package event

import (
	"encoding/json"
	"fmt"
	"time"

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

// Builder assembles a new event for this server to send.
//
// The caller fills in what the event says; Build fills in what the room
// version says — where the redaction target goes, whether the create event
// carries a room ID — then hashes, signs and validates the result. The
// roomserver (PLAN.md phase 3) is the only thing that should build events,
// because it is the only thing that knows the right prev_events, depth and
// auth_events.
type Builder struct {
	// RoomID is the room the event belongs to. It must be empty for an
	// m.room.create event in a room version where the room ID is derived
	// from that event; use RoomIDOfCreateEvent on the built event to find
	// out what the room ID turned out to be.
	RoomID string

	// Type is the event type, such as "m.room.message".
	Type string

	// StateKey makes this a state event when it is non-nil. The empty
	// string is a perfectly good state key, which is why this is a pointer.
	StateKey *string

	// Sender is the user ID sending the event. Its server name is the one
	// the event is signed as, since an event must be signed by its
	// sender's server.
	Sender string

	// Content is the event content. An empty Content becomes {}.
	Content json.RawMessage

	// PrevEvents and AuthEvents are the event IDs this event refers to.
	PrevEvents []string
	AuthEvents []string

	// Depth is one more than the greatest depth of the prev_events.
	Depth int64

	// Redacts is the event an m.room.redaction event targets. Build puts
	// it wherever this room version keeps it, so callers do not have to
	// know which versions moved it into the content.
	Redacts string

	// Unsigned is attached to the event but covered by neither its hashes
	// nor its signatures, so anything here can be changed in transit by
	// any server that passes the event on.
	Unsigned json.RawMessage
}

// Build hashes, signs and validates the event.
//
// now becomes origin_server_ts. It is passed in rather than read from the
// clock so that tests are deterministic and so that a caller sending a
// batch can stamp them all alike.
func (b *Builder) Build(now time.Time, key *signing.KeyPair, rules roomversion.Rules) (*Event, error) {
	serverName, err := id.ServerNameOf(b.Sender)
	if err != nil {
		return nil, fmt.Errorf("build event: sender: %w", err)
	}
	if b.Type == "" {
		return nil, fmt.Errorf("build event: type is required")
	}
	if b.Depth < 0 {
		return nil, fmt.Errorf("build event: depth %d is negative", b.Depth)
	}

	obj := map[string]json.RawMessage{}
	set := func(k string, v any) error {
		raw, err := json.Marshal(v)
		if err != nil {
			return fmt.Errorf("build event: %s: %w", k, err)
		}
		obj[k] = raw
		return nil
	}

	content := b.Content
	if len(content) == 0 {
		content = json.RawMessage("{}")
	}
	if b.Redacts != "" {
		if content, err = b.placeRedacts(content, obj, rules); err != nil {
			return nil, err
		}
	}

	if err := b.setRoomID(obj, rules); err != nil {
		return nil, err
	}
	for k, v := range map[string]any{
		"type":             b.Type,
		"sender":           b.Sender,
		"depth":            b.Depth,
		"origin_server_ts": now.UnixMilli(),
		// prev_events and auth_events are required, and must be [] rather
		// than absent when there are none — which is the case for the
		// create event and for nothing else.
		"prev_events": orEmpty(b.PrevEvents),
		"auth_events": orEmpty(b.AuthEvents),
	} {
		if err := set(k, v); err != nil {
			return nil, err
		}
	}
	obj["content"] = content
	if b.StateKey != nil {
		if err := set("state_key", *b.StateKey); err != nil {
			return nil, err
		}
	}
	if len(b.Unsigned) > 0 {
		obj["unsigned"] = b.Unsigned
	}

	raw, err := canonicaljson.Marshal(obj)
	if err != nil {
		return nil, fmt.Errorf("build event: %w", err)
	}
	signed, err := hashAndSign(raw, rules, serverName, key)
	if err != nil {
		return nil, fmt.Errorf("build event: %w", err)
	}
	// Parse is the one gate every event goes through, built here or
	// received over federation, so a bug in the builder surfaces as a
	// failure to build rather than as an invalid event in a room.
	return Parse(signed, rules)
}

// setRoomID writes the room_id property, or leaves it out for a create
// event in a room version that derives the room ID from it.
func (b *Builder) setRoomID(obj map[string]json.RawMessage, rules roomversion.Rules) error {
	if rules.RoomIDIsCreateEventID && b.Type == TypeCreate {
		if b.RoomID != "" {
			return fmt.Errorf("build event: an m.room.create event in a version %s room must not have a room_id; it becomes the room ID",
				rules.Version)
		}
		return nil
	}
	if err := id.ValidateRoomID(b.RoomID); err != nil {
		return fmt.Errorf("build event: room_id: %w", err)
	}
	raw, err := json.Marshal(b.RoomID)
	if err != nil {
		return fmt.Errorf("build event: room_id: %w", err)
	}
	obj["room_id"] = raw
	return nil
}

// placeRedacts puts Redacts where this room version keeps it: in the
// content from room version 11, at the top level before that.
func (b *Builder) placeRedacts(content json.RawMessage, obj map[string]json.RawMessage, rules roomversion.Rules) (json.RawMessage, error) {
	if err := id.ValidateEventID(b.Redacts); err != nil {
		return nil, fmt.Errorf("build event: redacts: %w", err)
	}
	if !rules.RedactsInContent {
		raw, err := json.Marshal(b.Redacts)
		if err != nil {
			return nil, fmt.Errorf("build event: redacts: %w", err)
		}
		obj["redacts"] = raw
		return content, nil
	}

	var parsed map[string]json.RawMessage
	if err := json.Unmarshal(content, &parsed); err != nil {
		return nil, fmt.Errorf("build event: content: %w", err)
	}
	if parsed == nil {
		parsed = map[string]json.RawMessage{}
	}
	raw, err := json.Marshal(b.Redacts)
	if err != nil {
		return nil, fmt.Errorf("build event: redacts: %w", err)
	}
	parsed["redacts"] = raw
	out, err := json.Marshal(parsed)
	if err != nil {
		return nil, fmt.Errorf("build event: content: %w", err)
	}
	return out, nil
}

// orEmpty turns a nil slice into an empty one, so that it encodes as []
// rather than null.
func orEmpty(s []string) []string {
	if s == nil {
		return []string{}
	}
	return s
}