Comparison of Unicode encodings: Difference between revisions

Revision as of 23:55, 15 August 2005

Revision as of 04:12, 19 August 2005

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This page compares Unicode encodings. Two situations are considered: eight-bit-clean environments and environments like [[Simple Mail Transfer Protocol]] that forbid use of byte values that have the high bit set. Originally such prohibitions were to allow for links that only used 7 data bits but they remain in the standards and so software must generate messages that comply with the restrictions. [[Standard Compression Scheme for Unicode]] and [[Binary Ordered Compression for Unicode]] are excluded from the comparison tables because it is difficult to simply quantify their size.

==Summary of size issues==

==In summary==

If space were the only consideration, UTF-32 would loseloses in almost every case since characters outside the [[basic multilingual plane]] are very rare, and one of the bytes of BMP characters in UTF-32 is always 0. For seven-bit environments UTF-7 clearly wins in terms of size over the combination of other Unicode encodings with [[quoted printable]] or [[base64]]. For eight-bit-clean environments things vary considerably depending on what code points are in the text.

==Considerations other than size==

===For processing===

For processing a format should be easy to search truncate and generally process safely. Fixed size characters can be helpfull but it should be remembered that even if there is a fixed width per code point (as in utf-32), there is not a fixed width per displayed character due to [[combining character]]s. Also if you are working with a particular API heavilly and that api has standardised on a particular unicode encoding it is generally a good idea to use the encoding that the API does. UTF-16 is popular because many apis date to the time when unicode was 16 bit fixed width. Unfortunately using UTF-16 encouranges code that does not properly handle code points outside the BMP.

===For communication===

Some protocols may limit you to a specific set of encodings but even when they don't some encodings may offer better compatibility than others with existing implementations. Also the cost of converting between your processing format and your communication format should be considered both in terms of program size (e.g. GB18030 requires a huge mapping table) and run-time requirements. It may simplify matters to use the same format for processing that you are communicating in especially for servers.

==In detail==