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 }