Base65536 encode/decode with Ruby

base65536.rb

module Base65536
  BLOCK_START = {
    0 => 13312,
    1 => 13568, 2 => 13824, 3 => 14080, 4 => 14336, 5 => 14592, 6 => 14848, 7 => 15104, 8 => 15360, 9 => 15616, 10 => 15872, 11 => 16128, 12 => 16384, 13 => 16640, 14 => 16896, 15 => 17152, 16 => 17408, 17 => 17664, 18 => 17920, 19 => 18176, 20 => 18432, 21 => 18688, 22 => 18944, 23 => 19200, 24 => 19456, 25 => 19968, 26 => 20224, 27 => 20480, 28 => 20736, 29 => 20992, 30 => 21248, 31 => 21504, 32 => 21760, 33 => 22016, 34 => 22272, 35 => 22528, 36 => 22784, 37 => 23040, 38 => 23296, 39 => 23552, 40 => 23808, 41 => 24064, 42 => 24320, 43 => 24576, 44 => 24832, 45 => 25088, 46 => 25344, 47 => 25600, 48 => 25856, 49 => 26112, 50 => 26368, 51 => 26624, 52 => 26880, 53 => 27136, 54 => 27392, 55 => 27648, 56 => 27904, 57 => 28160, 58 => 28416, 59 => 28672, 60 => 28928, 61 => 29184, 62 => 29440, 63 => 29696, 64 => 29952, 65 => 30208, 66 => 30464, 67 => 30720, 68 => 30976, 69 => 31232, 70 => 31488, 71 => 31744, 72 => 32000, 73 => 32256, 74 => 32512, 75 => 32768, 76 => 33024, 77 => 33280, 78 => 33536, 79 => 33792, 80 => 34048, 81 => 34304, 82 => 34560, 83 => 34816, 84 => 35072, 85 => 35328, 86 => 35584, 87 => 35840, 88 => 36096, 89 => 36352, 90 => 36608, 91 => 36864, 92 => 37120, 93 => 37376, 94 => 37632, 95 => 37888, 96 => 38144, 97 => 38400, 98 => 38656, 99 => 38912, 100 => 39168,
    101 => 39424, 102 => 39680, 103 => 39936, 104 => 40192, 105 => 40448, 106 => 41216, 107 => 41472, 108 => 41728, 109 => 42240, 110 => 67072, 111 => 73728, 112 => 73984, 113 => 74240, 114 => 77824, 115 => 78080, 116 => 78336, 117 => 78592, 118 => 82944, 119 => 83200, 120 => 92160, 121 => 92416, 122 => 131072, 123 => 131328, 124 => 131584, 125 => 131840, 126 => 132096, 127 => 132352, 128 => 132608, 129 => 132864, 130 => 133120, 131 => 133376, 132 => 133632, 133 => 133888, 134 => 134144, 135 => 134400, 136 => 134656, 137 => 134912, 138 => 135168, 139 => 135424, 140 => 135680, 141 => 135936, 142 => 136192, 143 => 136448, 144 => 136704, 145 => 136960, 146 => 137216, 147 => 137472, 148 => 137728, 149 => 137984, 150 => 138240, 151 => 138496, 152 => 138752, 153 => 139008, 154 => 139264, 155 => 139520, 156 => 139776, 157 => 140032, 158 => 140288, 159 => 140544, 160 => 140800, 161 => 141056, 162 => 141312, 163 => 141568, 164 => 141824, 165 => 142080, 166 => 142336, 167 => 142592, 168 => 142848, 169 => 143104, 170 => 143360, 171 => 143616, 172 => 143872, 173 => 144128, 174 => 144384, 175 => 144640, 176 => 144896, 177 => 145152, 178 => 145408, 179 => 145664, 180 => 145920, 181 => 146176, 182 => 146432, 183 => 146688, 184 => 146944, 185 => 147200, 186 => 147456, 187 => 147712, 188 => 147968, 189 => 148224, 190 => 148480, 191 => 148736, 192 => 148992, 193 => 149248, 194 => 149504, 195 => 149760, 196 => 150016, 197 => 150272, 198 => 150528, 199 => 150784, 200 => 151040,
    201 => 151296, 202 => 151552, 203 => 151808, 204 => 152064, 205 => 152320, 206 => 152576, 207 => 152832, 208 => 153088, 209 => 153344, 210 => 153600, 211 => 153856, 212 => 154112, 213 => 154368, 214 => 154624, 215 => 154880, 216 => 155136, 217 => 155392, 218 => 155648, 219 => 155904, 220 => 156160, 221 => 156416, 222 => 156672, 223 => 156928, 224 => 157184, 225 => 157440, 226 => 157696, 227 => 157952, 228 => 158208, 229 => 158464, 230 => 158720, 231 => 158976, 232 => 159232, 233 => 159488, 234 => 159744, 235 => 160000, 236 => 160256, 237 => 160512, 238 => 160768, 239 => 161024, 240 => 161280, 241 => 161536, 242 => 161792, 243 => 162048, 244 => 162304, 245 => 162560, 246 => 162816, 247 => 163072, 248 => 163328, 249 => 163584, 250 => 163840, 251 => 164096, 252 => 164352, 253 => 164608, 254 => 164864, 255 => 165120,
    -1 => 5376
  }.freeze
  B2 = BLOCK_START.invert.freeze

  class DecodeError < StandardError; end

  class << self
    def encode(str)
      bytes = str.bytes.lazy
      code_points = bytes.each_slice(2).map { |b1, b2| BLOCK_START[b2 || -1] + b1 }
      chars = code_points.map { _1.chr(Encoding::UTF_8) }

      chars.force.join
    end

    def decode(encoded)
      bytes =
        encoded.each_char.flat_map do |char|
          code_point = char.ord
          b1 = get_lower_8_bits(code_point)
          b2 = B2[code_point - b1]
          raise(DecodeError.new("Invalid Base65536 character: #{char}")) unless b2

          b2 == -1 ? b1 : [b1, b2]
        end

      bytes.pack('C*').force_encoding(Encoding::UTF_8)
    end

    private

    def get_lower_8_bits(n)
      n & 0xFF
    end
  end
end

if __FILE__ == $PROGRAM_NAME
  require 'minitest'
  require 'minitest/autorun'

  class Base65536Test < Minitest::Test
    def test_encode
      assert_equal('驈ꍬ慯𔔠𓁯饬ᔡ', Base65536.encode('Hello, world!'))
    end

    def test_decode
      assert_equal('Hello, world!', Base65536.decode('驈ꍬ慯𔔠𓁯饬ᔡ'))
      error = assert_raises(Base65536::DecodeError) do
        Base65536.decode('驈ꍬ慯𔔠𓁯饬ᔡ🐹')
      end
      assert_equal('Invalid Base65536 character: 🐹', error.message)
    end
  end
end

References

• qntm/base65536: Unicode's answer to Base64
• Parkayun/base65536: base65536 Python 2 and 3 implementation.
• Base encoding - この子の名はエビフライとする
• What makes a Unicode code point safe? @ Things Of Interest
• Base64 - MDN Web Docs 用語集: ウェブ関連用語の定義 | MDN