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66 lines
3.1 KiB
Markdown
66 lines
3.1 KiB
Markdown
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### Javascript porting of Markus Kuhn's wcwidth() implementation
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The following explanation comes from the original C implementation:
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This is an implementation of wcwidth() and wcswidth() (defined in
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IEEE Std 1002.1-2001) for Unicode.
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http://www.opengroup.org/onlinepubs/007904975/functions/wcwidth.html
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http://www.opengroup.org/onlinepubs/007904975/functions/wcswidth.html
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In fixed-width output devices, Latin characters all occupy a single
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"cell" position of equal width, whereas ideographic CJK characters
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occupy two such cells. Interoperability between terminal-line
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applications and (teletype-style) character terminals using the
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UTF-8 encoding requires agreement on which character should advance
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the cursor by how many cell positions. No established formal
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standards exist at present on which Unicode character shall occupy
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how many cell positions on character terminals. These routines are
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a first attempt of defining such behavior based on simple rules
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applied to data provided by the Unicode Consortium.
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For some graphical characters, the Unicode standard explicitly
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defines a character-cell width via the definition of the East Asian
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FullWidth (F), Wide (W), Half-width (H), and Narrow (Na) classes.
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In all these cases, there is no ambiguity about which width a
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terminal shall use. For characters in the East Asian Ambiguous (A)
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class, the width choice depends purely on a preference of backward
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compatibility with either historic CJK or Western practice.
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Choosing single-width for these characters is easy to justify as
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the appropriate long-term solution, as the CJK practice of
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displaying these characters as double-width comes from historic
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implementation simplicity (8-bit encoded characters were displayed
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single-width and 16-bit ones double-width, even for Greek,
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Cyrillic, etc.) and not any typographic considerations.
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Much less clear is the choice of width for the Not East Asian
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(Neutral) class. Existing practice does not dictate a width for any
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of these characters. It would nevertheless make sense
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typographically to allocate two character cells to characters such
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as for instance EM SPACE or VOLUME INTEGRAL, which cannot be
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represented adequately with a single-width glyph. The following
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routines at present merely assign a single-cell width to all
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neutral characters, in the interest of simplicity. This is not
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entirely satisfactory and should be reconsidered before
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establishing a formal standard in this area. At the moment, the
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decision which Not East Asian (Neutral) characters should be
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represented by double-width glyphs cannot yet be answered by
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applying a simple rule from the Unicode database content. Setting
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up a proper standard for the behavior of UTF-8 character terminals
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will require a careful analysis not only of each Unicode character,
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but also of each presentation form, something the author of these
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routines has avoided to do so far.
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http://www.unicode.org/unicode/reports/tr11/
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Markus Kuhn -- 2007-05-26 (Unicode 5.0)
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Permission to use, copy, modify, and distribute this software
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for any purpose and without fee is hereby granted. The author
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disclaims all warranties with regard to this software.
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Latest version: http://www.cl.cam.ac.uk/~mgk25/ucs/wcwidth.c
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