class_name SaveMigrationRegistry
extends RefCounted

## A deterministic, in-memory registry for adjacent save-format migrations.
##
## Every registered step migrates exactly N -> N + 1.  The callback receives a
## deep copy of the current value plus the two version numbers and must return
## either the migrated Dictionary, or { ok = true, data = Dictionary }.  A
## callback can reject a migration with { ok = false, code = ..., message = ... }.
## The caller-owned source Dictionary is never passed to migration code.

var _steps: Dictionary = {}
var _migration_ids: Dictionary = {}


func register_step(from_version: int, migration_id: StringName, migrator: Callable) -> Dictionary:
	if from_version < 0:
		return _failure(&"INVALID_FROM_VERSION", "from_version must be non-negative")
	if migration_id.is_empty():
		return _failure(&"INVALID_MIGRATION_ID", "migration_id must not be empty")
	if not migrator.is_valid():
		return _failure(&"INVALID_MIGRATOR", str(migration_id))
	if _steps.has(from_version):
		return _failure(&"DUPLICATE_MIGRATION_STEP", _required_step_id(from_version))
	if _migration_ids.has(migration_id):
		return _failure(&"DUPLICATE_MIGRATION_ID", str(migration_id))
	_steps[from_version] = {
		&"id": migration_id,
		&"migrator": migrator,
	}
	_migration_ids[migration_id] = from_version
	return {
		&"ok": true,
		&"code": &"OK",
		&"migration_id": migration_id,
		&"from_version": from_version,
		&"to_version": from_version + 1,
	}


func has_step(from_version: int) -> bool:
	return _steps.has(from_version)


func get_step_id(from_version: int) -> StringName:
	if not _steps.has(from_version):
		return StringName()
	return StringName(_steps[from_version][&"id"])


func get_registered_steps() -> Array[Dictionary]:
	var versions: Array = _steps.keys()
	versions.sort()
	var result: Array[Dictionary] = []
	for version_value: Variant in versions:
		var from_version := int(version_value)
		result.append({
			&"id": get_step_id(from_version),
			&"from_version": from_version,
			&"to_version": from_version + 1,
		})
	return result


func migrate(source: Dictionary, from_version: int, target_version: int) -> Dictionary:
	if from_version < 0 or target_version < 0 or target_version < from_version:
		return _failure(&"INVALID_VERSION_RANGE", "%d -> %d" % [from_version, target_version])
	var source_guard := source.duplicate(true)
	var working := source.duplicate(true)
	var applied_ids: Array[StringName] = []
	for version in range(from_version, target_version):
		if not _steps.has(version):
			return {
				&"ok": false,
				&"code": &"MIGRATION_STEP_MISSING",
				&"message": "missing adjacent migration %d -> %d" % [version, version + 1],
				&"failed_from_version": version,
				&"failed_to_version": version + 1,
				&"required_migration_id": _required_step_id(version),
				&"applied_migration_ids": applied_ids,
			}
		var step: Dictionary = _steps[version]
		var migration_id := StringName(step[&"id"])
		var migrator: Callable = step[&"migrator"]
		# Each callback receives its own deep copy so even a failing mutator cannot
		# contaminate the last accepted intermediate value.
		var callback_value: Variant = migrator.call(working.duplicate(true), version, version + 1)
		var unpacked := _unpack_callback_result(callback_value)
		if not unpacked[&"ok"]:
			return {
				&"ok": false,
				&"code": StringName(unpacked.get(&"code", &"MIGRATION_STEP_FAILED")),
				&"message": str(unpacked.get(&"message", migration_id)),
				&"failed_migration_id": migration_id,
				&"failed_from_version": version,
				&"failed_to_version": version + 1,
				&"applied_migration_ids": applied_ids,
			}
		working = unpacked[&"data"].duplicate(true)
		applied_ids.append(migration_id)
	if source != source_guard:
		# This should be unreachable because callbacks only see deep copies.  Keep
		# the guard explicit so purity regressions fail closed.
		return _failure(&"SOURCE_MUTATED", "migration source changed unexpectedly")
	return {
		&"ok": true,
		&"code": &"OK",
		&"data": working.duplicate(true),
		&"from_version": from_version,
		&"target_version": target_version,
		&"applied_migration_ids": applied_ids,
	}


func _unpack_callback_result(value: Variant) -> Dictionary:
	if not value is Dictionary:
		return _failure(&"MIGRATION_INVALID_RESULT", "migration callback must return a Dictionary")
	var result: Dictionary = value
	if result.has(&"ok") or result.has("ok"):
		var result_ok := bool(result.get(&"ok", result.get("ok", false)))
		if not result_ok:
			return _failure(
				StringName(result.get(&"code", result.get("code", &"MIGRATION_STEP_FAILED"))),
				str(result.get(&"message", result.get("message", "migration rejected")))
			)
		var data_value: Variant = result.get(&"data", result.get("data", null))
		if not data_value is Dictionary:
			return _failure(&"MIGRATION_INVALID_RESULT", "successful migration has no Dictionary data")
		return {&"ok": true, &"code": &"OK", &"data": data_value}
	return {&"ok": true, &"code": &"OK", &"data": result}


func _required_step_id(from_version: int) -> StringName:
	return StringName("format_%d_to_%d" % [from_version, from_version + 1])


func _failure(code: StringName, message: String) -> Dictionary:
	return {
		&"ok": false,
		&"code": code,
		&"message": message,
	}