/**
 * Pantry Forecast (spec §40): "Ni brukar använda mjölk var sjätte dag."
 * Enkel deterministisk konsumtionstakt ur transaktionshistorik.
 * Prognoser märks ALLTID som prognoser i UI.
 */

export interface ConsumptionEvent {
  /** ISO-datum */
  date: string;
  quantity: number;
}

export interface ForecastResult {
  /** Genomsnittlig förbrukning per dag (i postens enhet). */
  dailyRate: number;
  /** Dagar tills nuvarande saldo beräknas ta slut. */
  daysUntilEmpty: number | null;
  /** Föreslaget inköpsdatum (2 dagars marginal). */
  suggestedRestockDate: string | null;
  confidence: "low" | "medium" | "high";
}

export function forecastDepletion(
  currentBalance: number,
  consumption: ConsumptionEvent[],
  today: Date = new Date(),
): ForecastResult {
  if (consumption.length < 2) {
    return { dailyRate: 0, daysUntilEmpty: null, suggestedRestockDate: null, confidence: "low" };
  }
  const sorted = [...consumption].sort((a, b) => a.date.localeCompare(b.date));
  const first = Date.parse(sorted[0]!.date);
  const last = Date.parse(sorted[sorted.length - 1]!.date);
  const spanDays = Math.max(1, (last - first) / 86_400_000);
  const total = sorted.reduce((sum, e) => sum + Math.abs(e.quantity), 0);
  const dailyRate = total / spanDays;

  if (dailyRate <= 0) {
    return { dailyRate: 0, daysUntilEmpty: null, suggestedRestockDate: null, confidence: "low" };
  }

  const daysUntilEmpty = currentBalance / dailyRate;
  const restock = new Date(today.getTime() + Math.max(0, daysUntilEmpty - 2) * 86_400_000);
  const confidence = consumption.length >= 6 ? "high" : consumption.length >= 4 ? "medium" : "low";

  return {
    dailyRate: Math.round(dailyRate * 1000) / 1000,
    daysUntilEmpty: Math.round(daysUntilEmpty * 10) / 10,
    suggestedRestockDate: restock.toISOString().slice(0, 10),
    confidence,
  };
}