package event

import (
	"encoding/json"
	"fmt"

	"github.com/dariuszjaniak/warp/server/internal/matrix/canonicaljson"
	"github.com/dariuszjaniak/warp/server/internal/matrix/id"
	"github.com/dariuszjaniak/warp/server/internal/matrix/roomversion"
)

// The limits the spec puts on an event.
const (
	// MaxEventSize is the largest an event may be as canonical JSON,
	// signatures included. 64 KiB is generous for a message and small
	// enough that a room's state cannot be used to exhaust a server.
	MaxEventSize = 65536

	// MaxFieldLength caps the type and state_key properties.
	MaxFieldLength = 255

	// MaxPrevEvents and MaxAuthEvents cap the DAG references an event may
	// carry. The auth events selection algorithm never needs more than a
	// handful, so a longer list is a sign of something wrong rather than a
	// legitimate event.
	MaxPrevEvents = 20
	MaxAuthEvents = 10

	// MaxDepth is the largest depth an event may claim. Once a room gets
	// here every later event stays at the limit.
	MaxDepth = canonicaljson.MaxInt
)

// validate applies the structural rules: the properties the spec marks
// required are present and of the right type, the identifiers are
// well-formed, and nothing is over its size limit.
//
// It deliberately does not look at the room state. Everything checkable
// from the event alone is checked here so that the authorisation rules can
// assume a well-formed event and concern themselves only with permission.
func (e *Event) validate() error {
	if e.rules.EventIDFormat == roomversion.EventIDFormatExplicit {
		// Unreachable today: Get refuses to return rules for room versions
		// 1 and 2. The check is here so that adding them to the registry
		// fails loudly rather than computing nonsense event IDs.
		return fmt.Errorf("%w: room version %s carries event IDs in the event, which Warp cannot read yet",
			ErrInvalidEvent, e.rules.Version)
	}

	if n := len(e.raw); n > MaxEventSize {
		return fmt.Errorf("%w: event is %d bytes, the limit is %d", ErrInvalidEvent, n, MaxEventSize)
	}

	if e.fields.Type == "" {
		return fmt.Errorf("%w: type is required", ErrInvalidEvent)
	}
	if n := len(e.fields.Type); n > MaxFieldLength {
		return fmt.Errorf("%w: type is %d bytes, the limit is %d", ErrInvalidEvent, n, MaxFieldLength)
	}
	if err := id.ValidateUserID(e.fields.Sender); err != nil {
		return fmt.Errorf("%w: sender: %w", ErrInvalidEvent, err)
	}
	if sk, ok := e.StateKey(); ok {
		if n := len(sk); n > MaxFieldLength {
			return fmt.Errorf("%w: state_key is %d bytes, the limit is %d", ErrInvalidEvent, n, MaxFieldLength)
		}
	}

	if err := e.validateRoomID(); err != nil {
		return err
	}
	if err := e.validateContent(); err != nil {
		return err
	}
	if err := e.validateDAGReferences(); err != nil {
		return err
	}

	if e.fields.Depth > MaxDepth {
		return fmt.Errorf("%w: depth %d is above the limit of %d", ErrInvalidEvent, e.fields.Depth, MaxDepth)
	}
	if e.fields.Hashes.SHA256 == "" {
		return fmt.Errorf("%w: hashes.sha256 is required", ErrInvalidEvent)
	}
	if len(e.fields.Signatures) == 0 {
		return fmt.Errorf("%w: signatures is required", ErrInvalidEvent)
	}
	return nil
}

// validateRoomID checks the room_id property, which from room version 12 is
// absent on the create event because the room ID is derived from it.
func (e *Event) validateRoomID() error {
	isCreate := e.fields.Type == TypeCreate
	if e.rules.RoomIDIsCreateEventID && isCreate {
		if e.fields.RoomID != "" {
			// Rule 1.2 for this room version rejects it as well; failing
			// here too means we never compute an ID for an event whose
			// room is ambiguous.
			return fmt.Errorf("%w: an m.room.create event in a version %s room must not have a room_id",
				ErrInvalidEvent, e.rules.Version)
		}
		return nil
	}
	if err := id.ValidateRoomID(e.fields.RoomID); err != nil {
		return fmt.Errorf("%w: room_id: %w", ErrInvalidEvent, err)
	}
	return nil
}

// validateContent checks that content is present and is an object. The
// spec marks it required on every event, and the authorisation rules and
// the redaction algorithm both read into it.
func (e *Event) validateContent() error {
	if len(e.fields.Content) == 0 {
		return fmt.Errorf("%w: content is required", ErrInvalidEvent)
	}
	var obj map[string]json.RawMessage
	if err := json.Unmarshal(e.fields.Content, &obj); err != nil {
		return fmt.Errorf("%w: content must be an object: %w", ErrInvalidEvent, err)
	}
	if obj == nil {
		return fmt.Errorf("%w: content must be an object, not null", ErrInvalidEvent)
	}
	return nil
}

// validateDAGReferences checks prev_events and auth_events: within their
// limits, and every entry a well-formed event ID.
func (e *Event) validateDAGReferences() error {
	if n := len(e.fields.PrevEvents); n > MaxPrevEvents {
		return fmt.Errorf("%w: %d prev_events, the limit is %d", ErrInvalidEvent, n, MaxPrevEvents)
	}
	if n := len(e.fields.AuthEvents); n > MaxAuthEvents {
		return fmt.Errorf("%w: %d auth_events, the limit is %d", ErrInvalidEvent, n, MaxAuthEvents)
	}
	for _, eventID := range e.fields.PrevEvents {
		if err := id.ValidateEventID(eventID); err != nil {
			return fmt.Errorf("%w: prev_events: %w", ErrInvalidEvent, err)
		}
	}
	for _, eventID := range e.fields.AuthEvents {
		if err := id.ValidateEventID(eventID); err != nil {
			return fmt.Errorf("%w: auth_events: %w", ErrInvalidEvent, err)
		}
	}
	return nil
}