_index);
}

int main() {
  int CNT = 1;
  while (cin >> N, N) {
    vec.clear();
    vec.resize(N + 2);
    double x, y;
    rep(i, 2) {
      cin >> x >> y;
      vec[i].seg.p1 = vec[i].seg.p2 = Point(x, y);
      vec[i].type = 0;
    }
    rep(i, N) cin >> vec[2 + i].seg.p1.x >> vec[2 + i].seg.p1.y >>
        vec[2 + i].seg.p2.x >> vec[2 + i].seg.p2.y;
    cout << "Case " << CNT++ << ": ";
    compute();
  }
  return 0;
}

Fixed submission:
#include <algorithm>
#include <cassert>
#include <cmath>
#include <complex>
#include <iomanip>
#include <iostream>
#include <queue>
#include <vector>

#define REP(i, s, n) for (int i = s; i < n; i++)
#define rep(i, n) REP(i, 0, n)
#define inf (1 << 29)
#define EPS (1e-8)
#define all(n) (n).begin(), (n).end()
#define COUNTER_CLOCKWISE 1
#define CLOCKWISE -1
#define ONLINE_BACK 2
#define ONLINE_FRONT -2
#define ON_SEGMENT 0
#define equals(a, b) (fabs((a) - (b)) < EPS)

using namespace std;

//*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-

class Point {
public:
  double x, y;

  Point(double x = -inf, double y = -inf) : x(x), y(y) {}

  Point operator+(Point p) { return Point(x + p.x, y + p.y); }
  Point operator-(Point p) { return Point(x - p.x, y - p.y); }
  Point operator*(double a) { return Point(a * x, a * y); }
  Point operator/(double a) { return Point(x / a, y / a); }

  bool operator<(const Point &p) const {
    return !equals(x, p.x) ? x < p.x : y < p.y;
  }

  bool operator==(const Point &p) const {
    return fabs(x - p.x) < EPS && fabs(y - p.y) < EPS;
  }
};

struct Segment {
  Point p1, p2;
  Segment(Point p1 = Point(-inf, -inf), Point p2 = Point(-inf, -inf))
      : p1(p1), p2(p2) {}
  bool operator==(const Segment &p) const { return p.p1 == p1 && p.p2 == p2; }
};

typedef Point Vector;
typedef Segment Line;
typedef vector<Point> Polygon;

ostream &operator<<(ostream &os, const Point &a) {
  os << "(" << a.x << "," << a.y << ")";
}

ostream &operator<<(ostream &os, const Segment &a) {
  os << "( " << a.p1 << " , " << a.p2 << " )";
}

double dot(Point a, Point b) { return a.x * b.x + a.y * b.y; }

double cross(Point a, Point b) { return a.x * b.y - a.y * b.x; }

double norm(Point a) { return a.x * a.x + a.y * a.y; }

double abs(Point a) { return sqrt(norm(a)); }

// rad は角度をラジアンで持たせること
Point rotate(Point a, double rad) {
  return Point(cos(rad) * a.x - sin(rad) * a.y,
               sin(rad) * a.x + cos(rad) * a.y);
}

// 度をラジアンに変換
double toRad(double agl) { return agl * M_PI / 180.0; }

int ccw(Point p0, Point p1, Point p2) {
  Point a = p1 - p0;
  Point b = p2 - p0;
  if (cross(a, b) > EPS)
    return COUNTER_CLOCKWISE;
  if (cross(a, b) < -EPS)
    return CLOCKWISE;
  if (dot(a, b) < -EPS)
    return ONLINE_BACK;
  if (norm(a) < norm(b))
    return ONLINE_FRONT;
  return ON_SEGMENT;
}

bool intersectLL(Line l, Line m) {
  return abs(cross(l.p2 - l.p1, m.p2 - m.p1)) > EPS || // non-parallel
         abs(cross(l.p2 - l.p1, m.p1 - l.p1)) < EPS;   // same line
}
bool intersectLS(Line l, Line s) {
  return cross(l.p2 - l.p1, s.p1 - l.p1) * // s[0] is left of l
             cross(l.p2 - l.p1, s.p2 - l.p1) <
         EPS; // s[1] is right of l
}
bool intersectLP(Line l, Point p) {
  return abs(cross(l.p2 - p, l.p1 - p)) < EPS;
}
bool intersectSS(Line s, Line t) {
  return ccw(s.p1, s.p2, t.p1) * ccw(s.p1, s.p2, t.p2) <= 0 &&
         ccw(t.p1, t.p2, s.p1) * ccw(t.p1, t.p2, s.p2) <= 0;
}
bool intersectSP(Line s, Point p) {
  return abs(s.p1 - p) + abs(s.p2 - p) - abs(s.p2 - s.p1) <
         EPS; // triangle inequality
}

