Apply stripe conversion factor to gift badge amount check

service/src/main/java/org/whispersystems/textsecuregcm/stripe/StripeManager.java

@@ -33,6 +33,7 @@ import com.stripe.param.SubscriptionRetrieveParams;
 import com.stripe.param.SubscriptionUpdateParams;
 import com.stripe.param.SubscriptionUpdateParams.BillingCycleAnchor;
 import com.stripe.param.SubscriptionUpdateParams.ProrationBehavior;
+import java.math.BigDecimal;
 import java.nio.charset.StandardCharsets;
 import java.security.InvalidKeyException;
 import java.security.NoSuchAlgorithmException;
@@ -383,6 +384,19 @@ public class StripeManager {
         () -> Lists.newArrayList(invoice.getLines().autoPagingIterable(null, commonOptions())), executor);
   }
 
+  /**
+   * Takes an amount as configured; for instance USD 4.99 and turns it into an amount as Stripe expects to see it.
+   * Stripe appears to only support 0 and 2 decimal currencies, but also has some backwards compatibility issues with 0
+   * decimal currencies so this is not to any ISO standard but rather directly from Stripe's API doc page.
+   */
+  public BigDecimal convertConfiguredAmountToStripeAmount(String currency, BigDecimal configuredAmount) {
+    return switch (currency.toLowerCase(Locale.ROOT)) {
+      // Yuck, but this list was taken from https://stripe.com/docs/currencies?presentment-currency=US
+      case "bif", "clp", "djf", "gnf", "jpy", "kmf", "krw", "mga", "pyg", "rwf", "vnd", "vuv", "xaf", "xof", "xpf" -> configuredAmount;
+      default -> configuredAmount.scaleByPowerOfTen(2);
+    };
+  }
+
   /**
    * We use a client generated idempotency key for subscription updates due to not being able to distinguish between a
    * call to update to level 2, then back to level 1, then back to level 2. If this all happens within Stripe's