package web

import (
	"context"
	"crypto/hmac"
	"crypto/rand"
	"crypto/sha256"
	"encoding/base64"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"log/slog"
	"net/http"
	"net/url"
	"os"
	"strings"
	"time"

	"go-htmx-chat/internal/store"
)

type AuthMode string

const (
	AuthModeLocal AuthMode = "local"
	AuthModeOIDC  AuthMode = "oidc"
)

type AuthConfig struct {
	Mode             AuthMode
	Issuer           string
	ClientID         string
	ClientSecret     string
	RedirectURL      string
	Scopes           []string
	SessionSecret    []byte
	AdminMatchers    []string
	RoomCreatorRoles []store.Role
}

type oidcDiscovery struct {
	AuthorizationEndpoint string `json:"authorization_endpoint"`
	TokenEndpoint         string `json:"token_endpoint"`
	UserInfoEndpoint      string `json:"userinfo_endpoint"`
	Issuer                string `json:"issuer"`
}

type oauthTokenResponse struct {
	AccessToken string `json:"access_token"`
	TokenType   string `json:"token_type"`
	IDToken     string `json:"id_token"`
	ExpiresIn   int    `json:"expires_in"`
}

type userClaims struct {
	Subject           string `json:"sub"`
	PreferredUsername string `json:"preferred_username"`
	Name              string `json:"name"`
	Email             string `json:"email"`
}

type sessionData struct {
	Username string `json:"username"`
	Subject  string `json:"sub,omitempty"`
	Email    string `json:"email,omitempty"`
	Expires  int64  `json:"exp"`
}

func AuthConfigFromEnv(logger *slog.Logger) (AuthConfig, error) {
	mode := AuthMode(strings.ToLower(strings.TrimSpace(getenv("AUTH_MODE", "local"))))
	if mode != AuthModeLocal && mode != AuthModeOIDC {
		return AuthConfig{}, fmt.Errorf("unsupported AUTH_MODE %q", mode)
	}

	secret := strings.TrimSpace(os.Getenv("SESSION_SECRET"))
	var secretBytes []byte
	if secret == "" {
		secretBytes = make([]byte, 32)
		if _, err := rand.Read(secretBytes); err != nil {
			return AuthConfig{}, err
		}
		logger.Warn("SESSION_SECRET is not set; generated a temporary secret. Existing sessions will be invalid after restart")
	} else {
		decoded, err := base64.StdEncoding.DecodeString(secret)
		if err == nil && len(decoded) >= 32 {
			secretBytes = decoded
		} else {
			secretBytes = []byte(secret)
		}
		if len(secretBytes) < 32 {
			return AuthConfig{}, errors.New("SESSION_SECRET must be at least 32 bytes or a base64 encoded 32 byte value")
		}
	}

	cfg := AuthConfig{
		Mode:             mode,
		Issuer:           strings.TrimRight(strings.TrimSpace(os.Getenv("OIDC_ISSUER")), "/"),
		ClientID:         strings.TrimSpace(os.Getenv("OIDC_CLIENT_ID")),
		ClientSecret:     strings.TrimSpace(os.Getenv("OIDC_CLIENT_SECRET")),
		RedirectURL:      strings.TrimSpace(os.Getenv("OIDC_REDIRECT_URL")),
		Scopes:           splitScopes(getenv("OIDC_SCOPES", "openid profile email")),
		SessionSecret:    secretBytes,
		AdminMatchers:    splitCSV(os.Getenv("CHAT_ADMINS")),
		RoomCreatorRoles: parseRoles(getenv("ROOM_CREATE_ROLES", "admin,moderator")),
	}
	if cfg.Mode == AuthModeOIDC {
		if cfg.Issuer == "" || cfg.ClientID == "" || cfg.ClientSecret == "" || cfg.RedirectURL == "" {
			return AuthConfig{}, errors.New("AUTH_MODE=oidc requires OIDC_ISSUER, OIDC_CLIENT_ID, OIDC_CLIENT_SECRET and OIDC_REDIRECT_URL")
		}
	}
	return cfg, nil
}

func (s *Server) discoverOIDC() error {
	if s.auth.Mode != AuthModeOIDC {
		return nil
	}
	discoveryURL := s.auth.Issuer + "/.well-known/openid-configuration"
	req, err := http.NewRequest(http.MethodGet, discoveryURL, nil)
	if err != nil {
		return err
	}
	resp, err := http.DefaultClient.Do(req)
	if err != nil {
		return fmt.Errorf("OIDC discovery failed: %w", err)
	}
	defer resp.Body.Close()
	if resp.StatusCode < 200 || resp.StatusCode > 299 {
		return fmt.Errorf("OIDC discovery returned %s", resp.Status)
	}
	var d oidcDiscovery
	if err := json.NewDecoder(io.LimitReader(resp.Body, 1<<20)).Decode(&d); err != nil {
		return err
	}
	if d.AuthorizationEndpoint == "" || d.TokenEndpoint == "" || d.UserInfoEndpoint == "" {
		return errors.New("OIDC discovery response is missing required endpoints")
	}
	s.oidc = d
	return nil
}

