import { migrateSave, type SaveDocument } from "@deadhead/schema";

/**
 * Where a save lives (CR-014 §2): a main slot plus three rotating backups.
 * Each backend only stores text in named slots, writing a slot all-or-nothing;
 * `SaveStorage` does the rotation, validation and fallback once for all of them.
 */
export type Slot = "main" | "backup-1" | "backup-2" | "backup-3";
export const BACKUP_SLOTS = ["backup-1", "backup-2", "backup-3"] as const;

export interface SaveBackend {
  /** For debug info: "memory", "indexeddb" or "filesystem". */
  readonly kind: string;
  read(slot: Slot): Promise<string | null>;
  /** Replaces the slot's text completely, or leaves it as it was. */
  write(slot: Slot, text: string): Promise<void>;
  remove(slot: Slot): Promise<void>;
}

export type LoadResult =
  | { status: "empty" }
  | { status: "loaded"; save: SaveDocument }
  /** The main save was unreadable; the newest valid backup was loaded instead. */
  | { status: "restored-from-backup"; save: SaveDocument; backup: number; problem: string }
  /** Nothing readable: the main save and every backup failed. */
  | { status: "unreadable"; problem: string };

export interface BackupInfo {
  /** 1 is the newest. */
  index: number;
  lastTimeSeen: number | null;
  valid: boolean;
}

/** Text to a save, or the reason it isn't one. */
function parse(text: string) {
  let raw: unknown;
  try {
    raw = JSON.parse(text);
  } catch {
    return { ok: false as const, error: "The save isn't valid JSON." };
  }
  return migrateSave(raw);
}

export class SaveStorage {
  constructor(private readonly backend: SaveBackend) {}

  get kind(): string {
    return this.backend.kind;
  }

  /**
   * The current save. Missing main save: empty (a first launch). Unreadable
   * main save: the newest backup that validates, with the problem reported.
   */
  async load(): Promise<LoadResult> {
    const text = await this.backend.read("main");
    if (text === null) return { status: "empty" };
    const main = parse(text);
    if (main.ok) return { status: "loaded", save: main.save };
    for (const [i, slot] of BACKUP_SLOTS.entries()) {
      const backup = await this.backend.read(slot);
      if (backup === null) continue;
      const parsed = parse(backup);
      if (parsed.ok) {
        return {
          status: "restored-from-backup",
          save: parsed.save,
          backup: i + 1,
          problem: main.error,
        };
      }
    }
    return { status: "unreadable", problem: main.error };
  }

  /** Writes the save; the previous main save becomes backup 1, and so on, keeping three. */
  async save(save: SaveDocument): Promise<void> {
    const previous = await this.backend.read("main");
    if (previous !== null) {
      for (let i = BACKUP_SLOTS.length - 1; i > 0; i--) {
        const older = await this.backend.read(BACKUP_SLOTS[i - 1]!);
        if (older !== null) await this.backend.write(BACKUP_SLOTS[i]!, older);
      }
      await this.backend.write("backup-1", previous);
    }
    await this.backend.write("main", JSON.stringify(save));
  }

  async listBackups(): Promise<BackupInfo[]> {
    const found: BackupInfo[] = [];
    for (const [i, slot] of BACKUP_SLOTS.entries()) {
      const text = await this.backend.read(slot);
      if (text === null) continue;
      const parsed = parse(text);
      found.push({
        index: i + 1,
        valid: parsed.ok,
        lastTimeSeen: parsed.ok ? parsed.save.lastTimeSeen : null,
      });
    }
    return found;
  }

  async loadBackup(index: number) {
    const slot = BACKUP_SLOTS[index - 1];
    if (!slot) return { ok: false as const, error: `There is no backup ${index}.` };
    const text = await this.backend.read(slot);
    return text === null ? { ok: false as const, error: `Backup ${index} is empty.` } : parse(text);
  }

  /** Deletes the save and its backups (a new company or an import starts clean). */
  async clear(): Promise<void> {
    for (const slot of ["main", ...BACKUP_SLOTS] as const) await this.backend.remove(slot);
  }
}

