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Alphabetic Index

Status

Available as public ICU API
See also CLDR:

Original design

What we need from collation to make the indexing work without hardcoded data are the following:

1. Given a collation and a string, get the least character in a different script that is greater than that string.

I'm implementing this as a hack currently, just doing a binary search in the following list:

                "a", "α", "ⲁ", "а", "ⰰ", "ა", "ա", "א", "𐤀", "ࠀ", "ء", "ܐ", "ހ", "ߊ", "ⴰ", "ሀ", "ॐ", "অ", "ੴ", "ૐ", "ଅ", "ௐ",
                "అ", "ಅ", "അ", "අ", "ꯀ", "ꠀ", "ꢂ", "𑂃", "ᮃ", "𐨀", "ก", "ກ", "ꪀ", "ཀ", "ᰀ", "ꡀ", "ᤀ", "ᜀ", "ᜠ", "ᝀ", "ᝠ",
                "ᨀ", "ꤰ", "ꤊ", "က", "ក", "ᥐ", "ᦀ", "ᨠ", "ꨀ", "ᬅ", "ꦄ", "ᢀ", "ᱚ", "Ꭰ", "ᐁ", "ᚁ", "ᚠ", "𐰀", "ꔀ", "ꚠ", "ᄀ",
                "ぁ", "ァ", "ㄅ", "ꀀ", "ꓸ", "𐊀", "𐊠", "𐤠", "𐌀", "𐌰", "𐐨", "𐑐", "𐒀", "𐀀", "𐠀", "𐩠", "𐬀", "𐡀",
                "𐭀", "𐭠", "𐎀", "𐎠", "𒀀", "𓀀", "一"

Since we will have this information internally for the script blocks as part of the reordering, that should be fairly straightforward to implement, but will take some more data structure.


2. Han Collation

This collation is special, especially for Chinese. The ordering is based on mapping characters to some data, then ordering by that mapping data. The main ones are:
  1. Pinyin: compare according to the pinyin for each character. If the pinyin is the same, compare by stroke order.
  2. Stroke: compare according to the total strokes for each character. If the total strokes are the same, compare by radical-stroke order.
  3. Radical-Stroke: compare according to the radical-stroke for each character. If these are the same, compare by code point order.
That means that the index needs to be precisely aligned with the sorting method, and the index buckets need to be chosen to start on mapping data boundaries appropriate for each of the above. If they are out-of-sync, strange results occur. The labels also need to be based on the mapping data:
  1. Pinyin: A B C D ...
  2. Stroke: 1 2 3 ...
  3. Radical-Stroke: radicals
In actual display, the labels may be winnowed down, or presented differently (eg with Stroke the word for "strokes"). For pinyin, notice that both Latin and Chinese characters are in the same buckets. Ideally Latin is last; for that we'll wait until script-reordering is in.

This requires not only that the mapping data be available at collation-rule construction time, but that it also be available at runtime. Luckily, not all of the data needs to be available, just what the first character of each of the above buckets is. In an ideal world, that would be another method on the collator. The situation is complicated by the need to have alternative versions of the collators to cut down on size, so we can't depend on the first character of each group being stable for all the pinyin collations.

Rather than have some special syntax, the plan is to introduce special private-use codes, one for each normal index boundary. These will be added to the rules for each of the above collations.

... X < \U000F0000 < Y ...

The ranges for these will be hard-coded starting at \U000F0000.
  • 512 for the radicals (the extra bit is for the simplified vs traditional), then
  • 26 for A-Z for pinyin, then
  • 100 for stroke counts.
This has a very minor effect on the size of the collations, but also allows alternative tailorings and sizes of the tables, once we define the use of these codes.

3. Which Pinyin?

To generate the pinyin for each character, we are creating an ordered list from the following. The first item in the list is the one used for the collation and transliteration, so the ordering of the list is important. An item is not added to the list if it is already in the list.
  1. kHanyuPinlu
  2. kXHC1983
  3. kHanyuPinyin
  4. kMandarin
This was the priority order recommended by Richard Cook and John Jenkins.

Some additional pinyin data is added, and the data is closed under:
  1. radical<->Unified mapping (to give the radicals a sort order)
  2. kTraditionalVariant
  3. kSimplifiedVariant
As before, these are only added to the end of the list, so make no effective difference unless there was no pinyin already.

Any mappings from characters that are canonical equivalents are removed, since UCA already handles that.

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