Add pointers package

pointers/wallet.go

@@ -0,0 +1,35 @@
+package pointers
+
+import (
+	"errors"
+	"fmt"
+)
+
+type Bitcoin int
+
+type Wallet struct {
+	balance Bitcoin
+}
+
+func (w *Wallet) Deposit(a Bitcoin) {
+	w.balance += a
+}
+
+var ErrInsufficientFunds = errors.New("cannot withdraw, insufficient funds")
+
+func (w *Wallet) Withdraw(a Bitcoin) error {
+	if a > w.balance {
+		return ErrInsufficientFunds
+	}
+
+	w.balance -= a
+	return nil
+}
+
+func (w Wallet) Balance() Bitcoin {
+	return w.balance
+}
+
+func (b Bitcoin) String() string {
+	return fmt.Sprintf("%d BTC", b)
+}

pointers/wallet_test.go

@@ -0,0 +1,52 @@
+package pointers
+
+import (
+	"testing"
+)
+
+func TestWallet(t *testing.T) {
+	assertBalance := func(t testing.TB, w Wallet, want Bitcoin) {
+		t.Helper()
+		balance := w.Balance()
+
+		if balance != want {
+			t.Errorf("got %s want %s", balance, want)
+		}
+	}
+
+	t.Run("Deposit", func(t *testing.T) {
+		wallet := Wallet{}
+		wallet.Deposit(Bitcoin(10))
+		want := Bitcoin(10)
+
+		assertBalance(t, wallet, want)
+	})
+
+	t.Run("Withdraw", func(t *testing.T) {
+		wallet := Wallet{balance: Bitcoin(20)}
+		wallet.Withdraw(Bitcoin(10))
+
+		want := Bitcoin(10)
+		assertBalance(t, wallet, want)
+	})
+
+	assertError := func(t testing.TB, got, want error) {
+		t.Helper()
+
+		if got == nil {
+			t.Fatal("wanted an error but didn't get one")
+		}
+
+		if got != want {
+			t.Errorf("got %q, want %q", got, want)
+		}
+	}
+
+	t.Run("Withdraw insufficient funds", func(t *testing.T) {
+		startingBalance := Bitcoin(20)
+		wallet := Wallet{startingBalance}
+		err := wallet.Withdraw(Bitcoin(100))
+		assertError(t, err, ErrInsufficientFunds)
+		assertBalance(t, wallet, startingBalance)
+	})
+}