Mike,

I agree with your analysis.

re: "I suspect we should just provide our definitions rather than switching terms, but I'm open to it if we want to convert all uses of codepoint to "code unit"."
There is already a lot of confusion ( in the world of software ) around Unicode and its terminology. Our goal should be to use terminology that is consistent with Unicode's standard - otherwise I foresee a lot of opportunity for confusion, leading to divergent implementations of DFDL. I would prefer us to switch to the standard terms unless it's really painful.

regards,

Tim Kimber, DFDL Team,
Hursley, UK
Internet: kimbert@uk.ibm.com
Tel. 01962-816742
Internal tel. 37246742

From: Mike Beckerle <mbeckerle.dfdl@gmail.com>
To: dfdl-wg@ogf.org,
Date: 28/01/2013 15:57
Subject: [DFDL-WG] Glossary items needed (for a v12 errata?)
Sent by: dfdl-wg-bounces@ogf.org

We need to add some entries associated with character set and encoding terminology that we use quite a bit.

I would note that our usage of the term 'codepoint' differs somewhat from the Unicode Glossary: http://unicode.org/glossary. First, we use codepoint as one word not "code point" (there was some inconsistency on this that I have now fixed), second, what we call codepoint is closer to what Unicode Glossary calls 'code unit'. I suspect we should just provide our definitions rather than switching terms, but I'm open to it if we want to convert all uses of codepoint to "code unit".

Encoding - See Character Set Encoding

Codepoint - When a character set encoding uses differing variable width representations for characters, the units making up these variable width representations are called codepoints. For example the UTF-8 encoding uses between 1 and 4 codepoints to represent characters, and for UTF-8, the codepoints are single bytes. The UTF-16 encoding is either fixed or variable width. When dfdl:utf16Width='variable' this encoding uses either one or two codepoints per character and each codepoint is a 16-bit value. When a character set is fixed width, then there is no distinction between a codepoint and a character code.

Code page - An alternate identifier for a Character Set Encoding.

Character Code - The numeric value assigned to a character in a character set that is independent of any specific encoding of that character set. For any fixed-size encoding (all characters have the same size representation)

Character Set - An abstract set of characters independent of any specific encoding scheme: Examples are the Unicode character set, or the USASCII character set.

Character Set Encoding - A specific representation of a character set as bytes or bits of data. A character set encoding is usually identified by a standard character set name or a recognized alias name, or by a code page identifier. These identifiers are standardized by the IANA. Examples are UTF-8, USASCII, GB2312, ebcdic-cp-it, ISO-8859-5, UTF-16BE, Shift_JIS. The DFDL standard allows for implementation-specific character set encodings to be supported, and standardizes one name that is DFDL-specific which is USASCII-7bit-packed.

Character Width - The number of codepoints or bytes used to represent a character in a specific character set encoding is called the character width. Encodings are either fixed width (all characters encoded using the same width), or variable-width (different characters are encoded using different widths). For example the UTF-32 character set encoding has 4-byte character width, whereas USASCII has a 1-byte character width.

Fixed-Width Character Encoding - A character set encoding where all characters are encoded using a single codepoint for their representation. Note that a codepoint may take up one or more bytes.

Surrogate Pair - A Unicode character whose character code value is greater than 0xFFFF can be encoded into variable-width UTF-16BE or UTF-16LE which are variable-width encodings when the DFDL property utf16Width='variable'. In this case the representation uses two adjacent codepoints each of which is called a surrogate, and the pair of which is called a surrogate pair.

Variable-Width Character Encoding - A character set encoding where characters are encoded using one or more codepoints for their representation depending on which specific character is being encoded. An example is UTF-8 which uses from 1 to 4 bytes to encode a character.

...mike

--
Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology | www.tresys.com
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