package server

import (
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"net/http"
	"sort"
	"strings"
	"time"

	"cc.tiredsysadmin.keeporg/internal/store"
	"cc.tiredsysadmin.keeporg/internal/totp"
)

// A person's own account: the sessions it holds on each device, and its
// second factor. As with administration, every change is a function over the
// state taking the caller, and the JSON handlers below and the settings page
// in webaccount.go are both thin wrappers over them.

var (
	// errNoSuchSession is a session that does not exist or is not the
	// caller's; the two are not told apart.
	errNoSuchSession = errors.New("no such session")
	// errNoPassword is an account that signs in only through single
	// sign-on. Its second factor belongs at the identity provider, and there
	// is no password to change.
	errNoPassword = errors.New("this account signs in through single sign-on and has no password here")
	// errReauth asks for the password and, once enrolled, the one-time code,
	// before a change to how the account signs in. A live session is what
	// an unlocked laptop leaves lying around.
	errReauth = errors.New("confirm with your password and one-time code")
)

// sessionID is a session's public name: a one-way digest of its token, so a
// page or a response can point at a session without carrying the bearer
// token that would sign in as it.
func sessionID(token string) string {
	sum := sha256.Sum256([]byte(token))
	return hex.EncodeToString(sum[:8])
}

// sessionView is one signed-in device as its owner sees it.
type sessionView struct {
	ID         string    `json:"id"`
	Label      string    `json:"label"`
	Browser    bool      `json:"browser"`
	Current    bool      `json:"current"`
	CreatedAt  time.Time `json:"created_at"`
	LastSeenAt time.Time `json:"last_seen_at"`
	ExpiresAt  time.Time `json:"expires_at"`
}

// sessionsOf lists a user's live sessions, the caller's own first and the
// rest by most recent use.
func sessionsOf(st *store.State, userID, current string) []sessionView {
	out := []sessionView{}
	now := time.Now()
	for token, sess := range st.Sessions {
		if sess.UserID != userID || now.After(sess.ExpiresAt) {
			continue
		}
		seen := sess.LastSeenAt
		if seen.IsZero() {
			seen = sess.CreatedAt
		}
		out = append(out, sessionView{
			ID: sessionID(token), Label: sess.Label, Browser: sess.IsBrowser(),
			Current: token == current, CreatedAt: sess.CreatedAt,
			LastSeenAt: seen, ExpiresAt: sess.ExpiresAt,
		})
	}
	sort.Slice(out, func(i, j int) bool {
		if out[i].Current != out[j].Current {
			return out[i].Current
		}
		return out[i].LastSeenAt.After(out[j].LastSeenAt)
	})
	return out
}

// revokeSession signs one of the caller's sessions out. The device holding
// it is refused from its next request on.
func revokeSession(st *store.State, userID, id string) error {
	for token, sess := range st.Sessions {
		if sess.UserID == userID && sessionID(token) == id {
			delete(st.Sessions, token)
			return nil
		}
	}
	return errNoSuchSession
}

// revokeOtherSessions signs out every session of the caller but keep, and
// says how many went.
func revokeOtherSessions(st *store.State, userID, keep string) int {
	n := 0
	for token, sess := range st.Sessions {
		if sess.UserID == userID && token != keep {
			delete(st.Sessions, token)
			n++
		}
	}
	return n
}

// beginTOTP starts enrolling a second factor and returns its secret. Until
// confirmTOTP sees a code from it, the account signs in as before. The
// caller has re-proved the password (and any current code): turning a
// factor on from a stolen session would lock the owner out, and replacing
// one disarms the old.
func beginTOTP(st *store.State, userID string) (string, error) {
	u := st.Users[userID]
	if u == nil {
		return "", errNoSuchSession
	}
	if u.PassHash == "" {
		return "", errNoPassword
	}
	u.TOTPSecret, u.TOTPConfirmed, u.TOTPLastStep = totp.NewSecret(), false, 0
	return u.TOTPSecret, nil
}

// confirmTOTP turns the second factor on, once a code from it checks out.
// That code is spent: it cannot also sign in.
func confirmTOTP(st *store.State, userID, code string) error {
	u := st.Users[userID]
	if u == nil || u.TOTPSecret == "" {
		return errors.New("start setting up a second factor first")
	}
	if u.TOTPConfirmed {
		return errors.New("the second factor is already on")
	}
	step, ok := totp.Match(u.TOTPSecret, code, time.Now())
	if !ok {
		return errTOTPInvalid
	}
	u.TOTPConfirmed, u.TOTPLastStep = true, step
	return nil
}

// cancelTOTP abandons an enrolment that was never confirmed.
func cancelTOTP(st *store.State, userID string) {
	if u := st.Users[userID]; u != nil && !u.TOTPConfirmed {
		u.TOTPSecret = ""
	}
}

// disableTOTP turns the second factor off. The caller has re-proved the
// password and a current code.
func disableTOTP(st *store.State, userID string) error {
	u := st.Users[userID]
	if u == nil {
		return errNoSuchSession
	}
	if !u.TOTPConfirmed {
		return errors.New("no second factor is on")
	}
	u.TOTPSecret, u.TOTPConfirmed, u.TOTPLastStep = "", false, 0
	return nil
}

