wallet: check withdraw border cases

wallet/wallet.go

@@ -1,6 +1,9 @@
 package wallet
 
-import "fmt"
+import (
+	"errors"
+	"fmt"
+)
 
 type Bitcoin int
 
@@ -16,8 +19,12 @@ func (w *Wallet) Deposit(amount Bitcoin) {
 	w.balance += amount
 }
 
-func (w *Wallet) Withdraw(amount Bitcoin) {
+func (w *Wallet) Withdraw(amount Bitcoin) error {
+	if w.balance < amount {
+		return errors.New("not enough to withdraw")
+	}
 	w.balance -= amount
+	return nil
 }
 
 func (w *Wallet) Balance() Bitcoin {

wallet/wallet_test.go

@@ -11,14 +11,27 @@ func TestWallet(t *testing.T) {
 			t.Errorf("got %q want %q", got, want)
 		}
 	}
+
 	t.Run("deposit", func(t *testing.T) {
 		wallet := Wallet{}
 		wallet.Deposit(10)
 		assertBalance(t, wallet, Bitcoin(10))
 	})
+
 	t.Run("withdraw", func(t *testing.T) {
 		wallet := Wallet{balance: Bitcoin(20)}
 		wallet.Withdraw(10)
 		assertBalance(t, wallet, Bitcoin(10))
 	})
+
+	t.Run("withdraw insufficient funds", func(t *testing.T) {
+		startingBalance := Bitcoin(20)
+		wallet := Wallet{balance: startingBalance}
+		err := wallet.Withdraw(100)
+		assertBalance(t, wallet, startingBalance)
+
+		if err == nil {
+			t.Error("wanted an error but didn't get one")
+		}
+	})
 }