package auth

import (
	"context"
	"crypto/ed25519"
	"encoding/json"
	"fmt"

	"github.com/dariuszjaniak/warp/server/internal/matrix/event"
	"github.com/dariuszjaniak/warp/server/internal/matrix/id"
	"github.com/dariuszjaniak/warp/server/internal/matrix/signing"
)

// memberContent is the part of an m.room.member event's content the rules
// read. The pointer and json.RawMessage fields are there so that "absent"
// can be told from "empty", which several of the rules turn on.
type memberContent struct {
	Membership                   string            `json:"membership"`
	JoinAuthorisedViaUsersServer string            `json:"join_authorised_via_users_server"`
	ThirdPartyInvite             *thirdPartyInvite `json:"third_party_invite"`
}

type thirdPartyInvite struct {
	Signed json.RawMessage `json:"signed"`
}

// checkMember applies rule 5, the membership rules.
//
// These are the only rules a user who is not in the room may satisfy, so
// they carry the whole of who can get in: invitations, public rooms,
// knocking, restricted rooms, and who may throw whom out.
func checkMember(ctx context.Context, e *event.Event, st State, create *event.Event, pl *PowerLevels, verifier Verifier) error {
	rules := e.Rules()

	// Rule 5.1.
	target, ok := e.StateKey()
	if !ok {
		return reject("an m.room.member event must have a state_key")
	}
	var content memberContent
	if err := json.Unmarshal(e.Content(), &content); err != nil {
		return reject("m.room.member content: %v", err)
	}
	if content.Membership == "" {
		return reject("an m.room.member event must have a membership")
	}

	// Rule 5.2: a join authorised by another user must be signed by that
	// user's server, not merely by the joining user's. Without it, anybody
	// could claim to have been let into a restricted room.
	if rules.RestrictedJoinRule && content.JoinAuthorisedViaUsersServer != "" {
		if err := checkJoinAuthorisation(ctx, e, content.JoinAuthorisedViaUsersServer, verifier); err != nil {
			return err
		}
	}

	senderMembership := st.Membership(e.Sender())
	targetMembership := st.Membership(target)

	switch content.Membership {
	case event.MembershipJoin:
		return checkJoin(e, st, create, pl, target, targetMembership, content)
	case event.MembershipInvite:
		return checkInvite(e, st, pl, target, senderMembership, targetMembership, content)
	case event.MembershipLeave:
		return checkLeave(e, pl, target, senderMembership, targetMembership)
	case event.MembershipBan:
		return checkBan(e, pl, target, senderMembership)
	case event.MembershipKnock:
		if !rules.KnockJoinRule {
			return reject("room version %s does not allow knocking", rules.Version)
		}
		return checkKnock(e, st, target, senderMembership)
	default:
		// Rule 5.8.
		return reject("unknown membership %q", content.Membership)
	}
}

// checkJoin applies rule 5.3.
func checkJoin(e *event.Event, st State, create *event.Event, pl *PowerLevels, target, targetMembership string, content memberContent) error {
	rules := e.Rules()

	// Rule 5.3.1: the creator's own join, the second event in the room.
	// There is no state yet to authorise it against, so it is recognised
	// by its shape: it follows the create event and nothing else.
	prev := e.PrevEventIDs()
	if len(prev) == 1 && prev[0] == create.EventID() && target == create.Sender() {
		return nil
	}

	// Rule 5.3.2: you join yourself. Being dragged into a room is an
	// invite, which the invited user then accepts with a join of their own.
	if e.Sender() != target {
		return reject("the sender may not join %q on their behalf", target)
	}
	// Rule 5.3.3.
	if targetMembership == event.MembershipBan {
		return reject("the user is banned from the room")
	}

	joinRule := st.JoinRule()
	switch joinRule {
	case JoinRuleInvite, JoinRuleKnock:
		// Rule 5.3.4. A knock does not admit anybody by itself: it asks,
		// and somebody with the power to invite still has to answer.
		if targetMembership == event.MembershipInvite || targetMembership == event.MembershipJoin {
			return nil
		}
		return reject("the room's join rule is %q and the user has not been invited (membership is %q)",
			joinRule, targetMembership)

