add support for windows bitmap files

lib/dimensions/reader.rb

@@ -7,6 +7,7 @@ class Reader
     JPEG_HEADER   = [0xFF, 0xD8, 0xFF]
     TIFF_HEADER_I = [0x49, 0x49, 0x2A, 0x00]
     TIFF_HEADER_M = [0x4D, 0x4D, 0x00, 0x2A]
+    BMP_HEADER    = [0x42, 0x4D]
 
     attr_reader :type, :width, :height, :angle
 
@@ -45,12 +46,26 @@ def determine_type
           @type = :jpeg
         elsif match_header(TIFF_HEADER_I, bytes) || match_header(TIFF_HEADER_M, bytes)
           @type = :tiff
+        elsif match_header(BMP_HEADER, bytes)
+          @type = :bmp
         end
 
         process @type ? :"extract_#{type}_dimensions" : nil
       end
     end
 
+    def extract_bmp_dimensions
+      if @size >= 26
+        dib_header_size = @data.unpack("x14C").first
+        @width, @height = if dib_header_size == 12
+                            @data.unpack("x18v2")
+                          else
+                            @data.unpack("x18V2")
+                          end
+        process nil
+      end
+    end
+
     def extract_gif_dimensions
       if @size >= 10
         @width, @height = @data.unpack("x6v2")

test/test_dimensions.rb

@@ -30,6 +30,14 @@ def test_tiff_dimensions
     assert_dimensions "short.tif", 50, 20
   end
 
+  def test_windows_bmp_dimensions
+    assert_dimensions "windows.bmp", 127, 64
+  end
+
+  def test_os2_bmp_dimensions
+    assert_dimensions "os2.bmp", 127, 64
+  end
+
   def assert_dimensions(filename, expected_width, expected_height)
     actual_width, actual_height = Dimensions.dimensions(fixture_path(filename))
     assert_equal "#{expected_width}x#{expected_height}", "#{actual_width}x#{actual_height}"