class_name CoverCache
extends RefCounted
## Soundtrack covers kept on disk in the [code]covers[/code] folder next to the settings,
## by soundtrack UUID, so each one is downloaded only when it changed on the server (its
## [code]cover_modified[/code] time). See [FileCache].
##
## Covers are downloaded a few at a time, in the order they are asked for, so opening the
## statistics doesn't send a request for every soundtrack at once.
##
## It also remembers each soundtrack's cover time from the soundtrack lists it is given,
## so [method fetch_cover_of] finds a cover by the soundtrack alone, e.g. in the quiz.

## Emitted when the cover time of a soundtrack is new or changed, e.g. once the soundtrack
## list has loaded or a cover was changed.
signal cover_times_changed

const FOLDER_NAME: String = "covers"
## How many covers are downloaded at the same time.
const MAX_DOWNLOADS: int = 3

# The covers on disk and their index
var _files: FileCache = FileCache.new(FOLDER_NAME)
# Covers waiting to be downloaded: [uuid, modified]
var _waiting: Array[Array] = []
# Who is waiting for each cover that is asked for or downloading: uuid -> Array[Callable]
var _callbacks: Dictionary = {}
var _downloading: int = 0
# The server's cover time of each soundtrack seen: uuid -> int, 0 for none
var _times: Dictionary = {}


func _init() -> void:
	_files.load_index()


## The folder the covers are kept in.
static func folder() -> String:
	return Settings.config_dir().path_join(FOLDER_NAME)


## Deletes every cached cover. They are downloaded again when next asked for.
func clear() -> void:
	_files.clear()


## Deletes the covers of soundtracks that are not in [param covers] (UUID -> cover
## time), and those whose soundtrack no longer has a cover.
func keep_only(covers: Dictionary) -> void:
	set_cover_times(covers)
	var kept: Dictionary = {}
	for uuid: String in covers:
		if int(covers[uuid]) > 0:
			kept[uuid] = true
	_files.keep_only(kept)


## Remembers the cover times in [param covers] (UUID -> cover time, 0 for no cover) and
## emits [signal cover_times_changed] when any is new or changed.
func set_cover_times(covers: Dictionary) -> void:
	var changed: bool = false
	for uuid: String in covers:
		var modified: int = int(covers[uuid])
		if _times.get(uuid, -1) != modified:
			_times[uuid] = modified
			changed = true
	if changed:
		cover_times_changed.emit()


## The cover time of the soundtrack [param uuid], 0 when it has no cover or none is known.
func cover_time(uuid: String) -> int:
	return _times.get(uuid, 0)


## Like [method fetch_cover], with the cover time remembered for the soundtrack.
func fetch_cover_of(uuid: String, on_done: Callable) -> void:
	fetch_cover(uuid, cover_time(uuid), on_done)


## Asks the server for the cover time of the soundtrack [param uuid] again, e.g. after
## its cover was changed, then calls [param on_done] with the new time, 0 for no cover.
## A removed cover is deleted from disk too. When the server doesn't answer, it is called
## with the time it had.
func refresh(uuid: String, on_done: Callable) -> void:
	var with_soundtrack: Callable = func with_soundtrack(
			soundtrack: SoundtrackWithSongList) -> void:
		if soundtrack.cover_modified <= 0:
			_files.remove(uuid)
		set_cover_times({uuid: soundtrack.cover_modified})
		on_done.call(soundtrack.cover_modified)
	# Without an answer the cover is left as it was
	MusicServerClient.fetch_soundtrack(uuid, with_soundtrack,
			func() -> void: on_done.call(cover_time(uuid)))


## Calls [param on_done] with the cover of the soundtrack [param uuid] as an [Image]:
## from disk when the cached one has the server's time [param modified], otherwise once
## it has been downloaded. Nothing is called when the soundtrack has no cover
## ([param modified] 0) or the download fails.
func fetch_cover(uuid: String, modified: int, on_done: Callable) -> void:
	if modified <= 0 || uuid.is_empty():
		return
	if _files.has_file(uuid) && _files.modified(uuid) == modified:
		var cached: Image = CharacterCard.image_from_bytes(_files.read(uuid))
		if cached != null:
			on_done.call(cached)
			return
	if _callbacks.has(uuid):
		_callbacks[uuid].append(on_done)
		return
	_callbacks[uuid] = [on_done]
	_waiting.append([uuid, modified])
	_download_next()


## How many covers are waiting for their turn or downloading.
func busy() -> int:
	return _callbacks.size()


func _download_next() -> void:
	while _downloading < MAX_DOWNLOADS && !_waiting.is_empty():
		var next: Array = _waiting.pop_front()
		var uuid: String = next[0]
		_downloading += 1
		MusicServerClient.fetch_soundtrack_cover(uuid,
				_on_downloaded.bind(uuid, next[1]), _on_failed.bind(uuid))


func _on_downloaded(bytes: PackedByteArray, uuid: String, modified: int) -> void:
	_downloading -= 1
	var cover: Image = CharacterCard.image_from_bytes(bytes)
	if cover == null:
		GameLog.warning("The cover of soundtrack ", uuid, " could not be read")
	else:
		_files.store(uuid, bytes, modified)
	for on_done: Callable in _callbacks.get(uuid, []):
		# A card may be gone by the time its cover arrives
		if cover != null && on_done.is_valid():
			on_done.call(cover)
	_callbacks.erase(uuid)
	_download_next()


# A 404 means the server has no cover for it yet
func _on_failed(uuid: String) -> void:
	_downloading -= 1
	_callbacks.erase(uuid)
	_download_next()