type Capacity int64

const (
    J Capacity  = 1
    KJ          = J * 1000
    MJ          = KJ * 1000
    GJ          = MJ * 1000
)

func (e Capacity) ByStack(stack basic.Stack) Capacity {
    return Capacity(e.Int64() * stack.Int64())
}

func (e Capacity) Int64() int64 {
    return int64(e)
}

func (e Capacity) String() string {
    if e == 0 {
        return "0 J"
    } else {
        switch {
        case e%GJ == 0:
            return fmt.Sprintf("%d GJ", e/GJ)
        case e%MJ == 0:
            return fmt.Sprintf("%d MJ", e/MJ)
        case e%KJ == 0:
            return fmt.Sprintf("%d KJ", e/KJ)
        default:
            return fmt.Sprintf("%d J", e)
        }
    }
}