Repo created

libs/geometry/convex_hull.cpp

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+#include "geometry/convex_hull.hpp"
+
+#include "geometry/robust_orientation.hpp"
+
+#include "base/stl_helpers.hpp"
+
+#include <algorithm>
+
+namespace m2
+{
+namespace
+{
+// Checks whether (p1 - p) x (p2 - p) > 0.
+bool IsCCW(PointD const & p1, PointD const & p2, PointD const & p, double eps)
+{
+  return robust::OrientedS(p1, p2, p) > eps;
+}
+
+bool IsContinuedBy(std::vector<PointD> const & hull, PointD const & p, double eps)
+{
+  auto const n = hull.size();
+  if (n < 2)
+    return true;
+
+  auto const & p1 = hull[n - 2];
+  auto const & p2 = hull[n - 1];
+
+  // Checks whether (p2 - p1) x (p - p2) > 0.
+  return IsCCW(p, p1, p2, eps);
+}
+
+std::vector<PointD> BuildConvexHull(std::vector<PointD> points, double eps)
+{
+  base::SortUnique(points);
+
+  ASSERT(points.empty() || points.begin() == min_element(points.begin(), points.end()), ());
+
+  if (points.size() < 3)
+    return points;
+
+  auto const pivot = points[0];
+
+  std::sort(points.begin() + 1, points.end(), [&pivot, &eps](PointD const & lhs, PointD const & rhs)
+  {
+    if (IsCCW(lhs, rhs, pivot, eps))
+      return true;
+    if (IsCCW(rhs, lhs, pivot, eps))
+      return false;
+    return lhs.SquaredLength(pivot) < rhs.SquaredLength(pivot);
+  });
+
+  std::vector<PointD> hull;
+
+  for (auto const & p : points)
+  {
+    while (!IsContinuedBy(hull, p, eps))
+      hull.pop_back();
+    hull.push_back(p);
+  }
+
+  return hull;
+}
+}  // namespace
+
+ConvexHull::ConvexHull(std::vector<PointD> const & points, double eps) : m_hull(BuildConvexHull(points, eps)) {}
+}  // namespace m2