package app

import (
	"context"
	"crypto/pbkdf2"
	"crypto/rand"
	"crypto/sha256"
	"crypto/subtle"
	"database/sql"
	"encoding/base64"
	"encoding/hex"
	"fmt"
	"net/http"
	"strconv"
	"strings"
	"time"
)

const (
	passwordIterations = 310_000
	passwordSaltBytes  = 16
	passwordKeyBytes   = 32
	sessionCookie      = "pocketwatch_session"
)

// This is deliberately public and never authenticates an account. It only keeps
// failed-login password work roughly constant when a username does not exist.
const dummyPasswordHash = "pbkdf2-sha256$310000$cG9ja2V0d2F0Y2gtZHVtbXk$y2kfBbJVOW/xpojEt9AAGQfhF+Ul6iGKDKH09PSY7IY"

type session struct {
	User User
	CSRF string
}

type contextKey int

const sessionKey contextKey = 1

func hashPassword(password string) (string, error) {
	if len(password) < 10 {
		return "", fmt.Errorf("password must have at least 10 characters")
	}
	salt := make([]byte, passwordSaltBytes)
	if _, err := rand.Read(salt); err != nil {
		return "", err
	}
	key := pbkdf2SHA256([]byte(password), salt, passwordIterations, passwordKeyBytes)
	return fmt.Sprintf("pbkdf2-sha256$%d$%s$%s", passwordIterations, base64.RawStdEncoding.EncodeToString(salt), base64.RawStdEncoding.EncodeToString(key)), nil
}

func verifyPassword(encoded, password string) bool {
	parts := strings.Split(encoded, "$")
	if len(parts) != 4 || parts[0] != "pbkdf2-sha256" {
		return false
	}
	iter, err := strconv.Atoi(parts[1])
	if err != nil || iter < 100_000 || iter > 2_000_000 {
		return false
	}
	salt, err := base64.RawStdEncoding.DecodeString(parts[2])
	if err != nil {
		return false
	}
	want, err := base64.RawStdEncoding.DecodeString(parts[3])
	if err != nil || len(want) == 0 {
		return false
	}
	got := pbkdf2SHA256([]byte(password), salt, iter, len(want))
	return subtle.ConstantTimeCompare(got, want) == 1
}

// pbkdf2SHA256 wraps Go 1.26's standard-library PBKDF2 implementation.
func pbkdf2SHA256(password, salt []byte, iterations, keyLen int) []byte {
	key, err := pbkdf2.Key(sha256.New, string(password), salt, iterations, keyLen)
	if err != nil {
		panic(err) // callers use fixed, validated parameters
	}
	return key
}

func randomToken(n int) (string, error) {
	b := make([]byte, n)
	if _, err := rand.Read(b); err != nil {
		return "", err
	}
	return base64.RawURLEncoding.EncodeToString(b), nil
}

func tokenDigest(token string) []byte {
	s := sha256.Sum256([]byte(token))
	return s[:]
}

func newID() string {
	b := make([]byte, 16)
	if _, err := rand.Read(b); err != nil {
		panic(err)
	}
	b[6] = (b[6] & 0x0f) | 0x40
	b[8] = (b[8] & 0x3f) | 0x80
	h := hex.EncodeToString(b)
	return h[:8] + "-" + h[8:12] + "-" + h[12:16] + "-" + h[16:20] + "-" + h[20:]
}

func (a *App) createSession(ctx context.Context, userID string) (token, csrf string, expires time.Time, err error) {
	token, err = randomToken(32)
	if err != nil {
		return
	}
	csrf, err = randomToken(24)
	if err != nil {
		return
	}
	now := time.Now()
	expires = now.Add(a.cfg.SessionTTL)
	_, err = a.store.db.ExecContext(ctx, `INSERT INTO sessions(token_hash,user_id,csrf_token,created_at_ms,expires_at_ms) VALUES(?,?,?,?,?)`, tokenDigest(token), userID, csrf, now.UnixMilli(), expires.UnixMilli())
	return
}

func (a *App) sessionFromRequest(r *http.Request) (*session, error) {
	c, err := r.Cookie(sessionCookie)
	if err != nil || c.Value == "" {
		return nil, sql.ErrNoRows
	}
	var userID, csrf string
	var expires int64
	err = a.store.db.QueryRowContext(r.Context(), `SELECT user_id,csrf_token,expires_at_ms FROM sessions WHERE token_hash=?`, tokenDigest(c.Value)).Scan(&userID, &csrf, &expires)
	if err != nil {
		return nil, err
	}
	if expires <= time.Now().UnixMilli() {
		_, _ = a.store.db.ExecContext(r.Context(), `DELETE FROM sessions WHERE token_hash=?`, tokenDigest(c.Value))
		return nil, sql.ErrNoRows
	}
	u, err := a.store.userByID(r.Context(), userID)
	if err != nil || !u.Active {
		return nil, sql.ErrNoRows
	}
	return &session{User: u, CSRF: csrf}, nil
}

func (a *App) withSession(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		s, err := a.sessionFromRequest(r)
		if err != nil {
			if strings.HasPrefix(r.URL.Path, "/api/") {
				jsonError(w, http.StatusUnauthorized, "not_authenticated", "Bitte anmelden.")
				return
			}
			http.Redirect(w, r, "/login", http.StatusSeeOther)
			return
		}
		next.ServeHTTP(w, r.WithContext(context.WithValue(r.Context(), sessionKey, s)))
	})
}

func sessionOf(r *http.Request) *session {
	x, _ := r.Context().Value(sessionKey).(*session)
	return x
}

func requireCSRF(w http.ResponseWriter, r *http.Request) bool {
	if r.Method == http.MethodGet || r.Method == http.MethodHead || r.Method == http.MethodOptions {
		return true
	}
	s := sessionOf(r)
	if s == nil || subtle.ConstantTimeCompare([]byte(r.Header.Get("X-CSRF-Token")), []byte(s.CSRF)) != 1 {
		jsonError(w, http.StatusForbidden, "csrf", "Ungültiges CSRF-Token.")
		return false
	}
	return true
}

func requireAdmin(w http.ResponseWriter, r *http.Request) bool {
	s := sessionOf(r)
	if s == nil || s.User.Role != "admin" {
		jsonError(w, http.StatusForbidden, "forbidden", "Admin-Rechte erforderlich.")
		return false
	}
	return true
}

func (a *App) deleteCurrentSession(w http.ResponseWriter, r *http.Request) {
	if c, err := r.Cookie(sessionCookie); err == nil {
		_, _ = a.store.db.ExecContext(r.Context(), `DELETE FROM sessions WHERE token_hash=?`, tokenDigest(c.Value))
	}
	a.clearSessionCookie(w)
}

func (a *App) setSessionCookie(w http.ResponseWriter, token string, expires time.Time) {
	http.SetCookie(w, &http.Cookie{Name: sessionCookie, Value: token, Path: "/", HttpOnly: true, Secure: a.cfg.CookieSecure, SameSite: http.SameSiteLaxMode, Expires: expires, MaxAge: int(time.Until(expires).Seconds())})
}

func (a *App) clearSessionCookie(w http.ResponseWriter) {
	http.SetCookie(w, &http.Cookie{Name: sessionCookie, Value: "", Path: "/", HttpOnly: true, Secure: a.cfg.CookieSecure, SameSite: http.SameSiteLaxMode, MaxAge: -1, Expires: time.Unix(1, 0)})
}

func isUniqueConstraint(err error) bool {
	if err == nil {
		return false
	}
	s := strings.ToLower(err.Error())
	return strings.Contains(s, "unique constraint") || strings.Contains(s, "constraint failed")
}