package api

import (
	"errors"
	"math/rand"
	"net/http"

	"github.com/jackc/pgx/v5"

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

// Stage 2 of #415. GET /api/playlists/system/{variant}/shuffle returns
// the caller's system playlist with tracks in rotation-aware play
// order: tracks not yet heard this rotation first (shuffled), then
// already-heard tracks (shuffled). When the whole snapshot has been
// heard the rotation resets and the full list reshuffles.
//
// Same JSON shape as GET /api/playlists/{id} so clients reuse their
// existing track parsing. Intentionally a separate, uncached endpoint
// (Option A): the cached playlist-detail GET stays pure for the
// "open the playlist to look at it" path; this one varies per play.
//
// Two explicit static routes (one per variant) mirror the refresh
// handlers and sidestep chi static-vs-param ambiguity under
// /playlists/system/. Future kinds add their own thin route.

func (h *handlers) handleForYouShuffle(w http.ResponseWriter, r *http.Request) {
	h.serveSystemPlaylistShuffle(w, r, "for_you")
}

func (h *handlers) handleDiscoverShuffle(w http.ResponseWriter, r *http.Request) {
	h.serveSystemPlaylistShuffle(w, r, "discover")
}

func (h *handlers) serveSystemPlaylistShuffle(w http.ResponseWriter, r *http.Request, variant string) {
	user, ok := requireUser(w, r)
	if !ok {
		return
	}
	q := dbq.New(h.pool)

	v := variant
	pl, err := q.GetSystemPlaylistByVariantForUser(r.Context(),
		dbq.GetSystemPlaylistByVariantForUserParams{
			UserID:        user.ID,
			SystemVariant: &v,
		})
	if err != nil {
		if errors.Is(err, pgx.ErrNoRows) {
			// No snapshot yet (empty library / not built). Empty list,
			// not an error — the client just has nothing to play.
			writeJSON(w, http.StatusOK, playlistDetailView{
				Tracks: []playlistTrackView{},
			})
			return
		}
		h.logger.Error("system shuffle: lookup failed", "variant", variant, "err", err)
		writeErr(w, apierror.InternalMsg("lookup failed", err))
		return
	}

	detail, err := h.playlists.Get(r.Context(), user.ID, pl.ID)
	if err != nil {
		h.writePlaylistErr(w, err, "system shuffle")
		return
	}

	// Played set for the current rotation. ErrNoRows = nothing played
	// from this playlist yet → empty set, everything counts unplayed.
	played := map[[16]byte]bool{}
	st, rerr := q.GetRotationState(r.Context(), dbq.GetRotationStateParams{
		UserID: user.ID, PlaylistKind: variant,
	})
	if rerr != nil && !errors.Is(rerr, pgx.ErrNoRows) {
		h.logger.Error("system shuffle: rotation lookup failed", "variant", variant, "err", rerr)
		writeErr(w, apierror.InternalMsg("rotation lookup failed", rerr))
		return
	}
	if rerr == nil {
		for _, id := range st.PlayedTrackIds {
			if id.Valid {
				played[id.Bytes] = true
			}
		}
	}

	// Partition playable tracks. Rows whose upstream track was deleted
	// (TrackID nil) are dropped — nothing to enqueue.
	unplayed := make([]playlists.PlaylistTrack, 0, len(detail.Tracks))
	heard := make([]playlists.PlaylistTrack, 0)
	for _, t := range detail.Tracks {
		if t.TrackID == nil || !t.TrackID.Valid {
			continue
		}
		if played[t.TrackID.Bytes] {
			heard = append(heard, t)
		} else {
			unplayed = append(unplayed, t)
		}
	}

	// Whole snapshot heard → reset the rotation and treat everything
	// as fresh. Reset failure is non-fatal: we still return a
	// playable reshuffled list; the next play just re-resets.
	if len(unplayed) == 0 && len(heard) > 0 {
		if err := q.ResetRotationState(r.Context(), dbq.ResetRotationStateParams{
			UserID: user.ID, PlaylistKind: variant,
		}); err != nil {
			h.logger.Warn("system shuffle: rotation reset failed",
				"variant", variant, "err", err)
		}
		unplayed = heard
		heard = nil
	}

	rand.Shuffle(len(unplayed), func(i, j int) {
		unplayed[i], unplayed[j] = unplayed[j], unplayed[i]
	})
	rand.Shuffle(len(heard), func(i, j int) {
		heard[i], heard[j] = heard[j], heard[i]
	})

	detail.Tracks = append(unplayed, heard...)
	writeJSON(w, http.StatusOK, playlistDetailToView(detail))
}