// failAccount maps an account error onto a response. Re-authentication
// failures are 403, not 401: a client that drops its token on 401 must not
// sign itself out over a mistyped password.
func failAccount(w http.ResponseWriter, err error) {
	if writeThrottled(w, err) {
		return
	}
	switch {
	case errors.Is(err, errNoSuchSession):
		fail(w, http.StatusNotFound, "not_found", err.Error())
	case errors.Is(err, errReauth), errors.Is(err, errTOTPRequired):
		fail(w, http.StatusForbidden, "reauth_required", errReauth.Error())
	case errors.Is(err, errTOTPInvalid):
		fail(w, http.StatusForbidden, "totp_invalid", "the one-time code was rejected")
	case errors.Is(err, errBadCredentials):
		fail(w, http.StatusForbidden, "bad_credentials", "the password or one-time code is wrong")
	case errors.Is(err, errNoPassword):
		fail(w, http.StatusConflict, "no_password", err.Error())
	default:
		fail(w, http.StatusBadRequest, "bad_request", err.Error())
	}
}

// ---- API ----

func (s *Server) handleListSessions(w http.ResponseWriter, r *http.Request, u *store.User) {
	var out []sessionView
	s.st.Read(func(st *store.State) { out = sessionsOf(st, u.ID, bearer(r)) })
	writeJSON(w, http.StatusOK, map[string]any{"sessions": out})
}

func (s *Server) handleRevokeSession(w http.ResponseWriter, r *http.Request, u *store.User) {
	if err := s.st.Update(func(st *store.State) error {
		return revokeSession(st, u.ID, r.PathValue("id"))
	}); err != nil {
		failAccount(w, err)
		return
	}
	writeJSON(w, http.StatusOK, map[string]string{"status": "revoked"})
}

func (s *Server) handleRevokeOtherSessions(w http.ResponseWriter, r *http.Request, u *store.User) {
	var n int
	if err := s.st.Update(func(st *store.State) error {
		n = revokeOtherSessions(st, u.ID, bearer(r))
		return nil
	}); err != nil {
		fail(w, http.StatusInternalServerError, "internal", err.Error())
		return
	}
	writeJSON(w, http.StatusOK, map[string]int{"revoked": n})
}

// reauthRequest is the body of a change that needs re-authentication.
type reauthRequest struct {
	Password string `json:"password"`
	TOTP     string `json:"totp"`
}

func (s *Server) handleTOTPEnroll(w http.ResponseWriter, r *http.Request, u *store.User) {
	var req reauthRequest
	if r.ContentLength > 0 && !readJSON(w, r, &req) {
		return
	}
	var sec string
	if err := s.reconfirm(r, u, req.Password, req.TOTP, func(st *store.State, u *store.User) error {
		var err error
		sec, err = beginTOTP(st, u.ID)
		return err
	}); err != nil {
		failAccount(w, err)
		return
	}
	writeJSON(w, http.StatusOK, map[string]string{
		"secret": sec,
		"uri":    totp.URI("KeepOrg", u.Username, sec),
	})
}

func (s *Server) handleTOTPConfirm(w http.ResponseWriter, r *http.Request, u *store.User) {
	var req struct {
		Code string `json:"code"`
	}
	if !readJSON(w, r, &req) {
		return
	}
	if err := s.st.Update(func(st *store.State) error {
		return confirmTOTP(st, u.ID, req.Code)
	}); err != nil {
		if errors.Is(err, errTOTPInvalid) {
			fail(w, http.StatusBadRequest, "totp_invalid", "code rejected")
			return
		}
		failAccount(w, err)
		return
	}
	writeJSON(w, http.StatusOK, map[string]bool{"totp_enabled": true})
}

func (s *Server) handleTOTPDisable(w http.ResponseWriter, r *http.Request, u *store.User) {
	var req reauthRequest
	if !readJSON(w, r, &req) {
		return
	}
	if err := s.reconfirm(r, u, req.Password, req.TOTP, func(st *store.State, u *store.User) error {
		return disableTOTP(st, u.ID)
	}); err != nil {
		failAccount(w, err)
		return
	}
	writeJSON(w, http.StatusOK, map[string]bool{"totp_enabled": false})
}

// browserLabel names a browser session from its User-Agent, as "Firefox on
// macOS". It is a label for a person to recognise, not a fingerprint, so a
// handful of substrings is enough.
func browserLabel(ua string) string {
	browser := "Browser"
	for _, b := range []struct{ token, name string }{
		{"Edg/", "Edge"}, {"OPR/", "Opera"}, {"Firefox/", "Firefox"},
		{"Chrome/", "Chrome"}, {"Safari/", "Safari"},
	} {
		if strings.Contains(ua, b.token) {
			browser = b.name
			break
		}
	}
	for _, o := range []struct{ token, name string }{
		{"Android", "Android"}, {"iPhone", "iOS"}, {"iPad", "iPadOS"},
		{"CrOS", "ChromeOS"}, {"Mac OS X", "macOS"}, {"Windows", "Windows"},
		{"Linux", "Linux"}, {"BSD", "BSD"},
	} {
		if strings.Contains(ua, o.token) {
			return browser + " on " + o.name
		}
	}
	return browser
}