Backport from delaunator-js for

mapbox/delaunator#44
mapbox/delaunator#33

solves

#11
#16

include/delaunator.hpp

@@ -100,13 +100,12 @@ inline std::pair<double, double> circumcenter(
 
 struct compare {
 
+    std::vector<double> const& dists;
     std::vector<double> const& coords;
-    double cx;
-    double cy;
 
     bool operator()(std::size_t i, std::size_t j) {
-        const double d1 = dist(coords[2 * i], coords[2 * i + 1], cx, cy);
-        const double d2 = dist(coords[2 * j], coords[2 * j + 1], cx, cy);
+        const double d1 = dists[i];
+        const double d2 = dists[j];
         const double diff1 = d1 - d2;
         const double diff2 = coords[2 * i] - coords[2 * j];
         const double diff3 = coords[2 * i + 1] - coords[2 * j + 1];
@@ -302,8 +301,16 @@ Delaunator::Delaunator(std::vector<double> const& in_coords)
 
     std::tie(m_center_x, m_center_y) = circumcenter(i0x, i0y, i1x, i1y, i2x, i2y);
 
+    std::vector<double> dists;
+    dists.reserve(ids.size());
+
+	for (auto&& id : ids) {
+        const double d = dist(coords[2 * id], coords[2 * id + 1], m_center_x, m_center_y);
+        dists.push_back(d);
+	}
+
     // sort the points by distance from the seed triangle circumcenter
-    std::sort(ids.begin(), ids.end(), compare{ coords, m_center_x, m_center_y });
+    std::sort(ids.begin(), ids.end(), compare{ dists, coords });
 
     // initialize a hash table for storing edges of the advancing convex hull
     m_hash_size = static_cast<std::size_t>(std::llround(std::ceil(std::sqrt(n))));
@@ -508,7 +515,7 @@ std::size_t Delaunator::legalize(std::size_t a) {
                         hull_tri[e] = a;
                         break;
                     }
-                    e = hull_next[e];
+                    e = hull_prev[e];
                 } while (e != hull_start);
             }
             link(a, hbl);