Add multi-currency support

internal/currency/currency.go

@@ -0,0 +1,50 @@
+package currency
+
+import "fmt"
+
+// Rates holds exchange rates keyed by ISO 4217 currency code, relative to a
+// single base currency (rate of 1.0).
+type Rates map[string]float64
+
+// Converter converts amounts between currencies using a fixed rate table,
+// enabling payment streaming across different currencies.
+type Converter struct {
+	base  string
+	rates Rates
+}
+
+// NewConverter creates a Converter for the given base currency and rates.
+func NewConverter(base string, rates Rates) *Converter {
+	return &Converter{base: base, rates: rates}
+}
+
+// Convert converts amount (in the smallest currency unit, e.g. cents) from
+// one currency to another using the configured rate table.
+func (c *Converter) Convert(amount int64, from, to string) (int64, error) {
+	if from == to {
+		return amount, nil
+	}
+
+	fromRate, err := c.rateFor(from)
+	if err != nil {
+		return 0, err
+	}
+	toRate, err := c.rateFor(to)
+	if err != nil {
+		return 0, err
+	}
+
+	baseAmount := float64(amount) / fromRate
+	return int64(baseAmount * toRate), nil
+}
+
+func (c *Converter) rateFor(code string) (float64, error) {
+	if code == c.base {
+		return 1, nil
+	}
+	rate, ok := c.rates[code]
+	if !ok {
+		return 0, fmt.Errorf("currency: no rate configured for %q", code)
+	}
+	return rate, nil
+}