package main

import (
    "sync/atomic"
)

type WaitGroup interface {
    Add(delta int)
    Done()
    Wait()
}

type WG struct {
    count int32
}

func NewWaitGroup() *WG {
    return &WG{
        count: 0,
    }
}

func (wg *WG) Add(delta int) {
    atomic.AddInt32(&wg.count, int32(delta))
}

func (wg *WG) Done() {
    if atomic.LoadInt32(&wg.count) > 0 {
        atomic.AddInt32(&wg.count, -1)
    } else {
        panic("negative counter")
    }
}

func (wg *WG) Wait() {
    for {
        if atomic.LoadInt32(&wg.count) == 0 {
            break
        }
    }
}