feat(cbz): pure chapter-window helpers (chapterIndexAt/chapterWin) with table tests

frontend/test/virt.test.ts

@@ -1,5 +1,5 @@
 import { expect, it } from "vitest";
-import { PageHeights } from "../src/lib/virt";
+import { chapterIndexAt, chapterWin, PageHeights } from "../src/lib/virt";
 
 it("估算高度:offset/total/pageAt/range(带 overscan、夹边)", () => {
   const ph = new PageHeights(10, 100);
@@ -37,3 +37,35 @@ it("n=0 时一切归空", () => {
   expect(ph.range(0, 100, 2)).toEqual([0, 0]);
   expect(ph.set(0, 50)).toBe(0);
 });
+
+const CH4 = [{ start: 0 }, { start: 10 }, { start: 20 }, { start: 30 }];
+
+it("chapterIndexAt:无章/单章恒 0;页号归章;首章 start 之前的散页归 0", () => {
+  expect(chapterIndexAt(null, 7)).toBe(0);
+  expect(chapterIndexAt([{ start: 0 }], 7)).toBe(0);
+  expect(chapterIndexAt(CH4, 0)).toBe(0);
+  expect(chapterIndexAt(CH4, 9)).toBe(0);
+  expect(chapterIndexAt(CH4, 10)).toBe(1);
+  expect(chapterIndexAt(CH4, 999)).toBe(3);
+  expect(chapterIndexAt([{ start: 5 }, { start: 10 }], 3)).toBe(0); // 根散页
+});
+
+it("chapterWin:扁平/连读 = 全书窗口", () => {
+  expect(chapterWin(null, 40, 0, false, 2)).toEqual({ start: 0, end: 40, mountEnd: 40 });
+  expect(chapterWin(CH4, 40, 2, true, 2)).toEqual({ start: 0, end: 40, mountEnd: 40 });
+});
+
+it("chapterWin:锁章窗口 = 本章;mountEnd 按预读章数延伸、夹到 count", () => {
+  expect(chapterWin(CH4, 40, 1, false, 0)).toEqual({ start: 10, end: 20, mountEnd: 20 });
+  expect(chapterWin(CH4, 40, 1, false, 1)).toEqual({ start: 10, end: 20, mountEnd: 30 });
+  expect(chapterWin(CH4, 40, 1, false, 2)).toEqual({ start: 10, end: 20, mountEnd: 40 });
+  expect(chapterWin(CH4, 40, 1, false, 3)).toEqual({ start: 10, end: 20, mountEnd: 40 }); // 越界夹住
+  expect(chapterWin(CH4, 40, 3, false, 2)).toEqual({ start: 30, end: 40, mountEnd: 40 }); // 末章
+});
+
+it("chapterWin:ci 越界夹边;首章窗口含首章 start 之前的散页;prefetch 负数按 0", () => {
+  expect(chapterWin(CH4, 40, 9, false, 1)).toEqual({ start: 30, end: 40, mountEnd: 40 });
+  expect(chapterWin(CH4, 40, -2, false, 1)).toEqual({ start: 0, end: 10, mountEnd: 20 });
+  expect(chapterWin([{ start: 5 }, { start: 10 }], 20, 0, false, 0)).toEqual({ start: 0, end: 10, mountEnd: 10 });
+  expect(chapterWin(CH4, 40, 1, false, -1)).toEqual({ start: 10, end: 20, mountEnd: 20 });
+});