= Collation (Unicode Collation Algorithm) =
:author: Samantha McVey
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[abstract]
== Abstract ==
With Unicode applications widely deployed, multilingual data is the rule, not
the exception. Even when there was only ASCII, a pure sort by codepoint will
cause a capital `Z` to sort before a lowercase `a`x. With Unicode the number of
codepoints makes it even more clear that we cannot rely on the integer assigned
to each codepoint as a basis for how it should be sorted for text to be presented
to the user.

NOTE: The only op that uses the UCA is the `unicmp_s` op. Other forms of string
compare such as `cmp_s` go based on codepoint differences

In addition, due to Grapheme Cluster's, there may be multiple codepoints to
represent a single user visible character. It becomes clear that there must be
a solution for us to be able to sort this text in a way that makes sense to
the user. The Unicode Collation Algorithm was created to solve this problem.
It decouples both the _value_ and the _quantity_ of codepoints from the sort.


The form of the data in the UCA consists of <<CAE,Collation Array Element's>>
consisting of **Primary**, **Secondary** and **Tertiary** values of the form
`[* Primary | Secondary | Tertiary ]`.
The `*` represents whether it is something like punctuation that can be skipped
over. MoarVM does not yet support the ability to skip punctuation and spaces
using that value, although it supports customized sorting of each of the three
levels (Primary, Secondary, Tertiary). Levels can be enabled, reversed or disabled.

**Primary** level is the collation value for the character itself, so `a` and `A`
have the same Primary value. **Secondary** is used for diacritics and their counterparts
based on the script. **Tertiary** is for case as well as minor character variations.
This is a slight simplification, but this holds true for almost all `Latin` script
codepoints.

== Data Examples ==

=== DUCET Values ===

For example: with the UCA we are able to sort `æ` following `ae` and `ﬁ` following
`fi` despite them being a different number of codepoints. While most codepoints
map to only one Collation Array Element, some single codepoints map to
multiple. For example: `㌀` maps to 5:

```
㌀` [.3E71.0020.001C][.3E8B.0020.001C][.0000.0038.001C][.1C73.0020.001C][.3E85.0020.001C]
```
Compare this to the collation elements for the characters which are visually in
this “boxed” character:
```
ア U+30A2 [.3E71.0020.0011] # KATAKANA LETTER A
パ U+30D1 [.3E8B.0020.0011][.0000.0038.0002] # KATAKANA LETTER PA
ー U+30FC [.1C73.0020.0002] # KATAKANA-HIRAGANA PROLONGED SOUND MARK
ト U+30C8 [.3E85.0020.0011] # KATAKANA LETTER TO
```
As you may have guessed, this allows them to sort right next to each other, with
the exception of the tertiary collation values (as it is a letter variation).

*Multiple* codepoints can be assigned to a *single value* (as well as
*multiple* as well):

```
ꪵꪦ U+AAB5, U+AAA6 [.2EB6.0020.0002][.2EC5.0020.0002] # <TAI VIET VOWEL E, TAI VIET LETTER LOW RO>
```

=== Computed Collation Values ===
Some characters are on the other hand computed. This would include Unified Ideographs,
Tangut, Nushu and Unassigned.


== Implementation ==

This is an implementation of the Unicode Collation Algorithm using DUCET values.
We implement the standard “Non-ignorable” sort, as it does not ignore punctuation
or spaces while sorting.

We iterate by codepoint and put this into a ring buffer. The ring buffers hold the exact
number of codepoints which comprise the longest sequence of codepoints which
map to its own collation keys in the Unicode Collation Algorithm. As of Unicode
9.0 this number was 3. In case future versions contain longer series of codepoints,
`Generate-Collation-Data.p6` updates this number when generating the C data.

The iteration into the ringbuffer stops as soon as a non-matching codepoint, is
found. Whether the two codepoints are Less/More/Same compared to each other
is saved in a variable for later in case we end up needing to break a tie by codepoint.

The vast majority of the time we only need to use what is in the ring buffer to decide
whether one string is greater or less than the other string.
The elements in the ringbuffer are either passed into our function which finds
and pushes the collation arrays onto the stack or reordered to be first to last
and then pushed.

We then compare by primary levels, all the keys pushed so far.
If all the primaries match then we iterate more codepoints and push the
collation array elements onto a stack. This stack is malloced and can expand as needed,
but this should practically never be the case.

=== The Stack ===

The stack lets us do is a modified version of the UCA which lets us not
have to push all primaries from start to end of the string onto a one
dimensional array, and then after that push all the secondary, then all the
tertiary.

So: `[.3E8B.0020.0011][.0000.0038.0002]` would become
    `3E8B 0000 | 0020 0038 | 0011 0002` (`|` shown between the different levels).

Doing it like this would cause us to have to start pushing from our starting
position to the very end of the string if we flattened the collation arrays.
Instead we keep track of both are position in the stack, but also which level
we are on, moving further on the stack then pushing more arrays as needed.
If the primary values all tie, we wrap and go to the beginning of the stack but
on the subsequent level. String a and b are not necessarily on the same position
in the stack, or on the same collation level.

