/**
 * Availability: which dates and slots a specific cart can book at a specific
 * location with a specific method, right now.
 *
 * Rule precedence (each step only runs if the previous passed):
 *   1. Location / method eligibility (incl. product method & location rules)
 *   2. Product date restrictions (weekday/date blocks, windows, freshness)
 *   3. Holidays / closures / special openings
 *   4. Cutoffs (same-day, next-day, peak-day early cutoff)
 *   5. Preparation time (base + product rules + peak extra) vs slot start
 *   6. Daily and slot capacity (bookings + active holds), with waitlist on peak days
 */
import type {
  AvailabilityResult,
  Cart,
  DateResult,
  LocationConfig,
  Method,
  MethodConfig,
  PeakDay,
  ProductRule,
  Reason,
  ShopSettings,
  SlotResult,
  SlotTemplate,
  TimeRange,
  UsageSnapshot,
} from "./types";
import { type CartConstraints, checkFreshness, evaluateRules } from "./rules";
import { addDays, fromMinutes, iso, localParts, toMinutes, weekdayOf, zonedToUtc } from "./time";

export const usageKey = (locationId: string, method: Method, date: string) => `${locationId}|${method}|${date}`;
export const slotId = (start: string, end: string) => `${start}-${end}`;

export interface AvailabilityInput {
  location: LocationConfig;
  method: Method;
  cart: Cart;
  rules: ProductRule[];
  peakDays: PeakDay[];
  usage: UsageSnapshot;
  settings: Pick<ShopSettings, "mixedPrepPolicy">;
  now: number;
  constraints?: CartConstraints;
  /** Limit computed days (e.g. calendar month paging). Defaults to the method horizon. */
  maxDays?: number;
}

/** Hours for a date after applying closures and special openings. */
export function hoursFor(
  loc: LocationConfig,
  method: Method,
  date: string,
): { hours: TimeRange[]; exception?: Reason; special?: boolean } {
  const cfg = loc.methods[method];
  const ex = loc.exceptions.find((e) => e.date === date && (!e.methods?.length || e.methods.includes(method)));
  if (ex?.closed)
    return { hours: [], exception: { code: "CLOSED_EXCEPTION", message: ex.note || "Closed on this date" } };
  if (ex?.hours?.length) return { hours: ex.hours, special: true };
  return { hours: cfg?.weeklyHours[weekdayOf(date)] ?? [] };
}

export function peakFor(peaks: PeakDay[], locationId: string, method: Method, date: string): PeakDay | null {
  return (
    peaks.find(
      (p) =>
        p.dates.includes(date) &&
        (!p.locationIds?.length || p.locationIds.includes(locationId)) &&
        (!p.methods?.length || p.methods.includes(method)),
    ) ?? null
  );
}

function slotTemplates(cfg: MethodConfig, date: string, hours: TimeRange[], usingException: boolean): SlotTemplate[] {
  if (!hours.length) return [];
  if (cfg.slotMode === "dateOnly") return [{ start: hours[0].start, end: hours[hours.length - 1].end }];
  if (cfg.slotMode === "explicit" && !usingException) {
    const list = cfg.slots?.[weekdayOf(date)] ?? [];
    // Only keep explicit slots that fall within opening hours.
    return list.filter((s) =>
      hours.some((h) => toMinutes(s.start) >= toMinutes(h.start) && toMinutes(s.end) <= toMinutes(h.end)),
    );
  }
  const step = cfg.slotMinutes || 60;
  const out: SlotTemplate[] = [];
  for (const h of hours) {
    for (let t = toMinutes(h.start); t + step <= toMinutes(h.end); t += step)
      out.push({ start: fromMinutes(t), end: fromMinutes(t + step) });
  }
  return out;
}

