package daemon

import (
	"context"
	"fmt"
	"strconv"
	"strings"
	"sync"
)

const tapPoolPrefix = "tap-pool-"

// tapPool owns the idle TAP interface cache plus the monotonic index used to
// name new pool entries. All access goes through mu.
type tapPool struct {
	mu      sync.Mutex
	entries []string
	next    int
}

func (d *Daemon) initializeTapPool(ctx context.Context) error {
	if d.config.TapPoolSize <= 0 || d.store == nil {
		return nil
	}
	vms, err := d.store.ListVMs(ctx)
	if err != nil {
		return err
	}
	next := 0
	for _, vm := range vms {
		if index, ok := parseTapPoolIndex(d.vmHandles(vm.ID).TapDevice); ok && index >= next {
			next = index + 1
		}
	}
	d.tapPool.mu.Lock()
	d.tapPool.next = next
	d.tapPool.mu.Unlock()
	return nil
}

func (d *Daemon) ensureTapPool(ctx context.Context) {
	if d.config.TapPoolSize <= 0 {
		return
	}
	for {
		select {
		case <-ctx.Done():
			return
		case <-d.closing:
			return
		default:
		}

		d.tapPool.mu.Lock()
		if len(d.tapPool.entries) >= d.config.TapPoolSize {
			d.tapPool.mu.Unlock()
			return
		}
		tapName := fmt.Sprintf("%s%d", tapPoolPrefix, d.tapPool.next)
		d.tapPool.next++
		d.tapPool.mu.Unlock()

		if err := d.createTap(ctx, tapName); err != nil {
			if d.logger != nil {
				d.logger.Warn("tap pool warmup failed", "tap_device", tapName, "error", err.Error())
			}
			return
		}

		d.tapPool.mu.Lock()
		d.tapPool.entries = append(d.tapPool.entries, tapName)
		d.tapPool.mu.Unlock()

		if d.logger != nil {
			d.logger.Debug("tap added to idle pool", "tap_device", tapName)
		}
	}
}

func (d *Daemon) acquireTap(ctx context.Context, fallbackName string) (string, error) {
	d.tapPool.mu.Lock()
	if n := len(d.tapPool.entries); n > 0 {
		tapName := d.tapPool.entries[n-1]
		d.tapPool.entries = d.tapPool.entries[:n-1]
		d.tapPool.mu.Unlock()
		return tapName, nil
	}
	d.tapPool.mu.Unlock()

	if err := d.createTap(ctx, fallbackName); err != nil {
		return "", err
	}
	return fallbackName, nil
}

func (d *Daemon) releaseTap(ctx context.Context, tapName string) error {
	tapName = strings.TrimSpace(tapName)
	if tapName == "" {
		return nil
	}
	if isTapPoolName(tapName) {
		d.tapPool.mu.Lock()
		if len(d.tapPool.entries) < d.config.TapPoolSize {
			d.tapPool.entries = append(d.tapPool.entries, tapName)
			d.tapPool.mu.Unlock()
			return nil
		}
		d.tapPool.mu.Unlock()
	}
	_, err := d.runner.RunSudo(ctx, "ip", "link", "del", tapName)
	if err == nil {
		go d.ensureTapPool(context.Background())
	}
	return err
}

func isTapPoolName(tapName string) bool {
	return strings.HasPrefix(strings.TrimSpace(tapName), tapPoolPrefix)
}

func parseTapPoolIndex(tapName string) (int, bool) {
	if !isTapPoolName(tapName) {
		return 0, false
	}
	value, err := strconv.Atoi(strings.TrimPrefix(strings.TrimSpace(tapName), tapPoolPrefix))
	if err != nil {
		return 0, false
	}
	return value, true
}