package cart

// EvaluatedItem is one line in the per-line promotion breakdown exposed
// alongside a cart. It mirrors the line that was evaluated (sku,
// quantity, unit list price, optional OrgPrice for the strikethrough) and
// adds the per-line discount the engine applied (orgPrice-based +
// promotion-based, additive) plus the resulting effective per-unit and
// per-line totals.
//
// Lives in pkg/cart rather than pkg/promotions so CartGrain can carry the
// breakdown as a derived field (populated by the canonical promotion
// pipeline that already manages TotalDiscount/AppliedPromotions), and so
// every consumer of the grain — UCP cart response, legacy /cart HTTP
// handler, cart-mcp, AMQP mutation feed — naturally sees the same shape
// without ad-hoc wrappers at each call site. JSON tags are the canonical
// EvaluatedItem shape that /promotions/evaluate-with-cart has returned
// since the breakdown feature was introduced.
//
// Distributing the total cart discount down to the line level lets the
// storefront render "Item X: 100 kr → 80 kr" without re-doing the math
// on the client and lets verifiers confirm which promotion hit which
// line (per-line DiscountIncVat is the engine's authoritative answer).
type EvaluatedItem struct {
	Sku                  string `json:"sku"`
	Name                 string `json:"name,omitempty"`
	Quantity             uint16 `json:"qty"`
	PriceIncVat          int64  `json:"priceIncVat"`              // per-unit list price
	OrgPriceIncVat       int64  `json:"orgPriceIncVat,omitempty"` // per-unit pre-discount list price (for strikethrough)
	DiscountIncVat       int64  `json:"discountIncVat"`           // per-line TOTAL discount (orgPrice + promotion), incVat in öre
	EffectivePriceIncVat int64  `json:"effectivePriceIncVat"`     // per-unit, after discount, incVat in öre (rounded)
	EffectiveTotalIncVat int64  `json:"effectiveTotalIncVat"`     // per-line, after discount, incVat in öre (exact)
}

// MapEvaluatedItems walks the grain's items and produces the per-line
// EvaluatedItem list. The grain's item.Discount carries the TOTAL per-line
// discount (orgPrice + promotion — additive, set by UpdateTotals and the
// effects' per-line distribution). The effective totals are derived as
// (price * qty - discount), clamped to 0 (a promotion that over-discounted
// a line shows as free, not negative). The effective per-unit price is the
// per-line total divided by quantity, rounded to the nearest öre so the
// per-unit display matches the per-line total when multiplied back.
//
// Returns nil for a nil grain (callers that always pass a non-nil grain
// — the typical case — get a non-nil empty slice for an empty cart, which
// is the desired behavior for JSON omitempty at the consumer).
//
// Exported so the canonical promotion pipeline (pkg/promotions.EvaluateAndApply)
// and the manual /promotions/evaluate-with-cart preview handler can produce
// the same per-line shape.
func MapEvaluatedItems(g *CartGrain) []EvaluatedItem {
	if g == nil {
		return nil
	}
	out := make([]EvaluatedItem, 0, len(g.Items))
	for _, it := range g.Items {
		if it == nil {
			continue
		}
		var name string
		if it.Meta != nil {
			name = it.Meta.Name
		}
		var orgPrice int64
		if it.OrgPrice != nil {
			orgPrice = it.OrgPrice.IncVat.Int64()
		}
		var discount int64
		if it.Discount != nil {
			discount = it.Discount.IncVat.Int64()
		}
		priceRow := it.Price.IncVat.Int64() * int64(it.Quantity)
		effTotal := priceRow - discount
		if effTotal < 0 {
			effTotal = 0
		}
		var effUnit int64
		if it.Quantity > 0 {
			// Nearest-öre rounding so per-unit * qty matches the
			// per-line total (within 1 öre either way). Half-up:
			// (effTotal + qty/2) / qty.
			effUnit = (effTotal + int64(it.Quantity)/2) / int64(it.Quantity)
		}
		out = append(out, EvaluatedItem{
			Sku:                  it.Sku,
			Name:                 name,
			Quantity:             it.Quantity,
			PriceIncVat:          it.Price.IncVat.Int64(),
			OrgPriceIncVat:       orgPrice,
			DiscountIncVat:       discount,
			EffectivePriceIncVat: effUnit,
			EffectiveTotalIncVat: effTotal,
		})
	}
	return out
}