Add simple test for compare

The tests were initially super slow, as the PNG.sync.write took up to 20 seconds.
This seems to be related to jest [1][2]. I therefore added Math to the
sandboxInjectedGlobals to speed things up.

[1] pngjs/pngjs#96
[2] jestjs/jest#5163 (comment)

pixelpact/.dockerignore

@@ -0,0 +1 @@
+testdata

pixelpact/package.json

@@ -16,11 +16,17 @@
     "fastify": "^4.6.0",
     "pixelmatch": "^5.3.0",
     "playwright": "^1.26.1",
-    "pngjs": "^6.0.0"
+    "pngjs": "^7.0.0"
   },
   "devDependencies": {
     "jest": "^29.5.0",
     "prettier": "2.8.2",
     "supervisor": "^0.12.0"
+  },
+  "jest": {
+    "sandboxInjectedGlobals": [
+      "Math"
+    ],
+    "testEnvironment": "node"
   }
 }

pixelpact/src/compare.js

@@ -5,19 +5,21 @@ export async function compare(expected, actual, options = { threshold: 0.01 }) {
   const expectedPng = PNG.sync.read(expected);
   const actualPng = PNG.sync.read(actual);
   const { width, height } = expectedPng;
-  const diff = new PNG({ width, height });
+  const diffPng = new PNG({ width, height });
   const numDiffPixels = pixelmatch(
     expectedPng.data,
     actualPng.data,
-    diff.data,
+    diffPng.data,
     width,
     height,
     options
   );
+  const diff = PNG.sync.write(diffPng);
+
   return {
     numDiffPixels: numDiffPixels,
     expected: expected,
     actual: actual,
-    diff: PNG.sync.write(diff),
+    diff: diff,
   };
 }

pixelpact/src/compare.test.js

@@ -0,0 +1,17 @@
+import fs from "fs/promises";
+import { compare } from "./compare.js";
+
+describe("compare", () => {
+  it("returns the number of pixels that differ", async () => {
+    let expected = await fs.readFile("testdata/simple-expected.png");
+    let actual = await fs.readFile("testdata/simple-actual.png");
+    let expectedDiff = await fs.readFile("testdata/simple-diff.png");
+
+    let result = await compare(expected, actual);
+
+    expect(result.numDiffPixels).toBe(152);
+    expect(result.expected).toStrictEqual(expected);
+    expect(result.actual).toStrictEqual(actual);
+    expect(result.diff).toStrictEqual(expectedDiff);
+  });
+});