module INotify
  # Notifier wraps a single instance of inotify.
  # It's possible to have more than one instance,
  # but usually unnecessary.
  #
  # @example
  #   # Create the notifier
  #   notifier = INotify::Notifier.new
  #
  #   # Run this callback whenever the file path/to/foo.txt is read
  #   notifier.watch("path/to/foo.txt", :access) do
  #     puts "Foo.txt was accessed!"
  #   end
  #
  #   # Watch for any file in the directory being deleted
  #   # or moved out of the directory.
  #   notifier.watch("path/to/directory", :delete, :moved_from) do |event|
  #     # The #name field of the event object contains the name of the affected file
  #     puts "#{event.name} is no longer in the directory!"
  #   end
  #
  #   # Nothing happens until you run the notifier!
  #   notifier.run
  class Notifier
    # A hash from {Watcher} ids to the instances themselves.
    #
    # @private
    # @return [{Fixnum => Watcher}]
    attr_reader :watchers

    # The underlying file descriptor for this notifier.
    # This is a valid OS file descriptor, and can be used as such
    # (except under JRuby -- see \{#to\_io}).
    #
    # @return [Fixnum]
    attr_reader :fd

    # Creates a new {Notifier}.
    #
    # @return [Notifier]
    # @raise [SystemCallError] if inotify failed to initialize for some reason
    def initialize
      @fd = Native.inotify_init
      @watchers = {}
      return unless @fd < 0

      raise SystemCallError.new(
        "Failed to initialize inotify" +
        case FFI.errno
        when Errno::EMFILE::Errno; ": the user limit on the total number of inotify instances has been reached."
        when Errno::ENFILE::Errno; ": the system limit on the total number of file descriptors has been reached."
        when Errno::ENOMEM::Errno; ": insufficient kernel memory is available."
        else; ""
        end,
        FFI.errno)
    end

    # Returns a Ruby IO object wrapping the underlying file descriptor.
    # Since this file descriptor is fully functional (except under JRuby),
    # this IO object can be used in any way a Ruby-created IO object can.
    # This includes passing it to functions like `#select`.
    #
    # Note that this always returns the same IO object.
    # Creating lots of IO objects for the same file descriptor
    # can cause some odd problems.
    #
    # **This is not supported under JRuby**.
    # JRuby currently doesn't use native file descriptors for the IO object,
    # so we can't use this file descriptor as a stand-in.
    #
    # @return [IO] An IO object wrapping the file descriptor
    # @raise [NotImplementedError] if this is being called in JRuby
    def to_io
      raise NotImplementedError.new("INotify::Notifier#to_io is not supported under JRuby") if RUBY_PLATFORM =~ /java/
      @io ||= IO.new(@fd)
    end

