fix #188; floating point error in encloseBasis3

Author: mbostock

src/pack/enclose.js

@@ -114,7 +114,7 @@ function encloseBasis3(a, b, c) {
       A = xb * xb + yb * yb - 1,
       B = 2 * (r1 + xa * xb + ya * yb),
       C = xa * xa + ya * ya - r1 * r1,
-      r = -(A ? (B + Math.sqrt(B * B - 4 * A * C)) / (2 * A) : C / B);
+      r = -(Math.abs(A) > 1e-6 ? (B + Math.sqrt(B * B - 4 * A * C)) / (2 * A) : C / B);
   return {
     x: x1 + xa + xb * r,
     y: y1 + ya + yb * r,

test/pack/enclose-test.js

@@ -0,0 +1,18 @@
+import assert from "assert";
+import {packEnclose} from "../../src/index.js";
+
+// https://github.com/d3/d3-hierarchy/issues/188
+it("packEnclose(circles) handles a tricky case", () => {
+  assert.deepStrictEqual(
+    packEnclose([
+      {x: 14.5, y: 48.5, r: 7.585},
+      {x: 9.5, y: 79.5, r: 2.585},
+      {x: 15.5, y: 73.5, r: 8.585}
+    ]),
+    {
+      r: 20.790781637717107,
+      x: 12.80193548387092,
+      y: 61.59615384615385
+    }
+  );
+});