package clientapi

import (
	"crypto/rand"
	"errors"
	"net/http"

	"github.com/dariuszjaniak/warp/server/internal/httputil"
	"github.com/dariuszjaniak/warp/server/internal/userapi"
)

// registerRequest is the body of POST /register.
type registerRequest struct {
	Username string `json:"username"`
	Password string `json:"password"`

	DeviceID                 string `json:"device_id"`
	InitialDeviceDisplayName string `json:"initial_device_display_name"`

	// InhibitLogin asks for the account without a session, for a client
	// registering on someone else's behalf.
	InhibitLogin bool `json:"inhibit_login"`

	RefreshToken bool `json:"refresh_token"`

	Auth *uiaRequest `json:"auth"`
}

// registerResponse is the body of a successful POST /register.
type registerResponse struct {
	UserID string `json:"user_id"`

	// The session fields are absent when inhibit_login was set.
	AccessToken  string `json:"access_token,omitempty"`
	DeviceID     string `json:"device_id,omitempty"`
	RefreshToken string `json:"refresh_token,omitempty"`
	ExpiresInMS  int64  `json:"expires_in_ms,omitempty"`

	// HomeServer is deprecated and still read by older clients, which
	// treat its absence as a malformed response.
	HomeServer string `json:"home_server,omitempty"`
}

// handleRegister implements POST /_matrix/client/v3/register.
//
// The order of the checks matters and is not the obvious one. The spec has
// the registration flow start with a 401 listing the flows, so an empty
// body must get the challenge rather than a complaint about the missing
// password — but a username that is unusable or already taken has to be
// rejected *before* the challenge, because a client that has to complete a
// flow only to be told the name was taken cannot retry without starting
// over. Complement checks both halves of that.
func (c *ClientAPI) handleRegister(w http.ResponseWriter, r *http.Request) error {
	var req registerRequest
	if err := httputil.DecodeJSON(w, r, &req); err != nil {
		return err
	}

	// Guest registration needs the room-level restrictions that come with
	// the roomserver, so it is refused rather than half-implemented.
	if kind := r.URL.Query().Get("kind"); kind != "" && kind != "user" {
		if kind == "guest" {
			return httputil.Forbidden("Guest access is not supported by this server")
		}
		return httputil.InvalidParam("Unknown registration kind %q", kind)
	}

	if !c.settings.RegistrationEnabled() {
		return httputil.Forbidden("Registration is disabled on this server")
	}

	localpart := userapi.NormaliseLocalpart(req.Username)
	if localpart != "" {
		if err := userapi.ValidateLocalpart(localpart, c.serverName); err != nil {
			return httputil.InvalidUsername("%s", err.Error())
		}
		taken, err := c.users.AccountExists(r.Context(), localpart)
		if err != nil {
			return err
		}
		if taken {
			return httputil.UserInUse()
		}
	}

	flows := c.registrationFlows()
	sessionID, done, err := c.beginOrAdvanceUIA(w, flows, req.Auth, c.registrationStage)
	if err != nil || !done {
		return err
	}

	// Only now that the client has authenticated is the rest of the body
	// worth validating.
	if req.Password == "" {
		return httputil.MissingParam("A password is required to register an account")
	}
	if len(req.Password) > userapi.MaxPasswordLength {
		return httputil.NewError(http.StatusBadRequest, httputil.ErrCodeWeakPassword,
			"Password must be at most %d bytes", userapi.MaxPasswordLength)
	}
	if req.DeviceID != "" {
		if err := userapi.ValidateDeviceID(req.DeviceID); err != nil {
			return httputil.InvalidParam("%s", err.Error())
		}
	}

	if localpart == "" {
		// The spec lets a client leave the name to the server.
		if localpart, err = generateLocalpart(); err != nil {
			return err
		}
	}

	if _, err := c.users.CreateAccount(r.Context(), userapi.NewAccountRequest{
		Localpart: localpart,
		Password:  req.Password,
	}); err != nil {
		switch {
		case errors.Is(err, userapi.ErrUserExists):
			// Lost the race against another registration of the same name
			// between the check above and the insert.
			return httputil.UserInUse()
		case errors.Is(err, userapi.ErrInvalidLocalpart):
			return httputil.InvalidUsername("%s", err.Error())
		default:
			return err
		}
	}

