apply linear binning (#46)

* apply linear binning
closes #8

* a test for contourDensity & linear binning

* style

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

package.json

@@ -36,6 +36,7 @@
     "d3-array": "2 - 3"
   },
   "devDependencies": {
+    "d3-polygon": "1 - 2",
     "eslint": "7",
     "mocha": "8",
     "rollup": "2",

src/density.js

@@ -31,14 +31,22 @@ export default function() {
 
   function density(data) {
     var values0 = new Float32Array(n * m),
-        values1 = new Float32Array(n * m);
+        values1 = new Float32Array(n * m),
+        pow2k = Math.pow(2, -k);
 
     data.forEach(function(d, i, data) {
-      var xi = (+x(d, i, data) + o) >> k,
-          yi = (+y(d, i, data) + o) >> k,
+      var xi = (x(d, i, data) + o) * pow2k,
+          yi = (y(d, i, data) + o) * pow2k,
           wi = +weight(d, i, data);
       if (xi >= 0 && xi < n && yi >= 0 && yi < m) {
-        values0[xi + yi * n] += wi;
+        var x0 = Math.floor(xi),
+            y0 = Math.floor(yi),
+            xt = xi - x0 - 0.5,
+            yt = yi - y0 - 0.5;
+        values0[x0 + y0 * n] += (1 - xt) * (1 - yt) * wi;
+        values0[x0 + 1 + y0 * n] += xt * (1 - yt) * wi;
+        values0[x0 + 1 + (y0 + 1) * n] += xt * yt * wi;
+        values0[x0 + (y0 + 1) * n] += (1 - xt) * yt * wi;
       }
     });
 

test/asserts.js

@@ -0,0 +1,20 @@
+import assert from "assert";
+
+export function assertInDelta(actual, expected, delta = 1e-6) {
+  assert(inDelta(actual, expected, delta));
+}
+
+function inDelta(actual, expected, delta) {
+  return (Array.isArray(expected) ? inDeltaArray : inDeltaNumber)(actual, expected, delta);
+}
+
+function inDeltaArray(actual, expected, delta) {
+  const n = expected.length;
+  if (actual.length !== n) return false;
+  for (let i = 0; i < n; ++i) if (!inDelta(actual[i], expected[i], delta)) return false;
+  return true;
+}
+
+function inDeltaNumber(actual, expected, delta) {
+  return actual >= expected - delta && actual <= expected + delta;
+}

test/density-test.js

@@ -1,9 +1,29 @@
 import assert from "assert";
+import {polygonCentroid} from "d3-polygon";
 import {contourDensity} from "../src/index.js";
+import {assertInDelta} from "./asserts.js";
 
 it("density.size(…) validates the specified size", () => {
   assert.deepStrictEqual(contourDensity().size([1, 2]).size(), [1, 2]);
   assert.deepStrictEqual(contourDensity().size([0, 0]).size(), [0, 0]);
   assert.deepStrictEqual(contourDensity().size([1.5, 2.5]).size(), [1.5, 2.5]);
   assert.throws(() => void contourDensity().size([0, -1]), /invalid size/);
 });
+
+it("contourDensity(data) returns the expected result for empty data", () => {
+  const c = contourDensity();
+  assert.deepStrictEqual(c([]), []);
+});
+
+it("contourDensity(data) returns contours centered on a point", () => {
+  const c = contourDensity().thresholds([0.0001, 0.001]);
+  for (const p of [[100, 100], [100.5, 102]]) {
+    const contour = c([p]);
+    assert.strictEqual(contour.length, 2);
+    for (const b of contour) {
+      const a = polygonCentroid(b.coordinates[0][0]);
+      assertInDelta(a[0], p[0], 0.1);
+      assertInDelta(a[1], p[1], 0.1);
+    }
+  }
+});

yarn.lock

@@ -275,6 +275,11 @@ cross-spawn@^7.0.2:
   dependencies:
     internmap "1 - 2"
 
+"d3-polygon@1 - 2":
+  version "2.0.0"
+  resolved "https://registry.yarnpkg.com/d3-polygon/-/d3-polygon-2.0.0.tgz#13608ef042fbec625ba1598327564f03c0396d8e"
+  integrity sha512-MsexrCK38cTGermELs0cO1d79DcTsQRN7IWMJKczD/2kBjzNXxLUWP33qRF6VDpiLV/4EI4r6Gs0DAWQkE8pSQ==
+
 debug@4.3.1, debug@^4.0.1, debug@^4.1.1:
   version "4.3.1"
   resolved "https://registry.yarnpkg.com/debug/-/debug-4.3.1.tgz#f0d229c505e0c6d8c49ac553d1b13dc183f6b2ee"