export type WeightUnit = "kg" | "g" | "lb";

const UNIT_FACTORS: Record<WeightUnit, number> = {
  kg: 1,
  g: 1000,
  lb: 2.20462262,
};

/**
 * 将内部统一保存的千克值转换为界面单位，并固定显示两位小数。
 */
export function formatWeight(valueKg: number, unit: WeightUnit) {
  const safeValue = Number.isFinite(valueKg) ? Math.max(0, valueKg) : 0;
  return (safeValue * UNIT_FACTORS[unit]).toFixed(2);
}

/**
 * 把 ISO 时间转换为终端文件名使用的本地时间格式：YYYYMMDDHHmmss。
 */
export function createFileTimestamp(iso: string) {
  const date = new Date(iso);
  const pad = (value: number) => value < 10 ? `0${value}` : String(value);

  return [
    date.getFullYear(),
    pad(date.getMonth() + 1),
    pad(date.getDate()),
    pad(date.getHours()),
    pad(date.getMinutes()),
    pad(date.getSeconds()),
  ].join("");
}

/**
 * 兼容电子秤服务常见的消息格式：
 * - 纯数字，例如 `2.49`
 * - JSON，例如 `{"weight": 2490, "unit": "g"}`
 * - JSON 千克字段，例如 `{"weightKg": 2.49}`
 *
 * 返回值统一为千克；无效值、负数或不支持的单位返回 null。
 */
export function parseScaleMessage(message: string): number | null {
  const trimmed = message.trim();
  if (!trimmed) return null;

  let payload: unknown = trimmed;
  try {
    payload = JSON.parse(trimmed);
  } catch {
    // 非 JSON 消息继续按纯数字处理。
  }

  if (typeof payload === "number" || typeof payload === "string") {
    const value = Number(payload);
    return Number.isFinite(value) && value >= 0 ? value : null;
  }

  if (!payload || typeof payload !== "object") return null;

  const record = payload as Record<string, unknown>;
  const rawValue = record.weightKg ?? record.weight ?? record.value;
  const value = Number(rawValue);
  if (!Number.isFinite(value) || value < 0) return null;

  const unit = String(record.unit ?? (record.weightKg != null ? "kg" : "kg")).toLowerCase();
  if (unit === "kg") return value;
  if (unit === "g") return value / 1000;
  if (unit === "lb" || unit === "lbs") return value / UNIT_FACTORS.lb;
  return null;
}

export type StabilityResult = {
  stable: boolean;
  progress: number;
};

/**
 * 根据连续采样窗口判断重量是否稳定。
 * 至少需要 6 个有效样本，样本极差小于 8 克时判定稳定。
 */
export function evaluateStability(samples: number[]): StabilityResult {
  // 不使用 Array.prototype.at，确保兼容终端内置的 Chrome 52 WebView。
  const latest = samples.length ? samples[samples.length - 1] : 0;
  if (samples.length < 6 || latest < 0.01) {
    return {
      stable: false,
      progress: Math.min(85, samples.length * 14),
    };
  }

  const spread = Math.max(...samples) - Math.min(...samples);
  const stable = spread < 0.008;
  return {
    stable,
    progress: stable ? 100 : Math.max(25, 90 - spread * 500),
  };
}