package auth

import (
	"context"
	"crypto/hmac"
	"crypto/sha256"
	"crypto/subtle"
	"encoding/base64"
	"errors"
	"fmt"
	"log/slog"
	"net/http"
	"net/netip"
	"strings"
	"sync"
	"time"

	"github.com/owen/vibedns/internal/database"
	"github.com/owen/vibedns/internal/models"
	"github.com/owen/vibedns/internal/netutil"
)

// Realm is the HTTP Basic authentication realm.
const Realm = "vibedns management"

// PrincipalKind distinguishes an interactive administrator from automation.
type PrincipalKind string

const (
	KindAdmin PrincipalKind = "admin"
	KindToken PrincipalKind = "token"
)

// Principal is the authenticated identity attached to a request.
type Principal struct {
	Name     string
	Kind     PrincipalKind
	TokenID  int64
	ClientIP string
}

// IsAdmin reports whether the principal is the interactive administrator.
func (p Principal) IsAdmin() bool { return p.Kind == KindAdmin }

type ctxKey struct{}

// WithPrincipal stores a principal on a request context.
func WithPrincipal(ctx context.Context, p Principal) context.Context {
	return context.WithValue(ctx, ctxKey{}, p)
}

// PrincipalFrom retrieves the principal from a request context.
func PrincipalFrom(ctx context.Context) (Principal, bool) {
	p, ok := ctx.Value(ctxKey{}).(Principal)
	return p, ok
}

// Authenticator verifies credentials for the web UI and the REST API.
type Authenticator struct {
	db  *database.DB
	log *slog.Logger

	csrfKey  []byte
	verifier *credentialCache
	attempts *attemptLimiter

	// trusted lists proxies whose X-Forwarded-For header we believe.
	trustedMu sync.RWMutex
	trusted   *netutil.PrefixSet
}

// New creates an authenticator. csrfKey must be a stable secret; it is
// persisted so that tokens issued before a restart stay valid.
func New(db *database.DB, log *slog.Logger, csrfKey []byte) *Authenticator {
	return &Authenticator{
		db:       db,
		log:      log,
		csrfKey:  csrfKey,
		verifier: newCredentialCache(5 * time.Minute),
		attempts: newAttemptLimiter(10, 5*time.Minute),
		trusted:  netutil.NewPrefixSet(nil),
	}
}

// SetTrustedProxies configures which peers may set X-Forwarded-For.
func (a *Authenticator) SetTrustedProxies(cidrs []string) {
	a.trustedMu.Lock()
	a.trusted = netutil.NewPrefixSet(cidrs)
	a.trustedMu.Unlock()
}

// ClientIP resolves the client address, honouring X-Forwarded-For only when the
// immediate peer is a configured trusted proxy. Trusting the header
// unconditionally would let any client forge its own address and bypass the
// login rate limiter.
func (a *Authenticator) ClientIP(r *http.Request) string {
	peer, ok := netutil.AddrFromHostPort(r.RemoteAddr)
	if !ok {
		return r.RemoteAddr
	}
	a.trustedMu.RLock()
	trusted := a.trusted
	a.trustedMu.RUnlock()

	if trusted.Contains(peer) {
		if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
			// The left-most entry is the original client.
			first := strings.TrimSpace(strings.Split(xff, ",")[0])
			if addr, err := netip.ParseAddr(first); err == nil {
				return addr.Unmap().String()
			}
		}
		if xr := strings.TrimSpace(r.Header.Get("X-Real-IP")); xr != "" {
			if addr, err := netip.ParseAddr(xr); err == nil {
				return addr.Unmap().String()
			}
		}
	}
	return peer.String()
}

// Errors returned by credential verification.
var (
	ErrUnauthorised = errors.New("authentication required")
	ErrLockedOut    = errors.New("too many failed sign-in attempts")
	ErrNoAdmin      = errors.New("no administrator account exists")
)