func (s *Server) handleOIDCLogin(w http.ResponseWriter, r *http.Request) {
	if s.auth.Mode != AuthModeOIDC {
		http.Redirect(w, r, "/", http.StatusSeeOther)
		return
	}
	state, err := randomString(32)
	if err != nil {
		http.Error(w, "could not create login state", http.StatusInternalServerError)
		return
	}
	setCookie(w, &http.Cookie{Name: "oauth_state", Value: state, MaxAge: 10 * 60, Path: "/auth", HttpOnly: true, SameSite: http.SameSiteLaxMode, Secure: isSecure(r)})

	q := url.Values{}
	q.Set("response_type", "code")
	q.Set("client_id", s.auth.ClientID)
	q.Set("redirect_uri", s.auth.RedirectURL)
	q.Set("scope", strings.Join(s.auth.Scopes, " "))
	q.Set("state", state)
	authURL := s.oidc.AuthorizationEndpoint + "?" + q.Encode()
	http.Redirect(w, r, authURL, http.StatusFound)
}

func (s *Server) handleOIDCCallback(w http.ResponseWriter, r *http.Request) {
	if s.auth.Mode != AuthModeOIDC {
		http.Redirect(w, r, "/", http.StatusSeeOther)
		return
	}
	if errText := r.URL.Query().Get("error"); errText != "" {
		s.renderStatus(w, r, http.StatusUnauthorized, "login.html", map[string]any{"Title": "Login", "AuthMode": s.auth.Mode, "Error": "Login abgebrochen: " + errText})
		return
	}
	stateCookie, err := r.Cookie("oauth_state")
	if err != nil || stateCookie.Value == "" || stateCookie.Value != r.URL.Query().Get("state") {
		http.Error(w, "invalid oauth state", http.StatusBadRequest)
		return
	}
	clearCookie(w, "oauth_state", "/auth")
	code := r.URL.Query().Get("code")
	if code == "" {
		http.Error(w, "missing authorization code", http.StatusBadRequest)
		return
	}
	tok, err := s.exchangeCode(r.Context(), code)
	if err != nil {
		s.logger.Error("oauth token exchange", "error", err)
		http.Error(w, "token exchange failed", http.StatusBadGateway)
		return
	}
	claims, err := s.fetchUserInfo(r.Context(), tok.AccessToken)
	if err != nil {
		s.logger.Error("oauth userinfo", "error", err)
		http.Error(w, "could not fetch user info", http.StatusBadGateway)
		return
	}
	username := normalizeUsername(claims)
	if username == "" {
		http.Error(w, "identity provider did not return a usable username", http.StatusBadGateway)
		return
	}
	if err := s.setSession(w, r, sessionData{Username: username, Subject: claims.Subject, Email: claims.Email, Expires: time.Now().Add(30 * 24 * time.Hour).Unix()}); err != nil {
		http.Error(w, "could not create session", http.StatusInternalServerError)
		return
	}
	http.Redirect(w, r, "/rooms", http.StatusSeeOther)
}

func (s *Server) exchangeCode(ctx context.Context, code string) (oauthTokenResponse, error) {
	// Keep the public signature simple for Go 1.22 without adding external OAuth dependencies.
	values := url.Values{}
	values.Set("grant_type", "authorization_code")
	values.Set("code", code)
	values.Set("redirect_uri", s.auth.RedirectURL)
	values.Set("client_id", s.auth.ClientID)
	values.Set("client_secret", s.auth.ClientSecret)

	req, err := http.NewRequestWithContext(ctx, http.MethodPost, s.oidc.TokenEndpoint, strings.NewReader(values.Encode()))
	if err != nil {
		return oauthTokenResponse{}, err
	}
	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
	req.Header.Set("Accept", "application/json")
	resp, err := http.DefaultClient.Do(req)
	if err != nil {
		return oauthTokenResponse{}, err
	}
	defer resp.Body.Close()
	body, err := io.ReadAll(io.LimitReader(resp.Body, 1<<20))
	if err != nil {
		return oauthTokenResponse{}, err
	}
	if resp.StatusCode < 200 || resp.StatusCode > 299 {
		return oauthTokenResponse{}, fmt.Errorf("token endpoint returned %s: %s", resp.Status, string(body))
	}
	var tok oauthTokenResponse
	if err := json.Unmarshal(body, &tok); err != nil {
		return oauthTokenResponse{}, err
	}
	if tok.AccessToken == "" {
		return oauthTokenResponse{}, errors.New("token endpoint did not return access_token")
	}
	return tok, nil
}

