more collinear tests (#92)

Author: Fil

src/delaunay.js

@@ -16,17 +16,29 @@ function pointY(p) {
 function area(hull, points) {
   let n = hull.length, x0, y0,
       x1 = points[2 * hull[n - 1]],
-      y1 = points[2 * hull[n - 1] + 1],
-      area = 0;
+      y1 = points[2 * hull[n - 1] + 1];
 
+  const v = [];
   for (let i = 0; i < n; i ++) {
     x0 = x1, y0 = y1;
-    x1 = points[2 * hull[i]];
-    y1 = points[2 * hull[i] + 1];
-    area += y0 * x1 - x0 * y1;
+    x1 = points[2 * hull[i]] - points[2 * hull[0]];
+    y1 = points[2 * hull[i] + 1] - points[2 * hull[0] + 1];
+    v.push(y0 * x1, - x0 * y1);
   }
 
-  return area / 2;
+  return kahansum(v) / 2;
+}
+
+function kahansum(values) {
+  let summ = 0,
+    c = 0;
+  for (const num of values) {
+    let y = num - c,
+      t = summ + y;
+    c = t - summ - y;
+    summ = t;
+  }
+  return summ;
 }
 
 function jitter(x, y, r) {
@@ -50,7 +62,7 @@ export default class Delaunay {
     const d = this._delaunator, points = this.points;
 
     // check for collinear
-    if (d.hull && d.hull.length > 2 && area(d.hull, points) < 1e-10) {
+    if (d.hull && d.hull.length > 2 && (d.triangles.length === 0 || area(d.hull, points) < 1e-10)) {
       this.collinear = Int32Array.from({length: points.length/2}, (_,i) => i)
         .sort((i, j) => points[2 * i] - points[2 * j] || points[2 * i + 1] - points[2 * j + 1]); // for exact neighbors
       const e = this.collinear[0], f = this.collinear[this.collinear.length - 1],

test/delaunay-test.js

@@ -135,6 +135,18 @@ tape("delaunay.find(x, y) works with collinear points", test => {
   test.equal(points[delaunay.find(10, 10, 0)][1], 4);
 });
 
+tape("delaunay.find(x, y) works with collinear points 2", test => {
+  const points = Array.from({ length: 120 }, (_, i) => [i * 4, i / 3 + 100]);
+  const delaunay = Delaunay.from(points);
+  test.deepEqual([...delaunay.neighbors(2)], [ 1, 3 ]);
+});
+
+tape("delaunay.find(x, y) works with collinear points 3", test => {
+  const points = Array.from({ length: 120 }, (_, i) => [i * 4, i / 3 + 100]);
+  const delaunay = Delaunay.from(points);
+  test.deepEqual([...delaunay.neighbors(2)], [ 1, 3 ]);
+});
+
 tape("delaunay.find(x, y) works with collinear points (large)", test => {
   const points = Array.from({length: 2000}, (_,i) => [i**2,i**2]);
   const delaunay = Delaunay.from(points);