package syncapi

import (
	"context"

	"github.com/dariuszjaniak/warp/server/internal/roomserver"
	"github.com/dariuszjaniak/warp/server/internal/storage"
)

// GET /rooms/{roomID}/context/{eventID}: one event and what surrounds it.
//
// It is what a permalink opens, and what jumping to a search result or a
// reply does. The answer is the event, a few events either side of it, the
// tokens to go on paging in both directions, and the state of the room as
// of the event — because a client that jumps into the middle of a room's
// history has none of it.

// ContextRequest is one context read.
type ContextRequest struct {
	RoomID  string
	EventID string

	User      string
	Localpart string

	// Limit is how many events to return in total, including the event
	// itself. The spec's default is 10, and the events either side of the
	// target get half each.
	Limit int

	// Filter is the ?filter= parameter, a RoomEventFilter as for
	// /messages.
	Filter string
}

// ContextResponse is the event and its surroundings.
type ContextResponse struct {
	// Start and End are the tokens to page backwards and forwards from.
	Start string `json:"start,omitempty"`
	End   string `json:"end,omitempty"`

	// EventsBefore and EventsAfter are oldest-first and oldest-first
	// respectively — the spec says EventsBefore is in reverse
	// chronological order, which is what a client scrolling up consumes.
	EventsBefore []roomserver.ClientEvent `json:"events_before"`
	Event        roomserver.ClientEvent   `json:"event"`
	EventsAfter  []roomserver.ClientEvent `json:"events_after"`

	// State is the room's state as of the event, which is what a client
	// needs to render the names and avatars of the people in the chunk.
	State []roomserver.ClientEvent `json:"state"`
}

// DefaultContextLimit is the spec's default for /context.
const DefaultContextLimit = 10

// Context answers one context read.
func (s *API) Context(ctx context.Context, req ContextRequest) (ContextResponse, error) {
	filter, err := s.messageFilter(ctx, req.Localpart, req.Filter)
	if err != nil {
		return ContextResponse{}, err
	}

	limit := req.Limit
	if limit <= 0 {
		limit = DefaultContextLimit
	}
	limit = min(limit, MaxTimelineLimit)

	// The target event is read first, and through the roomserver, so that
	// the visibility check happens before anything else: an event the user
	// may not see has to come back as unknown rather than as a context
	// with an empty middle.
	target, err := s.rooms.Event(ctx, req.RoomID, req.User, req.EventID)
	if err != nil {
		return ContextResponse{}, err
	}

	// The target's own position. The roomserver's timeline reads are
	// bounded by position rather than by event ID, so it has to be found
	// before either side can be read — which is one extra read, and the
	// reason /context costs more than /messages.
	position, err := s.rooms.EventPosition(ctx, req.RoomID, req.User, req.EventID)
	if err != nil {
		return ContextResponse{}, err
	}

	// Half the budget each side, with the odd one going backwards: a
	// client opening a permalink scrolls up from it more often than down.
	after := limit / 2
	before := limit - after - 1
	if before < 0 {
		before = 0
	}

	out := ContextResponse{
		Event:        target,
		EventsBefore: []roomserver.ClientEvent{},
		EventsAfter:  []roomserver.ClientEvent{},
		State:        []roomserver.ClientEvent{},
	}

	if before > 0 {
		earlier, err := s.rooms.Timeline(ctx, roomserver.TimelineRequest{
			RoomID: req.RoomID,
			User:   req.User,
			// Strictly before the target: High is inclusive, and the
			// target itself is reported separately.
			High:      position - 1,
			Direction: storage.DirectionBackward,
			Limit:     before,
			Filter:    filter.TimelineFilter(),
		})
		if err != nil {
			return ContextResponse{}, err
		}
		// The spec wants this half newest first: it is what a client
		// prepends as the user scrolls up.
		out.EventsBefore = reversed(earlier.Events)
		out.Start = pduToken(earlier.Start).String()
	}
	if out.Start == "" {
		out.Start = pduToken(position - 1).String()
	}

	if after > 0 {
		later, err := s.rooms.Timeline(ctx, roomserver.TimelineRequest{
			RoomID: req.RoomID,
			User:   req.User,
			// Strictly after the target: Low is exclusive.
			Low:       position,
			Direction: storage.DirectionForward,
			Limit:     after,
			Filter:    filter.TimelineFilter(),
		})
		if err != nil {
			return ContextResponse{}, err
		}
		out.EventsAfter = later.Events
		out.End = pduToken(later.End).String()
	}
	if out.End == "" {
		out.End = pduToken(position).String()
	}

	state, err := s.contextState(ctx, req, filter, out)
	if err != nil {
		return ContextResponse{}, err
	}
	out.State = state
	return out, nil
}

// contextState is the state a context read reports.
//
// With lazy loading it is the member events of everybody in the chunk,
// which is all a client needs to render it. Without, it is the room's whole
// state as of the event — which for a large room is most of the response,
// and is why every client asks for lazy loading.
func (s *API) contextState(ctx context.Context, req ContextRequest, filter Filter, resp ContextResponse) ([]roomserver.ClientEvent, error) {
	if filter.LazyLoadMembers() {
		chunk := make([]roomserver.ClientEvent, 0, len(resp.EventsBefore)+len(resp.EventsAfter)+1)
		chunk = append(chunk, resp.EventsBefore...)
		chunk = append(chunk, resp.Event)
		chunk = append(chunk, resp.EventsAfter...)
		return s.rooms.MemberEvents(ctx, req.RoomID, sendersOf(chunk))
	}
	return s.rooms.StateBeforeEvent(ctx, req.RoomID, req.User, req.EventID)
}

// reversed returns a copy of events in the opposite order.
func reversed(events []roomserver.ClientEvent) []roomserver.ClientEvent {
	out := make([]roomserver.ClientEvent, len(events))
	for i, e := range events {
		out[len(events)-1-i] = e
	}
	return out
}