sunnymh-manga-site/lib/progress.ts

export type ReadingProgress = {
  chapter: number;
  page: number;
  ratio: number;
};

export type StorageLike = Pick<Storage, "getItem" | "setItem">;

export function storageKey(slug: string): string {
  return `sunnymh:last-read:${slug}`;
}

function clampRatio(n: unknown): number {
  const v = typeof n === "number" ? n : Number(n);
  if (!Number.isFinite(v)) return 0;
  if (v < 0) return 0;
  if (v > 1) return 1;
  return v;
}

export function parseProgress(raw: string | null): ReadingProgress | null {
  if (!raw) return null;
  if (raw.startsWith("{")) {
    try {
      const parsed = JSON.parse(raw) as Partial<ReadingProgress>;
      if (
        typeof parsed.chapter === "number" &&
        typeof parsed.page === "number" &&
        parsed.chapter > 0 &&
        parsed.page > 0
      ) {
        return {
          chapter: parsed.chapter,
          page: parsed.page,
          ratio: clampRatio(parsed.ratio),
        };
      }
    } catch {
      return null;
    }
    return null;
  }
  const n = Number(raw);
  return Number.isFinite(n) && n > 0
    ? { chapter: n, page: 1, ratio: 0 }
    : null;
}

export function readProgress(
  slug: string,
  storage: StorageLike | null = defaultStorage()
): ReadingProgress | null {
  if (!storage) return null;
  return parseProgress(storage.getItem(storageKey(slug)));
}

export function writeProgress(
  slug: string,
  progress: ReadingProgress,
  storage: StorageLike | null = defaultStorage()
): void {
  if (!storage) return;
  storage.setItem(storageKey(slug), JSON.stringify(progress));
}

function defaultStorage(): StorageLike | null {
  return typeof window === "undefined" ? null : window.localStorage;
}