// Package metrics exposes a lightweight Prometheus-compatible metrics endpoint
// for KubeSolo OS system and update status.
//
// Metrics exposed:
//
//	kubesolo_os_info{version, active_slot}           1 (gauge, labels identify the OS)
//	kubesolo_os_boot_success                         1 or 0 (gauge)
//	kubesolo_os_boot_counter                         0-3 (gauge)
//	kubesolo_os_uptime_seconds                       float (gauge)
//	kubesolo_os_update_available                     1 or 0 (gauge)
//	kubesolo_os_update_last_check_timestamp_seconds  unix timestamp (gauge)
//	kubesolo_os_memory_total_bytes                   total RAM (gauge)
//	kubesolo_os_memory_available_bytes               available RAM (gauge)
//
// This is a zero-dependency implementation — no Prometheus client library needed.
// It serves metrics in the Prometheus text exposition format.
package metrics

import (
	"fmt"
	"log/slog"
	"net/http"
	"os"
	"strconv"
	"strings"
	"sync"
	"time"
)

// Server is a lightweight Prometheus metrics HTTP server.
type Server struct {
	grubenvPath string
	listenAddr  string
	startTime   time.Time

	mu              sync.Mutex
	updateAvailable int
	lastCheckTime   float64
}

// NewServer creates a new metrics server.
func NewServer(listenAddr, grubenvPath string) *Server {
	return &Server{
		grubenvPath: grubenvPath,
		listenAddr:  listenAddr,
		startTime:   time.Now(),
	}
}

// SetUpdateAvailable records whether an update is available.
func (s *Server) SetUpdateAvailable(available bool) {
	s.mu.Lock()
	defer s.mu.Unlock()
	if available {
		s.updateAvailable = 1
	} else {
		s.updateAvailable = 0
	}
	s.lastCheckTime = float64(time.Now().Unix())
}

// ListenAndServe starts the metrics HTTP server.
func (s *Server) ListenAndServe() error {
	mux := http.NewServeMux()
	mux.HandleFunc("/metrics", s.handleMetrics)
	mux.HandleFunc("/healthz", s.handleHealthz)

	slog.Info("starting metrics server", "addr", s.listenAddr)
	return http.ListenAndServe(s.listenAddr, mux)
}

func (s *Server) handleHealthz(w http.ResponseWriter, r *http.Request) {
	w.WriteHeader(http.StatusOK)
	fmt.Fprint(w, "ok\n")
}

func (s *Server) handleMetrics(w http.ResponseWriter, r *http.Request) {
	w.Header().Set("Content-Type", "text/plain; version=0.0.4; charset=utf-8")

	var sb strings.Builder

	// OS info
	version := readFileString("/etc/kubesolo-os-version")
	activeSlot := s.readGrubenvVar("active_slot")
	sb.WriteString("# HELP kubesolo_os_info KubeSolo OS version and slot info.\n")
	sb.WriteString("# TYPE kubesolo_os_info gauge\n")
	sb.WriteString(fmt.Sprintf("kubesolo_os_info{version=%q,active_slot=%q} 1\n",
		version, activeSlot))

	// Boot status
	bootSuccess := s.readGrubenvVar("boot_success")
	bootCounter := s.readGrubenvVar("boot_counter")
	sb.WriteString("# HELP kubesolo_os_boot_success Whether the current boot was marked successful.\n")
	sb.WriteString("# TYPE kubesolo_os_boot_success gauge\n")
	sb.WriteString(fmt.Sprintf("kubesolo_os_boot_success %s\n", safeInt(bootSuccess, "0")))
	sb.WriteString("# HELP kubesolo_os_boot_counter Remaining boot attempts before rollback.\n")
	sb.WriteString("# TYPE kubesolo_os_boot_counter gauge\n")
	sb.WriteString(fmt.Sprintf("kubesolo_os_boot_counter %s\n", safeInt(bootCounter, "0")))

	// Uptime
	uptime := time.Since(s.startTime).Seconds()
	sb.WriteString("# HELP kubesolo_os_uptime_seconds Time since the metrics server started.\n")
	sb.WriteString("# TYPE kubesolo_os_uptime_seconds gauge\n")
	sb.WriteString(fmt.Sprintf("kubesolo_os_uptime_seconds %.1f\n", uptime))

	// Update status
	s.mu.Lock()
	updateAvail := s.updateAvailable
	lastCheck := s.lastCheckTime
	s.mu.Unlock()

	sb.WriteString("# HELP kubesolo_os_update_available Whether an OS update is available.\n")
	sb.WriteString("# TYPE kubesolo_os_update_available gauge\n")
	sb.WriteString(fmt.Sprintf("kubesolo_os_update_available %d\n", updateAvail))
	sb.WriteString("# HELP kubesolo_os_update_last_check_timestamp_seconds Unix timestamp of last update check.\n")
	sb.WriteString("# TYPE kubesolo_os_update_last_check_timestamp_seconds gauge\n")
	sb.WriteString(fmt.Sprintf("kubesolo_os_update_last_check_timestamp_seconds %.0f\n", lastCheck))

	// Memory
	memTotal, memAvail := readMemInfo()
	sb.WriteString("# HELP kubesolo_os_memory_total_bytes Total system memory in bytes.\n")
	sb.WriteString("# TYPE kubesolo_os_memory_total_bytes gauge\n")
	sb.WriteString(fmt.Sprintf("kubesolo_os_memory_total_bytes %d\n", memTotal))
	sb.WriteString("# HELP kubesolo_os_memory_available_bytes Available system memory in bytes.\n")
	sb.WriteString("# TYPE kubesolo_os_memory_available_bytes gauge\n")
	sb.WriteString(fmt.Sprintf("kubesolo_os_memory_available_bytes %d\n", memAvail))

	fmt.Fprint(w, sb.String())
}

// readGrubenvVar reads a single variable from grubenv using simple file parse.
func (s *Server) readGrubenvVar(key string) string {
	data, err := os.ReadFile(s.grubenvPath)
	if err != nil {
		return ""
	}
	for _, line := range strings.Split(string(data), "\n") {
		parts := strings.SplitN(line, "=", 2)
		if len(parts) == 2 && strings.TrimSpace(parts[0]) == key {
			return strings.TrimSpace(parts[1])
		}
	}
	return ""
}

// readFileString reads a file and returns trimmed content.
func readFileString(path string) string {
	data, err := os.ReadFile(path)
	if err != nil {
		return "unknown"
	}
	return strings.TrimSpace(string(data))
}

// readMemInfo parses /proc/meminfo for total and available memory.
func readMemInfo() (total, available int64) {
	data, err := os.ReadFile("/proc/meminfo")
	if err != nil {
		return 0, 0
	}
	for _, line := range strings.Split(string(data), "\n") {
		fields := strings.Fields(line)
		if len(fields) < 2 {
			continue
		}
		val, err := strconv.ParseInt(fields[1], 10, 64)
		if err != nil {
			continue
		}
		// /proc/meminfo values are in kB
		switch fields[0] {
		case "MemTotal:":
			total = val * 1024
		case "MemAvailable:":
			available = val * 1024
		}
	}
	return total, available
}

// safeInt returns the value if it's a valid integer, otherwise the default.
func safeInt(s, def string) string {
	if _, err := strconv.Atoi(s); err != nil {
		return def
	}
	return s
}