robust intercept (#139)

Co-authored-by: Mike Bostock <mbostock@gmail.com>

src/voronoi.js

@@ -152,17 +152,16 @@ export default class Voronoi {
     return this.delaunay._step(i, x, y) === i;
   }
   *neighbors(i) {
-    const epsilon = 1e-12;
     const ci = this._clip(i);
     if (ci) for (const j of this.delaunay.neighbors(i)) {
       const cj = this._clip(j);
       // find the common edge
       if (cj) loop: for (let ai = 0, li = ci.length; ai < li; ai += 2) {
         for (let aj = 0, lj = cj.length; aj < lj; aj += 2) {
-          if (Math.abs(ci[ai] - cj[aj]) < epsilon
-              && Math.abs(ci[ai + 1] - cj[aj + 1]) < epsilon
-              && Math.abs(ci[(ai + 2) % li] - cj[(aj + lj - 2) % lj]) < epsilon
-              && Math.abs(ci[(ai + 3) % li] - cj[(aj + lj - 1) % lj]) < epsilon) {
+          if (ci[ai] === cj[aj]
+              && ci[ai + 1] === cj[aj + 1]
+              && ci[(ai + 2) % li] === cj[(aj + lj - 2) % lj]
+              && ci[(ai + 3) % li] === cj[(aj + lj - 1) % lj]) {
             yield j;
             break loop;
           }
@@ -239,8 +238,11 @@ export default class Voronoi {
     return P;
   }
   _clipSegment(x0, y0, x1, y1, c0, c1) {
+    // for more robustness, always consider the segment in the same order
+    const flip = c0 < c1;
+    if (flip) [x0, y0, x1, y1, c0, c1] = [x1, y1, x0, y0, c1, c0];
     while (true) {
-      if (c0 === 0 && c1 === 0) return [x0, y0, x1, y1];
+      if (c0 === 0 && c1 === 0) return flip ? [x1, y1, x0, y0] : [x0, y0, x1, y1];
       if (c0 & c1) return null;
       let x, y, c = c0 || c1;
       if (c & 0b1000) x = x0 + (x1 - x0) * (this.ymax - y0) / (y1 - y0), y = this.ymax;

test/voronoi-test.js

@@ -88,3 +88,21 @@ it("voronoi.neighbors returns the correct neighbors", () => {
   const voronoi = Delaunay.from(points).voronoi([0, 0, 100, 90]);
   assert.deepStrictEqual([0, 1, 2, 3].map(i => [...voronoi.neighbors(i)].sort()), [[1], [0, 2, 3], [1, 3], [1, 2]]);
 });
+
+it("voronoi.neighbors returns the correct neighbors, flipped vertically", () => {
+  const points = [[10, -10], [36, -27], [90, -19], [50, -75]];
+  const voronoi = Delaunay.from(points).voronoi([0, -90, 100, 0]);
+  assert.deepStrictEqual([0, 1, 2, 3].map(i => [...voronoi.neighbors(i)].sort()), [[1], [0, 2, 3], [1, 3], [1, 2]]);
+});
+
+it("voronoi.neighbors returns the correct neighbors, flipped horizontally", () => {
+  const points = [[-10, 10], [-36, 27], [-90, 19], [-50, 75]];
+  const voronoi = Delaunay.from(points).voronoi([-100, 0, 0, 90]);
+  assert.deepStrictEqual([0, 1, 2, 3].map(i => [...voronoi.neighbors(i)].sort()), [[1], [0, 2, 3], [1, 3], [1, 2]]);
+});
+
+it("voronoi.neighbors returns the correct neighbors, rotated", () => {
+  const points = [[-10, -10], [-36, -27], [-90, -19], [-50, -75]];
+  const voronoi = Delaunay.from(points).voronoi([-100, -90, 0, 0]);
+  assert.deepStrictEqual([0, 1, 2, 3].map(i => [...voronoi.neighbors(i)].sort()), [[1], [0, 2, 3], [1, 3], [1, 2]]);
+});