Point projection(Line l, Point p) {
  double t = dot(p - l.p1, l.p1 - l.p2) / norm(l.p1 - l.p2);
  return l.p1 + (l.p1 - l.p2) * t;
}
Point reflection(Line l, Point p) { return p + (projection(l, p) - p) * 2; }
double distanceLP(Line l, Point p) { return abs(p - projection(l, p)); }
double distanceLL(Line l, Line m) {
  return intersectLL(l, m) ? 0 : distanceLP(l, m.p1);
}

double distanceLS(Line l, Line s) {
  if (intersectLS(l, s))
    return 0;
  return min(distanceLP(l, s.p1), distanceLP(l, s.p2));
}
double distanceSP(Line s, Point p) {
  Point r = projection(s, p);
  if (intersectSP(s, r))
    return abs(r - p);
  return min(abs(s.p1 - p), abs(s.p2 - p));
}

double distanceSS(Line s, Line t) {
  if (intersectSS(s, t))
    return 0;
  return min(min(distanceSP(s, t.p1), distanceSP(s, t.p2)),
             min(distanceSP(t, s.p1), distanceSP(t, s.p2)));
}
Point crosspoint(Line l, Line m) {
  double A = cross(l.p2 - l.p1, m.p2 - m.p1);
  double B = cross(l.p2 - l.p1, l.p2 - m.p1);
  if (abs(A) < EPS && abs(B) < EPS)
    return m.p1; // same line
  if (abs(A) < EPS)
    assert(false); // !!!PRECONDITION NOT SATISFIED!!!
  return m.p1 + (m.p2 - m.p1) * (B / A);
}

// -*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-|

struct SP {
  Segment seg;
  int type; // type 0 -> Point, type 1 -> Segment;
  SP(Segment seg = Segment(), int type = inf) : seg(seg), type(type) {}
};

struct Edge {
  int from, to;
  double cost;
  Edge(int from = inf, int to = inf, double cost = inf)
      : from(from), to(to), cost(cost) {}
  bool operator<(const Edge &a) const { return cost < a.cost; }
};

typedef vector<Edge> vE;
typedef vector<vE> vvE;

int N;
vector<SP> vec;

vector<vector<Edge>> segmentArrangementVer1(vector<Segment> vs,
                                            vector<Point> &ps) {

  rep(i, vs.size()) REP(j, i + 1, vs.size()) if (intersectSS(vs[i], vs[j]))
      ps.push_back(Point(crosspoint(vs[i], vs[j])));

  //
  ps.push_back(vec[0].seg.p1);
  ps.push_back(vec[1].seg.p2);
  //

  sort(all(ps));
  ps.erase(unique(all(ps)), ps.end());

  vector<vector<Edge>> ret(ps.size());

  for (int i = 0; i < vs.size(); i++) {
    vector<pair<double, int>> list;
    rep(j, ps.size()) if (intersectSP(vs[i], ps[j]))
        list.push_back(pair<double, int>(norm(vs[i].p1 - ps[j]), j));

    sort(all(list));

    // for(int j=0;j<list.size()-1;++j) is not good
    for (int j = 0; j + 1 < list.size(); ++j) {
      int from = list[j].second, to = list[j + 1].second;
      double cost = abs(ps[from] - ps[to]);
      ret[from].push_back(Edge(from, to, cost));
      ret[to].push_back(Edge(to, from, cost));
    }
  }
  return ret;
}

void toSegOnly(vector<Segment> &vs) {
  rep(i, 2) {
    REP(j, 2, 2 + N) {
      assert(vec[i].seg.p1 == vec[i].seg.p2);
      Point prj = projection(vec[j].seg, vec[i].seg.p1);
      // cout << i << "," << j << " : " << prj << endl;
      if (intersectSP(vec[j].seg, prj)) {
        if (vec[i].seg.p1 == prj)
          continue;
        // cout << vec[i].seg.p1 << " intersect : " << i << " and " << j << " "
        // << prj << endl;
        Point e = vec[i].seg.p1 - prj;
        // cout << "fe : " << e << endl;
        e = (e / abs(e)) * 1000;
        // cout << "e : " << e << endl;
        vs.push_back(Segment(vec[i].seg.p1 - e, prj + e));
      }
    }
  }
  /*
  cout << "VS =----------------- " << vs.size() << endl;
  rep(i,vs.size())
    cout << vs[i] << endl;
  */
  REP(j, 2, 2 + N) {
    Segment seg =
        Segment(vec[j].seg.p1 - vec[j].seg.p2, vec[j].seg.p2 - vec[j].seg.p2);
    Point rig = rotate(seg.p1, toRad(90.0));
    rig = (rig / abs(rig)) * 1000;
    vs.push_back(Segment(vec[j].seg.p1 - rig, vec[j].seg.p1 + rig));
    vs.push_back(Segment(vec[j].seg.p2 - rig, vec[j].seg.p2 + rig));
  }
}