/** For tests and as a fallback when the device offers no storage. */
export class MemoryBackend implements SaveBackend {
  readonly kind = "memory";
  readonly slots = new Map<Slot, string>();
  async read(slot: Slot) {
    return this.slots.get(slot) ?? null;
  }
  async write(slot: Slot, text: string) {
    this.slots.set(slot, text);
  }
  async remove(slot: Slot) {
    this.slots.delete(slot);
  }
}

const DB_NAME = "deadhead";
const STORE = "saves";

/** Web storage: one IndexedDB object store, one record per slot (each put is atomic). */
export class IndexedDbBackend implements SaveBackend {
  readonly kind = "indexeddb";
  #db: Promise<IDBDatabase> | null = null;

  constructor(private readonly factory: IDBFactory) {}

  #open(): Promise<IDBDatabase> {
    this.#db ??= new Promise((resolve, reject) => {
      const request = this.factory.open(DB_NAME, 1);
      request.onupgradeneeded = () => request.result.createObjectStore(STORE);
      request.onsuccess = () => resolve(request.result);
      request.onerror = () => reject(request.error);
    });
    return this.#db;
  }

  async #run<T>(
    mode: IDBTransactionMode,
    op: (store: IDBObjectStore) => IDBRequest<T>,
  ): Promise<T> {
    const db = await this.#open();
    return new Promise((resolve, reject) => {
      const tx = db.transaction(STORE, mode);
      const request = op(tx.objectStore(STORE));
      tx.oncomplete = () => resolve(request.result);
      tx.onerror = () => reject(tx.error);
      tx.onabort = () => reject(tx.error);
    });
  }

  async read(slot: Slot) {
    const value = await this.#run("readonly", (s) => s.get(slot) as IDBRequest<string | undefined>);
    return value ?? null;
  }
  async write(slot: Slot, text: string) {
    await this.#run("readwrite", (s) => s.put(text, slot));
  }
  async remove(slot: Slot) {
    await this.#run("readwrite", (s) => s.delete(slot));
  }
}

/**
 * The parts of the Capacitor Filesystem plugin this uses (`@capacitor/filesystem`),
 * so the engine doesn't depend on Capacitor and tests can pass a fake.
 */
export interface FilesystemPlugin {
  readFile(options: {
    path: string;
    directory: "DATA";
    encoding: "utf8";
  }): Promise<{ data: string | Blob }>;
  writeFile(options: {
    path: string;
    data: string;
    directory: "DATA";
    encoding: "utf8";
    recursive?: boolean;
  }): Promise<unknown>;
  rename(options: {
    from: string;
    to: string;
    directory: "DATA";
    toDirectory: "DATA";
  }): Promise<void>;
  deleteFile(options: { path: string; directory: "DATA" }): Promise<void>;
}

/**
 * Native storage: one JSON file per slot in the app's data directory (included
 * in iCloud and Android backups). A write goes to a temporary file that is then
 * renamed over the slot, so the slot is never half-written.
 */
export class FilesystemBackend implements SaveBackend {
  readonly kind = "filesystem";
  constructor(
    private readonly fs: FilesystemPlugin,
    private readonly folder = "deadhead",
  ) {}

  #path(name: string) {
    return `${this.folder}/${name}.json`;
  }

  async read(slot: Slot) {
    try {
      const { data } = await this.fs.readFile({
        path: this.#path(slot),
        directory: "DATA",
        encoding: "utf8",
      });
      return typeof data === "string" ? data : await data.text();
    } catch {
      // Capacitor rejects for a missing file.
      return null;
    }
  }

  async write(slot: Slot, text: string) {
    const temp = this.#path(`${slot}.tmp`);
    await this.fs.writeFile({
      path: temp,
      data: text,
      directory: "DATA",
      encoding: "utf8",
      recursive: true,
    });
    await this.fs.rename({
      from: temp,
      to: this.#path(slot),
      directory: "DATA",
      toDirectory: "DATA",
    });
  }

  async remove(slot: Slot) {
    try {
      await this.fs.deleteFile({ path: this.#path(slot), directory: "DATA" });
    } catch {
      // Already gone.
    }
  }
}