package api

import (
	"net/http"
	"time"

	"github.com/jackc/pgx/v5/pgtype"

	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
	"git.fabledsword.com/bvandeusen/minstrel/internal/reacquisition"
)

// missingTrackView is one track whose file the scan could not find.
//
// The text fields come from the tracks row, not from the filesystem, which is
// the point: the recording is still a known thing with a title, an artist and
// a play history — only its bytes are absent. LastPlayedAt is nullable because
// plenty of missing files were never played, and that is exactly the signal an
// operator wants when deciding whether to bother re-acquiring one.
type missingTrackView struct {
	TrackID      string  `json:"track_id"`
	Title        string  `json:"title"`
	ArtistID     string  `json:"artist_id"`
	ArtistName   string  `json:"artist_name"`
	AlbumID      string  `json:"album_id"`
	AlbumTitle   string  `json:"album_title"`
	FilePath     string  `json:"file_path"`
	DurationSec  int32   `json:"duration_sec"`
	MissingSince string  `json:"missing_since"`
	LastPlayedAt *string `json:"last_played_at"`
}

// reacquisitionStateView is what the sweeper has done about one album
// (milestone #290), attached to the group so the operator can tell "nothing
// has happened yet" from "asked twice, still nothing" without cross-checking
// the Requests queue.
//
// NextAttemptAt is computed rather than stored: the schedule is a function of
// the attempt count and the current settings, so persisting it would go stale
// the moment an operator edited the backoff.
type reacquisitionStateView struct {
	Attempts      int     `json:"attempts"`
	LastAttemptAt *string `json:"last_attempt_at"`
	NextAttemptAt *string `json:"next_attempt_at"`
	GaveUpAt      *string `json:"gave_up_at"`
}

// missingGroupView is a directory's worth of missing tracks.
//
// Grouping is the whole ergonomic argument for this surface. The case that
// produced #2523 was three reorganised albums showing up as ~40 individually
// missing files; presented flat that reads as forty problems, presented by
// folder it reads as three. MissingSince is the EARLIEST mark in the group,
// so a directory sorts and reads by when it first went away.
type missingGroupView struct {
	Directory    string             `json:"directory"`
	MissingSince string             `json:"missing_since"`
	Tracks       []missingTrackView `json:"tracks"`
	// Nil when nothing has been attempted for this group's album — the
	// common case for a folder that just went missing, and distinct from
	// an attempts:0 record, which cannot occur.
	Reacquisition *reacquisitionStateView `json:"reacquisition"`
}

// adminMissingResponse is the paged envelope. Total counts TRACKS, not
// groups — it is what the nav badge shows, and "12 files missing" is the
// honest number even when they happen to sit in two folders.
type adminMissingResponse struct {
	Total  int64              `json:"total"`
	Limit  int                `json:"limit"`
	Offset int                `json:"offset"`
	Groups []missingGroupView `json:"groups"`
}

// handleListMissingTracks implements GET /api/admin/library/missing.
//
// Read-only by design. Nothing on this surface deletes a track: a missing file
// keeps its row, its history and its likes because it may come back, and if it
// comes back renamed the scanner adopts it (#2528). The surface exists so an
// operator can SEE what the library has lost and act on it deliberately.
func (h *handlers) handleListMissingTracks(w http.ResponseWriter, r *http.Request) {
	limit, offset, err := parsePaging(r.URL.Query())
	if err != nil {
		writeAdminJSONErr(w, http.StatusBadRequest, "invalid_paging")
		return
	}

	q := dbq.New(h.pool)
	total, err := q.CountMissingTracks(r.Context())
	if err != nil {
		h.logger.Error("admin: count missing tracks", "err", err)
		writeAdminJSONErr(w, http.StatusInternalServerError, "server_error")
		return
	}
	rows, err := q.ListMissingTracks(r.Context(), dbq.ListMissingTracksParams{
		PageLimit:  int32(limit),
		PageOffset: int32(offset),
	})
	if err != nil {
		h.logger.Error("admin: list missing tracks", "err", err)
		writeAdminJSONErr(w, http.StatusInternalServerError, "server_error")
		return
	}

	groups := groupMissingByDirectory(rows)
	h.attachReacquisitionState(r, q, rows, groups)

	out := adminMissingResponse{
		Total:  total,
		Limit:  limit,
		Offset: offset,
		Groups: groups,
	}
	writeJSON(w, http.StatusOK, out)
}

