wallet: add wallet struct and balance funcs

wallet/wallet.go

@@ -0,0 +1,13 @@
+package wallet
+
+type Wallet struct {
+	balance int
+}
+
+func (w *Wallet) Deposit(units int) {
+	w.balance += units
+}
+
+func (w *Wallet) Balance() int {
+	return w.balance
+}

wallet/wallet_test.go

@@ -0,0 +1,16 @@
+package wallet
+
+import "testing"
+
+func TestWallet(t *testing.T) {
+	wallet := Wallet{}
+
+	wallet.Deposit(10)
+
+	got := wallet.Balance()
+	want := 10
+
+	if got != want {
+		t.Errorf("got %d want %d", got, want)
+	}
+}