metadata: move metadata parsing into ExifTool::Metadata.

Move the metadata parsing code from MediaAsset to ExifTool::Metadata so
we can use it outside the context of a MediaAsset, in particular when
dealing with a MediaFile that hasn't been saved to disk yet.

app/logical/exif_tool.rb

@@ -14,21 +14,112 @@ class ExifTool
   attr_reader :file
 
   # Open a file with ExifTool.
+  #
   # @param file [File, String] an image or video file
   def initialize(file)
     @file = file.is_a?(String) ? File.open(file) : file
   end
 
   # Get the file's metadata.
-  # @see https://exiftool.org/TagNames/index.html
+  #
   # @param options [String] the options to pass to exiftool
-  # @return [Hash] the file's metadata
+  # @return [ExifTool::Metadata] the file's metadata
   def metadata(options: DEFAULT_OPTIONS)
     output = %x(exiftool #{options} -json #{file.path.shellescape})
     json = JSON.parse(output).first
     json = json.except("SourceFile")
-    json.with_indifferent_access
+    ExifTool::Metadata.new(json.with_indifferent_access)
   end
-
   memoize :metadata
+
+  # A class representing the set of metadata returned by ExifTool for a file.
+  # Behaves like a Hash, but with extra helper methods for interpreting the metadata.
+  #
+  # @see https://exiftool.org/TagNames/index.html
+  class Metadata
+    attr_reader :metadata
+    delegate_missing_to :metadata
+
+    # @param [Hash] a hash of metadata as returned by ExifTool
+    def initialize(metadata)
+      @metadata = metadata
+    end
+
+    def is_animated?
+      frame_count.to_i > 1
+    end
+
+    def is_animated_gif?
+      file_ext == :gif && is_animated?
+    end
+
+    def is_animated_png?
+      file_ext == :png && is_animated?
+    end
+
+    # @see https://exiftool.org/TagNames/JPEG.html
+    # @see https://exiftool.org/TagNames/PNG.html
+    # @see https://danbooru.donmai.us/posts?tags=exif:File:ColorComponents=1
+    # @see https://danbooru.donmai.us/posts?tags=exif:PNG:ColorType=Grayscale
+    def is_greyscale?
+      metadata["File:ColorComponents"] == 1 ||
+        metadata["PNG:ColorType"] == "Grayscale" ||
+        metadata["PNG:ColorType"] == "Grayscale with Alpha"
+    end
+
+    # https://exiftool.org/TagNames/EXIF.html
+    def is_rotated?
+      file_ext == :jpg && metadata["IFD0:Orientation"].in?(["Rotate 90 CW", "Rotate 270 CW", "Rotate 180"])
+    end
+
+    # Some animations technically have a finite loop count, but loop for hundreds
+    # or thousands of times. Only count animations with a low loop count as non-repeating.
+    def is_non_repeating_animation?
+      loop_count.in?(0..10)
+    end
+
+    # @see http://www.vurdalakov.net/misc/gif/netscape-looping-application-extension
+    # @see https://wiki.mozilla.org/APNG_Specification#.60acTL.60:_The_Animation_Control_Chunk
+    # @see https://danbooru.donmai.us/posts?tags=-exif:GIF:AnimationIterations=Infinite+animated_gif
+    # @see https://danbooru.donmai.us/posts?tags=-exif:PNG:AnimationPlays=inf+animated_png
+    def loop_count
+      return Float::INFINITY if metadata["GIF:AnimationIterations"] == "Infinite"
+      return Float::INFINITY if metadata["PNG:AnimationPlays"] == "inf"
+      return metadata["GIF:AnimationIterations"] if has_key?("GIF:AnimationIterations")
+      return metadata["PNG:AnimationPlays"] if has_key?("PNG:AnimationPlays")
+
+      # If the AnimationIterations tag isn't present, then it's counted as a loop count of 0.
+      return 0 if is_animated_gif? && !has_key?("GIF:AnimationIterations")
+
+      nil
+    end
+
+    def frame_count
+      if file_ext == :gif
+        fetch("GIF:FrameCount", 1)
+      elsif file_ext == :png
+        fetch("PNG:AnimationFrames", 1)
+      else
+        nil
+      end
+    end
+
+    def file_ext
+      if has_key?("File:ColorComponents")
+        :jpg
+      elsif has_key?("PNG:ColorType")
+        :png
+      elsif has_key?("GIF:GIFVersion")
+        :gif
+      elsif has_key?("QuickTime:MovieHeaderVersion")
+        :mp4
+      elsif has_key?("Matroska:DocType")
+        :webm
+      elsif has_key?("Flash:FlashVersion")
+        :swf
+      elsif has_key?("ZIP:ZipCompression")
+        :ugoira
+      end
+    end
+  end
 end