/**
 * Add preparation minutes counting only operating hours of this method at this location.
 */
export function addOperatingMinutes(
  startMs: number,
  minutes: number,
  loc: LocationConfig,
  method: Method,
): number {
  if (minutes <= 0) return startMs;
  let remaining = minutes * 60000;
  let date = localParts(startMs, loc.timezone).date;
  for (let i = 0; i < 400; i++, date = addDays(date, 1)) {
    for (const h of hoursFor(loc, method, date).hours) {
      const rs = zonedToUtc(date, h.start, loc.timezone);
      const re = zonedToUtc(date, h.end, loc.timezone);
      if (re <= startMs) continue;
      const from = Math.max(rs, startMs);
      const avail = re - from;
      if (remaining <= avail) return from + remaining;
      remaining -= avail;
    }
  }
  return Number.POSITIVE_INFINITY;
}

export function computeAvailability(input: AvailabilityInput): AvailabilityResult {
  const { location: loc, method, now, usage } = input;
  const tz = loc.timezone;
  const cfg = loc.methods[method];
  const constraints =
    input.constraints ?? evaluateRules(input.cart, input.rules, input.settings.mixedPrepPolicy);

  const blockers: Reason[] = [];
  if (!loc.enabled) blockers.push({ code: "LOCATION_DISABLED", message: `${loc.name} isn't taking bookings right now` });
  if (!cfg?.enabled) blockers.push({ code: "METHOD_DISABLED", message: `${loc.name} doesn't offer ${method}` });
  if (!constraints.allowedMethods.has(method)) blockers.push(...(constraints.methodReasons.get(method) ?? []));
  if (constraints.allowedLocations && !constraints.allowedLocations.has(loc.id))
    blockers.push(...constraints.locationReasons);

  const basePrep = (cfg?.prepMinutes ?? 0) + constraints.extraPrepMinutes;
  const readyCache = new Map<number, number>();
  const readyAt = (prep: number) => {
    if (!readyCache.has(prep))
      readyCache.set(
        prep,
        cfg?.prepMode === "operating" ? addOperatingMinutes(now, prep, loc, method) : now + prep * 60000,
      );
    return readyCache.get(prep)!;
  };

  const result: AvailabilityResult = {
    locationId: loc.id,
    method,
    timezone: tz,
    dateOnly: cfg?.slotMode === "dateOnly",
    prepMinutes: basePrep,
    earliestReadyUtc: iso(readyAt(basePrep)),
    blockers,
    dates: [],
  };
  if (blockers.length || !cfg) return result;

  const nowLocal = localParts(now, tz);
  const days = Math.min(cfg.horizonDays, input.maxDays ?? cfg.horizonDays);

  for (let d = 0; d < days; d++) {
    const date = addDays(nowLocal.date, d);
    const weekday = weekdayOf(date);
    const peak = peakFor(input.peakDays, loc.id, method, date);
    const dr: DateResult = {
      date,
      weekday,
      available: false,
      remaining: null,
      peak: peak ? { id: peak.id, name: peak.name, message: peak.message } : null,
      surcharge: peak?.surcharge ?? 0,
      waitlist: false,
      reasons: [],
      slots: [],
    };
    result.dates.push(dr);

    // 2. product date restrictions
    const wd = constraints.blockedWeekdays.get(weekday);
    if (wd) dr.reasons.push(wd);
    const bd = constraints.blockedDates.get(date);
    if (bd) dr.reasons.push(bd);
    for (const w of constraints.windows)
      if ((w.from && date < w.from) || (w.to && date > w.to)) dr.reasons.push(w.reason);
    const fresh = checkFreshness(constraints.freshness, date, loc.id);
    if (fresh) dr.reasons.push(fresh);
    if (dr.reasons.length) continue;

    // 3. closures
    const { hours, exception, special } = hoursFor(loc, method, date);
    if (exception) {
      dr.reasons.push(exception);
      continue;
    }
    if (!hours.length) {
      dr.reasons.push({ code: "CLOSED_WEEKLY", message: "Closed on this day" });
      continue;
    }

    // 4. cutoffs
    if (d === 0 && cfg.sameDayCutoff && nowLocal.minutes >= toMinutes(cfg.sameDayCutoff)) {
      dr.reasons.push({ code: "PAST_CUTOFF", message: `Same-day orders close at ${cfg.sameDayCutoff}` });
      continue;
    }
    if (d === 1 && cfg.nextDayCutoff && nowLocal.minutes >= toMinutes(cfg.nextDayCutoff)) {
      dr.reasons.push({ code: "PAST_CUTOFF", message: `Next-day orders close at ${cfg.nextDayCutoff}` });
      continue;
    }
    if (peak?.cutoff) {
      const deadline = zonedToUtc(addDays(date, -peak.cutoff.daysBefore), peak.cutoff.time, tz);
      if (now >= deadline) {
        dr.reasons.push({
          code: "PEAK_CUTOFF",
          message: `${peak.name} orders closed on ${addDays(date, -peak.cutoff.daysBefore)} at ${peak.cutoff.time}`,
        });
        continue;
      }
    }

