Don't compress formats that are already compressed

src/virtual-machine.js

@@ -537,6 +537,22 @@ class VirtualMachine extends EventEmitter {
             file.date = date;
         }
 
+        // Tell JSZip to only compress file formats where there will be a significant gain.
+        const COMPRESSABLE_FORMATS = [
+            '.json',
+            '.svg',
+            '.wav',
+            '.ttf',
+            '.otf'
+        ];
+        for (const file of Object.values(zip.files)) {
+            if (COMPRESSABLE_FORMATS.some(ext => file.name.endsWith(ext))) {
+                file.options.compression = 'DEFLATE';
+            } else {
+                file.options.compression = 'STORE';
+            }
+        }
+
         return zip;
     }
 
@@ -546,9 +562,9 @@ class VirtualMachine extends EventEmitter {
      */
     saveProjectSb3 (type) {
         return this._saveProjectZip().generateAsync({
+            // Don't configure compression here. _saveProjectZip() will set it for each file.
             type: type || 'blob',
-            mimeType: 'application/x.scratch.sb3',
-            compression: 'DEFLATE'
+            mimeType: 'application/x.scratch.sb3'
         });
     }
 

test/integration/tw_compression_per_file_type.js

@@ -0,0 +1,31 @@
+const {test} = require('tap');
+const fs = require('fs');
+const path = require('path');
+const VM = require('../../src/virtual-machine');
+const makeTestStorage = require('../fixtures/make-test-storage');
+const JSZip = require('@turbowarp/jszip');
+
+test('saveProjectSb3() per-file compression', t => {
+    const vm = new VM();
+    vm.attachStorage(makeTestStorage());
+    const fixture = fs.readFileSync(path.join(__dirname, '../fixtures/tw-mixed-file-formats.sb3'));
+    vm.loadProject(fixture)
+        .then(() => vm.saveProjectSb3('arraybuffer'))
+        .then(buffer => JSZip.loadAsync(buffer))
+        .then(zip => {
+            const isCompressed = pathInZip => {
+                // note: uses JSZip private APIs, not very stable, but it should be okay...
+                const file = zip.files[pathInZip];
+                return file._data.compression.magic === '\x08\x00';
+            };
+
+            t.ok(isCompressed('project.json'));
+            t.ok(isCompressed('5cb46ddd903fc2c9976ff881df9273c9.wav'));
+            t.ok(isCompressed('cd21514d0531fdffb22204e0ec5ed84a.svg'));
+
+            t.notOk(isCompressed('0b2e50ca4107ce57416e2ceb840a6347.jpg'));
+            t.notOk(isCompressed('5c8826d846c06dddeb77590e8792fb7d.png'));
+
+            t.end();
+        });
+});