// Package license implements the NetCell MailGuard client for the NetCell
// Licensing server (license.netcell-it.com). It verifies a license key
// with Ed25519-signed responses, caches the result on disk (24 h TTL),
// falls back to a 30-day offline trial when no key is configured, and
// protects against clock manipulation via ServerTime drift checks.
//
// Pattern wholesale adopted from netcell-webpanel/management-api/internal/
// middleware/license.go; edgeguard uses `active_domains` (operator-defined domains) as the usage
// counter sent to the licensing server (the number of mail domains this
// cluster serves) instead of the webpanel's active_sites/active_servers.
//
// Cluster semantics: only one peer per cluster contacts license.netcell-it.com
// at a time — that peer is selected via a KeyDB lock (cluster:license-leader,
// 60-s TTL). The result ends up in KeyDB key cluster:license-status so every
// peer can read it locally. Leader election lives in internal/cluster; this
// package implements the verification + caching mechanics.
package license

import (
	"crypto/sha256"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"log/slog"
	"net"
	"net/http"
	"net/url"
	"os"
	"path/filepath"
	"strings"
	"time"
)

// Tunables. Kept package-private constants; override via internal/license/testing.go in tests.
const (
	DefaultServerURL      = "https://license.netcell-it.com"
	DefaultSelfServiceURL = "https://license.netcell-it.com/self-service"

	DefaultCacheDir    = "/var/lib/edgeguard"
	defaultCacheFile   = "license.cache"
	defaultTrialFile   = "trial.json"
	defaultHTTPTimeout = 10 * time.Second
	CacheMaxAge        = 24 * time.Hour
	VerifyInterval     = 24 * time.Hour
	TrialDuration      = 30 * 24 * time.Hour
	maxClockDrift      = 48 * time.Hour
	maxResponseBytes   = 64 * 1024

	// GracePeriod: wenn der Server eine valid:false-Antwort liefert,
	// behält der Client den letzten valid:true-Cache so lange bei,
	// solange der nicht älter als GracePeriod ist. Schützt vor
	// transienten Server-Bugs (z.B. activation_limit_exceeded falsch
	// gezählt) und kurzen Wartungsfenstern. Der User-sichtbare Effekt:
	// keine spontane „Lizenz fehlt"-UI-Zustand mid-Session.
	// Override per Env EDGEGUARD_LICENSE_GRACE_DAYS.
	GracePeriod = 7 * 24 * time.Hour
)

// Result is the license server response plus a few internal fields.
// JSON tags match the wire format from license.netcell-it.com.
type Result struct {
	Valid       bool             `json:"valid"`
	Reason      string           `json:"reason,omitempty"`
	Type        string           `json:"type,omitempty"`   // license | trial
	Status      string           `json:"status,omitempty"` // active | expired | revoked
	Product     string           `json:"product,omitempty"`
	ExpiresAt   *time.Time       `json:"expires_at,omitempty"`
	GracePeriod bool             `json:"grace_period,omitempty"`
	Features    map[string]bool  `json:"features,omitempty"`
	Limits      map[string]int64 `json:"limits,omitempty"`
	ServerTime  *time.Time       `json:"server_time,omitempty"` // authoritative timestamp (signed)
	CachedAt    *time.Time       `json:"cached_at,omitempty"`   // set when loaded from local cache
}

// Feature returns true if the given feature flag is enabled in this license.
func (r *Result) Feature(name string) bool { return r.Features[name] }

// Limit returns the numeric limit for a given key, or 0 if not set.
func (r *Result) Limit(name string) int64 { return r.Limits[name] }

// ActiveDomainsFn yields the mail-domain count for usage reporting
// to the license server. Optional — nil means send 0.
type ActiveDomainsFn func() int64

// Client performs verification calls. Construct with NewClient.
type Client struct {
	ServerURL       string
	CacheDir        string
	HTTPTimeout     time.Duration
	ActiveDomains   ActiveDomainsFn
	HTTPClient      *http.Client
	SignatureKeys   []SignatureKey // verifies the X-Signature header
	ClockNow        func() time.Time
	OSHostname      func() (string, error)
	ReadMachineID   func() ([]byte, error)
	InterfacesFn    func() ([]net.Interface, error)
}

// NewClient returns a client with production defaults.
func NewClient() *Client {
	return &Client{
		ServerURL:     DefaultServerURL,
		CacheDir:      DefaultCacheDir,
		HTTPTimeout:   defaultHTTPTimeout,
		HTTPClient:    &http.Client{Timeout: defaultHTTPTimeout},
		SignatureKeys: DefaultSigningKeys(),
		ClockNow:      func() time.Time { return time.Now().UTC() },
		OSHostname:    os.Hostname,
		ReadMachineID: func() ([]byte, error) { return os.ReadFile("/etc/machine-id") },
		InterfacesFn:  net.Interfaces,
	}
}

