import type { InventoryInfo, Product } from 'brainerce';

/**
 * KIT stock, for every surface that shows availability.
 *
 * A `KIT` is one purchasable product assembled from other catalog products, and
 * it carries **no `inventory` row of its own** — the component that runs out
 * first decides how many kits can be sold, and the API reports that as
 * `product.kitAvailable`:
 *
 *   - `null`      → unlimited (every component is untracked)
 *   - `0`         → not sellable (a component is out, or the kit has none)
 *   - a number    → that many kits
 *   - `undefined` → not a kit; the field is absent on SIMPLE / VARIABLE
 *
 * ⛔ `null` and `undefined` mean OPPOSITE things here. Treating `null` as "no
 * stock" renders an unlimited kit as sold out, which is the same bug as the one
 * this file exists to fix, only inverted.
 *
 * Why a helper rather than `product.inventory` at each call site: reading
 * `inventory` alone gave every kit two contradictory answers on one page — a red
 * "Out of stock" badge (because `!inventory`) beside an ENABLED add-to-cart
 * button (because `inventory?.canPurchase !== false` is `true` when inventory is
 * null). Both now read the same resolved value.
 */

/** The subset of a product this module needs. Keeps it usable with list rows. */
type StockSource = Pick<Product, 'type'> & {
  inventory?: InventoryInfo | null;
  kitAvailable?: number | null;
};

/**
 * Build the `InventoryInfo` a kit would have if it had one.
 *
 * Mirrors the shape the backend synthesizes for the sales-channel read, so a
 * storefront cannot disagree with the API about the same kit.
 */
export function kitInventory(kitAvailable: number | null): InventoryInfo {
  const unlimited = kitAvailable === null;
  const available = kitAvailable ?? 0;
  return {
    total: available,
    reserved: 0,
    available,
    trackingMode: unlimited ? 'UNLIMITED' : 'TRACKED',
    inStock: unlimited || available > 0,
    canPurchase: unlimited || available > 0,
    // A kit is never backorderable: its components decide, not the kit.
    backorderMode: 'NONE',
  };
}

/**
 * The stock signal to render for a product — kit or not.
 *
 * Pass the selected variant's inventory as the second argument on a product
 * page; it wins for VARIABLE products and is irrelevant for a kit, which takes
 * no variant at all.
 */
export function resolveStockInfo(
  product: StockSource | null | undefined,
  variantInventory?: InventoryInfo | null
): InventoryInfo | null {
  if (variantInventory) return variantInventory;
  if (!product) return null;
  if (product.type === 'KIT') {
    // `kitAvailable` first. Some reads also synthesize an `inventory` block for
    // kits, so fall back to it before giving up — and only treat the kit as
    // unresolved (`null`) when neither field arrived.
    if (product.kitAvailable !== undefined) return kitInventory(product.kitAvailable);
    return product.inventory ?? null;
  }
  return product.inventory ?? null;
}

/**
 * Can this product go in the cart right now?
 *
 * `!== false` rather than `=== true` on purpose: a missing stock signal must not
 * disable the button on a store that does not track inventory at all.
 */
export function canPurchaseProduct(
  product: StockSource | null | undefined,
  variantInventory?: InventoryInfo | null
): boolean {
  return resolveStockInfo(product, variantInventory)?.canPurchase !== false;
}