// groupMissingByDirectory folds the ordered rows into per-directory groups.
//
// It relies on ListMissingTracks ordering by directory, so a simple run-length
// fold is enough and no map is needed — which also preserves the query's
// ordering in the response instead of Go's random map iteration. A page
// boundary can split one directory across two pages; that is accepted rather
// than paging by group, because the alternative costs a second query to find
// the page's directories and this surface's realistic N is small.
func groupMissingByDirectory(rows []dbq.ListMissingTracksRow) []missingGroupView {
	groups := make([]missingGroupView, 0, 8)
	for _, row := range rows {
		t := missingTrackView{
			TrackID:      uuidToString(row.ID),
			Title:        row.Title,
			ArtistID:     uuidToString(row.ArtistID),
			ArtistName:   row.ArtistName,
			AlbumID:      uuidToString(row.AlbumID),
			AlbumTitle:   row.AlbumTitle,
			FilePath:     row.FilePath,
			DurationSec:  row.DurationMs / 1000,
			MissingSince: formatTimestamp(row.MissingSince),
		}
		if row.LastPlayedAt.Valid {
			s := formatTimestamp(row.LastPlayedAt)
			t.LastPlayedAt = &s
		}

		if n := len(groups); n > 0 && groups[n-1].Directory == row.Directory {
			groups[n-1].Tracks = append(groups[n-1].Tracks, t)
			continue
		}
		groups = append(groups, missingGroupView{
			Directory: row.Directory,
			// First row of a run carries the group's timestamp. Rows are
			// ordered within a directory by disc/track, not by mark time, so
			// this is "the mark on the first track" rather than the minimum —
			// they are the same value in the case that matters (a whole folder
			// vanishing at once) and close enough otherwise.
			MissingSince: t.MissingSince,
			Tracks:       []missingTrackView{t},
		})
	}
	return groups
}

// attachReacquisitionState decorates each group with what the sweeper has
// done about its album (milestone #290).
//
// Best-effort: this is context on a list whose primary job is showing what is
// missing, so a failure here leaves the groups bare rather than failing the
// page. One batched query for the whole page, not one per group.
//
// A group is keyed by directory while re-acquisition is keyed by album, and
// those line up in practice (an album's files live in one folder) but are not
// guaranteed to — a directory holding two albums takes the first album's
// state, which is the same album its first track belongs to.
func (h *handlers) attachReacquisitionState(
	r *http.Request,
	q *dbq.Queries,
	rows []dbq.ListMissingTracksRow,
	groups []missingGroupView,
) {
	if len(groups) == 0 {
		return
	}
	// Directory -> the album its first row belongs to, matching the order
	// groupMissingByDirectory folded them in.
	dirAlbum := make(map[string]pgtype.UUID, len(groups))
	ids := make([]pgtype.UUID, 0, len(groups))
	for _, row := range rows {
		if _, seen := dirAlbum[row.Directory]; seen {
			continue
		}
		dirAlbum[row.Directory] = row.AlbumID
		ids = append(ids, row.AlbumID)
	}

	states, err := q.GetReacquisitionForAlbums(r.Context(), ids)
	if err != nil {
		h.logger.Warn("admin missing: reacquisition state lookup failed", "err", err)
		return
	}
	byAlbum := make(map[string]dbq.MissingReacquisition, len(states))
	for _, s := range states {
		byAlbum[uuidToString(s.AlbumID)] = s
	}

	// The settings service is optional in contexts that only wire routing
	// (tests), and the backoff projection is decoration on a list whose real
	// job is elsewhere — so fall back to the shipped defaults rather than
	// making this a nil-pointer waiting to happen (rule #48).
	cfg := reacquisition.Defaults
	if h.reacqSettings != nil {
		cfg = h.reacqSettings.Get()
	}
	for i := range groups {
		albumID, ok := dirAlbum[groups[i].Directory]
		if !ok {
			continue
		}
		state, ok := byAlbum[uuidToString(albumID)]
		if !ok {
			continue
		}
		groups[i].Reacquisition = buildReacquisitionState(state, cfg.Backoff(state.Attempts))
	}
}

// buildReacquisitionState renders one album's attempt record, projecting the
// next attempt from the last one plus the backoff the current settings imply.
func buildReacquisitionState(
	state dbq.MissingReacquisition,
	backoff time.Duration,
) *reacquisitionStateView {
	out := &reacquisitionStateView{Attempts: int(state.Attempts)}
	if state.LastAttemptAt.Valid {
		s := formatTimestamp(state.LastAttemptAt)
		out.LastAttemptAt = &s
		// Only meaningful while more attempts remain; an album that has given
		// up has no next attempt to promise.
		if !state.GaveUpAt.Valid {
			next := formatTimestamp(pgtype.Timestamptz{
				Time:  state.LastAttemptAt.Time.Add(backoff),
				Valid: true,
			})
			out.NextAttemptAt = &next
		}
	}
	if state.GaveUpAt.Valid {
		s := formatTimestamp(state.GaveUpAt)
		out.GaveUpAt = &s
	}
	return out
}