export interface VolumeSnapshot {
  volumeId: string;
  totalBytes: number;
  usedBytes: number;
}

export interface InflightTask {
  taskId: string;
  planId?: string;
  volumeId: string;
  /** 种子选中文件总大小;未知(磁力未取到元数据)时为 undefined */
  sizeBytes?: number;
  /** 已占用磁盘的字节(已下载或已预分配,取二者较大者由调用方给出) */
  onDiskBytes: number;
  /** 是否在管理标签范围内 */
  managed: boolean;
  status: "queued" | "downloading" | "paused" | "completed" | "other";
}

export interface Reservation {
  planId: string;
  volumeId: string;
  bytes: number;
}

export interface ThresholdConfig {
  /** 硬阈值使用率(%),例如 90 */
  hardPct: number;
  /** 恢复水位(%),例如 85,必须低于 hardPct */
  resumePct: number;
}

export interface CacheVolumeInput {
  /** 读不到磁盘状态时传 null */
  volume: VolumeSnapshot | null;
  config: ThresholdConfig;
  inflight: readonly InflightTask[];
  reservations: readonly Reservation[];
  /** 待提交的新下载;不传表示只做巡检 */
  request?: { bytes?: number };
  /** 当前是否处于"因本阈值暂停"状态 */
  pausedByThreshold: boolean;
}

export interface CacheVolumeDecision {
  /** allow = 可提交;block = 阻止新增;pause_managed = 阻止新增并暂停纳管的下载中任务 */
  decision: "allow" | "block" | "pause_managed";
  /** 降到恢复水位:恢复"因本阈值暂停"的任务(不含用户手动暂停) */
  resumeThresholdPaused: boolean;
  pauseTaskIds: string[];
  reasons: string[];
  usedPct?: number;
  projectedPct?: number;
  pendingBytes: number;
  reservedBytes: number;
  unknownSizeTaskIds: string[];
  /** 未纳管任务仍在增长:不能声称暂停后磁盘不再增长 */
  unmanagedGrowthRisk: boolean;
}

/**
 * 缓存卷硬阈值判定(REQ §8.4.1)。
 * 预测占用 = 当前已用 + 在途任务剩余需求(size - 已落盘,已预分配不重复扣) + 未提交计划预留 + 本次请求;
 * 已提交的计划只按在途任务计算一次,不再算预留。多个 qB 共用卷时由调用方按 volumeId 汇总传入。
 */
export function evaluateCacheVolume(input: CacheVolumeInput): CacheVolumeDecision {
  const { config } = input;
  if (!(config.resumePct < config.hardPct)) throw new Error("resumePct 必须低于 hardPct");
  const base = {
    pauseTaskIds: [] as string[],
    pendingBytes: 0,
    reservedBytes: 0,
    unknownSizeTaskIds: [] as string[],
    unmanagedGrowthRisk: false,
  };
  const v = input.volume;
  if (!v || !(v.totalBytes > 0)) {
    return { ...base, decision: "block", resumeThresholdPaused: false, reasons: ["volume_state_unknown"] };
  }
  const tasks = input.inflight.filter((t) => t.volumeId === v.volumeId);
  let pending = 0;
  const unknownSize: string[] = [];
  let unmanagedGrowth = false;
  for (const t of tasks) {
    if (t.status === "completed") continue;
    if (t.sizeBytes === undefined) {
      unknownSize.push(t.taskId);
      continue;
    }
    const rest = Math.max(0, t.sizeBytes - t.onDiskBytes);
    pending += rest;
    if (!t.managed && rest > 0) unmanagedGrowth = true;
  }
  const submittedPlans = new Set(tasks.map((t) => t.planId).filter((p): p is string => p !== undefined));
  const reserved = input.reservations
    .filter((r) => r.volumeId === v.volumeId && !submittedPlans.has(r.planId))
    .reduce((s, r) => s + r.bytes, 0);
  const reqBytes = input.request?.bytes ?? 0;
  const usedPct = (v.usedBytes / v.totalBytes) * 100;
  const projectedPct = ((v.usedBytes + pending + reserved + reqBytes) / v.totalBytes) * 100;
  const out = {
    ...base,
    pendingBytes: pending,
    reservedBytes: reserved,
    unknownSizeTaskIds: unknownSize,
    unmanagedGrowthRisk: unmanagedGrowth,
    usedPct,
    projectedPct,
  };
  const reasons: string[] = [];
  if (usedPct >= config.hardPct) {
    return {
      ...out,
      decision: "pause_managed",
      resumeThresholdPaused: false,
      pauseTaskIds: tasks.filter((t) => t.managed && t.status === "downloading").map((t) => t.taskId),
      reasons: ["hard_threshold_reached"],
    };
  }
  const resume = input.pausedByThreshold && usedPct <= config.resumePct;
  if (input.pausedByThreshold && !resume) reasons.push("awaiting_resume_watermark");
  if (input.request && input.request.bytes === undefined) reasons.push("request_size_unknown");
  if (input.request && unknownSize.length > 0) reasons.push("inflight_size_unknown");
  if (projectedPct >= config.hardPct) reasons.push("projected_over_threshold");
  if (resume) reasons.push("below_resume_watermark");
  const blocked = reasons.some((r) => r !== "below_resume_watermark");
  return { ...out, decision: blocked ? "block" : "allow", resumeThresholdPaused: resume, reasons };
}