package main

import (
	"context"
	"fmt"
	"log"
	"net/http"
	"regexp"
	"strings"

	"github.com/docker/docker/api/types/image"
	"github.com/docker/docker/client"
	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/client_golang/prometheus/promhttp"
	"github.com/regclient/regclient"
	"github.com/regclient/regclient/types/ref"
)

// Hilfsfunktion: registrykompatibles Format aus repo:tag erzeugen
func toRegistryImage(imageTag string) (string, error) {
	r := regexp.MustCompile(`^(?:(?P<registry>[^/]+)/)?(?P<repo>[^:]+)(?::(?P<tag>.+))?$`)
	match := r.FindStringSubmatch(imageTag)
	if len(match) == 0 {
		return "", fmt.Errorf("Image-Tag nicht erkannt: %s", imageTag)
	}
	registry := match[r.SubexpIndex("registry")]
	repo := match[r.SubexpIndex("repo")]
	tag := match[r.SubexpIndex("tag")]
	if registry == "" {
		registry = "registry-1.docker.io"
	}
	if tag == "" {
		tag = "latest"
	}
	return fmt.Sprintf("%s/%s:%s", registry, repo, tag), nil
}

// Extrahiert nur den sha256:... Digest
func extractDigest(s string) string {
	for _, part := range strings.Split(s, "@") {
		if strings.HasPrefix(part, "sha256:") {
			return part
		}
	}
	return s
}

// Prometheus Collector-Struktur
type dockerImageUpdateCollector struct {
	desc *prometheus.Desc
}

func newDockerImageUpdateCollector() prometheus.Collector {
	return &dockerImageUpdateCollector{
		desc: prometheus.NewDesc(
			"docker_image_update_available",
			"Whether a new image digest is available for this local image tag (1=update, 0=current)",
			[]string{"image"},
			nil,
		),
	}
}

func (c *dockerImageUpdateCollector) Describe(ch chan<- *prometheus.Desc) {
	ch <- c.desc
}

func (c *dockerImageUpdateCollector) Collect(ch chan<- prometheus.Metric) {
	ctx := context.Background()
	cli, err := client.NewClientWithOpts(client.FromEnv)
	if err != nil {
		log.Println("Docker-Client-Fehler:", err)
		return
	}
	defer cli.Close()
	rc := regclient.New()

	images, err := cli.ImageList(ctx, image.ListOptions{All: true})
	if err != nil {
		log.Println("Docker-ImageList-Fehler:", err)
		return
	}

	for _, img := range images {
		for _, tag := range img.RepoTags {
			imageRef, err := toRegistryImage(tag)
			if err != nil {
				continue
			}
			var localDigest string
			if len(img.RepoDigests) > 0 {
				localDigest = extractDigest(img.RepoDigests[0])
			} else {
				localDigest = img.ID
			}
			refObj, err := ref.New(imageRef)
			if err != nil {
				continue
			}
			desc, err := rc.ManifestHead(ctx, refObj)
			if err != nil {
				continue
			}
			remoteDigest := desc.GetDigest().String()
			var value float64 = 0
			if localDigest != remoteDigest {
				value = 1
			}
			ch <- prometheus.MustNewConstMetric(
				c.desc,
				prometheus.GaugeValue,
				value,
				tag,
			)
		}
	}
}

func main() {
	prometheus.MustRegister(newDockerImageUpdateCollector())
	http.Handle("/metrics", promhttp.Handler())
	log.Println("Prometheus Exporter läuft auf :9888/metrics")
	log.Fatal(http.ListenAndServe(":9888", nil))
}