module RailsI18n::Transliteration::Russian

def lower
  @lower ||= begin
    lower_single = {
      "і"=>"i","ґ"=>"g","ё"=>"yo","№"=>"#","є"=>"e",
      "ї"=>"yi","а"=>"a","б"=>"b",
      "в"=>"v","г"=>"g","д"=>"d","е"=>"e","ж"=>"zh",
      "з"=>"z","и"=>"i","й"=>"y","к"=>"k","л"=>"l",
      "м"=>"m","н"=>"n","о"=>"o","п"=>"p","р"=>"r",
      "с"=>"s","т"=>"t","у"=>"u","ф"=>"f","х"=>"h",
      "ц"=>"ts","ч"=>"ch","ш"=>"sh","щ"=>"sch","ъ"=>"'",
      "ы"=>"y","ь"=>"","э"=>"e","ю"=>"yu","я"=>"ya",
    }
    (lower_single.merge(lower_multi)).freeze
  end
end

def lower_multi
  @lower_multi ||= { "ье"=>"ie", "ьё"=>"ie" }
end

def multi_keys
  @multi_keys ||= (lower_multi.merge(upper_multi)).keys.sort_by {|s| s.length}.reverse.freeze
end

def rule
  lambda do |string|
    next '' unless string
    chars = string.scan(%r{#{multi_keys.join '|'}|\w|.})
    result = +""
    chars.each_with_index do |char, index|
      if upper.has_key?(char) && lower.has_key?(chars[index+1])
        # combined case
        result << upper[char].downcase.capitalize
      elsif upper.has_key?(char)
        result << upper[char]
      elsif lower.has_key?(char)
        result << lower[char]
      else
        result << char
      end
    end
    result
  end
end

def upper
  @upper ||= begin
    upper_single = {
      "Ґ"=>"G","Ё"=>"YO","Є"=>"E","Ї"=>"YI","І"=>"I",
      "А"=>"A","Б"=>"B","В"=>"V","Г"=>"G",
      "Д"=>"D","Е"=>"E","Ж"=>"ZH","З"=>"Z","И"=>"I",
      "Й"=>"Y","К"=>"K","Л"=>"L","М"=>"M","Н"=>"N",
      "О"=>"O","П"=>"P","Р"=>"R","С"=>"S","Т"=>"T",
      "У"=>"U","Ф"=>"F","Х"=>"H","Ц"=>"TS","Ч"=>"CH",
      "Ш"=>"SH","Щ"=>"SCH","Ъ"=>"'","Ы"=>"Y","Ь"=>"",
      "Э"=>"E","Ю"=>"YU","Я"=>"YA",
    }
    (upper_single.merge(upper_multi)).freeze
  end
end

def upper_multi
  @upper_multi ||= { "ЬЕ"=>"IE", "ЬЁ"=>"IE" }
end