Fractional amount solidus-to-stripe conversion support class

app/models/solidus_stripe/gateway.rb

@@ -39,6 +39,70 @@ def void(transaction_id, source, options = {})
     def credit(money, source, transaction_id, options = {})
       ActiveMerchant::Billing::Response.new()
     end
+
+    module MoneyToStripeAmountConverter
+      extend self
+
+      ZERO_DECIMAL_CURRENCIES = %w[
+        BIF
+        CLP
+        DJF
+        GNF
+        JPY
+        KMF
+        KRW
+        MGA
+        PYG
+        RWF
+        UGX
+        VND
+        VUV
+        XAF
+        XOF
+        XPF
+      ].freeze
+
+      THREE_DECIMAL_CURRENCIES = %w[
+        BHD
+        JOD
+        KWD
+        OMR
+        TND
+      ].freeze
+
+      # special currencies that are represented in cents but
+      # should be divisible by 100, thus making them integer only.
+      DIVISIBLE_BY_100 = %w[
+        HUF
+        TWD
+        UGX
+      ].freeze
+
+      # Solidus will provide a "fractional" amount, that is specific for each currency
+      # following the configurationo defined in the Money gem.
+      #
+      # Stripe uses the "smallest currency unit",
+      # (e.g., 100 cents to charge $1.00 or 100 to charge ¥100, a zero-decimal currency)
+      # https://stripe.com/docs/currencies#zero-decimal
+      #
+      # We need to ensure the fractional amount is considering the same number of decimals.
+      def to_stripe_amount(fractional, currency)
+        money_subunit_to_unit = ::Money::Currency.new(currency).subunit_to_unit
+        stripe_subunit_to_unit =
+          case currency.to_s.upcase
+          when *ZERO_DECIMAL_CURRENCIES then 1
+          when *THREE_DECIMAL_CURRENCIES then 1000
+          when *DIVISIBLE_BY_100 then 100
+          else 100
+          end
+
+        if stripe_subunit_to_unit == money_subunit_to_unit
+          fractional
+        else
+          (fractional / money_subunit_to_unit.to_d) * stripe_subunit_to_unit
+        end
+      end
+    end
   end
 end
 # rubocop:enable Style/MethodCallWithoutArgsParentheses, Lint/UnusedMethodArgument

spec/models/solidus_stripe/gateway_spec.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+require 'solidus_stripe_spec_helper'
+
+# rubocop:disable Style/NumericLiterals
+RSpec.describe SolidusStripe::Gateway do
+  describe SolidusStripe::Gateway::MoneyToStripeAmountConverter do
+    describe '.to_stripe_amount' do
+      it 'converts to the fractional expected by stripe' do # rubocop:disable RSpec/MultipleExpectations
+        (described_class::ZERO_DECIMAL_CURRENCIES - %w[MGA]).each do |currency|
+          expect([currency, described_class.to_stripe_amount(12345, currency).to_i]).to eq([currency, 12345])
+        end
+
+        %w[USD EUR ILS MXN TWD].each do |currency| # 2 decimals on both sides, default case
+          expect([currency, described_class.to_stripe_amount(123_45, currency).to_i]).to eq([currency, 123_45])
+        end
+
+        described_class::THREE_DECIMAL_CURRENCIES.each do |currency|
+          expect([currency, described_class.to_stripe_amount(12_345, currency).to_i]).to eq([currency, 12_345])
+        end
+
+        # Special cases
+        expect(['MGA', described_class.to_stripe_amount(255, 'MGA').to_i]).to eq(['MGA', 51])
+        expect(['HUF', described_class.to_stripe_amount(1_2345, 'HUF').to_i]).to eq(['HUF', 12_345_00])
+        expect(['UGX', described_class.to_stripe_amount(12_3450_00, 'UGX').to_i]).to eq(['UGX', 123_450_00])
+        expect(['TWD', described_class.to_stripe_amount(1_2345_00, 'TWD').to_i]).to eq(['TWD', 12_345_00])
+      end
+    end
+  end
+end
+# rubocop:enable Style/NumericLiterals