package roomserver

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

	"github.com/dariuszjaniak/warp/server/internal/matrix/auth"
	"github.com/dariuszjaniak/warp/server/internal/matrix/event"
	"github.com/dariuszjaniak/warp/server/internal/matrix/roomversion"
	"github.com/dariuszjaniak/warp/server/internal/storage"
)

// SendEventRequest sends one event into a room on a user's behalf.
type SendEventRequest struct {
	RoomID string

	// Sender is the full user ID sending the event.
	Sender string

	Type string

	// StateKey makes this a state event when it is non-nil.
	StateKey *string

	Content json.RawMessage

	// Transaction, when set, makes the send idempotent: a retry with the
	// same transaction ID returns the event the first attempt created.
	// State events have no transaction ID in the Client-Server API, which
	// is why this is optional.
	Transaction *storage.TransactionRef
}

// SendEvent sends a message or state event into a room.
//
// It does no authorisation of its own, deliberately: whether a user may
// send a given type into a given room is a question about the room's power
// levels and membership, and the one place that is answered correctly is
// the authorisation rules inside the pipeline. A handler-level check here
// would be a second, worse copy of them.
//
// The one thing it does refuse up front is m.room.create. The rules would
// refuse it too — a second create event in a room is rejected — but the
// error a client gets for "you cannot re-create a room" is more use than
// the one for "rule 2 said no".
func (r *API) SendEvent(ctx context.Context, req SendEventRequest) (string, error) {
	if req.Type == event.TypeCreate {
		return "", fmt.Errorf("%w: a room's m.room.create event is sent by createRoom and cannot be replaced",
			ErrForbidden)
	}
	if req.Type == event.TypeMember {
		// A membership change through the state endpoint is legitimate and
		// the rules handle it, but the membership value still has to be one
		// the rules know: content that is not a membership at all would
		// otherwise become an event nobody can interpret.
		if err := checkMembershipContent(req.Content); err != nil {
			return "", err
		}
	}

	sent, err := r.send(ctx, sendRequest{
		RoomID:      req.RoomID,
		Type:        req.Type,
		StateKey:    req.StateKey,
		Sender:      req.Sender,
		Content:     req.Content,
		Transaction: req.Transaction,
	})
	if err != nil {
		return "", err
	}
	return sent.EventID(), nil
}

// checkMembershipContent rejects an m.room.member content whose membership
// is missing or not one of the five the spec defines.
func checkMembershipContent(content json.RawMessage) error {
	var body struct {
		Membership string `json:"membership"`
	}
	if err := json.Unmarshal(content, &body); err != nil {
		return fmt.Errorf("%w: an m.room.member content needs a membership", ErrInvalidContent)
	}
	switch body.Membership {
	case event.MembershipJoin, event.MembershipLeave, event.MembershipInvite,
		event.MembershipBan, event.MembershipKnock:
		return nil
	case "":
		return fmt.Errorf("%w: an m.room.member content needs a membership", ErrInvalidContent)
	default:
		return fmt.Errorf("%w: %q is not a membership", ErrInvalidContent, body.Membership)
	}
}

// RedactRequest asks to redact an event.
type RedactRequest struct {
	RoomID string

	// Sender is the full user ID sending the redaction.
	Sender string

	// EventID is the event to redact.
	EventID string

	Reason string

	Transaction *storage.TransactionRef
}

// Redact redacts an event.
//
// This is two decisions, not one, and the spec keeps them apart. The first
// is whether the m.room.redaction event may be in the room at all, which
// the authorisation rules answer and which is permissive: almost any member
// may send one, because a server receiving a redaction for an event it has
// never seen still has to be able to store it. The second is whether the
// redaction takes effect on its target, which needs both events in hand and
// is auth.ShouldApplyRedaction's job — either the sender has the redact
// level, or the redaction comes from the same server as the event, because
// a server may always take back its own users' events.
//
// Phase 3 only ever has both events, since every event in a room is local.
// A redaction whose target has not arrived is a federation case.
func (r *API) Redact(ctx context.Context, req RedactRequest) (string, error) {
	// The room is read first so that a room this server does not have is a
	// 404 before anything is built, and so that a room whose version is no
	// longer supported says so.
	room, err := r.store.Room(ctx, req.RoomID)
	if err != nil {
		return "", err
	}
	if _, err := r.rules(room); err != nil {
		return "", err
	}

	// The target is read before the redaction is built so that redacting
	// an event that does not exist is a 404 rather than an event in the
	// room that refers to nothing.
	if _, err := r.store.Event(ctx, req.RoomID, req.EventID); err != nil {
		return "", err
	}

	content := map[string]any{}
	if req.Reason != "" {
		content["reason"] = req.Reason
	}

	redaction, err := r.send(ctx, sendRequest{
		RoomID:      req.RoomID,
		Type:        event.TypeRedaction,
		Sender:      req.Sender,
		Content:     mustMarshal(content),
		Redacts:     req.EventID,
		Transaction: req.Transaction,
	})
	if err != nil {
		return "", err
	}
	return redaction.EventID(), nil
}

// prepareRedaction decides whether a redaction takes effect and, if it
// does, puts the target's redacted form into the write.
//
// It runs inside persist, within the same transaction as the redaction
// event itself, so that a client which gets an event ID back can always
// read the stripped event.
func (r *API) prepareRedaction(ctx context.Context, write *storage.RoomWrite, req persistRequest) error {
	storedTarget, err := r.store.Event(ctx, req.Room.ID, req.RedactsEventID)
	if isNotFound(err) {
		// Over federation a redaction can arrive before its target, and the
		// right answer is to keep it and apply it later. Locally the target
		// was read before the redaction was built, so this means it was
		// purged in between.
		r.log.Info("redaction target is not here",
			"room_id", req.Room.ID, "redacts", req.RedactsEventID)
		return nil
	} else if err != nil {
		return err
	}
	if storedTarget.RedactedBy != "" {
		// Already redacted. The event stays as it is: a second redaction
		// has nothing left to remove.
		return nil
	}

	target, err := parseEvent(storedTarget, req.Rules)
	if err != nil {
		return err
	}
	applies, err := r.redactionApplies(ctx, req.Room, req.Rules, req.Event, target)
	if err != nil {
		return err
	}
	if !applies {
		// The event is in the room and has no effect, which is what the
		// spec asks for. It is worth a log line: from the client's side a
		// redaction that returns an event ID and changes nothing looks
		// exactly like one that worked.
		r.log.Info("accepted a redaction that does not apply",
			"room_id", req.Room.ID, "event_id", req.Event.EventID(),
			"redacts", req.RedactsEventID, "sender", req.Event.Sender())
		return nil
	}

	redacted, err := target.Redact()
	if err != nil {
		return fmt.Errorf("redacting %s: %w", req.RedactsEventID, err)
	}
	write.Redacts = req.RedactsEventID
	write.RedactedJSON = redacted.JSON()
	return nil
}

// redactionApplies runs the spec's "should this redaction be applied" test.
func (r *API) redactionApplies(ctx context.Context, room storage.Room, rules roomversion.Rules, redaction, target *event.Event) (bool, error) {
	st, err := r.currentState(ctx, room, rules)
	if err != nil {
		return false, err
	}
	create := st.Create()
	if create == nil {
		return false, fmt.Errorf("%w: %s has no create event in its state", ErrRejected, room.ID)
	}
	powerLevels, err := auth.ResolvePowerLevels(st.PowerLevels(), create, rules)
	if err != nil {
		return false, fmt.Errorf("%w: %v", ErrRejected, err)
	}
	return auth.ShouldApplyRedaction(redaction, target, powerLevels), nil
}