module Rfc2047

# File lib/rfc_2047.rb, line 36
def decode(input)
  return input unless input.match?(ENCODED_WORD)

  input.gsub(ENCODED_WORD_SEQUENCE) do |match|
    result = +''
    match.scan(ENCODED_WORD) { result << decode_word($&) }
    if result.encoding == Encoding::UTF_7
      result.replace(
        decode_utf7(result.force_encoding(Encoding::BINARY))
      ).force_encoding(Encoding::UTF_8)
    else
      result.encode!(Encoding::UTF_8)
    end
    result
  end
end

# File lib/rfc_2047.rb, line 15
def encode(input, encoding: :B)
  return input if input.ascii_only?

  case encoding
  when :B
    size = 45
    chunks = Array.new(((input.bytesize + size - 1) / size)) { input.byteslice(_1 * size, size) }
    chunks.map! { "=?#{input.encoding}?B?#{[_1].pack('m0')}?=" }.join(' ')
  when :Q
    [input]
      .pack('M').each_line
      .map { "=?#{input.encoding}?Q?#{_1.chomp!.gsub(' ', '_')}?=" }
      .join(' ')
  else raise ":encoding should be either :B or :Q, got #{encoding}"
  end
end

# File lib/rfc_2047.rb, line 87
def decode_utf7(s)
  s.gsub(/&([^-]+)?-/n) do
    if Regexp.last_match(1)
      (Regexp.last_match(1).tr(',', '/') + '===').unpack1('m').encode(Encoding::UTF_8, Encoding::UTF_16BE)
    else
      '&'
    end
  end
end

# File lib/rfc_2047.rb, line 55
def decode_word(input)
  match_data = ENCODED_WORD.match(input)
  raise ArgumentError if match_data.nil?

  charset, encoding, encoded_text = match_data.captures
  charset = 'CP950' if charset == 'MS950'

  decoded =
    case encoding
    when 'Q', 'q' then encoded_text.gsub('_', '=20').unpack1('M')
    when 'B', 'b' then encoded_text.unpack1('m')
    end
  found_encoding = find_encoding(charset)
  found_encoding = Encoding::UTF_8 if found_encoding == Encoding::ASCII_8BIT
  decoded.force_encoding(found_encoding)
end

# File lib/rfc_2047.rb, line 72
def find_encoding(charset)
  case charset.downcase
  when 'utf-16' then Encoding::UTF_16BE
  when 'utf-32' then Encoding::UTF_32BE
  when 'ks_c_5601-1987' then Encoding::CP949
  when 'shift-jis' then Encoding::Shift_JIS
  when 'gb2312' then Encoding::GB18030
  when 'ms950' then Encoding::CP950
  when '8bit' then Encoding::ASCII_8BIT
  when 'latin2' then Encoding::ISO_8859_2
  else Encoding.find(charset)
  end
end