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Re: LWG ISSUE: Format's width estimation is too approximate and not forward compatible.

From: Corentin <corentin.jabot_at_[hidden]>
Date: Thu, 15 Sep 2022 19:11:55 +0200
On Thu, Sep 15, 2022 at 5:59 PM Tom Honermann <tom_at_[hidden]> wrote:

> Thank you for looking into this, Corentin. It would be great to have some
> kind of principled approach to how estimated widths are assigned.
>
> I don't think this should be handled as an LWG issue though. This is a
> design change, so should go through LEWG (and probably SG16 as well).
>
> I think it would be helpful to have a paper that includes screenshots (of
> select characters, not all 8500+ characters!) to demonstrate how common
> terminals display them today. The goal being to ensure that they are
> handled consistently across platforms before we standardize them.
>
> A paper would also present the opportunity to attribute an estimated width
> of 0 (or a negative width!) to some characters. Perhaps it is worth asking
> whether an estimated width of 1 really makes sense for CR, LF, NL, VT,
> etc... It looks like these changes still don't cover edge cases like ﷽
> (U+FDFD ARABIC LIGATURE BISMILLAH AR-RAHMAN AR-RAHEEM).
>

I'm not trying to change the design, nor am I trying to make case-by case
decisions, the original paper covered that.
I think these are questions worth asking, sure, at least for control
characters as, for an arbitrary non-cjk, non-emoji neither an information
nor a specification exist.
Note that however it's not straightforward
    * There is no spec so it's handled manually
https://github.com/termux/wcwidth/blob/master/wcwidth.c#L497
    * C1 control characters may be mapped differently on EBCDIC platforms
    * Combining characters not preceded by a grapheme starter are in fact
rendered
    * We already ignore non leading codepoints in graphemes
    * Negative width would be novel

I do not intend to explore these further but I'd be happy to review such
work :)

However, the original paper intended for CJK characters and emojis to be
covered, Unicode 14 and 15 added a number of such characters, I considered
this a bug.
But mostly, I consider specifying anything Unicode related in terms of
partially-assigned ranges to be a bug reminiscent of the identifiers of old.

Now, I expect SG16 will deal with this issue, but all the information is in
the issue.
The only design-ish question is whether we want to grandfather U+3248
CIRCLED NUMBER TEN ON BLACK SQUARE..U+324F CIRCLED NUMBER EIGHTY ON BLACK
SQUARE
for backward compat reasons.

Codepoints which would go from 1 to 2
https://gist.github.com/cor3ntin/e5731f77574b146d806e39283e8c7cb7
Screenshot of my terminal
[image: image.png]

