cart and checkout

pkg/cart/tax.go

@@ -0,0 +1,58 @@
+package cart
+
+// TaxProvider computes taxes for orders and line items.
+// It mirrors the PaymentProvider and ShippingProvider seam patterns: one
+// interface, interchangeable implementations.
+//
+// All tax rates are expressed as raw percent (e.g. 25 = 25 %). This matches the
+// OrderLine.tax_rate proto convention.
+type TaxProvider interface {
+	// Name returns the provider name for identification (logging, metrics).
+	Name() string
+
+	// ComputeTax computes the tax portion from a gross (inc-VAT) total and tax
+	// rate. Both total and return value are in minor currency units (ore).
+	// taxRate is expressed as raw percent (e.g. 25 = 25 %).
+	//
+	// Formula: tax = total x rate / (100 + rate)
+	//
+	// When taxRate <= 0 the result is 0 (untaxed / zero-rated items).
+	ComputeTax(total int64, taxRate int) int64
+
+	// DefaultTaxRate returns the default VAT rate for a country, expressed as
+	// raw percent (e.g. 25 = 25 %). Used for deliveries and items where no
+	// explicit rate was recorded at add-to-cart time.
+	//
+	// Return 0 if the country is unknown or untaxed.
+	DefaultTaxRate(country string) int
+}
+
+// ComputeTax is the shared tax formula: tax = total x rate / (100 + rate).
+// taxRate is raw percent (e.g. 25 = 25 %). When taxRate <= 0 the result is 0.
+//
+// This differs from GetTaxAmount which uses the CartItem convention
+// (percent x 100, e.g. 2500 = 25.00 %) with formula total x rate / (10000 + rate).
+func ComputeTax(total int64, taxRate int) int64 {
+	if taxRate <= 0 {
+		return 0
+	}
+	return total * int64(taxRate) / int64(100+taxRate)
+}
+
+// StaticTaxProvider is a stateless, always-available TaxProvider that simply
+// runs the formula without any country-specific rates. Use for local dev,
+// tests, and as a no-dependency default.
+type StaticTaxProvider struct{}
+
+var _ TaxProvider = (*StaticTaxProvider)(nil)
+
+func NewStaticTaxProvider() *StaticTaxProvider { return &StaticTaxProvider{} }
+
+func (p *StaticTaxProvider) Name() string { return "static" }
+
+func (p *StaticTaxProvider) ComputeTax(total int64, taxRate int) int64 {
+	return ComputeTax(total, taxRate)
+}
+
+// DefaultTaxRate returns 0 for any country (no awareness).
+func (p *StaticTaxProvider) DefaultTaxRate(_ string) int { return 0 }