/**
 * AgentGuard(TM) Spend: local Advisor forecast skeleton.
 *
 * Reads local decision logs only. No network calls are made.
 *
 * Patent notice: Protected by U.S. patent-pending technology
 * (App. Nos. 63/983,615; 63/983,621; 63/983,843; 63/984,626;
 * 64/071,781; 64/071,789).
 */

import type { AdvisorSpendPoint } from './anomaly';

export interface AdvisorForecast {
  daysObserved: number;
  monthEndCents: number;
  capCents: number | null;
  overCap: boolean;
  message: string;
}

export function forecastMonthEnd(points: AdvisorSpendPoint[], capCents: number | null = null, now = new Date()): AdvisorForecast {
  const daily = lastThirtyDaily(points, now);
  if (daily.length === 0) {
    return { daysObserved: 0, monthEndCents: 0, capCents, overCap: false, message: 'No local spend history found.' };
  }
  const slope = linearSlope(daily.map((row, index) => ({ x: index + 1, y: row.cents })));
  const monthDays = new Date(now.getFullYear(), now.getMonth() + 1, 0).getDate();
  const elapsed = now.getDate();
  const observedTotal = daily.reduce((sum, row) => sum + row.cents, 0);
  const avg = observedTotal / Math.max(1, daily.length);
  const projectedRemaining = Math.max(0, monthDays - elapsed) * Math.max(0, avg + slope);
  const monthEndCents = Math.round(observedTotal + projectedRemaining);
  const overCap = capCents !== null && monthEndCents > capCents;
  return {
    daysObserved: daily.length,
    monthEndCents,
    capCents,
    overCap,
    message: overCap ? 'Projected spend is above cap. Consider a lower fallback model or a tighter per_day cap.' : 'Projected spend is within the current cap.',
  };
}

function lastThirtyDaily(points: AdvisorSpendPoint[], now: Date): Array<{ day: string; cents: number }> {
  const cutoff = now.getTime() - 30 * 24 * 60 * 60 * 1000;
  const buckets = new Map<string, number>();
  for (const point of points) {
    const t = Date.parse(point.ts);
    if (!Number.isFinite(t) || t < cutoff) continue;
    const day = new Date(t).toISOString().slice(0, 10);
    buckets.set(day, (buckets.get(day) ?? 0) + point.cents);
  }
  return [...buckets.entries()].map(([day, cents]) => ({ day, cents })).sort((a, b) => a.day.localeCompare(b.day));
}

function linearSlope(points: Array<{ x: number; y: number }>): number {
  if (points.length < 2) return 0;
  const n = points.length;
  const sx = points.reduce((sum, point) => sum + point.x, 0);
  const sy = points.reduce((sum, point) => sum + point.y, 0);
  const sxx = points.reduce((sum, point) => sum + point.x * point.x, 0);
  const sxy = points.reduce((sum, point) => sum + point.x * point.y, 0);
  const denom = n * sxx - sx * sx;
  return denom === 0 ? 0 : (n * sxy - sx * sy) / denom;
}