From 7e1cbba9eacbe0edcdff89f85455dcb658e0fc71 Mon Sep 17 00:00:00 2001
From: oppiliappan <me@oppi.li>
Date: Fri, 23 Oct 2026 09:50:05 +0100
Subject: [PATCH] web/repo: utils to find hidden diff lines

Signed-off-by: oppiliappan <me@oppi.li>
---
 web/src/lib/components/repo/fileDiff.test.ts | 37 +++++++++++++++++++-
 web/src/lib/components/repo/fileDiff.ts      | 37 +++++++++++++++++++-
 2 files changed, 72 insertions(+), 2 deletions(-)

diff --git a/web/src/lib/components/repo/fileDiff.test.ts b/web/src/lib/components/repo/fileDiff.test.ts
index 8035e313a..d5903576a 100644
--- a/web/src/lib/components/repo/fileDiff.test.ts
+++ b/web/src/lib/components/repo/fileDiff.test.ts
@@ -1,5 +1,6 @@
+import type { Hunk } from "@pierre/diffs";
 import { describe, expect, it } from "vitest";
-import { reservedRows } from "#lib/components/repo/fileDiff.ts";
+import { hiddenLine, newSideLine, reservedRows } from "#lib/components/repo/fileDiff.ts";
 
 describe("reservedRows", () => {
 	const file = (lines: number, insertions: number, deletions: number) => ({
@@ -25,3 +26,37 @@ describe("reservedRows", () => {
 		expect(reservedRows({ stat: { insertions: 10, deletions: 6 } }, false)).toBe(22);
 	});
 });
+
+const hunk = (
+	deletionStart: number,
+	deletionCount: number,
+	additionStart: number,
+	additionCount: number
+): Hunk => ({ deletionStart, deletionCount, additionStart, additionCount }) as Hunk;
+
+const hunks = [hunk(10, 5, 10, 7), hunk(40, 3, 41, 0)];
+
+describe("hiddenLine", () => {
+	it("shows lines inside a hunk", () => {
+		expect(hiddenLine(hunks, 12, "additions")).toBeUndefined();
+		expect(hiddenLine(hunks, 14, "deletions")).toBeUndefined();
+		expect(hiddenLine(hunks, 41, "deletions")).toBeUndefined();
+	});
+
+	it("maps hidden lines to the new side", () => {
+		expect(hiddenLine(hunks, 3, "additions")).toBe(3);
+		expect(hiddenLine(hunks, 3, "deletions")).toBe(3);
+		expect(hiddenLine(hunks, 20, "deletions")).toBe(22);
+		expect(hiddenLine(hunks, 30, "additions")).toBe(30);
+		expect(hiddenLine(hunks, 50, "deletions")).toBe(49);
+		expect(hiddenLine(hunks, 49, "additions")).toBe(49);
+	});
+});
+
+describe("newSideLine", () => {
+	it("puts a removed line where its hunk starts", () => {
+		expect(newSideLine(hunks, 41, "deletions")).toBe(42);
+		expect(newSideLine(hunks, 20, "deletions")).toBe(22);
+		expect(newSideLine(hunks, 7, "additions")).toBe(7);
+	});
+});
diff --git a/web/src/lib/components/repo/fileDiff.ts b/web/src/lib/components/repo/fileDiff.ts
index e0969dfea..9f94f0e63 100644
--- a/web/src/lib/components/repo/fileDiff.ts
+++ b/web/src/lib/components/repo/fileDiff.ts
@@ -1,4 +1,4 @@
-import type { FileDiffMetadata } from "@pierre/diffs";
+import type { FileDiffMetadata, Hunk, SelectionSide } from "@pierre/diffs";
 
 export type DiffEntry = FileDiffMetadata | { note: string };
 
@@ -10,3 +10,38 @@ export const reservedRows = (
 		? (file.lines ?? file.stat.insertions + file.stat.deletions)
 		: (file.lines ?? file.stat.insertions + file.stat.deletions) +
 			Math.min(file.stat.insertions, file.stat.deletions);
+
+const sideStart = (start: number, count: number): number => (count === 0 ? start + 1 : start);
+
+export const hiddenLine = (
+	hunks: readonly Hunk[],
+	line: number,
+	side: SelectionSide
+): number | undefined => {
+	let shift = 0;
+	for (const hunk of hunks) {
+		const deletionFrom = sideStart(hunk.deletionStart, hunk.deletionCount);
+		const additionFrom = sideStart(hunk.additionStart, hunk.additionCount);
+		const from = side === "deletions" ? deletionFrom : additionFrom;
+		const count = side === "deletions" ? hunk.deletionCount : hunk.additionCount;
+		const delta = additionFrom - deletionFrom;
+		if (line < from) return side === "deletions" ? line + delta : line;
+		if (line < from + count) return undefined;
+		shift = delta + hunk.additionCount - hunk.deletionCount;
+	}
+	return side === "deletions" ? line + shift : line;
+};
+
+export const newSideLine = (hunks: readonly Hunk[], line: number, side: SelectionSide): number => {
+	if (side === "additions") return line;
+	const hidden = hiddenLine(hunks, line, side);
+	if (hidden !== undefined) return hidden;
+	const hunk = hunks.find(
+		(candidate) =>
+			line >= sideStart(candidate.deletionStart, candidate.deletionCount) &&
+			line <
+				sideStart(candidate.deletionStart, candidate.deletionCount) +
+					candidate.deletionCount
+	);
+	return hunk ? sideStart(hunk.additionStart, hunk.additionCount) : line;
+};
-- 
2.54.0