memby/server/internal/api/viewer_rows.go

package api

import (
	"context"
	"encoding/json"
	"net/url"
	"strings"

	"github.com/ponzischeme89/memby/server/internal/emby"
	"github.com/ponzischeme89/memby/server/internal/store"
	"github.com/ponzischeme89/memby/server/internal/timing"
)

// viewerRowLookupLimit bounds one row's worth of ids. A row is a shelf on a television and
// nothing draws more than a screenful plus what the D-pad can reach.
const viewerRowLookupLimit = 60

// itemsByID fetches a named set of titles and returns them **in the order asked for**.
//
// Emby answers an Ids= query in its own order, and for a shadow viewer the order is the
// whole answer: Continue Watching is "what am I in the middle of, most recent first", and
// that ranking was decided in Postgres out of this viewer's own history. Handing back
// Emby's order would keep the right titles and throw away the reason they were chosen.
//
// The metadata is still Emby's. Only the viewing state belongs to Memby, which is why this
// asks for the ordinary row fields and lets decorateItems replace the UserData afterwards.
func (s *Server) itemsByID(
	ctx context.Context, cred emby.Credentials, ids []string, fields string,
) (*emby.ItemsResult, error) {
	if len(ids) == 0 {
		return &emby.ItemsResult{}, nil
	}
	if len(ids) > viewerRowLookupLimit {
		ids = ids[:viewerRowLookupLimit]
	}
	result, err := s.emby.Items(ctx, cred, rowParams(url.Values{
		"Ids":       {strings.Join(ids, ",")},
		"Recursive": {"true"},
		"Limit":     {itoa(len(ids))},
	}, fields))
	if err != nil {
		return nil, err
	}
	result.Items = orderItemsByID(result.Items, ids)
	return result, nil
}

// orderItemsByID puts a set of items back into the order they were asked for.
//
// A title the catalogue still names but Emby no longer answers for is dropped rather than
// left as a gap — the row is drawn from what comes back, and a missing card is better than
// one that cannot be opened. A title Emby volunteers that was not asked for is dropped too:
// the ids are the answer, and anything else in the response is not part of it.
func orderItemsByID(items []json.RawMessage, ids []string) []json.RawMessage {
	byID := make(map[string]json.RawMessage, len(items))
	for _, raw := range items {
		if id := itemIDOf(raw); id != "" {
			if _, seen := byID[id]; !seen {
				byID[id] = raw
			}
		}
	}
	ordered := make([]json.RawMessage, 0, len(ids))
	for _, id := range ids {
		if raw, ok := byID[id]; ok {
			ordered = append(ordered, raw)
			// Removed so a repeated id cannot draw the same card twice. The television
			// keys its rows by item id and throws on a duplicate.
			delete(byID, id)
		}
	}
	return ordered
}

// viewerContinueRow is a shadow viewer's Continue Watching, built from their own playheads.
func (s *Server) viewerContinueRow(
	ctx context.Context, cred emby.Credentials, viewerID string, limit int,
) (*emby.ItemsResult, error) {
	ids, err := s.store.ViewerResumeItems(ctx, viewerID, limit)
	if err != nil {
		return nil, err
	}
	return s.itemsByID(timing.WithLabel(ctx, "emby.viewer_resume"), cred, ids, fieldsContinue)
}

// viewerNextUpRow is the next unwatched episode of each series this viewer is part-way
// through. The ranking is Postgres's; Emby is only asked to describe the titles.
func (s *Server) viewerNextUpRow(
	ctx context.Context, cred emby.Credentials, viewerID string, limit int,
) (*emby.ItemsResult, error) {
	ids, err := s.store.ViewerNextUp(ctx, viewerID, limit)
	if err != nil {
		return nil, err
	}
	return s.itemsByID(timing.WithLabel(ctx, "emby.viewer_nextup"), cred, ids, fieldsContinue)
}

// viewerFavouritesRow is this viewer's own favourites rather than the account's.
//
// It is deliberately *not* re-sorted by name the way the Emby row is. A shadow viewer's
// favourites are the ones they marked, and the order they marked them in is the only
// ordering Memby has that means anything.
func (s *Server) viewerFavouritesRow(
	ctx context.Context, cred emby.Credentials, viewerID string, limit int,
) (*emby.ItemsResult, error) {
	ids, err := s.store.ViewerFavouriteItems(ctx, viewerID, limit)
	if err != nil {
		return nil, err
	}
	return s.itemsByID(timing.WithLabel(ctx, "emby.viewer_favourites"), cred, ids, fieldsRow)
}

// firstUnwatchedEpisodeFor is where a shadow viewer's series starts.
//
// It reads the series' episodes once from Emby and walks them against this viewer's own
// played set, rather than asking Emby's NextUp — which answers for the account and is the
// whole reason a shadow viewer pressing Play on a show they have never seen was being
// dropped into the middle of somebody else's season.
//
// A part-watched episode wins over the first unwatched one: somebody eleven minutes into
// an episode wants that episode, which is the same judgement the Continue Watching merge
// makes.
func (s *Server) firstUnwatchedEpisodeFor(
	ctx context.Context, cred emby.Credentials, viewer store.Viewer, seriesID string,
) (*emby.Summary, error) {
	states, err := s.store.ViewerPlayedInSeries(ctx, viewer.ID, seriesID)
	if err != nil {
		return nil, err
	}
	if len(states) == 0 {
		// Never watched. Emby's own first episode is the right answer and costs the
		// caller nothing extra to ask for.
		return nil, nil
	}
	episodes, err := s.emby.Episodes(timing.WithLabel(ctx, "emby.viewer_series"), cred, seriesID, url.Values{
		"Fields":                 {"Overview,RunTimeTicks,SeriesName,ParentIndexNumber,IndexNumber"},
		"EnableUserData":         {"false"},
		"EnableTotalRecordCount": {"false"},
	})
	if err != nil {
		return nil, err
	}
	resume, err := s.store.ViewerStates(ctx, viewer.ID, episodeIDsOf(episodes.Items))
	if err != nil {
		return nil, err
	}
	var firstUnplayed *emby.Summary
	for _, raw := range episodes.Items {
		summary, err := emby.Summarise(raw)
		if err != nil || summary.ID == "" {
			continue
		}
		state := resume[summary.ID]
		if state.PositionTicks > 0 && !state.Played {
			summary.UserData.PlaybackPositionTicks = state.PositionTicks
			return &summary, nil
		}
		if !state.Played && firstUnplayed == nil {
			episode := summary
			firstUnplayed = &episode
		}
	}
	return firstUnplayed, nil
}

func episodeIDsOf(items []json.RawMessage) []string {
	ids := make([]string, 0, len(items))
	for _, raw := range items {
		if id := itemIDOf(raw); id != "" {
			ids = append(ids, id)
		}
	}
	return ids
}