	// The flow has been acted on; a replay of the same request must not
	// create a second account.
	c.uia.remove(sessionID)

	resp := registerResponse{
		UserID:     c.users.UserID(localpart),
		HomeServer: c.serverName,
	}
	if req.InhibitLogin {
		httputil.WriteJSON(w, http.StatusOK, resp)
		return nil
	}

	session, err := c.users.CreateSession(r.Context(), userapi.SessionRequest{
		Localpart:        localpart,
		DeviceID:         req.DeviceID,
		DisplayName:      req.InitialDeviceDisplayName,
		WithRefreshToken: req.RefreshToken,
		IP:               c.clientIP(r),
		UserAgent:        r.UserAgent(),
	})
	if err != nil {
		return err
	}
	resp.AccessToken = session.AccessToken
	resp.DeviceID = session.DeviceID
	resp.RefreshToken = session.RefreshToken
	resp.ExpiresInMS = session.ExpiresInMS

	httputil.WriteJSON(w, http.StatusOK, resp)
	return nil
}

// registrationFlows is what a client has to complete to register.
//
// m.login.dummy is a stage that is satisfied by being attempted. It exists
// so that the flow is never empty: the spec's registration is always
// interactive, and a server with no real requirements still has to offer
// something for the client to complete.
func (c *ClientAPI) registrationFlows() []uiaFlow {
	if c.registration.RequiresToken() {
		return []uiaFlow{{Stages: []string{stageRegistrationToken, stageDummy}}}
	}
	return []uiaFlow{{Stages: []string{stageDummy}}}
}

// registrationStage checks one stage of the registration flow.
func (c *ClientAPI) registrationStage(stage string, auth *uiaRequest) (*httputil.MatrixError, error) {
	switch stage {
	case stageDummy:
		return nil, nil
	case stageRegistrationToken:
		if auth.Token == "" {
			return httputil.MissingParam("A registration token is required"), nil
		}
		if !c.registration.TokenValid(auth.Token) {
			return httputil.Forbidden("Invalid registration token"), nil
		}
		return nil, nil
	default:
		// beginOrAdvanceUIA only calls this for stages it offered, so
		// reaching here means the flow list and this switch disagree.
		return nil, httputil.InternalError()
	}
}

// handleRegisterAvailable implements GET /register/available, the live check
// a client runs while the user is typing a name.
func (c *ClientAPI) handleRegisterAvailable(w http.ResponseWriter, r *http.Request) error {
	username := r.URL.Query().Get("username")
	if username == "" {
		return httputil.MissingParam("The username query parameter is required")
	}

	localpart := userapi.NormaliseLocalpart(username)
	if err := userapi.ValidateLocalpart(localpart, c.serverName); err != nil {
		return httputil.InvalidUsername("%s", err.Error())
	}
	taken, err := c.users.AccountExists(r.Context(), localpart)
	if err != nil {
		return err
	}
	if taken {
		return httputil.UserInUse()
	}

	httputil.WriteJSON(w, http.StatusOK, map[string]bool{"available": true})
	return nil
}

// handleRegistrationTokenValidity implements
// GET /_matrix/client/v1/register/m.login.registration_token/validity, which
// lets a client tell the user their token is wrong before they fill in the
// rest of the form.
func (c *ClientAPI) handleRegistrationTokenValidity(w http.ResponseWriter, r *http.Request) error {
	token := r.URL.Query().Get("token")
	if token == "" {
		return httputil.MissingParam("The token query parameter is required")
	}
	httputil.WriteJSON(w, http.StatusOK, map[string]bool{"valid": c.registration.TokenValid(token)})
	return nil
}

// generatedLocalpartAlphabet is the character set for a server-chosen user
// name: lowercase letters and digits, which are valid everywhere in a
// localpart and unambiguous when read out.
const generatedLocalpartAlphabet = "abcdefghijklmnopqrstuvwxyz0123456789"

// generatedLocalpartLength gives roughly 62 bits of entropy, enough that a
// server-generated name will not collide with an existing one in practice.
const generatedLocalpartLength = 12

// generateLocalpart invents a user name for a registration that did not ask
// for one.
func generateLocalpart() (string, error) {
	raw := make([]byte, generatedLocalpartLength)
	if _, err := rand.Read(raw); err != nil {
		return "", err
	}
	out := make([]byte, generatedLocalpartLength)
	for i, b := range raw {
		out[i] = generatedLocalpartAlphabet[int(b)%len(generatedLocalpartAlphabet)]
	}
	return string(out), nil
}