	case JoinRuleRestricted, JoinRuleKnockRestricted:
		if joinRule == JoinRuleRestricted && !rules.RestrictedJoinRule {
			return reject("room version %s does not have the restricted join rule", rules.Version)
		}
		if joinRule == JoinRuleKnockRestricted && !rules.KnockRestrictedJoinRule {
			return reject("room version %s does not have the knock_restricted join rule", rules.Version)
		}
		// Rule 5.3.5.1: an existing invite or join still works, so a
		// restricted room is never *harder* to enter than an invite room.
		if targetMembership == event.MembershipInvite || targetMembership == event.MembershipJoin {
			return nil
		}
		// Rule 5.3.5.2. The authorising user has to be in the room and
		// able to invite, because what they are really doing is vouching
		// that the joining user is a member of one of the allowed rooms —
		// something this server may be unable to see for itself.
		authoriser := content.JoinAuthorisedViaUsersServer
		if authoriser == "" {
			return reject("the room's join rule is %q and the join is not authorised by any user", joinRule)
		}
		if m := st.Membership(authoriser); m != event.MembershipJoin {
			return reject("the authorising user %q is not joined to the room (membership is %q)", authoriser, m)
		}
		if level := pl.UserLevel(authoriser); level < pl.Invite {
			return reject("the authorising user %q has power level %s, below the invite level %d",
				authoriser, levelString(level), pl.Invite)
		}
		// Rule 5.3.5.3.
		return nil

	case JoinRulePublic:
		// Rule 5.3.6.
		return nil

	default:
		// Rule 5.3.7. "private" lands here, and so does any join rule
		// from a future room version: a rule we cannot evaluate must keep
		// people out rather than let them in.
		return reject("the room's join rule %q does not allow joining", joinRule)
	}
}

// checkInvite applies rule 5.4.
func checkInvite(e *event.Event, st State, pl *PowerLevels, target, senderMembership, targetMembership string, content memberContent) error {
	// Rule 5.4.1: a third-party invite is an invitation issued to an email
	// address or phone number before the server knew which user it was.
	// The identity server's signature stands in for the inviter's power.
	if content.ThirdPartyInvite != nil {
		return checkThirdPartyInvite(e, st, target, targetMembership, content)
	}

	// Rule 5.4.2.
	if senderMembership != event.MembershipJoin {
		return reject("the sender is not joined to the room (membership is %q)", senderMembership)
	}
	// Rule 5.4.3.
	if targetMembership == event.MembershipJoin || targetMembership == event.MembershipBan {
		return reject("the user is already %q in the room", targetMembership)
	}
	// Rule 5.4.4.
	if level := pl.UserLevel(e.Sender()); level < pl.Invite {
		return reject("the sender's power level %s is below the invite level %d", levelString(level), pl.Invite)
	}
	// Rule 5.4.5 is the negation of the above.
	return nil
}

// checkThirdPartyInvite applies rule 5.4.1.
func checkThirdPartyInvite(e *event.Event, st State, target, targetMembership string, content memberContent) error {
	// Rule 5.4.1.1.
	if targetMembership == event.MembershipBan {
		return reject("the user is banned from the room")
	}
	// Rule 5.4.1.2.
	if len(content.ThirdPartyInvite.Signed) == 0 {
		return reject("the third_party_invite has no signed property")
	}
	var signed struct {
		MXID  string `json:"mxid"`
		Token string `json:"token"`
	}
	if err := json.Unmarshal(content.ThirdPartyInvite.Signed, &signed); err != nil {
		return reject("the third_party_invite signed property: %v", err)
	}
	// Rule 5.4.1.3.
	if signed.MXID == "" || signed.Token == "" {
		return reject("the third_party_invite signed property needs both mxid and token")
	}
	// Rule 5.4.1.4.
	if signed.MXID != target {
		return reject("the third_party_invite names %q but the event invites %q", signed.MXID, target)
	}
	// Rule 5.4.1.5.
	inviteEvent := st.ThirdPartyInvite(signed.Token)
	if inviteEvent == nil {
		return reject("there is no m.room.third_party_invite in the room state for token %q", signed.Token)
	}
	// Rule 5.4.1.6: the same user who issued the invitation must be the
	// one redeeming it.
	if inviteEvent.Sender() != e.Sender() {
		return reject("the third-party invite was issued by %q, not by %q", inviteEvent.Sender(), e.Sender())
	}
	// Rules 5.4.1.7 and 5.4.1.8.
	if !VerifyThirdPartyInviteSignature(content.ThirdPartyInvite.Signed, inviteEvent) {
		return reject("no signature on the third-party invite matches a public key in %s", inviteEvent.EventID())
	}
	return nil
}

// checkLeave applies rule 5.5, which covers both leaving and being kicked.
func checkLeave(e *event.Event, pl *PowerLevels, target, senderMembership, targetMembership string) error {
	rules := e.Rules()

	// Rule 5.5.1: leaving on your own account, which also covers turning
	// down an invitation and withdrawing a knock.
	if e.Sender() == target {
		switch targetMembership {
		case event.MembershipInvite, event.MembershipJoin:
			return nil
		case event.MembershipKnock:
			if rules.KnockJoinRule {
				return nil
			}
		}
		return reject("the user cannot leave from membership %q", targetMembership)
	}