// Verify performs a live check against the license server. Non-nil Result
// means we got a valid, signed response. On network failure returns an error;
// callers should fall back to the cache (LoadCache) or to Trial (see Check).
func (c *Client) Verify(key string) (*Result, error) {
	if key == "" {
		return nil, errors.New("license key is empty")
	}
	fp := c.SystemFingerprint()
	hostname, _ := c.OSHostname()

	var activeDomains int64
	if c.ActiveDomains != nil {
		activeDomains = c.ActiveDomains()
	}

	endpoint := fmt.Sprintf(
		"%s/api/v1/licenses/%s/verify?system_id=%s&system_name=%s&active_domains=%d",
		c.ServerURL,
		url.PathEscape(key),
		url.QueryEscape(fp),
		url.QueryEscape(hostname),
		activeDomains,
	)

	resp, err := c.HTTPClient.Get(endpoint)
	if err != nil {
		return nil, fmt.Errorf("license server unreachable: %w", err)
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("license server returned status %d", resp.StatusCode)
	}

	body, err := io.ReadAll(io.LimitReader(resp.Body, maxResponseBytes))
	if err != nil {
		return nil, fmt.Errorf("read license response: %w", err)
	}

	sig := resp.Header.Get("X-Signature")
	if sig == "" {
		return nil, errors.New("license server response missing X-Signature header")
	}
	if !VerifySignature(c.SignatureKeys, body, sig) {
		return nil, errors.New("license server response signature invalid")
	}

	var result Result
	if err := json.Unmarshal(body, &result); err != nil {
		return nil, fmt.Errorf("decode license response: %w", err)
	}

	if result.ServerTime != nil {
		drift := c.ClockNow().Sub(*result.ServerTime)
		if drift < 0 {
			drift = -drift
		}
		if drift > maxClockDrift {
			return nil, fmt.Errorf("system clock drift %s exceeds %s (check NTP)", drift, maxClockDrift)
		}
	}

	return &result, nil
}

// Check returns the currently effective license, trying in order:
// live verify (valid:true) -> live verify (valid:false) but cached
// valid:true within grace period -> local cache (24 h TTL) -> trial
// (7 days) -> expired. Never returns an error.
//
// Grace-Period-Verhalten (1.6.66+):
//   - Server-Antwort valid:true → SaveCache + return r
//   - Server-Antwort valid:false → cache laden:
//       * cache.Valid && cached_at innerhalb GracePeriod → cache nutzen
//         (typische Ursache: transientes Server-Problem wie
//         activation_limit_exceeded nach mehreren verify-Calls)
//       * sonst → server-Antwort durchreichen, cache NICHT überschreiben
//   - Verify-Fehler (Netzwerk/HTTP) → cache nutzen (wie pre-1.6.66)
//
// Die SaveCache-Aufrufe werden ausschließlich für valid:true-Results
// gemacht. Damit überschreibt eine vorübergehende valid:false-Antwort
// niemals einen vorher gespeicherten gültigen Stand. Operator hat 7
// Tage Zeit zu reagieren ohne dass die UI sofort Lizenz-Eingabe fordert.
func (c *Client) Check(key string) *Result {
	if key != "" {
		if r, err := c.Verify(key); err == nil {
			if r.Valid {
				_ = c.SaveCache(r)
				slog.Info("license verified", "product", r.Product, "type", r.Type, "valid", true)
				return r
			}
			// Server says valid:false. Vor wir das durchreichen,
			// prüfen wir ob ein gültiger Cache existiert der noch
			// in der Grace-Period liegt — dann nutzen wir den.
			if cached, cerr := c.LoadCache(); cerr == nil && cached.Valid {
				age := graceAge(cached)
				if age <= GracePeriod {
					slog.Warn("license server said invalid, using cached valid result (grace period)",
						"server_reason", r.Reason, "cache_age", age, "grace", GracePeriod)
					return cached
				}
				slog.Warn("license server said invalid, cached valid result is too old",
					"server_reason", r.Reason, "cache_age", age, "grace", GracePeriod)
			}
			slog.Warn("license invalid", "reason", r.Reason)
			return r
		} else {
			slog.Warn("license verify failed, falling back to cache", "error", err)
		}
		if r, err := c.LoadCache(); err == nil {
			slog.Warn("using cached license", "cached_at", r.CachedAt)
			return r
		}
		return &Result{Valid: false, Reason: "verify_failed_no_cache", Type: "license"}
	}

	// No key — trial mode.
	r, err := c.Trial()
	if err != nil {
		slog.Error("trial check failed", "error", err)
		return &Result{Valid: false, Reason: "trial_check_failed", Type: "trial"}
	}
	return r
}

// graceAge berechnet das Alter des Cache-Eintrags. Fallback auf weit-
// in-der-Zukunft wenn cached_at fehlt — dann gilt die Cache als zu alt
// und Grace greift NICHT (defensiv: lieber einmal Auth-Fragen als
// dauerhaft falsche Lizenz nutzen).
func graceAge(r *Result) time.Duration {
	if r == nil || r.CachedAt == nil {
		return 100 * 365 * 24 * time.Hour
	}
	return time.Since(*r.CachedAt)
}

