Add a CurrencyCount type with GrandTotal method

This hold totals in different currencies (currently expected to just be
USD and EUR), and GrandTotal provides a total in USD given a
conversion rate from EUR to USD.

summary.go

@@ -0,0 +1,8 @@
+package main
+
+// Use a map because of multiple currencies
+type CurrencyCount map[string]float32
+
+func (c CurrencyCount) GrandTotal(eurToUsdRate float32) float32 {
+	return c["USD"] + c["EUR"]*eurToUsdRate
+}

summary_test.go

@@ -0,0 +1,38 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+//==============================================================================
+// CurrencyCount.GrandTotal
+//==============================================================================
+
+const eurToUsdRate = 1.25
+
+func TestGrandTotalWithEmptyMap(t *testing.T) {
+	cc := make(CurrencyCount)
+
+	assert.Equal(t, 0, cc.GrandTotal(eurToUsdRate))
+}
+
+func TestGrandTotalWithJustUSD(t *testing.T) {
+	cc := CurrencyCount{
+		"USD": 34.56,
+	}
+
+	assert.Equal(t, 34.56, cc.GrandTotal(eurToUsdRate))
+}
+
+func TestGrandTotalWithJustUSDAndEUR(t *testing.T) {
+	cc := CurrencyCount{
+		"USD": 34.56,
+		"EUR": 10.00,
+	}
+
+	assert.Equal(t, 47.06, cc.GrandTotal(eurToUsdRate))
+}
+
+//==============================================================================