// Authenticate verifies the credentials on a request.
//
// It accepts either HTTP Basic credentials (the interactive administrator) or
// a bearer API token. Tokens are rejected for the HTML interface by the caller,
// which passes allowTokens=false.
func (a *Authenticator) Authenticate(r *http.Request, allowTokens bool) (Principal, error) {
	clientIP := a.ClientIP(r)
	if !a.attempts.allow(clientIP) {
		return Principal{}, ErrLockedOut
	}

	if allowTokens {
		if secret, ok := bearerToken(r); ok {
			p, err := a.verifyToken(r.Context(), secret, clientIP)
			if err != nil {
				a.attempts.fail(clientIP)
				return Principal{}, err
			}
			a.attempts.succeed(clientIP)
			return p, nil
		}
	}

	username, password, ok := r.BasicAuth()
	if !ok {
		return Principal{}, ErrUnauthorised
	}
	p, err := a.verifyPassword(r.Context(), username, password, clientIP)
	if err != nil {
		a.attempts.fail(clientIP)
		a.log.Warn("failed sign-in attempt", "username", username, "client", clientIP)
		return Principal{}, err
	}
	a.attempts.succeed(clientIP)
	return p, nil
}

func (a *Authenticator) verifyPassword(ctx context.Context, username, password, clientIP string) (Principal, error) {
	admin, err := a.db.Admin(ctx)
	if errors.Is(err, database.ErrNotFound) {
		return Principal{}, ErrNoAdmin
	}
	if err != nil {
		return Principal{}, fmt.Errorf("load administrator: %w", err)
	}

	// Compare the username in constant time so it cannot be probed by timing.
	userOK := subtle.ConstantTimeCompare([]byte(username), []byte(admin.Username)) == 1

	// HTTP Basic sends credentials on every request, including every page load.
	// Running Argon2id each time would cost 64 MiB and tens of milliseconds per
	// request, so a successful verification is remembered briefly, keyed by a
	// MAC of the password rather than the password itself.
	if userOK && a.verifier.valid(username, password, admin.PasswordHash) {
		return Principal{Name: admin.Username, Kind: KindAdmin, ClientIP: clientIP}, nil
	}

	passOK, err := VerifyPassword(admin.PasswordHash, password)
	if err != nil {
		a.log.Error("stored administrator password hash is unusable", "error", err)
		return Principal{}, ErrUnauthorised
	}
	if !userOK || !passOK {
		return Principal{}, ErrUnauthorised
	}

	a.verifier.store(username, password, admin.PasswordHash)
	_ = a.db.TouchAdminLogin(ctx)
	return Principal{Name: admin.Username, Kind: KindAdmin, ClientIP: clientIP}, nil
}

func (a *Authenticator) verifyToken(ctx context.Context, secret, clientIP string) (Principal, error) {
	prefix, err := TokenPrefix(secret)
	if err != nil {
		return Principal{}, ErrUnauthorised
	}
	candidates, err := a.db.APITokensByPrefix(ctx, prefix)
	if err != nil {
		return Principal{}, fmt.Errorf("look up API token: %w", err)
	}
	for _, c := range candidates {
		if VerifyToken(c.Hash, secret) {
			go func(id int64) {
				tctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
				defer cancel()
				_ = a.db.TouchAPIToken(tctx, id)
			}(c.ID)
			return Principal{Name: c.Name, Kind: KindToken, TokenID: c.ID, ClientIP: clientIP}, nil
		}
	}
	return Principal{}, ErrUnauthorised
}

func bearerToken(r *http.Request) (string, bool) {
	h := r.Header.Get("Authorization")
	if strings.HasPrefix(h, "Bearer ") {
		return strings.TrimSpace(strings.TrimPrefix(h, "Bearer ")), true
	}
	if v := r.Header.Get("X-API-Token"); v != "" {
		return strings.TrimSpace(v), true
	}
	return "", false
}

// InvalidateCredentials clears the verification cache. It is called after a
// password change so the old password stops working immediately.
func (a *Authenticator) InvalidateCredentials() { a.verifier.reset() }

