package writeconcurrencylimiter

import (
	"flag"
	"fmt"
	"io"
	"net/http"
	"sync"
	"time"

	"github.com/VictoriaMetrics/VictoriaMetrics/lib/cgroup"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/httpserver"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/timerpool"
	"github.com/VictoriaMetrics/metrics"
)

var (
	maxConcurrentInserts = flag.Int("maxConcurrentInserts", 2*cgroup.AvailableCPUs(), "The maximum number of concurrent insert requests. "+
		"Default value depends on the number of CPU cores and should work for most cases since it minimizes the memory usage. The default value can be increased when clients send data over slow networks. "+
		"See also -insert.maxQueueDuration")
	maxQueueDuration = flag.Duration("insert.maxQueueDuration", time.Minute, "The maximum duration to wait in the queue when -maxConcurrentInserts "+
		"concurrent insert requests are executed")
)

// Reader is a reader, which increases the concurrency after the first Read() call
//
// The concurrency can be reduced by calling DecConcurrency().
// Then the concurrency is increased after the next Read() call.
type Reader struct {
	r                    io.Reader
	increasedConcurrency bool
}

// GetReader returns the Reader for r.
//
// The PutReader() must be called when the returned Reader is no longer needed.
func GetReader(r io.Reader) *Reader {
	v := readerPool.Get()
	if v == nil {
		return &Reader{
			r: r,
		}
	}
	rr := v.(*Reader)
	rr.r = r
	return rr
}

// PutReader returns the r to the pool.
//
// It decreases the concurrency if r has increased concurrency.
func PutReader(r *Reader) {
	r.DecConcurrency()
	r.r = nil
	readerPool.Put(r)
}

var readerPool sync.Pool

// Read implements io.Reader.
//
// It increases concurrency after the first call or after the next call after DecConcurrency() call.
func (r *Reader) Read(p []byte) (int, error) {
	n, err := r.r.Read(p)
	if !r.increasedConcurrency {
		if !incConcurrency() {
			err = &httpserver.ErrorWithStatusCode{
				Err: fmt.Errorf("cannot process insert request for %.3f seconds because %d concurrent insert requests are executed. "+
					"Possible solutions: to reduce workload; to increase compute resources at the server; "+
					"to increase -insert.maxQueueDuration; to increase -maxConcurrentInserts",
					maxQueueDuration.Seconds(), *maxConcurrentInserts),
				StatusCode: http.StatusServiceUnavailable,
			}
			return 0, err
		}
		r.increasedConcurrency = true
	}
	return n, err
}

// DecConcurrency decreases the concurrency, so it could be increased again after the next Read() call.
func (r *Reader) DecConcurrency() {
	if r.increasedConcurrency {
		decConcurrency()
		r.increasedConcurrency = false
	}
}

func initConcurrencyLimitCh() {
	concurrencyLimitCh = make(chan struct{}, *maxConcurrentInserts)
}

var (
	concurrencyLimitCh     chan struct{}
	concurrencyLimitChOnce sync.Once
)

func incConcurrency() bool {
	concurrencyLimitChOnce.Do(initConcurrencyLimitCh)

	select {
	case concurrencyLimitCh <- struct{}{}:
		return true
	default:
	}

	concurrencyLimitReached.Inc()
	t := timerpool.Get(*maxQueueDuration)
	select {
	case concurrencyLimitCh <- struct{}{}:
		timerpool.Put(t)
		return true
	case <-t.C:
		timerpool.Put(t)
		concurrencyLimitTimeout.Inc()
		return false
	}
}

func decConcurrency() {
	<-concurrencyLimitCh
}

var (
	concurrencyLimitReached = metrics.NewCounter(`vm_concurrent_insert_limit_reached_total`)
	concurrencyLimitTimeout = metrics.NewCounter(`vm_concurrent_insert_limit_timeout_total`)

	_ = metrics.NewGauge(`vm_concurrent_insert_capacity`, func() float64 {
		concurrencyLimitChOnce.Do(initConcurrencyLimitCh)
		return float64(cap(concurrencyLimitCh))
	})
	_ = metrics.NewGauge(`vm_concurrent_insert_current`, func() float64 {
		concurrencyLimitChOnce.Do(initConcurrencyLimitCh)
		return float64(len(concurrencyLimitCh))
	})
)