Allow serializing and deserializing infinite length comments

Goes along with TurboWarp/scratch-blocks@78177be

src/serialization/sb3.js

@@ -95,6 +95,9 @@ const primitiveOpcodeInfoMap = {
     data_listcontents: [LIST_PRIMITIVE, 'LIST']
 };
 
+// We don't enforce this limit, but Scratch does, so we need to handle it for compatibility.
+const UPSTREAM_MAX_COMMENT_LENGTH = 8000;
+
 /**
  * Serializes primitives described above into a more compact format
  * @param {object} block the block to serialize
@@ -555,7 +558,16 @@ const serializeComments = function (comments) {
         serializedComment.width = comment.width;
         serializedComment.height = comment.height;
         serializedComment.minimized = comment.minimized;
-        serializedComment.text = comment.text;
+
+        if (comment.text.length > UPSTREAM_MAX_COMMENT_LENGTH) {
+            // Upstream's scratch-parser will refuse to load projects if the text is too long, so to maximize
+            // compatibility and minimize redundancy we'll store a truncated version in .text and the rest in
+            // another field
+            serializedComment.text = comment.text.substring(0, UPSTREAM_MAX_COMMENT_LENGTH);
+            serializedComment.extraText = comment.text.substring(UPSTREAM_MAX_COMMENT_LENGTH);
+        } else {
+            serializedComment.text = comment.text;
+        }
 
         obj[commentId] = serializedComment;
     }
@@ -1213,7 +1225,8 @@ const parseScratchObject = function (object, runtime, extensions, zip, assets) {
             const comment = object.comments[commentId];
             const newComment = new Comment(
                 commentId,
-                comment.text,
+                // text has a length limit, so anything extra got saved in extraText
+                comment.text + (typeof comment.extraText === 'string' ? comment.extraText : ''),
                 comment.x,
                 comment.y,
                 comment.width,

test/integration/tw_very_long_comments.js

@@ -0,0 +1,82 @@
+const {test} = require('tap');
+const fs = require('fs');
+const VirtualMachine = require('../../src/virtual-machine');
+const Runtime = require('../../src/engine/runtime');
+const sb3 = require('../../src/serialization/sb3');
+const RenderedTarget = require('../../src/sprites/rendered-target');
+const Sprite = require('../../src/sprites/sprite');
+const path = require('path');
+
+test('serializes very long comments', t => {
+    const rt = new Runtime();
+    const sprite = new Sprite();
+    const target = new RenderedTarget(sprite);
+    rt.addTarget(target);
+
+    target.createComment('id_short', null, 'short comment', 0, 0, 20, 20, false);
+    target.createComment('id_max_length', null, `start${'0'.repeat(8000 - 5)}`, 0, 0, 20, 20, false);
+    target.createComment('id_max_length_plus_1', null, `start${'0'.repeat(8000 - 5)}a`, 0, 0, 20, 20, false);
+    target.createComment(
+        'id_way_too_long',
+        null,
+        `start${'0'.repeat(8000 - 5 - 3)}endthis should be the truncated part${'0'.repeat(25000)}`,
+        0,
+        0,
+        20,
+        20,
+        false
+    );
+
+    const serialized = sb3.serialize(rt, target.id);
+    const common = {
+        // same for every block, already tested elsewhere, not interesting for here
+        blockId: null,
+        x: 0,
+        y: 0,
+        width: 20,
+        height: 20,
+        minimized: false
+    };
+    t.same(serialized.comments, {
+        id_short: {
+            ...common,
+            text: 'short comment'
+        },
+        id_max_length: {
+            ...common,
+            text: `start${'0'.repeat(8000 - 5)}`
+        },
+        id_max_length_plus_1: {
+            ...common,
+            text: `start${'0'.repeat(8000 - 5)}`,
+            extraText: 'a'
+        },
+        id_way_too_long: {
+            ...common,
+            text: `start${'0'.repeat(8000 - 5 - 3)}end`,
+            extraText: `this should be the truncated part${'0'.repeat(25000)}`
+        }
+    });
+
+    t.end();
+});
+
+test('deserializes very long comments', t => {
+    const vm = new VirtualMachine();
+    const fixture = fs.readFileSync(path.resolve(__dirname, '../fixtures/tw-very-long-comments.sb3'));
+    vm.loadProject(fixture).then(() => {
+        const comments = vm.runtime.targets[0].comments;
+
+        // note that comment IDs may change each time the test project is saved
+        t.equal(comments.b.text, '');
+        t.equal(comments.a.text, 'short');
+        t.equal(comments.c.text, `exactly length limit${'0'.repeat(8000 - 'exactly length limit'.length)}`);
+        t.equal(comments.d.text, `length limit + 1${':'.repeat(8000 - 'length limit + 1'.length)}1`);
+        t.equal(
+            comments.e.text,
+            `unreasonably long${'!'.repeat(8000 - 'unreasonably long'.length)}${'123456789'.repeat(2000)}`
+        );
+
+        t.end();
+    });
+});