Add snapshot regression tests

I'm about to make some changes to the alignArrays module. Before I do
that, I wanted to make sure we have a good testing setup in place.

This will make it easier for outside contributors when reporting bugs.
They can add a new test case and either fix or ask for a fix.

Author: trotzig

snapshots/logo/after.png

@@ -0,0 +1,58 @@
+const crypto = require('crypto');
+const childProcess = require('child_process');
+const fs = require('fs');
+const path = require('path');
+
+const Jimp = require('jimp');
+
+const imageDiff = require('../');
+
+function hashFunction(data) {
+  return crypto
+    .createHash('md5')
+    .update(data)
+    .digest('hex');
+}
+
+const snapshots = childProcess
+  .execSync('ls snapshots', { encoding: 'utf-8' })
+  .split(/\n/)
+  .filter(Boolean);
+
+jest.setTimeout(60000);
+
+describe('snapshot tests', () => {
+  snapshots.forEach(snapshot => {
+    it(snapshot, async () => {
+      console.log('Starting', snapshot);
+      const [image1, image2] = await Promise.all([
+        Jimp.read(path.resolve('snapshots', snapshot, 'before.png')),
+        Jimp.read(path.resolve('snapshots', snapshot, 'after.png')),
+      ]);
+      console.log('Images ready', snapshot);
+      const diffImage = imageDiff(image1.bitmap, image2.bitmap, {
+        hashFunction,
+      });
+
+      console.log('Created diff image', snapshot);
+      const pathToDiff = path.resolve('snapshots', snapshot, 'diff.png');
+
+      if (!fs.existsSync(pathToDiff)) {
+        console.log(
+          `No previous diff image for ${snapshot} found -- saving diff.png.`,
+        );
+        const newDiffImage = await new Jimp(diffImage);
+        await newDiffImage.write(pathToDiff);
+      }
+      const expectedDiffImage = (await Jimp.read(pathToDiff)).bitmap;
+      const diffHash = hashFunction(diffImage.data);
+      const expectedHash = hashFunction(expectedDiffImage.data);
+      if (diffHash !== expectedHash) {
+        console.log(
+          `Diff image did not match existing diff image. Remove this image and run again to re-generate:\n${pathToDiff}`,
+        );
+      }
+      expect(diffHash).toEqual(expectedHash);
+    });
+  });
+});