From 2d016c44bd321dcd4cdfb10cb304979afa95227e Mon Sep 17 00:00:00 2001
From: Andras Schmelczer <andras@schmelczer.dev>
Date: Sun, 17 Aug 2025 15:01:38 +0100
Subject: [PATCH] Add local selection update

---
 .../views/cursors/update-selection.test.ts    | 111 ++++++++++++++++++
 .../src/views/cursors/update-selection.ts     |  37 ++++++
 2 files changed, 148 insertions(+)
 create mode 100644 frontend/obsidian-plugin/src/views/cursors/update-selection.test.ts
 create mode 100644 frontend/obsidian-plugin/src/views/cursors/update-selection.ts

diff --git a/frontend/obsidian-plugin/src/views/cursors/update-selection.test.ts b/frontend/obsidian-plugin/src/views/cursors/update-selection.test.ts
new file mode 100644
index 00000000..991ff76b
--- /dev/null
+++ b/frontend/obsidian-plugin/src/views/cursors/update-selection.test.ts
@@ -0,0 +1,111 @@
+import { updateSelection } from "./update-selection";
+
+describe("Selection update", () => {
+	it("should handle span fully before - insert", () => {
+		const spans = [{ start: 3, end: 5 }];
+		updateSelection({
+			fromA: 0,
+			toA: 0,
+			fromB: 0,
+			toB: 2,
+			spans
+		});
+		expect(spans).toEqual([{ start: 5, end: 7 }]);
+	});
+
+	it("should handle span fully before - delete", () => {
+		const spans = [{ start: 3, end: 5 }];
+		updateSelection({
+			fromA: 0,
+			toA: 2,
+			fromB: 0,
+			toB: 0,
+			spans
+		});
+		expect(spans).toEqual([{ start: 1, end: 3 }]);
+	});
+
+	it("should handle span fully after - insert", () => {
+		const spans = [{ start: 3, end: 5 }];
+		updateSelection({
+			fromA: 6,
+			toA: 6,
+			fromB: 6,
+			toB: 10,
+			spans
+		});
+		expect(spans).toEqual([{ start: 3, end: 5 }]);
+	});
+
+	it("should handle span fully after - delete", () => {
+		const spans = [{ start: 3, end: 5 }];
+		updateSelection({
+			fromA: 6,
+			toA: 10,
+			fromB: 6,
+			toB: 6,
+			spans
+		});
+		expect(spans).toEqual([{ start: 3, end: 5 }]);
+	});
+
+	it("should handle span fully within - insert", () => {
+		const spans = [{ start: 3, end: 5 }];
+		updateSelection({
+			fromA: 4,
+			toA: 4,
+			fromB: 4,
+			toB: 6,
+			spans
+		});
+		expect(spans).toEqual([{ start: 3, end: 7 }]);
+	});
+
+	it("should handle span fully within - delete", () => {
+		const spans = [{ start: 3, end: 5 }];
+		updateSelection({
+			fromA: 4,
+			toA: 5,
+			fromB: 4,
+			toB: 4,
+			spans
+		});
+		expect(spans).toEqual([{ start: 3, end: 4 }]);
+	});
+
+	it("should handle span overlapping with start", () => {
+		const spans = [{ start: 3, end: 5 }];
+		updateSelection({
+			fromA: 2,
+			toA: 4,
+			fromB: 2,
+			toB: 2,
+			spans
+		});
+		expect(spans).toEqual([{ start: 2, end: 4 }]);
+	});
+
+	it("should handle span overlapping with end", () => {
+		const spans = [{ start: 3, end: 5 }];
+		updateSelection({
+			fromA: 4,
+			toA: 6,
+			fromB: 4,
+			toB: 4,
+			spans
+		});
+		expect(spans).toEqual([{ start: 3, end: 4 }]);
+	});
+
+	it("delete entire selection", () => {
+		const spans = [{ start: 3, end: 5 }];
+		updateSelection({
+			fromA: 0,
+			toA: 10,
+			fromB: 0,
+			toB: 0,
+			spans
+		});
+		expect(spans).toEqual([{ start: 0, end: 0 }]);
+	});
+});
diff --git a/frontend/obsidian-plugin/src/views/cursors/update-selection.ts b/frontend/obsidian-plugin/src/views/cursors/update-selection.ts
new file mode 100644
index 00000000..2551f863
--- /dev/null
+++ b/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;
+		}
+	});
+};