=== The Data ===

Codepoints which have single collation array elements get the data from the MVM
UCD database created with `ucd2c.pl`. Any codepoints which have more than one
collation array element or if it is a sequence of codepoints, that uses the data
in `src/strings/unicode_uca.c`. The data in `unicode_uca.c` is this:
`main_nodes` contains a linked list representation. Although all the nodes
are in the same `main_nodes` struct array, we `#define` how many root nodes there
are, and this number lets us do a binary search of the root nodes. If we get a
match, we check if there are any sub node elements, and if none, we then use
the `collation_link` and collation_elems` values to push the specified number of
collation elements from the correct location in the `special_collation` struct
array onto the stack. If there _are_ more possibilities, if we don't have anymore
codepoints passed to `collation_push_cp`, we grab more and then use `sub_node_link`
and `sub_node_elems` to do a linear search, stopping if we see any codepointns which
are higher than the one we are looking for. The reason linear search is used is
because we have 1-18 or so subnodes from each parent node, and binary search
is slower for small numbers of elements.

Tangut, Ideographs, Nushu and Unassigned codepoints have collation values which
are generated algorithmically based on their codepoint. Hangul characters
decompose before they are pushed onto the array.

=== Configuration ===

We support the ability to configure collation so you can reverse or
disable levels as you wish. The trick to this is knowing that for all collation
values: `_tertiary_ *<* _secondary_ *<* _primary_`

We use `level_eval_settings` to store the settings for each level, which we set
up based on the bitmask of the collation_mode argument to the function. If the
two levels are the same we are able to compare them based on the setting. If the
levels are not equal, we do not need to do this, since tertiary < secondary <
primary for all values.

Some info on our collation values. They are all 1 higher than those listed for
DUCET (Default Unicode Collation Element Table). The reason for this is that a 0
counts as 0 while a 1 is skipped and ignorable. This corresponds to things
listed as 0 in DUCET, which our implementation gives a value of 1. We only use 0
for the tertiary value of the level separator to ensure that longer strings win
(though we also have a fallback to ensure this happens in certain cases which
this isn't enough).

== Return Value/Bitmask ==

MoarVM function: `MVM_unicode_string_compare`
[source,c]
MVMint64 MVM_unicode_string_compare(MVMThreadContext *tc, MVMString *a, MVMString *b,
         MVMint64 collation_mode, MVMint64 lang_mode, MVMint64 country_mode)

Op: `unicmp_s`
[source,perl6]
unicmp_s(str a, str b, int collation_mode, int lang_mode, int country_mode)


.Return values:
[width="75",cols="0,1"]
|==============
|    0 |  The strings are identical for the collation levels requested
| -1/1  | String a is less than string b/String a is greater than string b
|==============

`collation_mode` acts like a bitmask. Each of primary, secondary and tertiary
collation levels can be either: disabled, enabled, reversed.
In the table below, where + designates sorting normal direction and
- indicates reversed sorting for that collation level.

[options="header",width="0"]
|==================
|Collation level | bitfield value
|        Primary+ |   1
|        Primary− |   2
|      Secondary+ |   4
|      Secondary− |   8
|       Tertiary+ |  16
|       Tertiary− |  32
|     Quaternary+ |  64
|     Quaternary- | 128
|==================


== The Future ==

=== Language Specific Sort ===
:CLDRlatest: http://unicode.org/Public/cldr/latest
:CLDRcore:   http://unicode.org/Public/cldr/latest/core.zip

In the future we may support language specific sort. This data will have to be
taken from the Unicode CLDR (Common Language Data Repository), as it is not
part of DUCET. {CLDRcore}[`core.zip`] contains a folder `./core/collation`
which contains XML files with notes for different languages. To read the specs
of how to interpret these files, see the {CLDRSpec}[CLDR Spec page].

=== Natural Sorting/Number based sorting ===
:nat-sort: https://en.wikipedia.org/wiki/Natural_sort_order

This is another possible addition, called {nat-sort}[Natural Sorting].
We can sort `<12 9>` as `9, 12` instead of
`12, 9`. Since we use a ring buffer to find where codepoints differ. I think we
will not have to backtrack any, we only have to care about codepoints _including_
and _after_ the differing codepoint. Since we know all codepoints before must
have matched before this point, we should only have to see how long each number
is from that point on.

[glossary]
== Glossary ==

[[CAE]] Collation Array Element::
    Made up of primary, secondary, tertiary and a boolean for ignorable (whether
    it should be ignored when ignoring punctuation is wanted).
DUCET::
    Default Unicode Collation Element Table. This data is provided by Unicode and
    provides us with the collation arrays we use. See <<TR10>> for more information.
Grapheme::
    Short for Grapheme Cluster. See <<TR29>> for more information.
Synthetic::
    In MoarVM, a special representative to store a grapheme containing more than
    one codepoint using the same space as a standard codepoint. Internally
    stored using negative numbers in the C string data array.

[bibliography]
== References
- [[[TR10]]] **Unicode Technical Report 10**. _Unicode Collation Algorithm_. http://unicode.org/reports/tr10/
- [[[TR29]]] **Unicode Technical Report 29**. _Unicode Text Segmentation_. http://unicode.org/reports/tr29/
