Editing Shift JIS (section)

=== Compatibility ===
Shift JIS is fully [[backward compatibility|backwards compatible]] with the {{nowrap|[[JIS X 0201]]}} [[single-byte encoding]], meaning that any valid {{nowrap|JIS X 0201}} string is also a valid Shift JIS string.

Double-byte characters in {{nowrap|JIS X 0208}} need to be transformed in order to be encoded in Shift JIS. For a double-byte JIS X 0208 sequence <math>j_1 j_2</math>,{{efn|In JIS X 0208, ''j''<sub>1</sub> and ''j''<sub>2</sub> are each in the range 33 (0x21) to 126 (0x7e) inclusive (i.e., 7-bit character values excluding control characters (0&ndash;31 (0x1f) and 127 (0x7f)) and space).}} the transformation to the corresponding Shift JIS bytes <math>s_1 s_2</math> is:

:<math>s_1 = \begin{cases} \left \lfloor \frac{j_1 + 1}{2} \right \rfloor + 112  & \mbox{if } 33 \le j_1 \le 94 \\
                           \left \lfloor \frac{j_1 + 1}{2} \right \rfloor + 176  & \mbox{if } 95 \le j_1 \le 126             \end{cases}</math>
:<math>s_2 = \begin{cases} j_2 + 31 + \left \lfloor \frac{j_2}{96} \right \rfloor & \mbox{if } j_1 \mbox{ is odd }\\
                           j_2 + 126 & \mbox{if } j_1 \mbox{ is even }
             \end{cases}</math>

The competing 8-bit format [[Extended Unix Code#EUC-JP|EUC-JP]], which does not support single-byte halfwidth katakana, allows for a cleaner and more direct conversion to and from JIS X 0208 [[code point]]s, as all high-bit-set bytes are parts of a double-byte character and all codes from ASCII range represent single-byte characters.