USACO 2015 February Contest, Gold Problem 3. Fencing the Herd

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USACO2015FEB-G3

答案

(Analysis by Richard Peng)

This problem is a dynamic (insertion of points) version of "a highway and seven dwarfs" from CEOI 2002. It asks to support a point set (the set of cows) under insertion, and queries of whether a line (the fence) separates the point set.

One way to solve it is by a systematic reduction of decomposable problems under insertion to O(logn) static versions. A problem is decomposable if for any partition of the input data (points here), solutions on the two halves can be pieced together to form an overall solution.

Whether a point set is separated by a line is not decomposable under splitting of point sets: we can always split the point set into those above the line, and those below. However, the question of whether there is a point above the line is decomposable: there is a point above the line if and only if there is one in either half of the data that we split into.

This subproblem has a solution that runs in O(nlogn) time preprocessing and O(logn) time per query via convex hulls. It is described in details in the solution to the CEOI 02 problem mentioned above.

A simple way to use this data structure in an incremental setting is to store the last say, 500, new points in a buffer, and rebuild the entire data structure when the buffer is full. The overall structure is rebuilt 100,000 / 500 = 200 times, while each query takes O(logn) for the points in the hull, and scanning through the buffer linearly. This suffices for full points, and theoretically optimizing the buffer size at n^0.5 gives a total performance of about O(n^1.5).

To obtain an even faster solution, instead of having a buffer, we use another copy of this data structure. When unrolled, this corresponds to having copies of data structures storing exponentially larger point sets. Whenever a smaller set fills up, it can be merged with the set that's one larger than it. A systematic treatment of this setup can be found in pages 1-14 of this report, with more details in the rest of Section 3.

Below is Mark Gordon's code that implements this sequence of copies idea.

#include <iostream>
#include <algorithm>
#include <vector>
#include <complex>
#include <cmath>
#include <cstdio>

using namespace std;

// #define USE_FLOAT

// #define USE_RELATIVE_ERROR

#ifdef USE_FLOAT
#define EPS 1e-9
typedef double num;
#else
#define EPS 0
typedef long long num;
#endif

inline num nabs(num x) {
  return x < 0 ? -x : x;
}

bool num_lt(num X, num Y) {
#ifdef USE_RELATIVE_ERROR
  return X + max(num(1), nabs(Y)) * EPS < Y;
#else
  return X + EPS < Y;
#endif
}

bool num_lteq(num X, num Y) {
#ifdef USE_RELATIVE_ERROR
  return X <= Y + max(num(1), nabs(Y)) * EPS;
#else
  return X <= Y + EPS;
#endif
}

bool num_eq(num X, num Y) {
#ifdef USE_FLOAT
  return num_lteq(X, Y) && num_lteq(Y, X);
#else
  return X == Y;
#endif
}

typedef complex<num> point;
typedef vector<point> poly;

num cp(point A, point B, point C = point(0, 0)) {
  return imag(conj(A - C) * (B - C));
}

/* Returns true if C is strictly counter-clockwise AB in cartesian coordinates.
 * In other words as you walk from A to B, C would be on your left.  */
bool ccw(point A, point B, point C) {
  return num_lt(0, cp(A, B, C));
}

bool ccweq(point A, point B, point C) {
  return num_lteq(0, cp(A, B, C));
}

num dot(point A, point B, point C = point(0, 0)) {
  return real(conj(A - C) * (B - C));
}

point pivot;
bool pointCmp(point A, point B) {
  return make_pair(A.real(), A.imag()) < make_pair(B.real(), B.imag());
}
bool angleCmp(point A, point B) {
  num c = cp(pivot, A, B);
  return num_eq(c, 0) && dot(A, A, pivot) < dot(B, B, pivot) || num_lt(0, c);
}
void unwind(poly& P, point A) {
  int sz = P.size();
  while(sz > 1 && ccweq(A, P[sz - 1], P[sz - 2])) --sz;
  P.resize(sz);
}

/* Computes the convex hull of the list of points P.  Returns the points
 * defining the convex hull in ccw order. */
poly hull(poly P) {
  swap(P[0], *min_element(P.begin(), P.end(), pointCmp));
  pivot = P[0];
  sort(P.begin() + 1, P.end(), angleCmp);

  poly ret(1, pivot);
  for(int i = 1; i < P.size(); i++) {
    unwind(ret, P[i]);
    ret.push_back(P[i]);
  }
  if(ret.size() > 2) {
    unwind(ret, pivot);
  }
  return ret;
}

/* Returns true if x comes before y when doing a radially sweep around the
 * origin starting from the direction pointed by base.
 */
bool radial_compare(point x, point y, point base = point(1, 0)) {
  num cx = cp(base, x);
  num cy = cp(base, y);
  if (num_eq(cx, 0)) {
    cx = dot(base, x);
  }
  if (num_eq(cy, 0)) {
    cy = dot(base, y);
  }
  if ((cx < 0) == (cy < 0)) {
    return ccw(0, x, y);
  }
  return cy < 0;
}

/* Given a convex poly in ccw order find the max dot product of any point within
 * it with pt. */
num convex_max_dot(const poly& P, point pt) {
  int lo = 0;
  int hi = P.size() - 1;
  point base = P[0] - P.back();
  while (lo < hi) {
    int md = (lo + hi + 1) / 2;
    point v = P[md] - P[md ? md - 1 : P.size() - 1];

    if (radial_compare(v, pt * point(0, 1), base)) {
      lo = md;
    } else {
      hi = md - 1;
    }
  }
  return dot(P[lo], pt);
}

struct dyn_hull {
  dyn_hull() {
  }

  void add(point pt) {
    poly p(1, pt);
    for (int i = 0; i < hulls.size(); i++) {
      if (hulls[i].empty()) {
        hulls[i] = hull(p);
        return;
      }
      for (int j = 0; j < hulls[i].size(); j++) {
        p.push_back(hulls[i][j]);
      }
      hulls[i].clear();
    }
    hulls.push_back(hull(p));
  }

  num max_dot(point pt) {
    num ret = 0;
    bool init = false;
    for (int i = 0; i < hulls.size(); i++) {
      if (hulls[i].empty()) {
        continue;
      }
      num res = convex_max_dot(hulls[i], pt);
      if (!init || res > ret) {
        init = true;
        ret = res;
      }
    }
    return ret;
  }

  bool empty() {
    return hulls.empty();
  }

  vector<poly> hulls;
};

int main() {
  ios_base::sync_with_stdio(false);
  freopen("fencing.in", "r", stdin);
  freopen("fencing.out", "w", stdout);

  int N, Q;
  cin >> N >> Q;

  poly h0;
  for (int i = 0; i < N; i++) {
    num x, y;
    cin >> x >> y;
    h0.push_back(point(x, y));
  }
  h0 = hull(h0);

  dyn_hull h;
  for (int i = 0; i < Q; i++) {
    int cmd;
    cin >> cmd;
    if (cmd == 1) {
      num x, y;
      cin >> x >> y;
      h.add(point(x, y));
    } else {
      num a, b, c;
      cin >> a >> b >> c;
      point pt(a, b);
      
      if ((
            dot(h0[0], pt) <= c ||
            -convex_max_dot(h0, -pt) <= c ||
            (!h.empty() && -h.max_dot(-pt) <= c)
          ) && (
            dot(h0[0], pt) >= c ||
            convex_max_dot(h0, pt) >= c ||
            (!h.empty() && h.max_dot(pt) >= c)
          )
        ) {
        cout << "NO\n";
      } else {
        cout << "YES\n";
      }
    }
  }

  return 0;
}
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