Account: Add account.Balance.Available() method (thrasher-corp#777)

* add account.Balance.Available() method that returns the amount of an asset not locked/held

* account: comment Balance.Available()

* account: test Balance.Available()

exchanges/account/account.go

@@ -93,3 +93,9 @@ func (s *Service) Update(a *Holdings) error {
 
 	return s.mux.Publish([]uuid.UUID{acc.ID}, acc.h)
 }
+
+// Available returns the amount you can use immediately.  E.g. if you have $100, but $20
+// are held (locked) because of a limit buy order, your available balance is $80.
+func (b Balance) Available() float64 {
+	return b.TotalValue - b.Hold
+}

exchanges/account/account_test.go

@@ -134,3 +134,29 @@ func TestHoldings(t *testing.T) {
 
 	wg.Wait()
 }
+
+func TestBalance_Available(t *testing.T) {
+	t.Parallel()
+
+	b := Balance{
+		CurrencyName: currency.BTC,
+		TotalValue:   16,
+		Hold:         0,
+	}
+
+	if have := b.Available(); have != 16 {
+		t.Errorf("have %f, want 16", have)
+	}
+
+	b.Hold = 8
+
+	if have := b.Available(); have != 8 {
+		t.Errorf("have %f, want 8", have)
+	}
+
+	b.Hold = 16
+
+	if have := b.Available(); have != 0 {
+		t.Errorf("have %f, want 0", have)
+	}
+}