Change how hashtags are normalized (#18795)

* Change how hashtags are normalized

* Fix tests

app/lib/ascii_folding.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+class ASCIIFolding
+  NON_ASCII_CHARS        = 'ÀÁÂÃÄÅàáâãäåĀāĂ㥹ÇçĆćĈĉĊċČčÐðĎďĐđÈÉÊËèéêëĒēĔĕĖėĘęĚěĜĝĞğĠġĢģĤĥĦħÌÍÎÏìíîïĨĩĪīĬĭĮįİıĴĵĶķĸĹĺĻļĽľĿŀŁłÑñŃńŅņŇňʼnŊŋÒÓÔÕÖØòóôõöøŌōŎŏŐőŔŕŖŗŘřŚśŜŝŞşŠšſŢţŤťŦŧÙÚÛÜùúûüŨũŪūŬŭŮůŰűŲųŴŵÝýÿŶŷŸŹźŻżŽž'
+  EQUIVALENT_ASCII_CHARS = 'AAAAAAaaaaaaAaAaAaCcCcCcCcCcDdDdDdEEEEeeeeEeEeEeEeEeGgGgGgGgHhHhIIIIiiiiIiIiIiIiIiJjKkkLlLlLlLlLlNnNnNnNnnNnOOOOOOooooooOoOoOoRrRrRrSsSsSsSssTtTtTtUUUUuuuuUuUuUuUuUuUuWwYyyYyYZzZzZz'
+
+  def fold(str)
+    str.tr(NON_ASCII_CHARS, EQUIVALENT_ASCII_CHARS)
+  end
+end

app/lib/hashtag_normalizer.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+class HashtagNormalizer
+  def normalize(str)
+    remove_invalid_characters(ascii_folding(lowercase(cjk_width(str))))
+  end
+
+  private
+
+  def remove_invalid_characters(str)
+    str.gsub(/[^[:alnum:]#{Tag::HASHTAG_SEPARATORS}]/, '')
+  end
+
+  def ascii_folding(str)
+    ASCIIFolding.new.fold(str)
+  end
+
+  def lowercase(str)
+    str.mb_chars.downcase.to_s
+  end
+
+  def cjk_width(str)
+    str.unicode_normalize(:nfkc)
+  end
+end