import { eq, ledgerEvents, siteConfig, type Db } from "@parking/db";

// Occupancy = a FOLD over the signed ledger: the count of vehicle_entry events
// with no matching vehicle_exit. Never a hand-maintained counter (which is
// editable + drifts) — the chain is the truth. See wiki/concepts/capacity-occupancy.md.

export interface Occupancy {
  /** Cars currently inside (open sessions). */
  readonly count: number;
  /** Admin-set nominal capacity, or null = no limit. */
  readonly capacity: number | null;
  /** capacity − count, or null when uncapped. Can read 0 (or below) when full. */
  readonly free: number | null;
  /** True when count ≥ capacity (always false when uncapped). */
  readonly full: boolean;
}

/** Count cars inside: entries minus exits, per identity, over the ledger. */
export function occupancyCount(db: Db): number {
  const rows = db
    .select({ type: ledgerEvents.type, identity: ledgerEvents.identity })
    .from(ledgerEvents)
    .all();
  const balance = new Map<string, number>();
  for (const r of rows) {
    if (r.type === "vehicle_entry") balance.set(r.identity ?? "", (balance.get(r.identity ?? "") ?? 0) + 1);
    else if (r.type === "vehicle_exit") balance.set(r.identity ?? "", (balance.get(r.identity ?? "") ?? 0) - 1);
  }
  let open = 0;
  for (const v of balance.values()) if (v > 0) open += 1;
  return open;
}

/** Admin-set capacity (null = uncapped). */
export function siteCapacity(db: Db): number | null {
  const row = db.select().from(siteConfig).where(eq(siteConfig.id, 1)).get();
  return row?.capacity ?? null;
}

export function getOccupancy(db: Db): Occupancy {
  const count = occupancyCount(db);
  const capacity = siteCapacity(db);
  return {
    count,
    capacity,
    free: capacity == null ? null : capacity - count,
    full: capacity != null && count >= capacity,
  };
}