app/models/media_asset.rb

@@ -1,6 +1,7 @@
 class MediaAsset < ApplicationRecord
   has_one :media_metadata, dependent: :destroy
   delegate :metadata, to: :media_metadata
+  delegate :is_animated?, :is_animated_gif?, :is_animated_png?, :is_non_repeating_animation?, :is_greyscale?, :is_rotated?, to: :metadata
 
   def self.search(params)
     q = search_attributes(params, :id, :created_at, :updated_at, :md5, :file_ext, :file_size, :image_width, :image_height)
@@ -18,59 +19,4 @@ class MediaAsset < ApplicationRecord
     self.image_height = media_file.height
     self.media_metadata = MediaMetadata.new(file: media_file)
   end
-
-  def is_animated?
-    is_animated_gif? || is_animated_png?
-  end
-
-  def is_animated_gif?
-    file_ext == "gif" && metadata.fetch("GIF:FrameCount", 1) > 1
-  end
-
-  def is_animated_png?
-    file_ext == "png" && metadata.fetch("PNG:AnimationFrames", 1) > 1
-  end
-
-  # @see https://exiftool.org/TagNames/JPEG.html
-  # @see https://exiftool.org/TagNames/PNG.html
-  # @see https://danbooru.donmai.us/posts?tags=exif:File:ColorComponents=1
-  # @see https://danbooru.donmai.us/posts?tags=exif:PNG:ColorType=Grayscale
-  def is_greyscale?
-    metadata["File:ColorComponents"] == 1 ||
-    metadata["PNG:ColorType"] == "Grayscale" ||
-    metadata["PNG:ColorType"] == "Grayscale with Alpha"
-
-    # Not always accurate:
-    # metadata["ICC-header:ColorSpaceData"] == "GRAY" ||
-    # metadata["XMP-photoshop:ColorMode"] == "Grayscale" ||
-    # metadata["XMP-photoshop:ICCProfileName"] == "EPSON Gray - Gamma 2.2" ||
-    # metadata["XMP-photoshop:ICCProfileName"] == "Gray Gamma 2.2"
-  end
-
-  # https://exiftool.org/TagNames/EXIF.html
-  def is_rotated?
-    file_ext == "jpg" && metadata["IFD0:Orientation"].in?(["Rotate 90 CW", "Rotate 270 CW", "Rotate 180"])
-  end
-
-  # Some animations technically have a finite loop count, but loop for hundreds
-  # or thousands of times. Only count animations with a low loop count as non-repeating.
-  def is_non_repeating_animation?
-    loop_count.in?(0..10)
-  end
-
-  # @see https://exiftool.org/TagNames/GIF.html
-  # @see https://exiftool.org/TagNames/PNG.html
-  # @see https://danbooru.donmai.us/posts?tags=-exif:GIF:AnimationIterations=Infinite+animated_gif
-  # @see https://danbooru.donmai.us/posts?tags=-exif:PNG:AnimationPlays=inf+animated_png
-  def loop_count
-    return Float::INFINITY if metadata["GIF:AnimationIterations"] == "Infinite"
-    return Float::INFINITY if metadata["PNG:AnimationPlays"] == "inf"
-    return metadata["GIF:AnimationIterations"] if metadata["GIF:AnimationIterations"].present?
-    return metadata["PNG:AnimationPlays"] if metadata["PNG:AnimationPlays"].present?
-
-    # If the AnimationIterations tag isn't present, then it's counted as a loop count of 0.
-    return 0 if is_animated_gif? && metadata["GIF:AnimationIterations"].nil?
-
-    nil
-  end
 end

app/models/media_metadata.rb

@@ -23,4 +23,8 @@ class MediaMetadata < ApplicationRecord
   def file=(file_or_path)
     self.metadata = MediaFile.open(file_or_path).metadata
   end
+
+  def metadata
+    ExifTool::Metadata.new(self[:metadata])
+  end
 end