package services

import (
	"fmt"
	"sort"
)

// Podcast episode ordering — THE single source of order truth (§0 item
// 3 + item 5a, 2026-10-30). Every surface (sequence JSON, reader chain
// context, SSR page list, download queueing) routes through
// OrderEpisodes; per-surface sort blocks were the drift risk (three
// near-identical comparators existed by hand).

// PodcastEpisodeOrder values:
//   - native:  feed position ASC, date ASC tiebreak (fetch-maintained order)
//   - reverse: feed position DESC, date DESC tiebreak ("Reverse feed
//     order" — no pub dates involved, so Scribl's messy/all-equal
//     migrated podiobooks dates can't break it)
//   - oldest / newest: chronological by pub date string (2006-01-02 or
//     anything lexical-comparable), position as same-date tiebreak
func ValidPodcastEpisodeOrder(o string) bool {
	return o == "native" || o == "reverse" || o == "oldest" || o == "newest"
}

// OrderEpisodes sorts eps in place into the given order. Accessor
// functions supply each episode's feed position and pub date key
// (LEN lexical-comparable, e.g. RFC3339 or 2006-01-02).
func OrderEpisodes[T any](eps []T, order string, pos func(T) int32, date func(T) string) {
	switch order {
	case "reverse":
		sort.SliceStable(eps, func(i, j int) bool {
			if pos(eps[i]) != pos(eps[j]) {
				return pos(eps[i]) > pos(eps[j])
			}
			return date(eps[i]) > date(eps[j])
		})
	case "oldest", "newest":
		sort.SliceStable(eps, func(i, j int) bool {
			ka := date(eps[i]) + episodePosKey(pos(eps[i]))
			kb := date(eps[j]) + episodePosKey(pos(eps[j]))
			if order == "newest" {
				return ka > kb
			}
			return ka < kb
		})
	default: // native
		sort.SliceStable(eps, func(i, j int) bool {
			if pos(eps[i]) != pos(eps[j]) {
				return pos(eps[i]) < pos(eps[j])
			}
			return date(eps[i]) < date(eps[j])
		})
	}
}

// episodePosKey: zero-padded position so lexical date keys tiebreak
// numerically.
func episodePosKey(pos int32) string {
	return fmt.Sprintf("%06d", pos)
}