feat: 🎸 update remote.scanBwd() method

Author: streamich

src/__demos__/json-crdt-server/services/blocks/BlocksServices.ts

@@ -106,10 +106,10 @@ export class BlocksServices {
     if (!limit || Math.round(limit) !== limit) throw RpcError.badRequest('INVALID_LIMIT');
     if (limit > 0) {
       min = Number(offset) || 0;
-      max = min + limit;
+      max = min + limit - 1;
     } else {
       max = Number(offset) || 0;
-      min = max - limit;
+      min = max - limit + 1;
     }
     if (min < 0) {
       min = 0;

src/json-crdt-repo/remote/DemoServerRemoteHistory.ts

@@ -31,7 +31,7 @@ export class DemoServerRemoteHistory
     const limit = 100;
     const res = await this.client.call('block.scan', {
       id,
-      cur: cursor,
+      cur: cursor + 1,
       limit,
     });
     return res;
@@ -40,14 +40,20 @@ export class DemoServerRemoteHistory
   public async scanBwd(
     id: string,
     cursor: Cursor,
-  ): Promise<{snapshot: DemoServerSnapshot; patches: DemoServerPatch[]}> {
-    throw new Error('Method not implemented.');
-    // const res = await this.client.call('block.scan', {
-    //   id,
-    //   seq: cursor,
-    //   limit: -100,
-    //   model: true,
-    // });
+  ): Promise<{snapshot?: DemoServerSnapshot; patches: DemoServerPatch[]}> {
+    if (cursor <= 0) {
+      return {
+        patches: [],
+      };
+    }
+    const res = await this.client.call('block.scan', {
+      id,
+      cur: 0,
+      limit: cursor,
+    });
+    return {
+      patches: res.patches,
+    };
   }
 
   public async create(

src/json-crdt-repo/remote/__tests__/DemoServerRemoteHistory.spec.ts

@@ -133,13 +133,43 @@ describe('.scanFwd()', () => {
     const patch2 = model1.api.flush();
     const blob2 = patch2.toBinary();
     await remote.update(id, [{blob: blob2}]);
-    const scan1 = await remote.scanFwd(id, read1.block.snapshot.cur + 1);
+    const scan1 = await remote.scanFwd(id, read1.block.snapshot.cur);
     expect(scan1).toMatchObject({
       patches: [{
         blob: expect.any(Uint8Array),
         ts: expect.any(Number),
       }]
     });
+    expect(scan1.patches[0].blob).toEqual(blob2);
+  });
+});
+
+describe('.scanBwd()', () => {
+  test('can scan patches backward', async () => {
+    const {remote} = await setup();
+    const id = genId();
+    const model1 = Model.withLogicalClock();
+    model1.api.root({score: 42});
+    const patch1 = model1.api.flush();
+    const blob1 = patch1.toBinary();
+    await remote.create(id, [{blob: blob1}]);
+    const read1 = await remote.read(id);
+    model1.api.obj([]).set({
+      foo: 'bar',
+    });
+    const patch2 = model1.api.flush();
+    const blob2 = patch2.toBinary();
+    await remote.update(id, [{blob: blob2}]);
+    const read2 = await remote.read(id);
+    const scan1 = await remote.scanBwd(id, read2.block.snapshot.cur);
+    expect(scan1.patches.length).toBe(1);
+    expect(scan1).toMatchObject({
+      patches: [{
+        blob: expect.any(Uint8Array),
+        ts: expect.any(Number),
+      }]
+    });
+    expect(scan1.patches[0].blob).toEqual(blob1);
   });
 });
 

src/json-crdt-repo/remote/types.ts

@@ -59,7 +59,7 @@ export interface RemoteHistory<
    * @param id ID of the block.
    * @param cursor The cursor until which to scan.
    */
-  scanBwd(id: string, cursor: Cursor): Promise<{snapshot: S; patches: P[]}>;
+  scanBwd(id: string, cursor: Cursor): Promise<{patches: P[]; snapshot?: S}>;
 
   /**
    * Create a new block with the given patches.