# FastImage finds the size or type of an image given its uri.
# It is careful to only fetch and parse as much of the image as is needed to determine the result.
# It does this by using a feature of Net::HTTP that yields strings from the resource being fetched
# as soon as the packets arrive.
#
# No external libraries such as ImageMagick are used here, this is a very lightweight solution to 
# finding image information.
#
# FastImage knows about GIF, JPEG, BMP and PNG files.
#
# FastImage can also read files from the local filesystem by supplying the path instead of a uri.
# In this case FastImage uses the open-uri library to read the file in chunks of 256 bytes until
# it has enough. This is possibly a useful bandwidth-saving feature if the file is on a network
# attached disk rather than truly local.
#
# === Examples
#   require 'fastimage'
#
#   FastImage.size("http://stephensykes.com/images/ss.com_x.gif")
#   => [266, 56]
#   FastImage.type("http://stephensykes.com/images/pngimage")
#   => :png
#   FastImage.type("/some/local/file.gif")
#   => :gif
#
# === References
# * http://snippets.dzone.com/posts/show/805
# * http://www.anttikupila.com/flash/getting-jpg-dimensions-with-as3-without-loading-the-entire-file/
# * http://pennysmalls.com/2008/08/19/find-jpeg-dimensions-fast-in-ruby/
# * http://imagesize.rubyforge.org/
#
require 'net/https'
require 'open-uri'

module Middleman
  class FastImage
    attr_reader :size, :type

    class FastImageException < StandardError # :nodoc:
    end
    class MoreCharsNeeded < FastImageException # :nodoc:
    end
    class UnknownImageType < FastImageException # :nodoc:
    end
    class ImageFetchFailure < FastImageException # :nodoc:
    end
    class SizeNotFound < FastImageException # :nodoc:
    end

    DefaultTimeout = 2
  
    LocalFileChunkSize = 256

    # Returns an array containing the width and height of the image.
    # It will return nil if the image could not be fetched, or if the image type was not recognised.
    #
    # By default there is a timeout of 2 seconds for opening and reading from a remote server.
    # This can be changed by passing a :timeout => number_of_seconds in the options.
    #
    # If you wish FastImage to raise if it cannot size the image for any reason, then pass
    # :raise_on_failure => true in the options.
    #
    # FastImage knows about GIF, JPEG, BMP and PNG files.
    #
    # === Example
    #
    #   require 'fastimage'
    #
    #   FastImage.size("http://stephensykes.com/images/ss.com_x.gif")
    #   => [266, 56]
    #   FastImage.size("http://stephensykes.com/images/pngimage")
    #   => [16, 16]
    #   FastImage.size("http://farm4.static.flickr.com/3023/3047236863_9dce98b836.jpg")
    #   => [500, 375]
    #   FastImage.size("http://www-ece.rice.edu/~wakin/images/lena512.bmp")
    #   => [512, 512]
    #   FastImage.size("test/fixtures/test.jpg")
    #   => [882, 470]
    #   FastImage.size("http://pennysmalls.com/does_not_exist")
    #   => nil
    #   FastImage.size("http://pennysmalls.com/does_not_exist", :raise_on_failure=>true)
    #   => raises FastImage::ImageFetchFailure
    #   FastImage.size("http://stephensykes.com/favicon.ico", :raise_on_failure=>true)
    #   => raises FastImage::UnknownImageType
    #   FastImage.size("http://stephensykes.com/favicon.ico", :raise_on_failure=>true, :timeout=>0.01)
    #   => raises FastImage::ImageFetchFailure
    #   FastImage.size("http://stephensykes.com/images/faulty.jpg", :raise_on_failure=>true)
    #   => raises FastImage::SizeNotFound
    #
    # === Supported options
    # [:timeout]
    #   Overrides the default timeout of 2 seconds.  Applies both to reading from and opening the http connection.
    # [:raise_on_failure]
    #   If set to true causes an exception to be raised if the image size cannot be found for any reason.
    #
    def self.size(uri, options={})
      new(uri, options).size
    end

