/**
 * Cadence optimizer — Plan §3.6 (Analyst gap 3).
 *
 * Pure function `propose(history, config) → [Proposal]`. Clock and RNG injected
 * for deterministic tests.
 *
 * Model: Beta posterior over engagement rate per (day_of_week, hour_of_day) bucket.
 * Prior: Beta(1, 19) — weakly informative, centred on 5% baseline.
 * Threshold: ≥8 posts per bucket AND ≥4 weeks of data.
 * Change trigger: alt-bucket posterior mean > current * 1.2 AND 95% CIs don't overlap.
 * Output: Proposals in `/metrics` UI with Approve / Reject / Snooze 4 weeks.
 * Never auto-applied (RBR).
 */

export type DayOfWeek = 0 | 1 | 2 | 3 | 4 | 5 | 6;
export type HourOfDay =
  | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11
  | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23;

export interface PostObservation {
  day: DayOfWeek;
  hour: HourOfDay;
  publishedAt: Date;
  impressions: number;
  reactions: number;
  comments: number;
  shares: number;
}

export interface CadenceConfig {
  prior: { alpha: number; beta: number };
  minSamplesPerBucket: number;
  minWeeksOfData: number;
  changeRatio: number;
  ciAlpha: number; // e.g. 0.05 for 95% CI
  current: { day: DayOfWeek; hour: HourOfDay };
}

export const DEFAULT_CADENCE_CONFIG: CadenceConfig = {
  prior: { alpha: 1, beta: 19 },
  minSamplesPerBucket: 8,
  minWeeksOfData: 4,
  changeRatio: 1.2,
  ciAlpha: 0.05,
  current: { day: 2, hour: 8 }, // Tue 08:30 AM AST → bucket Tue/8
};

export interface BucketStats {
  day: DayOfWeek;
  hour: HourOfDay;
  n: number;
  posteriorAlpha: number;
  posteriorBeta: number;
  posteriorMean: number;
  ci95: { lo: number; hi: number };
}

export interface Proposal {
  current: BucketStats;
  proposed: BucketStats;
  ratio: number;
  rationale: string;
}

const engagementRate = (o: PostObservation): number => {
  if (o.impressions <= 0) return 0;
  return Math.min(1, (o.reactions + o.comments + o.shares) / o.impressions);
};

const weeksSpanned = (history: readonly PostObservation[], now: Date): number => {
  if (history.length === 0) return 0;
  const oldest = history.reduce((acc, o) => Math.min(acc, o.publishedAt.getTime()), now.getTime());
  return Math.max(0, (now.getTime() - oldest) / (7 * 24 * 60 * 60 * 1000));
};

/**
 * Beta CDF inverse approximation via the Wilson score interval — sufficient
 * for proposal-vs-current comparisons; not for high-precision inference.
 */
const betaCi = (alpha: number, beta: number, ciAlpha: number): { lo: number; hi: number } => {
  const n = alpha + beta - 2;
  const p = (alpha - 1) / Math.max(1, n);
  const z = ciAlpha <= 0.05 ? 1.96 : 1.64;
  const denom = 1 + (z * z) / Math.max(1, n);
  const centre = (p + (z * z) / (2 * Math.max(1, n))) / denom;
  const margin =
    (z * Math.sqrt((p * (1 - p)) / Math.max(1, n) + (z * z) / (4 * Math.max(1, n) * Math.max(1, n)))) /
    denom;
  return { lo: Math.max(0, centre - margin), hi: Math.min(1, centre + margin) };
};

const bucketStats = (
  obs: readonly PostObservation[],
  cfg: CadenceConfig,
  day: DayOfWeek,
  hour: HourOfDay,
): BucketStats => {
  let alpha = cfg.prior.alpha;
  let beta = cfg.prior.beta;
  let n = 0;
  for (const o of obs) {
    if (o.day !== day || o.hour !== hour) continue;
    const e = engagementRate(o);
    alpha += e;
    beta += 1 - e;
    n++;
  }
  const total = alpha + beta;
  const mean = alpha / total;
  return {
    day,
    hour,
    n,
    posteriorAlpha: alpha,
    posteriorBeta: beta,
    posteriorMean: mean,
    ci95: betaCi(alpha, beta, cfg.ciAlpha),
  };
};

const ciOverlap = (a: { lo: number; hi: number }, b: { lo: number; hi: number }): boolean =>
  !(a.hi < b.lo || b.hi < a.lo);

export interface ProposeArgs {
  history: readonly PostObservation[];
  config?: CadenceConfig;
  now?: Date;
  candidateBuckets?: ReadonlyArray<{ day: DayOfWeek; hour: HourOfDay }>;
}

export const propose = (args: ProposeArgs): readonly Proposal[] => {
  const cfg = args.config ?? DEFAULT_CADENCE_CONFIG;
  const now = args.now ?? new Date();
  const weeks = weeksSpanned(args.history, now);
  if (weeks < cfg.minWeeksOfData) return [];

  const candidates =
    args.candidateBuckets ??
    ([
      { day: 1, hour: 8 },
      { day: 2, hour: 8 },
      { day: 3, hour: 8 },
      { day: 4, hour: 8 },
      { day: 5, hour: 8 },
      { day: 1, hour: 12 },
      { day: 2, hour: 12 },
      { day: 3, hour: 12 },
      { day: 4, hour: 12 },
      { day: 5, hour: 12 },
    ] as const);

  const cur = bucketStats(args.history, cfg, cfg.current.day, cfg.current.hour);
  if (cur.n < cfg.minSamplesPerBucket) return [];

  const proposals: Proposal[] = [];
  for (const c of candidates) {
    if (c.day === cfg.current.day && c.hour === cfg.current.hour) continue;
    const alt = bucketStats(args.history, cfg, c.day, c.hour);
    if (alt.n < cfg.minSamplesPerBucket) continue;
    const ratio = alt.posteriorMean / cur.posteriorMean;
    if (ratio < cfg.changeRatio) continue;
    if (ciOverlap(cur.ci95, alt.ci95)) continue;
    proposals.push({
      current: cur,
      proposed: alt,
      ratio,
      rationale: `Alt bucket day=${c.day} hour=${c.hour} mean=${alt.posteriorMean.toFixed(3)} > current mean=${cur.posteriorMean.toFixed(3)} by ${(ratio * 100 - 100).toFixed(1)}%, 95% CIs disjoint.`,
    });
  }
  proposals.sort((a, b) => b.ratio - a.ratio);
  return proposals;
};