package web

import (
	"crypto/hmac"
	"crypto/sha256"
	"crypto/subtle"
	"encoding/base64"
	"encoding/hex"
	"encoding/json"
	"errors"
	"log"
	"net/http"
	"regexp"
	"strings"
	"time"

	"git.veillax.com/Veillax/basking/internal/config"
	"git.veillax.com/Veillax/basking/internal/oidc"
)

const (
	sessionCookie = "basking_session"
	flowCookie    = "basking_signin"
	sessionTTL    = 14 * 24 * time.Hour
	flowTTL       = 10 * time.Minute
)

// A cookie is "payload.signature": the payload is JSON, the signature is an HMAC with the secret from
// the settings file. Nothing is kept on the server, so a restart does not sign anyone out.
func seal(c config.Config, kind string, v any) string {
	raw, _ := json.Marshal(v)
	payload := base64.RawURLEncoding.EncodeToString(raw)
	return payload + "." + mac(c, kind, payload)
}

func mac(c config.Config, kind, payload string) string {
	m := hmac.New(sha256.New, []byte(c.Secret))
	m.Write([]byte(kind + "|" + payload))
	return hex.EncodeToString(m.Sum(nil))
}

func unseal(c config.Config, kind, value string, v any) bool {
	payload, sig, ok := strings.Cut(value, ".")
	if !ok || !hmac.Equal([]byte(sig), []byte(mac(c, kind, payload))) {
		return false
	}
	raw, err := base64.RawURLEncoding.DecodeString(payload)
	return err == nil && json.Unmarshal(raw, v) == nil
}

type sessionData struct {
	Sub   string `json:"s"`
	Name  string `json:"n"`
	Email string `json:"e"`
	User  string `json:"u"`
	Exp   int64  `json:"x"`
}

type flowData struct {
	State    string `json:"s"`
	Nonce    string `json:"n"`
	Verifier string `json:"v"`
	Next     string `json:"t"`
	Exp      int64  `json:"x"`
}

// session reads the sign-in cookie. The allowed list is checked on every request, so taking someone
// off it takes effect at once.
func (s *Server) session(c config.Config, r *http.Request) (sessionData, bool) {
	var d sessionData
	ck, err := r.Cookie(sessionCookie)
	if err != nil || !unseal(c, "session", ck.Value, &d) || d.Sub == "" || time.Now().Unix() > d.Exp {
		return sessionData{}, false
	}
	return d, allowed(c, d)
}

func allowed(c config.Config, d sessionData) bool {
	if len(c.Allowed) == 0 {
		return true
	}
	for _, a := range c.Allowed {
		for _, who := range []string{d.Email, d.User, d.Sub} {
			if who != "" && strings.EqualFold(strings.TrimSpace(a), who) {
				return true
			}
		}
	}
	return false
}

func setCookie(w http.ResponseWriter, c config.Config, name, value, path string, ttl time.Duration) {
	ck := &http.Cookie{Name: name, Value: value, Path: path, HttpOnly: true, Secure: c.Secure(), SameSite: http.SameSiteLaxMode}
	if value == "" {
		ck.MaxAge = -1
	} else {
		ck.MaxAge = int(ttl.Seconds())
	}
	http.SetCookie(w, ck)
}

// Where to go after signing in: only a place inside Basking, such as #/p/abc.
var nextRe = regexp.MustCompile(`^#/[A-Za-z0-9/_.~%-]{0,300}$`)

func (s *Server) authLogin(w http.ResponseWriter, r *http.Request) {
	c := s.cfg.Get()
	next := r.URL.Query().Get("next")
	if !nextRe.MatchString(next) {
		next = ""
	}
	f := flowData{State: oidc.Random(18), Nonce: oidc.Random(18), Verifier: oidc.Random(40), Next: next, Exp: time.Now().Add(flowTTL).Unix()}
	to, err := s.oidc.AuthURL(r.Context(), f.State, f.Nonce, oidc.Challenge(f.Verifier))
	if err != nil {
		log.Printf("sign-in: %v", err)
		http.Redirect(w, r, "/?signin=unavailable", http.StatusSeeOther)
		return
	}
	setCookie(w, c, flowCookie, seal(c, "flow", f), "/auth/", flowTTL)
	http.Redirect(w, r, to, http.StatusSeeOther)
}

func (s *Server) authCallback(w http.ResponseWriter, r *http.Request) {
	c := s.cfg.Get()
	back := func(why string) { http.Redirect(w, r, "/?signin="+why, http.StatusSeeOther) }

	var f flowData
	ck, err := r.Cookie(flowCookie)
	setCookie(w, c, flowCookie, "", "/auth/", 0) // a sign-in attempt is used once
	if err != nil || !unseal(c, "flow", ck.Value, &f) || time.Now().Unix() > f.Exp {
		back("expired")
		return
	}
	q := r.URL.Query()
	if e := q.Get("error"); e != "" {
		log.Printf("sign-in: the provider answered %q: %s", e, q.Get("error_description"))
		back("denied")
		return
	}
	if subtle.ConstantTimeCompare([]byte(q.Get("state")), []byte(f.State)) != 1 || q.Get("code") == "" {
		back("failed")
		return
	}
	id, err := s.oidc.Exchange(r.Context(), q.Get("code"), f.Verifier, f.Nonce)
	if err != nil {
		log.Printf("sign-in: %v", err)
		if errors.Is(err, oidc.ErrUnavailable) {
			back("unavailable")
		} else {
			back("failed")
		}
		return
	}
	d := sessionData{Sub: id.Sub, Name: firstOf(id.Name, id.Username, id.Email, "You"), Email: id.Email, User: id.Username,
		Exp: time.Now().Add(sessionTTL).Unix()}
	if !allowed(c, d) {
		log.Printf("sign-in: %s (%s) is not on the allowed list", d.Name, firstOf(d.Email, d.User, d.Sub))
		back("denied")
		return
	}
	setCookie(w, c, sessionCookie, seal(c, "session", d), "/", sessionTTL)
	http.Redirect(w, r, "/"+f.Next, http.StatusSeeOther)
}

func firstOf(s ...string) string {
	for _, x := range s {
		if strings.TrimSpace(x) != "" {
			return x
		}
	}
	return ""
}

func (s *Server) logout(w http.ResponseWriter, r *http.Request, p principal) {
	setCookie(w, s.cfg.Get(), sessionCookie, "", "/", 0)
	writeJSON(w, map[string]bool{"ok": true})
}

func (s *Server) me(w http.ResponseWriter, r *http.Request, p principal) {
	writeJSON(w, map[string]any{"name": p.Name, "email": p.Email, "version": s.version})
}