Tom.
> On 9/15/22 5:02 AM, Corentin via SG16 wrote:
>
>
> Affected clause [format.string.std]
>
> Issue:
> For the purpose of width estimation, format considers ranges of
> codepoints initially derived from an implementation of wcwidth with
> modifications.
> (See https://wg21.link/p1868r1)
>
> This however present a number of challenges:
>
> - From a reading of the spec, it is not clear how these ranges were
> selected.
> - Poor forward compatibility with future Unicode versions. The list
> will become less and less meaningful overtime or require active maintenance
> at each Unicode release (which we have not done for Unicode 14 already)
> - Some of these codepoints are unassigned or otherwise reserved, which
> is another forward compatibility concern.
>
> Instead, we propose to
>
> - Rely on UAX-11 https://www.unicode.org/reports/tr11/ for most of the
> codepoints)
> - Grand-father specific and fully assigned, blocks of codepoints to
> support additional pictograms per the original intent of the paper and
> existing practices. We add the name of these blocks in the wording for
> clarity.
>
> Note that per UAX-11
>
> - Most emojis are considered East_Asian_Width="W"
> - By design, East_Asian_Width="W" includes specific unassigned ranges,
> which should always be treated as Wide.
>
> This change:
>
> - Considers 8477 extra codepoints as having a width 2 (as of Unicode
> 15) (mostly Tangut Ideographs)
> - Change the width of 85 unassigned code points from 2 to 1
> - Change the width of 8 codepoints (in the range U+3248 CIRCLED NUMBER
> TEN ON BLACK SQUARE ..U+324F CIRCLED NUMBER EIGHTY ON BLACK SQUARE) from 2
> to 1, because it seems questionable to make an exception for those without
> input from Unicode
>
>
> Proposed wording:
>
> Modify [format.string.std]/p12
>
> For a string in a Unicode encoding, implementations should estimate the
> width of a string as the sum of estimated widths of the first code points
> in its extended grapheme clusters. The extended grapheme clusters of a
> string are defined by UAX #29. The estimated width of the following code
> points is 2:
>
> - U+1100 – U+115F
> - U+2329 – U+232A
> - U+2E80 – U+303E
> - U+3040 – U+A4CF
> - U+AC00 – U+D7A3
> - U+F900 – U+FAFF
> - U+FE10 – U+FE19
> - U+FE30 – U+FE6F
> - U+FF00 – U+FF60
> - U+FFE0 – U+FFE6
> - U+1F300 – U+1F64F
> - U+1F900 – U+1F9FF
> - U+20000 – U+2FFFD
> - U+30000 – U+3FFFD
> - *Any codepoint with the East_Asian_Width="W"
> or East_Asian_Width="F" Derived Extracted Property as described by UAX #44*
> -
> *U+4DC0 - U+4DFF (Yijing Hexagram Symbols) *
> - *U+1F300 - U+1F5FF (Miscellaneous Symbols and Pictographs)*
> - *U+1F900 - U+1F9FF (Supplemental Symbols and Pictographs)*
>
> The estimated width of other code points is 1.
>
>
> ---- END OF WORDING ----
>
> Annex: Differences introduced by this change
>
> // Missing from the standard (count: 8477)
> // Used to be of width 1, becomes 2
>
> U+231A WATCH ..U+231B HOURGLASS
> U+23E9 BLACK RIGHT-POINTING DOUBLE TRIANGLE ..U+23EC BLACK DOWN-POINTING
> DOUBLE TRIANGLE
> U+23F0 ALARM CLOCK
> U+23F3 HOURGLASS WITH FLOWING SAND
> U+25FD WHITE MEDIUM SMALL SQUARE ..U+25FE BLACK MEDIUM SMALL SQUARE
> U+2614 UMBRELLA WITH RAIN DROPS ..U+2615 HOT BEVERAGE
> U+2648 ARIES ..U+2653 PISCES
> U+267F WHEELCHAIR SYMBOL
> U+2693 ANCHOR
> U+26A1 HIGH VOLTAGE SIGN
> U+26AA MEDIUM WHITE CIRCLE ..U+26AB MEDIUM BLACK CIRCLE
> U+26BD SOCCER BALL ..U+26BE BASEBALL
