Add local selection update

frontend/obsidian-plugin/src/views/cursors/update-selection.ts

@@ -0,0 +1,37 @@
+import type { CursorSpan } from "sync-client";
+
+export const updateSelection = ({
+	fromA,
+	toA,
+	toB,
+	spans
+}: {
+	fromA: number;
+	toA: number;
+	fromB: number;
+	toB: number;
+	spans: CursorSpan[];
+}): void => {
+	spans.forEach((span) => {
+		if (fromA <= span.start) {
+			// The change covers the entirety of the selection
+			if (toA > span.end) {
+				span.start = toB;
+				span.end = toB;
+				return;
+			}
+
+			let change = toB - toA;
+			if (change < 0) {
+				change = Math.max(change, fromA - span.start);
+			}
+
+			span.start += change;
+			span.end += change;
+		} else if (toA <= span.end) {
+			span.end += toB - toA;
+		} else if (toB <= span.end) {
+			span.end = toB;
+		}
+	});
+};