	// Rule 5.5.2.
	if senderMembership != event.MembershipJoin {
		return reject("the sender is not joined to the room (membership is %q)", senderMembership)
	}

	senderLevel := pl.UserLevel(e.Sender())

	// Rule 5.5.3: unbanning is a leave over a ban, and it takes the power
	// to have banned in the first place.
	if targetMembership == event.MembershipBan && senderLevel < pl.Ban {
		return reject("the sender's power level %s is below the ban level %d needed to unban",
			levelString(senderLevel), pl.Ban)
	}
	// Rule 5.5.4.
	if senderLevel < pl.Kick {
		return reject("the sender's power level %s is below the kick level %d", levelString(senderLevel), pl.Kick)
	}
	if targetLevel := pl.UserLevel(target); targetLevel >= senderLevel {
		return reject("the sender's power level %s does not exceed %q's power level %s",
			levelString(senderLevel), target, levelString(targetLevel))
	}
	// Rule 5.5.5 is the negation of the above.
	return nil
}

// checkBan applies rule 5.6.
func checkBan(e *event.Event, pl *PowerLevels, target, senderMembership string) error {
	// Rule 5.6.1.
	if senderMembership != event.MembershipJoin {
		return reject("the sender is not joined to the room (membership is %q)", senderMembership)
	}
	// Rule 5.6.2.
	senderLevel := pl.UserLevel(e.Sender())
	if senderLevel < pl.Ban {
		return reject("the sender's power level %s is below the ban level %d", levelString(senderLevel), pl.Ban)
	}
	if targetLevel := pl.UserLevel(target); targetLevel >= senderLevel {
		return reject("the sender's power level %s does not exceed %q's power level %s",
			levelString(senderLevel), target, levelString(targetLevel))
	}
	// Rule 5.6.3 is the negation of the above.
	return nil
}

// checkKnock applies rule 5.7.
func checkKnock(e *event.Event, st State, target, senderMembership string) error {
	// Rule 5.7.1.
	joinRule := st.JoinRule()
	if joinRule != JoinRuleKnock && joinRule != JoinRuleKnockRestricted {
		return reject("the room's join rule is %q, so it cannot be knocked on", joinRule)
	}
	// Rule 5.7.2.
	if e.Sender() != target {
		return reject("the sender may not knock on behalf of %q", target)
	}
	// Rule 5.7.3: knocking is allowed from anything but banned, invited or
	// joined — there is no point knocking on a door that is already open.
	switch senderMembership {
	case event.MembershipBan, event.MembershipInvite, event.MembershipJoin:
		// Rule 5.7.4.
		return reject("the user cannot knock while their membership is %q", senderMembership)
	default:
		return nil
	}
}

// checkJoinAuthorisation applies rule 5.2.
func checkJoinAuthorisation(ctx context.Context, e *event.Event, authoriser string, verifier Verifier) error {
	serverName, err := id.ServerNameOf(authoriser)
	if err != nil {
		return reject("join_authorised_via_users_server: %v", err)
	}
	if verifier == nil {
		return fmt.Errorf("%w: %s is authorised by %q, whose server's signature cannot be checked",
			ErrNoVerifier, e.EventID(), authoriser)
	}
	if err := verifier.VerifyEventSignature(ctx, serverName, e); err != nil {
		return reject("the join is authorised by %q but carries no valid signature from %q: %v",
			authoriser, serverName, err)
	}
	return nil
}

// thirdPartyInviteKeys returns the public keys an m.room.third_party_invite
// event published, from either of the two places the spec allows.
func thirdPartyInviteKeys(e *event.Event) ([]ed25519.PublicKey, error) {
	var content struct {
		PublicKey  string `json:"public_key"`
		PublicKeys []struct {
			PublicKey string `json:"public_key"`
		} `json:"public_keys"`
	}
	if err := json.Unmarshal(e.Content(), &content); err != nil {
		return nil, reject("m.room.third_party_invite content: %v", err)
	}

	encoded := make([]string, 0, len(content.PublicKeys)+1)
	if content.PublicKey != "" {
		encoded = append(encoded, content.PublicKey)
	}
	for _, k := range content.PublicKeys {
		if k.PublicKey != "" {
			encoded = append(encoded, k.PublicKey)
		}
	}

	keys := make([]ed25519.PublicKey, 0, len(encoded))
	for _, s := range encoded {
		raw, err := signing.DecodeBase64(s)
		if err != nil || len(raw) != ed25519.PublicKeySize {
			// A key we cannot read cannot match a signature, and the rule
			// asks whether *any* key matches — so a bad one is skipped
			// rather than fatal.
			continue
		}
		keys = append(keys, ed25519.PublicKey(raw))
	}
	return keys, nil
}