> U+26C4 SNOWMAN WITHOUT SNOW ..U+26C5 SUN BEHIND CLOUD
> U+26CE OPHIUCHUS
> U+26D4 NO ENTRY
> U+26EA CHURCH
> U+26F2 FOUNTAIN ..U+26F3 FLAG IN HOLE
> U+26F5 SAILBOAT
> U+26FA TENT
> U+26FD FUEL PUMP
> U+2705 WHITE HEAVY CHECK MARK
> U+270A RAISED FIST ..U+270B RAISED HAND
> U+2728 SPARKLES
> U+274C CROSS MARK
> U+274E NEGATIVE SQUARED CROSS MARK
> U+2753 BLACK QUESTION MARK ORNAMENT ..U+2755 WHITE EXCLAMATION MARK
> ORNAMENT
> U+2757 HEAVY EXCLAMATION MARK SYMBOL
> U+2795 HEAVY PLUS SIGN ..U+2797 HEAVY DIVISION SIGN
> U+27B0 CURLY LOOP
> U+27BF DOUBLE CURLY LOOP
> U+2B1B BLACK LARGE SQUARE ..U+2B1C WHITE LARGE SQUARE
> U+2B50 WHITE MEDIUM STAR
> U+2B55 HEAVY LARGE CIRCLE
> U+A960 HANGUL CHOSEONG TIKEUT-MIEUM ..U+A97C HANGUL CHOSEONG
> SSANGYEORINHIEUH
> U+16FE0 TANGUT ITERATION MARK ..U+16FE4 KHITAN SMALL SCRIPT FILLER
> U+16FF0 VIETNAMESE ALTERNATE READING MARK CA ..U+16FF1 VIETNAMESE
> ALTERNATE READING MARK NHAY
> U+17000 TANGUT IDEOGRAPH-# ..U+187F7 TANGUT IDEOGRAPH-#
> U+18800 TANGUT COMPONENT-001 ..U+18CD5 KHITAN SMALL SCRIPT CHARACTER-#
> U+18D00 TANGUT IDEOGRAPH-# ..U+18D08 TANGUT IDEOGRAPH-#
> U+1AFF0 KATAKANA LETTER MINNAN TONE-2 ..U+1AFF3 KATAKANA LETTER MINNAN
> TONE-5
> U+1AFF5 KATAKANA LETTER MINNAN TONE-7 ..U+1AFFB KATAKANA LETTER MINNAN
> NASALIZED TONE-5
> U+1AFFD KATAKANA LETTER MINNAN NASALIZED TONE-7 ..U+1AFFE KATAKANA LETTER
> MINNAN NASALIZED TONE-8
> U+1B000 KATAKANA LETTER ARCHAIC E ..U+1B122 KATAKANA LETTER ARCHAIC WU
> U+1B132 HIRAGANA LETTER SMALL KO
> U+1B150 HIRAGANA LETTER SMALL WI ..U+1B152 HIRAGANA LETTER SMALL WO
> U+1B155 KATAKANA LETTER SMALL KO
> U+1B164 KATAKANA LETTER SMALL WI ..U+1B167 KATAKANA LETTER SMALL N
> U+1B170 NUSHU CHARACTER-# ..U+1B2FB NUSHU CHARACTER-#
> U+1F004 MAHJONG TILE RED DRAGON
> U+1F0CF PLAYING CARD BLACK JOKER
> U+1F18E NEGATIVE SQUARED AB
> U+1F191 SQUARED CL ..U+1F19A SQUARED VS
> U+1F200 SQUARE HIRAGANA HOKA ..U+1F202 SQUARED KATAKANA SA
> U+1F210 SQUARED CJK UNIFIED IDEOGRAPH-624B ..U+1F23B SQUARED CJK UNIFIED
> IDEOGRAPH-914D
> U+1F240 TORTOISE SHELL BRACKETED CJK UNIFIED IDEOGRAPH-672C ..U+1F248
> TORTOISE SHELL BRACKETED CJK UNIFIED IDEOGRAPH-6557
> U+1F250 CIRCLED IDEOGRAPH ADVANTAGE ..U+1F251 CIRCLED IDEOGRAPH ACCEPT
> U+1F260 ROUNDED SYMBOL FOR FU ..U+1F265 ROUNDED SYMBOL FOR CAI
> U+1F680 ROCKET ..U+1F6C5 LEFT LUGGAGE
> U+1F6CC SLEEPING ACCOMMODATION
> U+1F6D0 PLACE OF WORSHIP ..U+1F6D2 SHOPPING TROLLEY
> U+1F6D5 HINDU TEMPLE ..U+1F6D7 ELEVATOR
> U+1F6DC WIRELESS ..U+1F6DF RING BUOY
> U+1F6EB AIRPLANE DEPARTURE ..U+1F6EC AIRPLANE ARRIVING
> U+1F6F4 SCOOTER ..U+1F6FC ROLLER SKATE
> U+1F7E0 LARGE ORANGE CIRCLE ..U+1F7EB LARGE BROWN SQUARE
> U+1F7F0 HEAVY EQUALS SIGN
> U+1FA70 BALLET SHOES ..U+1FA7C CRUTCH
> U+1FA80 YO-YO ..U+1FA88 FLUTE
> U+1FA90 RINGED PLANET ..U+1FABD WING
> U+1FABF GOOSE ..U+1FAC5 PERSON WITH CROWN
> U+1FACE MOOSE ..U+1FADB PEA POD
> U+1FAE0 MELTING FACE ..U+1FAE8 SHAKING FACE
> U+1FAF0 HAND WITH INDEX FINGER AND THUMB CROSSED ..U+1FAF8 RIGHTWARDS
> PUSHING HAND
>
> // Reserved (count: 85) - Become used to be of width 2, becomes 1
>
> U+2E9A
> U+2EF4 ..U+2EFF
> U+2FD6 ..U+2FEF
> U+2FFC ..U+2FFF
> U+3040
> U+3097 ..U+3098
> U+3100 ..U+3104
> U+3130
> U+318F
> U+31E4 ..U+31EF
> U+321F
> U+A48D ..U+A48F
> U+A4C7 ..U+A4CF
> U+FE53
> U+FE67
> U+FE6C ..U+FE6F
> U+FF00
>
> // Used to be of width 2, becomes 1
> U+3248 CIRCLED NUMBER TEN ON BLACK SQUARE ..U+324F CIRCLED NUMBER EIGHTY
> ON BLACK SQUARE
>
>
>
>

Received on 2022-09-15 17:12:10