// --- credential cache ---------------------------------------------------

type cachedCred struct {
	mac      []byte
	hashSeen string
	expires  time.Time
}

type credentialCache struct {
	mu    sync.RWMutex
	key   []byte
	ttl   time.Duration
	items map[string]cachedCred
}

func newCredentialCache(ttl time.Duration) *credentialCache {
	key := make([]byte, 32)
	// A failure here is not fatal: an all-zero key only weakens the cache
	// index, which never leaves this process and is not a stored secret.
	if s, err := RandomKey(32); err == nil {
		copy(key, s)
	}
	return &credentialCache{key: key, ttl: ttl, items: map[string]cachedCred{}}
}

func (c *credentialCache) mac(password string) []byte {
	h := hmac.New(sha256.New, c.key)
	h.Write([]byte(password))
	return h.Sum(nil)
}

func (c *credentialCache) valid(username, password, currentHash string) bool {
	c.mu.RLock()
	item, ok := c.items[username]
	c.mu.RUnlock()
	if !ok || time.Now().After(item.expires) {
		return false
	}
	// A changed stored hash means the password was rotated; the cache entry is
	// no longer authoritative.
	if item.hashSeen != currentHash {
		return false
	}
	return hmac.Equal(item.mac, c.mac(password))
}

func (c *credentialCache) store(username, password, currentHash string) {
	c.mu.Lock()
	c.items[username] = cachedCred{
		mac:      c.mac(password),
		hashSeen: currentHash,
		expires:  time.Now().Add(c.ttl),
	}
	c.mu.Unlock()
}

func (c *credentialCache) reset() {
	c.mu.Lock()
	c.items = map[string]cachedCred{}
	c.mu.Unlock()
}

// --- failed attempt limiting -------------------------------------------

type attemptState struct {
	failures int
	until    time.Time
	last     time.Time
}

// attemptLimiter slows down credential guessing per source address.
type attemptLimiter struct {
	mu      sync.Mutex
	items   map[string]*attemptState
	max     int
	lockout time.Duration
	lastGC  time.Time
}

func newAttemptLimiter(max int, lockout time.Duration) *attemptLimiter {
	return &attemptLimiter{items: map[string]*attemptState{}, max: max, lockout: lockout}
}

func (l *attemptLimiter) allow(key string) bool {
	l.mu.Lock()
	defer l.mu.Unlock()
	l.gcLocked()
	st, ok := l.items[key]
	if !ok {
		return true
	}
	if time.Now().Before(st.until) {
		return false
	}
	return true
}

func (l *attemptLimiter) fail(key string) {
	l.mu.Lock()
	defer l.mu.Unlock()
	st, ok := l.items[key]
	if !ok {
		st = &attemptState{}
		l.items[key] = st
	}
	st.failures++
	st.last = time.Now()
	if st.failures >= l.max {
		st.until = time.Now().Add(l.lockout)
		st.failures = 0
	}
}

func (l *attemptLimiter) succeed(key string) {
	l.mu.Lock()
	delete(l.items, key)
	l.mu.Unlock()
}

// gcLocked drops stale entries so the map cannot grow without bound.
func (l *attemptLimiter) gcLocked() {
	now := time.Now()
	if now.Sub(l.lastGC) < time.Minute {
		return
	}
	l.lastGC = now
	for k, st := range l.items {
		if now.After(st.until) && now.Sub(st.last) > l.lockout {
			delete(l.items, k)
		}
	}
}

// --- CSRF ---------------------------------------------------------------

// CSRFCookieName is the double-submit cookie the browser echoes back.
const CSRFCookieName = "vibedns_csrf"

// CSRFFieldName is the form field carrying the token.
const CSRFFieldName = "_csrf"

