all the refactor

cmd/cart/projection_overlay.go

@@ -0,0 +1,97 @@
+package main
+
+import (
+	messages "git.k6n.net/mats/go-cart-actor/proto/cart"
+	"git.k6n.net/mats/platform/catalog"
+)
+
+// taxClassToBp maps the canonical TaxClass string identifiers published on
+// catalog.Projection to basis-points-of-a-percent (rate × 100) — the single
+// platform-wide rate scale used by AddItem.Tax (int32 basis points), the
+// mutation registry, and the tax.Provider Compute formula.
+//
+// Values reflect the standard tariff names used by the Nordic configuration.
+// Per-country / per-tenant numeric resolution (e.g. Finland 25.5%, Ireland
+// 13.5%) is delegated to platform/tax and is a follow-up: this static map
+// gets the common case right (2500 / 1200 / 600 / 0) and lets the cache path
+// ship while the platform/tax lookup is wired into the cart pool server.
+var taxClassToBp = map[string]int{
+	"standard":  2500,
+	"reduced":   1200, // common reduced rate (e.g. SE/NO food)
+	"lowered":   600,  // super-reduced
+	"zero":      0,    // zero-rated (export, healthcare, ...)
+	"exempt":    0,    // similarly untaxed, separate nominal class
+}
+
+// resolveTaxBp returns the basis-point rate for a TaxClass string, falling
+// back to platform "standard" (25%) when the class is unrecognised. The
+// default matches the grain's existing behaviour (AddItem falls back to 2500
+// when m.Tax == 0 — see pkg/cart/mutation_add_item.go).
+func resolveTaxBp(class string) int {
+	if bp, ok := taxClassToBp[class]; ok {
+		return bp
+	}
+	return 2500
+}
+
+// ApplyProjectionOverlay overlays the cache's authoritative catalog facts onto
+// an AddItem message that was just produced by a PRODUCT_BASE_URL HTTP fetch.
+//
+// Authoritative-from-cache fields (these WIN over the HTTP-fetched value when
+// both are present, because the bus is the live stream):
+//
+//	Price          (bus has post-class-overrides; product service has static)
+//	Tax            (TaxClass → bp via resolveTaxBp; only when TaxClass is set —
+//	                  an empty TaxClass means the producer didn't classify,
+//	                  so we leave the HTTP-fetched rate intact rather than
+//	                  quietly swapping in a default 2500 and miscategorising)
+//	Title          (canonical, post-trim)
+//	Image          (canonical)
+//	InventoryTracked (only flips to true; false is the zero value in proto
+//	                  and may be unset on the wire, so we don't second-guess)
+//	DropShip       (only flips to true; same reasoning)
+//
+// The HTTP-fetched values remain authoritative for fields the cache does not
+// carry: SellerId / SellerName (marketplace split), Stock (per docs/inventory-
+// shape.md stock is queried synchronously from inventory, not cached),
+// OrgPrice / Discount (not yet on the projection schema), the dynamic
+// ExtraJson product data (configurator options — width/height used by
+// accessory child-pricing), and the catalog uint32 ItemId (the product
+// service's integer id which messages.AddItem.ItemId dedupes on — Projection
+// only carries the opaque string ID, so a cache-only build path is blocked
+// and the HTTP fetch is preserved).
+//
+// Tombstoned SKUs (catalog.projection_published with deleted=true) → the
+// caller checks idx.IsDeleted(sku) before this helper and short-circuits.
+func ApplyProjectionOverlay(msg *messages.AddItem, p catalog.Projection) {
+	if msg == nil {
+		return
+	}
+	// Price: positive bus values override HTTP. A zero PriceIncVat is treated
+	// as "no value" rather than "free".
+	if p.PriceIncVat.Int64() > 0 {
+		msg.Price = p.PriceIncVat.Int64()
+	}
+	// Tax: only override when the bus has classified the SKU. An empty TaxClass
+	// means the producer had nothing to say about VAT; the HTTP rate is more
+	// trustworthy in that case than a guessed 2500.
+	if p.TaxClass != "" {
+		msg.Tax = int32(resolveTaxBp(p.TaxClass))
+	}
+	// Display fields: only override if non-empty so the HTTP-fanned value
+	// remains in place when the cache hasn't populated them yet.
+	if p.Title != "" {
+		msg.Name = p.Title
+	}
+	if p.Image != "" {
+		msg.Image = p.Image
+	}
+	// Flags: only flip to true (positive boolean signals). A false is the
+	// proto-zero value and not worth overriding.
+	if p.InventoryTracked {
+		msg.InventoryTracked = true
+	}
+	if p.DropShip {
+		msg.DropShip = true
+	}
+}