/** 下载器实例看到的路径 ↔ 本服务宿主看到的路径(前缀替换,支持多实例) */
export interface PathMapping {
  instanceId: string;
  downloaderPath: string;
  hostPath: string;
}

export type MapResult =
  | { ok: true; path: string; mapping: PathMapping }
  | { ok: false; reason: "no_mapping" | "ambiguous" };

const norm = (p: string) => {
  const s = p.replace(/\\/g, "/").replace(/\/{2,}/g, "/");
  return s.length > 1 ? s.replace(/\/+$/, "") : s;
};

function mapPrefix(
  path: string,
  instanceId: string,
  mappings: readonly PathMapping[],
  from: "downloaderPath" | "hostPath",
  to: "downloaderPath" | "hostPath",
): MapResult {
  const p = norm(path);
  let best: PathMapping[] = [];
  let bestLen = -1;
  for (const m of mappings) {
    if (m.instanceId !== instanceId) continue;
    const prefix = norm(m[from]);
    const hit = p === prefix || p.startsWith(prefix === "/" ? "/" : `${prefix}/`);
    if (!hit) continue;
    if (prefix.length > bestLen) {
      best = [m];
      bestLen = prefix.length;
    } else if (prefix.length === bestLen) best.push(m);
  }
  if (best.length === 0) return { ok: false, reason: "no_mapping" };
  const targets = new Set(best.map((m) => norm(m[to])));
  if (targets.size > 1) return { ok: false, reason: "ambiguous" };
  const m = best[0]!;
  const rest = p.slice(bestLen).replace(/^\//, "");
  const base = norm(m[to]);
  const out = rest ? (base === "/" ? `/${rest}` : `${base}/${rest}`) : base;
  return { ok: true, path: out, mapping: m };
}

/** 最长前缀匹配、按路径段边界(/downloads 不匹配 /downloads2);找不到映射不猜测 */
export function toHostPath(path: string, instanceId: string, mappings: readonly PathMapping[]): MapResult {
  return mapPrefix(path, instanceId, mappings, "downloaderPath", "hostPath");
}

export function toDownloaderPath(path: string, instanceId: string, mappings: readonly PathMapping[]): MapResult {
  return mapPrefix(path, instanceId, mappings, "hostPath", "downloaderPath");
}

/** 同一实例同一前缀指向不同目标 → 配置错误 */
export function validatePathMappings(mappings: readonly PathMapping[]): string[] {
  const errors: string[] = [];
  const seen = new Map<string, string>();
  for (const m of mappings) {
    const k = `${m.instanceId}\u0000${norm(m.downloaderPath)}`;
    const prev = seen.get(k);
    if (prev !== undefined && prev !== norm(m.hostPath))
      errors.push(`实例 ${m.instanceId} 的 ${m.downloaderPath} 映射到多个宿主路径`);
    seen.set(k, norm(m.hostPath));
  }
  return errors;
}