package main

import (
	"context"
	"encoding/json"
	"errors"
	"math/rand"
	"runtime"
	"strconv"
	"time"

	"github.com/SevereCloud/vksdk/v2/object"
	"github.com/allegro/bigcache/v3"
	"go.uber.org/atomic"
)

type UsersUser struct {
	ID int64
}

var UserUpdateInterval = 24

const (
	maxUsersPerChannel      = 50
	fetcherInterval         = time.Millisecond
	fetcherBatchingInterval = time.Millisecond * 20
)

type Fetcher interface {
	Enqueue(userID int64)
	WaitFor(userID int64, sleepBetweenAttempts time.Duration, attemptsCount int) UsersUser
}

type fetcher struct {
	rnd     *rand.Rand
	input   chan int64
	offload *bigcache.BigCache
	output  chan []int64
	result  *bigcache.BigCache
	stop    atomic.Bool
	token   string
}

func newFetcher(token string) *fetcher {
	input, _ := bigcache.New(context.Background(), bigcache.DefaultConfig(time.Minute*10))
	output, _ := bigcache.New(context.Background(), bigcache.DefaultConfig(time.Minute))
	uf := &fetcher{
		rnd:     rand.New(rand.NewSource(time.Now().UnixNano())),
		input:   make(chan int64, maxUsersPerChannel),
		output:  make(chan []int64),
		offload: input,
		result:  output,
		token:   token,
	}
	runtime.SetFinalizer(uf, usersFetcherDestructor)
	go uf.consumeOffload()
	go uf.assembleBatches()
	go uf.doWork()
	return uf
}

func usersFetcherDestructor(uf *fetcher) {
	uf.stop.Store(true)
}

func (u *fetcher) Enqueue(userID int64) {
	if u.stop.Load() {
		return
	}
	if userID == 0 {
		return
	}
	if len(u.input) < 50 {
		u.input <- userID
		return
	}
	_ = u.offload.Set(strconv.FormatInt(userID, 10), []byte{})
}

func (u *fetcher) WaitFor(userID int64, sleepBetweenAttempts time.Duration, attemptsCount int) object.UsersUser {
	attempt := 0
	uid := strconv.FormatInt(userID, 10)
retryFetching:
	val, err := u.result.Get(uid)
	if err != nil {
		if errors.Is(err, bigcache.ErrEntryNotFound) {
			if attempt == attemptsCount {
				return object.UsersUser{}
			}
			time.Sleep(sleepBetweenAttempts)
			attempt++
			goto retryFetching
		}
		return object.UsersUser{}
	}

	var user object.UsersUser
	if err := json.Unmarshal(val, &user); err != nil {
		return object.UsersUser{}
	}
	return user
}

func (u *fetcher) consumeOffload() {
	for {
		if u.stop.Load() {
			break
		}
		it := u.offload.Iterator()
		for it.SetNext() {
			entry, err := it.Value()
			if err != nil {
				break
			}
			uid, _ := strconv.ParseInt(entry.Key(), 10, 64)
			if uid == 0 {
				u.offload.Delete(entry.Key())
				continue
			}
			u.input <- uid
			u.offload.Delete(entry.Key())
			time.Sleep(fetcherInterval)
		}
	}
}

func (u *fetcher) assembleBatches() {
	cnt := 0
	items := make([]int64, 50)
	for {
		if u.stop.Load() {
			break
		}

		select {
		case item := <-u.input:
			items[cnt] = item
			cnt++
			if cnt == 50 {
				cnt = 0
				u.output <- items
				items = make([]int64, 50)
				continue
			}
		case <-time.After(fetcherBatchingInterval):
			if len(items) > 0 && items[0] != 0 {
				cnt = 0
				u.output <- items
				items = make([]int64, 50)
			}
		}
	}
}

func (u *fetcher) doWork() {
	defer func() {
		if r := recover(); r != nil {
			time.Sleep(fetcherInterval)
			go u.doWork()
		}
	}()

	for batch := range u.output {
		time.Sleep(time.Millisecond * time.Duration(u.rnd.Intn(150)+200))

		for _, user := range batch {
			data, err := json.Marshal(UsersUser{ID: user})
			if err != nil {
				continue
			}
			_ = u.result.Set(strconv.FormatInt(user, 10), data)
		}
	}

	close(u.output)
	close(u.input)
}