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Script Reordering

We would like to add script reordering as a new collation setting. This will allow, for example, sorting Greek before Latin, and digits after all letters, without listing all affected characters in the rules. This can be implemented via a permutation table for primary collation weights.

See the original (somewhat outdated) ICU collation design doc for reference: Script_Order. See also CLDR ticket 2267/3081 and the CLDR design doc for proposed collation additions.

For building the permutation table, we will need to map from a script code to a range of lead bytes of collation elements, and reorder whole ranges. For many scripts, there is only one lead byte. For Latn, there is a range (to optimize Latin collation performance). For Hani, we should probably consider two ranges: The "top" range after the [last non-ignorable] character, which is normally used for CJK tailorings to provide 2-byte primary weights for thousands of characters, as well as the immediately following range of CJK implicit primary weights.

Below are the current UCA values when translated into a Fractional UCA table, listed by lead bytes. For reference, also look at the UCA Charts.

Status

Fully implemented in ICU 4.8 with public API.

Was already mostly implemented in ICU 4.6 with incomplete "@internal" API and a couple of implementation issues.

Issues

  1. We need to have rule syntax and locale code syntax for the reordering, so that we can put them into the rules as defaults for a language.
  2. We have to make sure that when you tailor, the script doesn't change. That is putting something before alpha needs to still be in the Greek block, and putting something after omega still needs to be in the greek block. Done: FractionalUCA.txt 6.0 (ICU 4.6) adds special weights that appear only in invuca.icu not ucadata.icu and serve as weight allocation boundaries.
  3. Variable-top can occur in the middle of a lead byte. But we can't shift half a byte. For comparison, the current variables are: chart_Variable. And do we reorder before or after we do the variable detection? (ICU ticket #8065)
  4. We need to make sure that tailored characters get treated consistently. That is, we have to decide the "script" of the bytes that can be filled by tailored characters: 04, 28, 29, 2B, 2C, CF-DF below. We have two choices.
    1. Fixed. The script code assigned to those bytes is invariant. For example, we could assign CF-DF to be Hani. (I'm leaning towards this.) Done
    2. Dynamic. The script code depends on the tailoring. That means that we would recompute the code and store it for each set of rules.
  5. I think we want to consider reordering other categories, not just scripts. In particular, people seem to want to do numbers (Nd) and currency marks (Sc) separately. These are currently in byte 2A, together with some non-variable gorp (see chart_Common). Byte 05 (controlish whitespace) and 06 (spaces) also may want to be separately reorderable.
    1. In Fractional UCA, we may want to separate byte 2A into three lead bytes: gorp, then currency symbols (Sc), then numbers (Nd/some Nl). That would allow them to be reordered separately.
    2. Done: FractionalUCA.txt 6.0 (ICU 4.6) separates space/punct/symbol/currency/digit groups into separate lead bytes and the script reordering supports them.
    3. TODO: We should split off and support unassigned-implicit weights as a group with separate lead bytes, separate from Han implicits. (ICU ticket #8067)
    4. Trailing weights are not reordered, just like separators (bytes 00..02) and specials (F0..FF).
  6. The UCA has some peculiarities exposed by script reordering. We may want to have a tailoring in CLDR for this and/or take it to the UTC.
    1. There are two Sc characters that are oddly sorted with other blocks (4F{Latn}=1 6A{Arab}=1). This would be easy to fix with a tailoring. Done
    2. Ogham space mark is not sorted with the same primary as the other spaces. We make Space a single byte for space and speed (06). If we tailored the Ogham space mark to be the same as Space, reordering 06 would reorder all spaces. On the other hand, who cares much about Ogham space mark sorting... Not done
    3. It appears that people generally want to treat punctuation marks different than symbols. It is more common for punctuation to be ignored, while not symbols. So ideally, all the punctuation marks would come before all symbols. Unfortunately, they are interleaved with symbols between 07 and 27 (see  chart_Variable). While it would be a pain to tailor them all so as to have separate lead bytes, it could be done. Done
    4. There are some oddities that, while not an issue for reordering, we might also call attention to:
      1. Po has 95{Talu}=2 Not done
      2. There are a bunch of So's that are all over the map. Done

Fractional UCA

(UCA 5.2 data)
There are two sheets: lead-byte => script/category, and the reverse.

Script Reordering


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