// 本项目新增(REQ §8.1/§8.2 分批增量扫描与中断续扫);MP 整理监控为全量目录遍历,无游标。
import type { Dirent } from "node:fs";
import { lstat, readdir } from "node:fs/promises";
import { join } from "node:path";
import { errnoOf } from "./errors";
import { checkRoot, DEFAULT_ROOT_SENTINEL } from "./stat";

export interface ScanEntry {
  /** 相对根目录的路径,/ 分隔 */
  relPath: string;
  size: number;
  mtimeMs: number;
  ino: number;
  dev: number;
  nlink: number;
  isSymlink: boolean;
  change: "new" | "changed" | "unchanged";
}

export interface ScanOptions {
  /** 上次返回的游标;空则从头扫 */
  cursor?: string | null;
  /** 本批最多 lstat 的文件数(I/O 上限),默认 2000 */
  maxStats?: number;
  /** 本批最多 readdir 的目录数,默认不限 */
  maxDirs?: number;
  /** 哨兵文件名;false 表示不检查(不推荐) */
  sentinel?: string | false;
  /** 已知记录,用于标注 new/changed/unchanged */
  previous?: (relPath: string) => { size: number; mtimeMs: number } | undefined;
  /** 跳过的文件名判定(默认跳过 .uhd-partial 与 .!qB 未完成文件) */
  skip?: (name: string) => boolean;
  signal?: AbortSignal;
}

export interface ScanResult {
  status: "complete" | "partial" | "root_offline" | "aborted";
  entries: ScanEntry[];
  /** 继续扫描用的游标;complete 时为 null */
  cursor: string | null;
  statCount: number;
  dirCount: number;
  errors: { relPath: string; code: string }[];
}

const defaultSkip = (n: string) => n.endsWith(".uhd-partial") || n.endsWith(".!qB");

function encodeCursor(after: string[]): string {
  return Buffer.from(JSON.stringify({ v: 1, after })).toString("base64url");
}
function decodeCursor(c: string | null | undefined): string[] | null {
  if (!c) return null;
  try {
    const j = JSON.parse(Buffer.from(c, "base64url").toString()) as { v: number; after: string[] };
    return j.v === 1 && Array.isArray(j.after) ? j.after : null;
  } catch {
    return null;
  }
}
function cmp(a: string, b: string): number {
  return a < b ? -1 : a > b ? 1 : 0;
}
/** 段序比较(深度优先前序、名称排序的遍历顺序) */
function cmpSegs(a: string[], b: string[]): number {
  for (let i = 0; i < Math.min(a.length, b.length); i++) {
    const c = cmp(a[i]!, b[i]!);
    if (c) return c;
  }
  return a.length - b.length;
}
function isPrefix(p: string[], of: string[]): boolean {
  return p.length < of.length && p.every((s, i) => s === of[i]);
}

/**
 * 分批增量扫描:确定性顺序(名称排序、深度优先),每批受 maxStats/maxDirs 限制,
 * 返回游标供下一批续扫;进程中断后用最后持久化的游标续扫,不必从头遍历。
 * 目录只 readdir 不 stat;文件 lstat(不跟随符号链接)。
 */
export async function scanIncremental(root: string, opts: ScanOptions = {}): Promise<ScanResult> {
  const result: ScanResult = { status: "complete", entries: [], cursor: null, statCount: 0, dirCount: 0, errors: [] };
  if (opts.sentinel !== false) {
    const h = await checkRoot(root, opts.sentinel ?? DEFAULT_ROOT_SENTINEL);
    if (h.kind === "offline") return { ...result, status: "root_offline" };
  }
  const maxStats = opts.maxStats ?? 2000;
  const maxDirs = opts.maxDirs ?? Number.POSITIVE_INFINITY;
  const skip = opts.skip ?? defaultSkip;
  const sentinelName = opts.sentinel === false ? null : (opts.sentinel ?? DEFAULT_ROOT_SENTINEL);
  const after = decodeCursor(opts.cursor);
  let last: string[] | null = after;
  let stopped: ScanResult["status"] | null = null;

  const walk = async (dirSegs: string[]): Promise<void> => {
    if (result.dirCount >= maxDirs) {
      stopped = "partial";
      return;
    }
    let ents: Dirent[];
    try {
      ents = await readdir(join(root, ...dirSegs), { withFileTypes: true });
      result.dirCount++;
    } catch (e) {
      result.errors.push({ relPath: dirSegs.join("/"), code: errnoOf(e) ?? "UNKNOWN" });
      return;
    }
    ents.sort((a, b) => cmp(a.name, b.name));
    for (const d of ents) {
      if (opts.signal?.aborted) {
        stopped = "aborted";
        return;
      }
      const segs = [...dirSegs, d.name];
      if (dirSegs.length === 0 && d.name === sentinelName) continue;
      if (after && cmpSegs(segs, after) <= 0 && !isPrefix(segs, after)) continue;
      if (d.isDirectory()) {
        await walk(segs);
        if (stopped) return;
        continue;
      }
      if (skip(d.name)) continue;
      if (result.statCount >= maxStats) {
        stopped = "partial";
        return;
      }
      const rel = segs.join("/");
      try {
        const st = await lstat(join(root, ...segs));
        result.statCount++;
        if (st.isDirectory()) continue;
        const prev = opts.previous?.(rel);
        result.entries.push({
          relPath: rel,
          size: st.size,
          mtimeMs: st.mtimeMs,
          ino: st.ino,
          dev: st.dev,
          nlink: st.nlink,
          isSymlink: st.isSymbolicLink(),
          change: !prev ? "new" : prev.size === st.size && prev.mtimeMs === st.mtimeMs ? "unchanged" : "changed",
        });
      } catch (e) {
        result.statCount++;
        result.errors.push({ relPath: rel, code: errnoOf(e) ?? "UNKNOWN" });
      }
      last = segs;
    }
  };

  await walk([]);
  if (stopped) {
    result.status = stopped;
    result.cursor = last ? encodeCursor(last) : (opts.cursor ?? null);
  }
  return result;
}