// segment merge--------
bool merge_if_able(Segment &s, Segment t) {
  if (abs(cross(s.p2 - s.p1, t.p2 - t.p1)) > EPS)
    return false;
  if (ccw(s.p1, t.p1, s.p2) == +1 || ccw(s.p1, t.p1, s.p2) == -1)
    return false; // not on the same line
  if (ccw(s.p1, s.p2, t.p1) == -2 || ccw(t.p1, t.p2, s.p1) == -2)
    return false; // separated
  s = Segment(min(s.p1, t.p1), max(s.p2, t.p2));
  return true;
}
void merge_segments(vector<Segment> &segs) {
  for (int i = 0; i < segs.size(); ++i)
    if (segs[i].p2 < segs[i].p1)
      swap(segs[i].p2, segs[i].p1);
  for (int i = 0; i < segs.size(); ++i)
    for (int j = i + 1; j < segs.size(); ++j)
      if (merge_if_able(segs[i], segs[j]))
        segs[j--] = segs.back(), segs.pop_back();
}
// -------------------

struct Pox {
  int cur;
  double cost;
  Pox(int cur = inf, double cost = inf) : cur(cur), cost(cost) {}
  bool operator<(const Pox &a) const { return cost > a.cost; }
};

void dijkstra(vvE &G, int st, int ed) {
  int n = G.size();
  double mincost[n];
  rep(i, n) mincost[i] = inf;
  mincost[st] = 0;
  priority_queue<Pox> Q;
  Q.push(Pox(st, 0));

  while (!Q.empty()) {
    Pox pox = Q.top();
    Q.pop();

    if (pox.cur == ed) {
      cout << setiosflags(ios::fixed) << setprecision(5) << pox.cost << endl;
      return;
    }

    rep(i, G[pox.cur].size()) {
      int next = G[pox.cur][i].to;
      double cost = G[pox.cur][i].cost;
      if (!equals(mincost[next], pox.cost + cost) &&
          mincost[next] > pox.cost + cost) {
        mincost[next] = pox.cost + cost;
        Q.push(Pox(next, pox.cost + cost));
      }
    }
  }
  cout << "Impossible" << endl;
}

void compute() {
  vector<Segment> vs;
  toSegOnly(vs);

  /*
  cout << "vs------bef" << endl;
  rep(i,vs.size())
    {
      cout << vs[i] << endl;
    }


  rep(i,vs.size())
    if(!equals(vs[i].p1.x,vs[i].p2.x) && vs[i].p1.x > vs[i].p2.x)
      swap(vs[i].p1,vs[i].p2);
  */
  merge_segments(vs);
  /*
  cout << "vs------aft" << endl;
  rep(i,vs.size())
    {
      cout << vs[i] << endl;
    }
  */

  vector<Point> ps;
  vvE G = segmentArrangementVer1(vs, ps);

  int st_index, ed_index;
  rep(i, ps.size()) {
    // cout << "ps[" << i << "] = "<< ps[i] << endl;
    if (ps[i] == vec[0].seg.p1)
      st_index = i;
    if (ps[i] == vec[1].seg.p1)
      ed_index = i;
  }

  // cout << "st = " << st_index << endl;
  // cout << "ed = " << ed_index << endl;

  /*
  cout << "G-----" << endl;
  rep(i,G.size())
    {
      cout << "node " << i << endl;
      rep(j,G[i].size())
        {
          cout << "(" << G[i][j].from << "," << G[i][j].to << "," <<
  G[i][j].cost << ") ";
        }
      cout << endl;
    }
  */
  dijkstra(G, st_index, ed_index);
}

int main() {
  int CNT = 1;
  while (cin >> N, N) {
    vec.clear();
    vec.resize(N + 2);
    double x, y;
    rep(i, 2) {
      cin >> x >> y;
      vec[i].seg.p1 = vec[i].seg.p2 = Point(x, y);
      vec[i].type = 0;
    }
    rep(i, N) cin >> vec[2 + i].seg.p1.x >> vec[2 + i].seg.p1.y >>
        vec[2 + i].seg.p2.x >> vec[2 + i].seg.p2.y;
    cout << "Case " << CNT++ << ": ";
    compute();
  }
  return 0;
}