package syncapi

import (
	"context"

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

// GET /rooms/{roomID}/messages: a room's history, a page at a time.
//
// It is the other half of /sync. A sync hands a client the live stream and,
// when a client has been away, a `limited` timeline with a gap in it;
// /messages is how the client closes that gap, and how it scrolls back
// through a room it has just opened. The tokens are the same ones — a sync's
// prev_batch is a valid ?from= here, and an ?end= from here is a valid
// ?since= for /sync — which is what makes the two compose.

// MessagesRequest is one page of a room's history.
type MessagesRequest struct {
	RoomID string

	// User is the full user ID asking, and Localpart is their localpart,
	// for resolving a filter ID.
	User      string
	Localpart string

	// From is where to page from. Empty means the end of the room for a
	// backwards page and the start of it for a forwards one, which is what
	// a client opening a room sends.
	From string

	// To bounds the far end of the page. Empty is unbounded.
	To string

	// Direction is "b" to page backwards, which is what scrolling up does,
	// or "f" to page forwards. The spec requires it.
	Direction storage.TimelineDirection

	// Limit caps the events returned.
	Limit int

	// Filter is the ?filter= parameter: a filter ID or an inline
	// RoomEventFilter as JSON.
	Filter string
}

// MessagesResponse is one page.
type MessagesResponse struct {
	// Start is the token this page began at, which is the ?from= the
	// client sent back to it.
	Start string `json:"start"`

	// End is the token for the next page. It is left out when the page
	// reached the end of what the client may see, which is how a client
	// knows to stop scrolling.
	End string `json:"end,omitempty"`

	Chunk []roomserver.ClientEvent `json:"chunk"`

	// State is the state of the events in the chunk: the member events of
	// their senders, when the filter asked for lazy loading. A client
	// needs them to render a name against a message from somebody who is
	// no longer in the room.
	State []roomserver.ClientEvent `json:"state,omitempty"`
}

// Messages answers one page of a room's history.
func (s *API) Messages(ctx context.Context, req MessagesRequest) (MessagesResponse, error) {
	from, err := ParseToken(req.From)
	if err != nil {
		return MessagesResponse{}, errInvalidTokenf("from: %v", err)
	}
	to, err := ParseToken(req.To)
	if err != nil {
		return MessagesResponse{}, errInvalidTokenf("to: %v", err)
	}

	filter, err := s.messageFilter(ctx, req.Localpart, req.Filter)
	if err != nil {
		return MessagesResponse{}, err
	}

	direction := req.Direction
	if direction != storage.DirectionForward {
		// Backwards is the default: the spec makes `dir` required, but
		// every client that leaves it out means "scroll up".
		direction = storage.DirectionBackward
	}

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

	// Which end of the window the client's two tokens are depends on which
	// way it is paging: scrolling up, `from` is the ceiling and `to` the
	// floor, and scrolling down it is the other way round. An unset bound
	// is zero, which the read already takes as "unbounded at that end" —
	// so paging backwards from the end of the room needs nothing special.
	low, high := from.PDU, to.PDU
	if direction == storage.DirectionBackward {
		low, high = to.PDU, from.PDU
	}

	timeline, err := s.rooms.Timeline(ctx, roomserver.TimelineRequest{
		RoomID:    req.RoomID,
		User:      req.User,
		Low:       low,
		High:      high,
		Direction: direction,
		Limit:     limit,
		Filter:    filter.TimelineFilter(),
	})
	if err != nil {
		return MessagesResponse{}, err
	}

	// The chunk is in the direction asked for: the spec says a backwards
	// page is in reverse chronological order, because that is the order a
	// client scrolling up consumes it in. The read returns events oldest
	// first — which is what /sync wants — so a backwards page is turned
	// round here.
	chunk := timeline.Events
	if direction == storage.DirectionBackward {
		chunk = reversed(chunk)
	}
	if chunk == nil {
		chunk = []roomserver.ClientEvent{}
	}

	out := MessagesResponse{
		Start: pduToken(from.PDU).String(),
		Chunk: chunk,
	}
	// `from` is optional, and without it `start` has to name the end of
	// the room the page actually began at rather than position zero: a
	// client that paged backwards from the end and then used `start` to
	// page forwards again would otherwise be handed the whole room.
	if req.From == "" {
		if direction == storage.DirectionBackward {
			out.Start = pduToken(timeline.End).String()
		} else {
			out.Start = pduToken(timeline.Start).String()
		}
	}

	// `end` is left out at the end of the room, which is how a client
	// knows to stop asking. Reading backwards the far end is Start, the
	// position below the oldest event; reading forwards it is End.
	//
	// The read fetches one more row than the limit, so `Limited` is the
	// whole answer to "is there more": a page that came back exactly full
	// and happened to be the last one still correctly has no `end`.
	if timeline.Limited {
		if direction == storage.DirectionBackward {
			out.End = pduToken(timeline.Start).String()
		} else {
			out.End = pduToken(timeline.End).String()
		}
	}

	if filter.LazyLoadMembers() {
		state, err := s.rooms.MemberEvents(ctx, req.RoomID, sendersOf(chunk))
		if err != nil {
			return MessagesResponse{}, err
		}
		out.State = state
	}
	return out, nil
}

// messageFilter resolves the ?filter= of /messages.
//
// The parameter is a RoomEventFilter here rather than the whole filter that
// /sync takes — the spec defines it that way, because a filter about
// presence or account data would mean nothing to a room's history. A filter
// ID, though, names a full filter, so both shapes have to be accepted: an
// inline one is wrapped into the room's timeline slot, and a stored one is
// used as it stands.
func (s *API) messageFilter(ctx context.Context, localpart, raw string) (Filter, error) {
	if raw == "" {
		return Filter{}, nil
	}
	if raw[0] != '{' {
		return s.filterFor(ctx, localpart, raw)
	}

	var roomEvents RoomEventFilter
	if err := unmarshalStrict([]byte(raw), &roomEvents); err != nil {
		return Filter{}, errInvalidParamf("filter: %v", err)
	}
	return Filter{Room: &RoomFilter{Timeline: &roomEvents}}, nil
}

// sendersOf lists the distinct senders of a chunk, in the order they first
// appear.
func sendersOf(events []roomserver.ClientEvent) []string {
	seen := make(map[string]struct{}, len(events))
	out := make([]string, 0, len(events))
	for _, e := range events {
		if _, dup := seen[e.Sender]; dup {
			continue
		}
		seen[e.Sender] = struct{}{}
		out = append(out, e.Sender)
	}
	return out
}