feat(frontend): cbz page/spread view math (TDD)

frontend/src/lib/cbzPages.ts

@@ -0,0 +1,21 @@
+export type PageMode = "page" | "spread";
+
+export function viewCount(count: number, mode: PageMode): number {
+  return mode === "page" ? count : Math.ceil(count / 2);
+}
+export function pageIndexToView(i: number, mode: PageMode): number {
+  return mode === "page" ? i : Math.floor(i / 2);
+}
+export function viewToPageIndex(v: number, mode: PageMode): number {
+  return mode === "page" ? v : v * 2;
+}
+/** 一个 spread 视图要渲染的页,按显示顺序（LTR=[左,右];RTL=[右,左] 即先读页在前）。 */
+export function spreadPages(first: number, count: number, rtl: boolean): number[] {
+  const pages = [first, ...(first + 1 < count ? [first + 1] : [])];
+  return rtl ? [...pages].reverse() : pages;
+}
+/** 翻一屏/页；rtl 时方向翻转；越界钳制。 */
+export function turnView(v: number, d: number, rtl: boolean, views: number): number {
+  const step = rtl ? -d : d;
+  return Math.max(0, Math.min(views - 1, v + step));
+}