func (s *Server) fetchUserInfo(ctx context.Context, accessToken string) (userClaims, error) {
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, s.oidc.UserInfoEndpoint, nil)
	if err != nil {
		return userClaims{}, err
	}
	req.Header.Set("Authorization", "Bearer "+accessToken)
	req.Header.Set("Accept", "application/json")
	resp, err := http.DefaultClient.Do(req)
	if err != nil {
		return userClaims{}, err
	}
	defer resp.Body.Close()
	body, err := io.ReadAll(io.LimitReader(resp.Body, 1<<20))
	if err != nil {
		return userClaims{}, err
	}
	if resp.StatusCode < 200 || resp.StatusCode > 299 {
		return userClaims{}, fmt.Errorf("userinfo endpoint returned %s: %s", resp.Status, string(body))
	}
	var claims userClaims
	if err := json.Unmarshal(body, &claims); err != nil {
		return userClaims{}, err
	}
	return claims, nil
}

func (s *Server) setSession(w http.ResponseWriter, r *http.Request, data sessionData) error {
	b, err := json.Marshal(data)
	if err != nil {
		return err
	}
	payload := base64.RawURLEncoding.EncodeToString(b)
	sig := sign(payload, s.auth.SessionSecret)
	setCookie(w, &http.Cookie{Name: "chat_session", Value: payload + "." + sig, Path: "/", MaxAge: 60 * 60 * 24 * 30, HttpOnly: true, SameSite: http.SameSiteLaxMode, Secure: isSecure(r)})
	return nil
}

func (s *Server) readSession(r *http.Request) (sessionData, bool) {
	c, err := r.Cookie("chat_session")
	if err != nil || c.Value == "" {
		return sessionData{}, false
	}
	parts := strings.Split(c.Value, ".")
	if len(parts) != 2 || !verify(parts[0], parts[1], s.auth.SessionSecret) {
		return sessionData{}, false
	}
	b, err := base64.RawURLEncoding.DecodeString(parts[0])
	if err != nil {
		return sessionData{}, false
	}
	var data sessionData
	if err := json.Unmarshal(b, &data); err != nil {
		return sessionData{}, false
	}
	if data.Expires < time.Now().Unix() || strings.TrimSpace(data.Username) == "" {
		return sessionData{}, false
	}
	return data, true
}

func sign(payload string, secret []byte) string {
	mac := hmac.New(sha256.New, secret)
	_, _ = mac.Write([]byte(payload))
	return base64.RawURLEncoding.EncodeToString(mac.Sum(nil))
}

func verify(payload, got string, secret []byte) bool {
	expected := sign(payload, secret)
	return hmac.Equal([]byte(expected), []byte(got))
}

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

func normalizeUsername(c userClaims) string {
	for _, v := range []string{c.PreferredUsername, c.Name, c.Email, c.Subject} {
		v = strings.TrimSpace(v)
		if v != "" {
			if strings.Contains(v, "@") {
				v = strings.Split(v, "@")[0]
			}
			if len(v) > 40 {
				v = v[:40]
			}
			return v
		}
	}
	return ""
}

func splitScopes(s string) []string {
	fields := strings.Fields(s)
	if len(fields) == 0 {
		return []string{"openid", "profile", "email"}
	}
	return fields
}

func getenv(key, fallback string) string {
	if v := os.Getenv(key); v != "" {
		return v
	}
	return fallback
}

func setCookie(w http.ResponseWriter, c *http.Cookie) { http.SetCookie(w, c) }

func clearCookie(w http.ResponseWriter, name, path string) {
	http.SetCookie(w, &http.Cookie{Name: name, Value: "", Path: path, MaxAge: -1, HttpOnly: true, SameSite: http.SameSiteLaxMode})
}

func isSecure(r *http.Request) bool {
	return r.TLS != nil || strings.EqualFold(r.Header.Get("X-Forwarded-Proto"), "https")
}

func splitCSV(s string) []string {
	parts := strings.Split(s, ",")
	out := make([]string, 0, len(parts))
	for _, p := range parts {
		p = strings.TrimSpace(p)
		if p != "" {
			out = append(out, p)
		}
	}
	return out
}

func parseRoles(s string) []store.Role {
	parts := strings.Split(s, ",")
	out := make([]store.Role, 0, len(parts))
	for _, p := range parts {
		switch store.Role(strings.ToLower(strings.TrimSpace(p))) {
		case store.RoleAdmin:
			out = append(out, store.RoleAdmin)
		case store.RoleModerator:
			out = append(out, store.RoleModerator)
		case store.RoleUser:
			out = append(out, store.RoleUser)
		}
	}
	if len(out) == 0 {
		return []store.Role{store.RoleAdmin}
	}
	return out
}