    # Returns an symbol indicating the image type fetched from a uri.
    # It will return nil if the image could not be fetched, or if the image type was not recognised.
    #
    # By default there is a timeout of 2 seconds for opening and reading from a remote server.
    # This can be changed by passing a :timeout => number_of_seconds in the options.
    #
    # If you wish FastImage to raise if it cannot find the type of the image for any reason, then pass
    # :raise_on_failure => true in the options.
    #
    # === Example
    #
    #   require 'fastimage'
    #
    #   FastImage.type("http://stephensykes.com/images/ss.com_x.gif")
    #   => :gif
    #   FastImage.type("http://stephensykes.com/images/pngimage")
    #   => :png
    #   FastImage.type("http://farm4.static.flickr.com/3023/3047236863_9dce98b836.jpg")
    #   => :jpeg
    #   FastImage.type("http://www-ece.rice.edu/~wakin/images/lena512.bmp")
    #   => :bmp
    #   FastImage.type("test/fixtures/test.jpg")
    #   => :jpeg
    #   FastImage.type("http://pennysmalls.com/does_not_exist")
    #   => nil
    #
    # === Supported options
    # [:timeout]
    #   Overrides the default timeout of 2 seconds.  Applies both to reading from and opening the http connection.
    # [:raise_on_failure]
    #   If set to true causes an exception to be raised if the image type cannot be found for any reason.
    #
    def self.type(uri, options={})
      new(uri, options.merge(:type_only=>true)).type
    end

    def initialize(uri, options={})
      @property = options[:type_only] ? :type : :size
      @timeout = options[:timeout] || DefaultTimeout
      @uri = uri
      @parsed_uri = URI.parse(uri.gsub(/\s/, "%20"))
      if @parsed_uri.scheme == "http" || @parsed_uri.scheme == "https"
        fetch_using_http
      else
        fetch_using_open_uri
      end
      raise SizeNotFound if options[:raise_on_failure] && @property == :size && !@size
    rescue Timeout::Error, SocketError, Errno::ECONNREFUSED, Errno::EHOSTUNREACH, Errno::ECONNRESET, ImageFetchFailure
      raise ImageFetchFailure if options[:raise_on_failure]
    rescue Errno::ENOENT
      raise ImageFetchFailure if options[:raise_on_failure]    
    rescue UnknownImageType
      raise UnknownImageType if options[:raise_on_failure]
    end

    private

    def fetch_using_http
      setup_http
      @http.request_get(@parsed_uri.request_uri) do |res|
        raise ImageFetchFailure unless res.is_a?(Net::HTTPSuccess)
        res.read_body do |str|
          break if parse_packet(str)
        end
      end
    end

    def setup_http
      @http = Net::HTTP.new(@parsed_uri.host, @parsed_uri.port)
      @http.use_ssl = (@parsed_uri.scheme == "https")
      @http.verify_mode = OpenSSL::SSL::VERIFY_NONE
      @http.open_timeout = @timeout
      @http.read_timeout = @timeout
    end

    def fetch_using_open_uri
      open(@uri) do |s|
        while str = s.read(LocalFileChunkSize)
          break if parse_packet(str)
        end
      end
    end

    # returns true once result is achieved
    #
    def parse_packet(str)
      @str = (@unused_str || "") + str
      @strpos = 0
      begin
        result = send("parse_#{@property}")
        if result 
          instance_variable_set("@#{@property}", result)
          true
        end
      rescue MoreCharsNeeded
        false
      end
    end

    def parse_size
      @type = parse_type unless @type
      send("parse_size_for_#{@type}")
    end

    def get_chars(n)
      if @strpos + n - 1 >= @str.size
        @unused_str = @str[@strpos..-1]
        raise MoreCharsNeeded
      else
        result = @str[@strpos..(@strpos + n - 1)]
        @strpos += n
        result
      end
    end

    def get_byte
      get_chars(1).unpack("C")[0]
    end

    def read_int(str)
      size_bytes = str.unpack("CC")
      (size_bytes[0] << 8) + size_bytes[1]
    end

    def parse_type
      case get_chars(2)
      when "BM"
        :bmp
      when "GI"
        :gif
      when 0xff.chr + 0xd8.chr
        :jpeg
      when 0x89.chr + "P"
        :png
      else
        raise UnknownImageType
      end
    end

    def parse_size_for_gif
      get_chars(9)[4..8].unpack('SS')
    end

    def parse_size_for_png
      get_chars(23)[14..22].unpack('NN')
    end

    def parse_size_for_jpeg
      loop do
        @state = case @state
        when nil
          get_chars(2)
          :started
        when :started
          get_byte == 0xFF ? :sof : :started          
        when :sof
          c = get_byte
          if (0xe0..0xef).include?(c)
            :skipframe
          elsif [0xC0..0xC3, 0xC5..0xC7, 0xC9..0xCB, 0xCD..0xCF].detect {|r| r.include? c}
            :readsize
          else
            :skipframe
          end
        when :skipframe
          @skip_chars = read_int(get_chars(2)) - 2
          :do_skip
        when :do_skip
          get_chars(@skip_chars)
          :started
        when :readsize
          s = get_chars(7)
          return [read_int(s[5..6]), read_int(s[3..4])]
        end
      end
    end

    def parse_size_for_bmp
      d = get_chars(27)[12..26]
      d[0] == 40 ? d[4..-1].unpack('LL') : d[4..8].unpack('SS')
    end
  end
end