// StartPeriodicVerification re-checks the license every VerifyInterval.
// Writes each result back to the cache; does not exit on failure.
func (c *Client) StartPeriodicVerification(key string) {
	go func() {
		ticker := time.NewTicker(VerifyInterval)
		defer ticker.Stop()
		for range ticker.C {
			r := c.Check(key)
			if !r.Valid {
				slog.Warn("periodic license check: invalid",
					"reason", r.Reason, "self_service", DefaultSelfServiceURL)
			}
		}
	}()
}

// SystemFingerprint is a stable SHA-256 over machine-id, the first active
// non-loopback MAC and the hostname. Identical bytes on the same machine
// across reboots; changes when network hardware or hostname changes.
func (c *Client) SystemFingerprint() string {
	var parts []string
	if b, err := c.ReadMachineID(); err == nil {
		parts = append(parts, strings.TrimSpace(string(b)))
	}
	ifaces, _ := c.InterfacesFn()
	for _, i := range ifaces {
		if len(i.HardwareAddr) > 0 && i.Flags&net.FlagLoopback == 0 && i.Flags&net.FlagUp != 0 {
			parts = append(parts, i.HardwareAddr.String())
			break
		}
	}
	if h, err := c.OSHostname(); err == nil {
		parts = append(parts, h)
	}
	sum := sha256.Sum256([]byte(strings.Join(parts, "|")))
	return fmt.Sprintf("%x", sum)
}

// Cache + Trial helpers --------------------------------------------------------

func (c *Client) cachePath() string { return filepath.Join(c.CacheDir, defaultCacheFile) }
func (c *Client) trialPath() string { return filepath.Join(c.CacheDir, defaultTrialFile) }

type cacheEntry struct {
	Result   *Result   `json:"result"`
	CachedAt time.Time `json:"cached_at"`
}

// SaveCache writes a verified result to disk. Errors are logged, not fatal.
func (c *Client) SaveCache(r *Result) error {
	if err := os.MkdirAll(c.CacheDir, 0o700); err != nil {
		return err
	}
	entry := cacheEntry{Result: r, CachedAt: c.ClockNow()}
	data, err := json.Marshal(entry)
	if err != nil {
		return err
	}
	return os.WriteFile(c.cachePath(), data, 0o600)
}

// LoadCache reads the cached result; errors if missing, malformed or older
// than CacheMaxAge.
func (c *Client) LoadCache() (*Result, error) {
	data, err := os.ReadFile(c.cachePath())
	if err != nil {
		return nil, err
	}
	var entry cacheEntry
	if err := json.Unmarshal(data, &entry); err != nil {
		return nil, err
	}
	if c.ClockNow().Sub(entry.CachedAt) > CacheMaxAge {
		return nil, errors.New("cache expired")
	}
	if entry.Result != nil {
		entry.Result.CachedAt = &entry.CachedAt
	}
	return entry.Result, nil
}

type trialInfo struct {
	StartedAt time.Time `json:"started_at"`
}

// Trial returns an active trial result while inside TrialDuration, else
// a Valid=false "trial_expired" result. Creates the marker file on first call.
func (c *Client) Trial() (*Result, error) {
	var info trialInfo
	data, err := os.ReadFile(c.trialPath())
	if err != nil {
		if !os.IsNotExist(err) {
			return nil, err
		}
		info = trialInfo{StartedAt: c.ClockNow()}
		if err := os.MkdirAll(c.CacheDir, 0o700); err != nil {
			return nil, err
		}
		raw, _ := json.Marshal(info)
		if err := os.WriteFile(c.trialPath(), raw, 0o600); err != nil {
			return nil, err
		}
		slog.Info("nmg trial started",
			"expires_at", info.StartedAt.Add(TrialDuration))
	} else {
		if err := json.Unmarshal(data, &info); err != nil {
			return nil, err
		}
	}

	remaining := TrialDuration - c.ClockNow().Sub(info.StartedAt)
	if remaining <= 0 {
		return &Result{Valid: false, Reason: "trial_expired", Type: "trial"}, nil
	}
	daysLeft := int(remaining.Hours()/24) + 1
	expiresAt := info.StartedAt.Add(TrialDuration)

	// Trial schaltet alle Pro-Features frei, Limits bleiben 0 (= Skip-
	// Enforcement-Konvention). nmg-Backend prüft `lim > 0 && count >= lim`
	// und blockiert in Trial-Mode niemanden.
	return &Result{
		Valid:       true,
		Type:        "trial",
		Status:      "active",
		Product:     "NetCell MailGuard",
		ExpiresAt:   &expiresAt,
		GracePeriod: daysLeft <= 2,
		Features: map[string]bool{
			"reporting":      true,
			"eu_portal":      true,
			"digest":         true,
			"whitelabel":     true,
			"rest_api_write": true,
		},
		Limits: map[string]int64{
			"max_domains":     0, // 0 = Skip-Enforcement (Trial-Konvention)
			"max_nodes":       0,
			"max_activations": 0,
		},
	}, nil
}