    # Watches a file or directory for changes,
    # calling the callback when there are.
    # This is only activated once \{#process} or \{#run} is called.
    #
    # **Note that by default, this does not recursively watch subdirectories
    # of the watched directory**.
    # To do so, use the `:recursive` flag.
    #
    # ## Flags
    #
    # `:access`
    # : A file is accessed (that is, read).
    #
    # `:attrib`
    # : A file's metadata is changed (e.g. permissions, timestamps, etc).
    #
    # `:close_write`
    # : A file that was opened for writing is closed.
    #
    # `:close_nowrite`
    # : A file that was not opened for writing is closed.
    #
    # `:modify`
    # : A file is modified.
    #
    # `:open`
    # : A file is opened.
    #
    # ### Directory-Specific Flags
    #
    # These flags only apply when a directory is being watched.
    #
    # `:moved_from`
    # : A file is moved out of the watched directory.
    #
    # `:moved_to`
    # : A file is moved into the watched directory.
    #
    # `:create`
    # : A file is created in the watched directory.
    #
    # `:delete`
    # : A file is deleted in the watched directory.
    #
    # `:delete_self`
    # : The watched file or directory itself is deleted.
    #
    # `:move_self`
    # : The watched file or directory itself is moved.
    #
    # ### Helper Flags
    #
    # These flags are just combinations of the flags above.
    #
    # `:close`
    # : Either `:close_write` or `:close_nowrite` is activated.
    #
    # `:move`
    # : Either `:moved_from` or `:moved_to` is activated.
    #
    # `:all_events`
    # : Any event above is activated.
    #
    # ### Options Flags
    #
    # These flags don't actually specify events.
    # Instead, they specify options for the watcher.
    #
    # `:onlydir`
    # : Only watch the path if it's a directory.
    #
    # `:dont_follow`
    # : Don't follow symlinks.
    #
    # `:mask_add`
    # : Add these flags to the pre-existing flags for this path.
    #
    # `:oneshot`
    # : Only send the event once, then shut down the watcher.
    #
    # `:recursive`
    # : Recursively watch any subdirectories that are created.
    #   Note that this is a feature of rb-inotify,
    #   rather than of inotify itself, which can only watch one level of a directory.
    #   This means that the {Event#name} field
    #   will contain only the basename of the modified file.
    #   When using `:recursive`, {Event#absolute_name} should always be used.
    #
    # @param path [String] The path to the file or directory
    # @param flags [Array<Symbol>] Which events to watch for
    # @yield [event] A block that will be called
    #   whenever one of the specified events occur
    # @yieldparam event [Event] The Event object containing information
    #   about the event that occured
    # @return [Watcher] A Watcher set up to watch this path for these events
    # @raise [SystemCallError] if the file or directory can't be watched,
    #   e.g. if the file isn't found, read access is denied,
    #   or the flags don't contain any events
    def watch(path, *flags, &callback)
      return Watcher.new(self, path, *flags, &callback) unless flags.include?(:recursive)
      Dir[File.join(path, '*/')].each {|d| watch(d, *flags, &callback)}

      rec_flags = [:create, :moved_to]
      return watch(path, *((flags - [:recursive]) | rec_flags)) do |event|
        callback.call(event) unless (flags & event.flags).empty?
        next if (rec_flags & event.flags).empty? || !event.flags.include?(:isdir)
        watch(event.absolute_name, *flags, &callback)
      end
    end

    # Starts the notifier watching for filesystem events.
    # Blocks forever and never returns.
    #
    # @see #process
    def run
      @stop = false
      process until @stop
    end

    # Stop watching for filesystem events.
    # That is, if we're in a \{#run} loop,
    # exit out as soon as we finish handling the events.
    def stop
      @stop = true
    end

    # Blocks until there are one or more filesystem events
    # that this notifier has watchers registered for.
    # Once there are events, the appropriate callbacks are called
    # and this function returns.
    #
    # @see #run
    def process
      read_events.each {|event| event.callback!}
    end

    # Blocks until there are one or more filesystem events
    # that this notifier has watchers registered for.
    # Once there are events, returns their {Event} objects.
    #
    # @private
    def read_events
      size = 64 * Native::Event.size
      tries = 1

      begin
        data = readpartial(size)
      rescue SystemCallError => er
        # EINVAL means that there's more data to be read
        # than will fit in the buffer size
        raise er unless er.errno == EINVAL || tries == 5
        size *= 2
        tries += 1
        retry
      end

      events = []
      cookies = {}
      while ev = Event.consume(data, self)
        events << ev
        next if ev.cookie == 0
        cookies[ev.cookie] ||= []
        cookies[ev.cookie] << ev
      end
      cookies.each {|c, evs| evs.each {|ev| ev.related.replace(evs - [ev]).freeze}}
      events
    end

    private

    # Same as IO#readpartial, or as close as we need.
    def readpartial(size)
      buffer = FFI::MemoryPointer.new(:char, size)
      size_read = Native.read(fd, buffer, size)
      return buffer.read_string(size_read) if size_read >= 0

      raise SystemCallError.new("Error reading inotify events" +
        case FFI.errno
        when Errno::EAGAIN::Errno; ": no data available for non-blocking I/O"
        when Errno::EBADF::Errno; ": invalid or closed file descriptor"
        when Errno::EFAULT::Errno; ": invalid buffer"
        when Errno::EINVAL::Errno; ": invalid file descriptor"
        when Errno::EIO::Errno; ": I/O error"
        when Errno::EISDIR::Errno; ": file descriptor is a directory"
        else; ""
        end,
        FFI.errno)
    end
  end
end