post replacement: leave a system comment after replacement.

app/presenters/post_presenter.rb

@@ -279,4 +279,53 @@ class PostPresenter < Presenter
     pool_html << "</li>"
     pool_html
   end
+
+  def comment_replacement_message(replacer = CurrentUser.user)
+    "@#{replacer.name} replaced this post with a new image:\n\n#{replacement_message}"
+  end
+
+  def modaction_replacement_message
+    "replaced post ##{@post.id}:\n\n#{replacement_message}"
+  end
+
+  def replacement_message
+    linked_source = linked_source(@post.source)
+    linked_source_was = linked_source(@post.source_was)
+
+    <<-EOS.strip_heredoc
+      [table]
+        [tbody]
+          [tr]
+            [th]Old[/th]
+            [td]#{linked_source_was}[/td]
+            [td]#{@post.md5_was}[/td]
+            [td]#{@post.file_ext_was}[/td]
+            [td]#{@post.image_width_was} x #{@post.image_height_was}[/td]
+            [td]#{@post.file_size_was.to_s(:human_size, precision: 4)}[/td]
+          [/tr]
+          [tr]
+            [th]New[/th]
+            [td]#{linked_source}[/td]
+            [td]#{@post.md5}[/td]
+            [td]#{@post.file_ext}[/td]
+            [td]#{@post.image_width} x #{@post.image_height}[/td]
+            [td]#{@post.file_size.to_s(:human_size, precision: 4)}[/td]
+          [/tr]
+        [/tbody]
+      [/table]
+    EOS
+  end
+
+protected
+
+  def linked_source(source)
+    # truncate long sources in the middle: "www.pixiv.net...lust_id=23264933"
+    truncated_source = source.gsub(%r{\Ahttps?://}, "").truncate(64, omission: "...#{source.last(32)}")
+
+    if source =~ %r{\Ahttps?://}i
+      %("#{truncated_source}":[#{source}])
+    else
+      truncated_source
+    end
+  end
 end