feat(json-crdt-extensions): 🎸 implement overlay "ref" concept

src/json-crdt-extensions/peritext/overlay/refs.ts

@@ -0,0 +1,21 @@
+import type {SplitSlice} from '../slice/SplitSlice';
+import type {Slice} from '../slice/types';
+
+/**
+ * On overlay "ref" is a reference from the {@link Overlay} to a {@link Slice}.
+ * In case of a *marker* slice, the reference is to the slice itself. In case of
+ * a regular annotation slice, two references are needed: one to the start slice
+ * and one to the end slice.
+ */
+export type OverlayRef =
+  | SplitSlice // Ref to a *marker* slice
+  | OverlayRefSliceStart // Ref to the start of an annotation slice
+  | OverlayRefSliceEnd; // Ref to the end of an annotation slice
+
+export class OverlayRefSliceStart {
+  constructor(public readonly slice: Slice) {}
+}
+
+export class OverlayRefSliceEnd {
+  constructor(public readonly slice: Slice) {}
+}