package services

import "nearle/models"

// InStockOnly drops the products an outlet cannot actually sell.
//
// Being on a shelf is not the same as being in stock. Suriya Store's Peelamedu
// branch listed three products in the customer app of which exactly one had any
// stock, so a shopper could add a chocolate bar to their basket and order
// something the shop did not have. The rule the business wants is the simple
// one: if we have the stock, it can be ordered — otherwise it is not on offer.
//
// ── Why here and not in the SQL ──────────────────────────────────────────────
//
// The obvious implementation is another correlated subquery in GetProducts'
// WHERE clause. This is better for a reason that outlives the convenience:
// `Productstock` is the number the app DISPLAYS, computed by that query as the
// live SUM(in)-SUM(out) balance. Filtering on the same value that is shown makes
// it impossible for the two to disagree. A second copy of the balance
// expression in a WHERE clause is a copy that can drift from the one in the
// SELECT, and the failure it produces — a product listed as "3 in stock" that
// the filter considers empty, or worse the reverse — is very hard to see.
//
// It also filters on the ledger rather than productlocations.status. That flag
// is derived and admits drift: SyncProductLocationStatus repairs it on the next
// ledger entry, so between an order and that entry it can still read
// "available" for an empty shelf. The ledger cannot drift from itself.
//
// ── The locationID guard ─────────────────────────────────────────────────────
//
// Not defensive padding. GetProducts computes the stock balance scoped to
// params.LocationID, and with no outlet the subquery matches nothing, so EVERY
// product comes back with Productstock 0. Filtering that would turn an unscoped
// browse into an empty catalogue rather than an unfiltered one.
//
// The console is deliberately unaffected: its inventory screens read
// GetLocationProducts, a different query, and they must keep showing the empty
// lines — restocking them is the entire point of that screen.
func InStockOnly(products []models.Products, locationID int) []models.Products {
	if locationID <= 0 {
		return products
	}

	// Rebuilt rather than filtered in place: the caller's slice is the
	// repository's own result and nothing here should be writing through it.
	inStock := make([]models.Products, 0, len(products))
	for _, p := range products {
		if p.Productstock > 0 {
			inStock = append(inStock, p)
		}
	}
	return inStock
}