geometry/CircleLine.h

• View this file on GitHub
• Last update: 2025-11-21 16:12:02+07:00

Depends on

• geometry/Point.h

Code

#include "Point.h"

template <class P>
vector<P> circleLine(P c, ld r, P a, P b) {
  P ab = b - a;
  ld s = a.cross(b, c);

  // calculate intersection, return vector<P>
  P p = a + ab * (c - a).dot(ab) / ab.dist2();
  ld h2 = r * r - s * s / ab.dist2();
  if (h2 < 0) return {};
  if (h2 == 0) return {p};
  P h = ab.unit() * sqrt(h2);
  return {p - h, p + h};

  // calculate smaller part area, return ld
  // ld dist = fabs(s) / sqrt(ab.dist2());
  // assert(dist <= r);
  // ld theta = 2.0 * acos(dist / r);
  // ld area = 0.5 * r * r * (theta - sin(theta));
  // return area;
}

#line 2 "geometry/Point.h"

template <class T>
int sgn(T x) { return (x > 0) - (x < 0); }
template <class T>
struct Point {
  typedef Point P;
  T x, y;
  explicit Point(T x = 0, T y = 0) : x(x), y(y) {}
  bool operator<(P p) const { return tie(x, y) < tie(p.x, p.y); }
  bool operator==(P p) const { return tie(x, y) == tie(p.x, p.y); }
  P operator+(P p) const { return P(x + p.x, y + p.y); }
  P operator-(P p) const { return P(x - p.x, y - p.y); }
  P operator*(T d) const { return P(x * d, y * d); }
  P operator/(T d) const { return P(x / d, y / d); }
  T dot(P p) const { return x * p.x + y * p.y; }
  T cross(P p) const { return x * p.y - y * p.x; }
  T cross(P a, P b) const { return (a - *this).cross(b - *this); }
  T dist2() const { return x * x + y * y; }
  T dist() const { return sqrt(dist2()); }
  // angle to x-axis in interval [-pi, pi]
  T angle() const { return atan2l(y, x); }
  P unit() const { return *this / dist(); }  // makes dist()=1
  P perp() const { return P(-y, x); }        // rotates +90 degrees
  P normal() const { return perp().unit(); }
  // returns point rotated 'a' radians ccw around the origin
  P rotate(ld a) const {
    return P(x * cos(a) - y * sin(a), x * sin(a) + y * cos(a));
  }
  friend ostream& operator<<(ostream& os, P p) {
    return os << "(" << p.x << "," << p.y << ")";
  }
};
#line 2 "geometry/CircleLine.h"

template <class P>
vector<P> circleLine(P c, ld r, P a, P b) {
  P ab = b - a;
  ld s = a.cross(b, c);

  // calculate intersection, return vector<P>
  P p = a + ab * (c - a).dot(ab) / ab.dist2();
  ld h2 = r * r - s * s / ab.dist2();
  if (h2 < 0) return {};
  if (h2 == 0) return {p};
  P h = ab.unit() * sqrt(h2);
  return {p - h, p + h};

  // calculate smaller part area, return ld
  // ld dist = fabs(s) / sqrt(ab.dist2());
  // assert(dist <= r);
  // ld theta = 2.0 * acos(dist / r);
  // ld area = 0.5 * r * r * (theta - sin(theta));
  // return area;
}

Back to top page