import Foundation

public enum WatchPayloadValidationError: Error, Equatable {
    case emptyPayload
    case payloadTooLarge(actualBytes: Int, maxBytes: Int)
    case decodeFailure
}

public enum WatchPayloadValidation {
    public static let defaultMaxPayloadBytes = 256 * 1024

    public static func validate(_ payload: Data, maxBytes: Int = defaultMaxPayloadBytes) -> WatchPayloadValidationError? {
        guard payload.isEmpty == false else {
            return .emptyPayload
        }

        guard payload.count <= maxBytes else {
            return .payloadTooLarge(actualBytes: payload.count, maxBytes: maxBytes)
        }

        return nil
    }

    public static func decode<T: Decodable>(
        _ type: T.Type,
        from payload: Data,
        maxBytes: Int = defaultMaxPayloadBytes,
        decoder: JSONDecoder = JSONDecoder()
    ) throws -> T {
        if let validationError = validate(payload, maxBytes: maxBytes) {
            throw validationError
        }

        do {
            return try decoder.decode(T.self, from: payload)
        } catch {
            throw WatchPayloadValidationError.decodeFailure
        }
    }
}