// CSRFHeaderName is the header carrying the token for fetch() calls.
const CSRFHeaderName = "X-CSRF-Token"

const csrfTokenTTL = 12 * time.Hour

// IssueCSRFToken mints a token bound to a user and an expiry.
//
// HTTP Basic credentials are replayed by the browser on every request,
// including cross-site form posts, so Basic auth alone does not protect
// state-changing requests. The token is signed, tied to the account, and
// double-submitted: an attacker on another origin can neither read the cookie
// nor forge the signature.
func (a *Authenticator) IssueCSRFToken(username string) string {
	expiry := time.Now().Add(csrfTokenTTL).Unix()
	payload := fmt.Sprintf("%s|%d", username, expiry)
	mac := a.csrfMAC(payload)
	return base64.RawURLEncoding.EncodeToString([]byte(payload + "|" + mac))
}

func (a *Authenticator) csrfMAC(payload string) string {
	h := hmac.New(sha256.New, a.csrfKey)
	h.Write([]byte(payload))
	return base64.RawURLEncoding.EncodeToString(h.Sum(nil))
}

// ValidateCSRFToken checks a token's signature, expiry and account binding.
func (a *Authenticator) ValidateCSRFToken(token, username string) bool {
	raw, err := base64.RawURLEncoding.DecodeString(token)
	if err != nil {
		return false
	}
	parts := strings.Split(string(raw), "|")
	if len(parts) != 3 {
		return false
	}
	payload := parts[0] + "|" + parts[1]
	if !hmac.Equal([]byte(a.csrfMAC(payload)), []byte(parts[2])) {
		return false
	}
	if parts[0] != username {
		return false
	}
	var expiry int64
	if _, err := fmt.Sscanf(parts[1], "%d", &expiry); err != nil {
		return false
	}
	return time.Now().Unix() < expiry
}

// SetCSRFCookie writes the double-submit cookie.
func SetCSRFCookie(w http.ResponseWriter, r *http.Request, token string) {
	http.SetCookie(w, &http.Cookie{
		Name:     CSRFCookieName,
		Value:    token,
		Path:     "/",
		HttpOnly: false, // the page's JavaScript reads it for fetch() calls
		Secure:   r.TLS != nil,
		SameSite: http.SameSiteLaxMode,
		MaxAge:   int(csrfTokenTTL / time.Second),
	})
}

// CheckCSRF validates a state-changing browser request.
//
// The submitted token must be present, correctly signed for this account, and
// identical to the cookie value.
func (a *Authenticator) CheckCSRF(r *http.Request, p Principal) error {
	// API tokens are not sent automatically by browsers, so a request
	// authenticated by one cannot be cross-site forged.
	if p.Kind == KindToken {
		return nil
	}
	switch r.Method {
	case http.MethodGet, http.MethodHead, http.MethodOptions:
		return nil
	}

	submitted := r.Header.Get(CSRFHeaderName)
	if submitted == "" {
		submitted = r.PostFormValue(CSRFFieldName)
	}
	if submitted == "" {
		return errors.New("this request is missing its CSRF token; reload the page and try again")
	}
	cookie, err := r.Cookie(CSRFCookieName)
	if err != nil || cookie.Value == "" {
		return errors.New("the CSRF cookie is missing; make sure cookies are enabled, then reload the page")
	}
	if subtle.ConstantTimeCompare([]byte(submitted), []byte(cookie.Value)) != 1 {
		return errors.New("the CSRF token does not match; reload the page and try again")
	}
	if !a.ValidateCSRFToken(submitted, p.Name) {
		return errors.New("the CSRF token has expired; reload the page and try again")
	}
	return nil
}

// EnsureAdmin creates the administrator account if one does not exist,
// returning the generated password when it had to invent one.
func (a *Authenticator) EnsureAdmin(ctx context.Context, username, password string) (created bool, generated string, err error) {
	if _, err := a.db.Admin(ctx); err == nil {
		return false, "", nil
	} else if !errors.Is(err, database.ErrNotFound) {
		return false, "", err
	}

	mustChange := false
	if password == "" {
		password, err = GeneratePassword(20)
		if err != nil {
			return false, "", err
		}
		generated = password
		mustChange = true
	}
	hash, err := HashPassword(password)
	if err != nil {
		return false, "", err
	}
	if err := a.db.CreateAdmin(ctx, username, hash, mustChange); err != nil {
		return false, "", err
	}
	return true, generated, nil
}

// Admin returns the administrator record.
func (a *Authenticator) Admin(ctx context.Context) (models.Admin, error) { return a.db.Admin(ctx) }