    // 5 + 6. slots
    const ready = readyAt(basePrep + (peak?.extraPrepMinutes ?? 0));
    const key = usageKey(loc.id, method, date);
    const used = usage[key] ?? { day: 0, slots: {} };
    const dayCap = peak?.dailyCapacity ?? cfg.dailyCapacity ?? null;
    const dayLeft = dayCap == null ? null : Math.max(0, dayCap - used.day);
    dr.remaining = dayLeft;
    const templates = slotTemplates(cfg, date, hours, !!special);
    if (!templates.length) {
      dr.reasons.push({ code: "CLOSED_WEEKLY", message: "No time slots on this day" });
      continue;
    }

    for (const t of templates) {
      const startUtc = zonedToUtc(date, t.start, tz);
      const endUtc = zonedToUtc(date, t.end, tz);
      const id = slotId(t.start, t.end);
      const sr: SlotResult = {
        id,
        start: t.start,
        end: t.end,
        startUtc: iso(startUtc),
        endUtc: iso(endUtc),
        available: false,
        remaining: null,
        surcharge: t.surcharge ?? 0,
        waitlist: false,
        reasons: [],
      };
      dr.slots.push(sr);

      if (result.dateOnly) {
        // Dispatch date: the order must be ready before the location closes.
        if (endUtc <= now) sr.reasons.push({ code: "SLOT_IN_PAST", message: "Closed for today" });
        else if (ready > endUtc) sr.reasons.push({ code: "PREP_TIME", message: "Not enough preparation time" });
      } else if (startUtc <= now) {
        sr.reasons.push({ code: "SLOT_IN_PAST", message: "This time has passed" });
      } else if (ready > startUtc) {
        sr.reasons.push({ code: "PREP_TIME", message: "Not enough preparation time" });
      }
      if (sr.reasons.length) continue;

      const slotCap = peak?.slotCapacity ?? t.capacity ?? cfg.slotCapacity ?? null;
      const slotUsed = used.slots[id] ?? 0;
      const slotLeft = slotCap == null ? null : Math.max(0, slotCap - slotUsed);
      sr.remaining = slotLeft == null ? dayLeft : dayLeft == null ? slotLeft : Math.min(slotLeft, dayLeft);
      if (dayLeft === 0) sr.reasons.push({ code: "DAY_FULL", message: "Fully booked for this day" });
      else if (slotLeft === 0) sr.reasons.push({ code: "SLOT_FULL", message: "This time slot is full" });
      if (sr.reasons.length) {
        sr.waitlist = !!peak?.waitlist;
        continue;
      }
      sr.available = true;
    }

    dr.available = dr.slots.some((s) => s.available);
    dr.waitlist = !dr.available && dr.slots.some((s) => s.waitlist);
    if (!dr.available) {
      const seen = new Set<string>();
      for (const s of dr.slots)
        for (const r of s.reasons)
          if (!seen.has(r.code)) {
            seen.add(r.code);
            dr.reasons.push(r);
          }
    }
  }
  return result;
}

export function firstAvailable(r: AvailabilityResult): { date: string; slot: SlotResult } | null {
  for (const d of r.dates) {
    const s = d.slots.find((x) => x.available);
    if (s) return { date: d.date, slot: s };
  }
  return null;
}

/**
 * Re-validate a specific selection (used before hold, at order creation and on reschedule).
 */
export function validateSelection(
  input: AvailabilityInput,
  date: string,
  slot: string,
): { ok: true; slot: SlotResult; date: DateResult } | { ok: false; reasons: Reason[] } {
  const r = computeAvailability(input);
  if (r.blockers.length) return { ok: false, reasons: r.blockers };
  const dr = r.dates.find((d) => d.date === date);
  if (!dr) return { ok: false, reasons: [{ code: "BEYOND_HORIZON", message: "This date can't be booked" }] };
  const sr = dr.slots.find((s) => s.id === slot);
  if (!sr) return { ok: false, reasons: dr.reasons.length ? dr.reasons : [{ code: "CLOSED_WEEKLY", message: "Slot not offered" }] };
  if (!sr.available) return { ok: false, reasons: sr.reasons.length ? sr.reasons : dr.reasons };
  return { ok: true, slot: sr, date: dr };
}