type Batcher struct {
    valueSlow *slow.Value
    value interface{}
    mu sync.Mutex
    con *sync.Cond
    done, running bool
}

func (b *Batcher) Load() interface{} {
    b.mu.Lock()
    defer b.mu.Unlock()

    if b.done {
        b.done = false

        b.mu.Unlock()
        b.value = b.valueSlow.Load()
        b.mu.Lock()

        b.con.Broadcast()
        return b.value
    }

    if b.running {
        b.con.Wait()

        if !b.done {
            b.con.Wait()
            return b.value
        }
    }
    b.running = true
    b.mu.Unlock()

    b.value = b.valueSlow.Load()

    b.mu.Lock()
    b.done = true
    b.con.Broadcast()

    return b.value
}

func NewBatcher(v *slow.Value) *Batcher {
    mu := sync.Mutex{}
    return &Batcher{valueSlow: v, mu: mu,  con: sync.NewCond(&mu)}
}