feat(json-crdt): 🎸 implement .prevId() utility

Author: streamich

src/json-crdt/nodes/rga/AbstractRga.ts

@@ -755,7 +755,7 @@ export abstract class AbstractRga<T> {
     this.count--;
   }
 
-  public findById(after: ITimestampStruct): undefined | Chunk<T> {
+  public findById(after: ITimestampStruct): Chunk<T> | undefined {
     const afterSid = after.sid;
     const afterTime = after.time;
     let curr: Chunk<T> | undefined = this.ids;
@@ -793,6 +793,18 @@ export abstract class AbstractRga<T> {
     return chunk;
   }
 
+  public prevId(id: ITimestampStruct): ITimestampStruct | undefined {
+    let chunk = this.findById(id);
+    if (!chunk) return;
+    const time = id.time;
+    if (chunk.id.time < time) return new Timestamp(id.sid, time - 1);
+    chunk = prev(chunk);
+    if (!chunk) return;
+    const prevId = chunk.id;
+    const span = chunk.span;
+    return span > 1 ? new Timestamp(prevId.sid, prevId.time + chunk.span - 1) : prevId;
+  }
+
   public spanView(span: ITimespanStruct): T[] {
     const view: T[] = [];
     let remaining = span.span;

src/json-crdt/nodes/str/__tests__/StrNode.spec.ts

@@ -1354,6 +1354,46 @@ describe('StrNode', () => {
     });
   });
 
+  describe('.prevId()', () => {
+    test('can iterate through IDs in reverse', () => {
+      const type = new StrNode(ts(1, 1));
+      type.ins(ts(1, 1), ts(1, 2), '123456789');
+      type.ins(ts(1, 4), ts(2, 5), 'xxx');
+      type.ins(ts(1, 7), ts(3, 5), 'yyy');
+      let id = ts(1, 10);
+      id = type.prevId(id)!;
+      expect(id).toStrictEqual(ts(1, 9));
+      id = type.prevId(id)!;
+      expect(id).toStrictEqual(ts(1, 8));
+      id = type.prevId(id)!;
+      expect(id).toStrictEqual(ts(3, 7));
+      id = type.prevId(id)!;
+      expect(id).toStrictEqual(ts(3, 6));
+      id = type.prevId(id)!;
+      expect(id).toStrictEqual(ts(3, 5));
+      id = type.prevId(id)!;
+      expect(id).toStrictEqual(ts(1, 7));
+      id = type.prevId(id)!;
+      expect(id).toStrictEqual(ts(1, 6));
+      id = type.prevId(id)!;
+      expect(id).toStrictEqual(ts(1, 5));
+      id = type.prevId(id)!;
+      expect(id).toStrictEqual(ts(2, 7));
+      id = type.prevId(id)!;
+      expect(id).toStrictEqual(ts(2, 6));
+      id = type.prevId(id)!;
+      expect(id).toStrictEqual(ts(2, 5));
+      id = type.prevId(id)!;
+      expect(id).toStrictEqual(ts(1, 4));
+      id = type.prevId(id)!;
+      expect(id).toStrictEqual(ts(1, 3));
+      id = type.prevId(id)!;
+      expect(id).toStrictEqual(ts(1, 2));
+      id = type.prevId(id)!;
+      expect(id).toBe(undefined);
+    });
+  });
+
   describe('export / import', () => {
     type Entry = [ITimestampStruct, number, string];
     const exp = (type: StrNode) => {