gistfile1.txt

image_format.rb

# image_format.rb
#
# Copyright (c) 2010 Ross Paffett
# 
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
# 
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
# 
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
# THE SOFTWARE.

require "stringio"

class ImageFormat
  FORMATS = [ :jpeg, :png, :gif ]
  
  MIME_TYPES = {
    :jpeg => "image/jpeg",
    :png  => "image/png",
    :gif  => "image/gif"
  }
  
  class << self
    def of_blob(blob)
      sio = ::StringIO.new(blob)
      obj = self.new(sio)
      obj.format
      obj
    end
  end
  
  def initialize(source)
    @image = source
    @format = nil
    @mime_type = nil
    @header = nil
  end
  
  def format
    @format ||= FORMATS.detect do |format|
      self.send("#{format}?")
    end
  end
  
  def mime_type
    @mime_type ||= MIME_TYPES[format]
  end
  
  def gif?
    header[0...6] =~ /GIF8[79]a/
  end
  
  def png?
    header[0...8] == "\x89PNG\r\n\x1a\n"
  end
  
  def jpeg?
    header[0...2] == "\xff\xd8"
  end
  
  protected
    def header
      @header unless @header.nil?
      @image.rewind
      @header = @image.read(100)
    end
end