package server

import (
	"context"
	"fmt"
	"sync"
	"time"

	"github.com/netbirdio/netbird/management/server/telemetry"
)

type UpdateBuffer struct {
	mu             sync.Mutex
	cond           *sync.Cond
	update         *UpdateMessage
	closed         bool
	metrics        *telemetry.UpdateChannelMetrics
	overwriteCount int       // Number of overwrites since last Pop
	lastPopTime    time.Time // Time of last Pop
}

func NewUpdateBuffer(metrics *telemetry.UpdateChannelMetrics) *UpdateBuffer {
	ub := &UpdateBuffer{metrics: metrics}
	ub.cond = sync.NewCond(&ub.mu)
	return ub
}

func (b *UpdateBuffer) Push(update *UpdateMessage) {
	b.mu.Lock()
	defer b.mu.Unlock()

	if b.update == nil || update.Update.NetworkMap.Serial > b.update.Update.NetworkMap.Serial || b.update.Update.NetworkMap.Serial == 0 {
		if b.update == nil {
			b.metrics.CountBufferPush()
		} else {
			b.metrics.CountBufferOverwrite()
			b.overwriteCount++
		}

		b.update = update
		b.cond.Signal()

		return
	}

	b.metrics.CountBufferIgnore()
}

func (b *UpdateBuffer) Pop(ctx context.Context) (*UpdateMessage, int, time.Duration, error) {
	b.mu.Lock()
	defer b.mu.Unlock()

	for b.update == nil && !b.closed {
		select {
		case <-ctx.Done():
			return nil, 0, 0, fmt.Errorf("context cancelled")
		default:
		}

		waitCh := make(chan struct{})
		go func() {
			select {
			case <-ctx.Done():
				b.cond.Broadcast()
			case <-waitCh:
				// noop
			}
		}()
		b.cond.Wait()
		close(waitCh)

		select {
		case <-ctx.Done():
			return nil, 0, 0, fmt.Errorf("context cancelled")
		default:
		}
	}

	if b.closed {
		return nil, 0, 0, fmt.Errorf("buffer closed")
	}

	msg := b.update
	overwrites := b.overwriteCount

	// Calculate time since last pop
	now := time.Now()
	var timeSinceLastPop time.Duration
	if !b.lastPopTime.IsZero() {
		timeSinceLastPop = now.Sub(b.lastPopTime)
	}

	// Reset counters
	b.update = nil
	b.overwriteCount = 0
	b.lastPopTime = now

	return msg, overwrites, timeSinceLastPop, nil
}

func (b *UpdateBuffer) Close() {
	b.mu.Lock()
	b.closed = true
	b.cond.Broadcast()
	b.mu.Unlock()
}