From df4904d2fd08ef4ed21aa0b79f2be38b7ed2d948 Mon Sep 17 00:00:00 2001
From: Uruk <contact@uruk.dev>
Date: Fri, 22 May 2026 11:47:51 +0200
Subject: [PATCH] feat(chapters): add pure chapter helpers

---
 utils/chapters.test.ts | 57 +++++++++++++++++++++++++++++++++++++++
 utils/chapters.ts      | 60 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 117 insertions(+)
 create mode 100644 utils/chapters.test.ts
 create mode 100644 utils/chapters.ts

diff --git a/utils/chapters.test.ts b/utils/chapters.test.ts
new file mode 100644
index 00000000..d1ab690a
--- /dev/null
+++ b/utils/chapters.test.ts
@@ -0,0 +1,57 @@
+import { describe, expect, test } from "bun:test";
+import {
+  chapterMarkers,
+  currentChapterIndex,
+  formatChapterTime,
+} from "./chapters";
+
+// Helper: a ChapterInfo with a start in milliseconds.
+const ch = (ms: number, name?: string) => ({
+  StartPositionTicks: ms * 10000,
+  Name: name,
+});
+
+describe("chapterMarkers", () => {
+  test("maps chapters to position + percent", () => {
+    expect(chapterMarkers([ch(0), ch(30_000), ch(60_000)], 120_000)).toEqual([
+      { positionMs: 0, percent: 0 },
+      { positionMs: 30_000, percent: 25 },
+      { positionMs: 60_000, percent: 50 },
+    ]);
+  });
+
+  test("drops chapters past the duration", () => {
+    expect(chapterMarkers([ch(0), ch(200_000)], 120_000)).toEqual([
+      { positionMs: 0, percent: 0 },
+    ]);
+  });
+
+  test("returns [] when duration is 0 or chapters missing", () => {
+    expect(chapterMarkers([ch(0)], 0)).toEqual([]);
+    expect(chapterMarkers(null, 120_000)).toEqual([]);
+    expect(chapterMarkers(undefined, 120_000)).toEqual([]);
+  });
+});
+
+describe("currentChapterIndex", () => {
+  const chapters = [ch(0), ch(30_000), ch(60_000)];
+  test("returns the chapter containing the position", () => {
+    expect(currentChapterIndex(0, chapters)).toBe(0);
+    expect(currentChapterIndex(15_000, chapters)).toBe(0);
+    expect(currentChapterIndex(30_000, chapters)).toBe(1);
+    expect(currentChapterIndex(90_000, chapters)).toBe(2);
+  });
+  test("returns -1 before the first chapter and for no chapters", () => {
+    expect(currentChapterIndex(-5, chapters)).toBe(-1);
+    expect(currentChapterIndex(10_000, [])).toBe(-1);
+    expect(currentChapterIndex(10_000, null)).toBe(-1);
+  });
+});
+
+describe("formatChapterTime", () => {
+  test("formats m:ss and h:mm:ss", () => {
+    expect(formatChapterTime(65_000)).toBe("1:05");
+    expect(formatChapterTime(3_725_000)).toBe("1:02:05");
+    expect(formatChapterTime(-100)).toBe("0:00");
+  });
+});
diff --git a/utils/chapters.ts b/utils/chapters.ts
new file mode 100644
index 00000000..bab4a8c5
--- /dev/null
+++ b/utils/chapters.ts
@@ -0,0 +1,60 @@
+/**
+ * Pure helpers for Jellyfin chapter markers. Dependency-free so they are
+ * unit-testable under `bun test`.
+ */
+
+import type { ChapterInfo } from "@jellyfin/sdk/lib/generated-client/models";
+
+const TICKS_PER_MS = 10000;
+
+export interface ChapterMarker {
+  /** Chapter start, in milliseconds. */
+  positionMs: number;
+  /** Chapter start as a percentage (0-100) of the media duration. */
+  percent: number;
+}
+
+/** Chapter start positions in milliseconds, ascending. */
+export const chapterStartsMs = (
+  chapters: ChapterInfo[] | null | undefined,
+): number[] =>
+  (chapters ?? [])
+    .map((c) => (c.StartPositionTicks ?? 0) / TICKS_PER_MS)
+    .sort((a, b) => a - b);
+
+/** Chapter markers within [0, durationMs]; empty when duration is unknown. */
+export const chapterMarkers = (
+  chapters: ChapterInfo[] | null | undefined,
+  durationMs: number,
+): ChapterMarker[] => {
+  if (durationMs <= 0) return [];
+  return chapterStartsMs(chapters)
+    .filter((ms) => ms >= 0 && ms <= durationMs)
+    .map((ms) => ({ positionMs: ms, percent: (ms / durationMs) * 100 }));
+};
+
+/** Index of the chapter containing `positionMs`, or -1 if before the first. */
+export const currentChapterIndex = (
+  positionMs: number,
+  chapters: ChapterInfo[] | null | undefined,
+): number => {
+  const starts = chapterStartsMs(chapters);
+  let index = -1;
+  for (let i = 0; i < starts.length; i++) {
+    if (positionMs >= starts[i]) index = i;
+    else break;
+  }
+  return index;
+};
+
+/** `m:ss` (or `h:mm:ss` past an hour) label for a millisecond position. */
+export const formatChapterTime = (positionMs: number): string => {
+  const total = Math.max(0, Math.floor(positionMs / 1000));
+  const hours = Math.floor(total / 3600);
+  const minutes = Math.floor((total % 3600) / 60);
+  const seconds = total % 60;
+  const pad = (n: number) => String(n).padStart(2, "0");
+  return hours > 0
+    ? `${hours}:${pad(minutes)}:${pad(seconds)}`
+    : `${minutes}:${pad(seconds)}`;
+};