# icu_collator [![crates.io](https://img.shields.io/crates/v/icu_collator)](https://crates.io/crates/icu_collator)

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Comparing strings according to language-dependent conventions.

This module is published as its own crate ([`icu_collator`](https://docs.rs/icu_collator/latest/icu_collator/))
and as part of the [`icu`](https://docs.rs/icu/latest/icu/) crate. See the latter for more details on the ICU4X project.
`Collator` is the main structure of the component. It accepts a set of arguments
which allow it to collect necessary data from the data provider, and once
instantiated, can be used to compare strings.

Refer to the ICU User Guide sections for Collation that give an
[introduction](https://unicode-org.github.io/icu/userguide/collation/) and explain
[basic concepts](https://unicode-org.github.io/icu/userguide/collation/concepts.html).

## Examples

As its most basic purpose, `Collator` offers locale-aware ordering:

```rust
use core::cmp::Ordering;
use icu::collator::{options::*, *};
use icu::locale::locale;

let mut options = CollatorOptions::default();
options.strength = Some(Strength::Primary);
let collator_es =
    Collator::try_new(locale!("es-u-co-trad").into(), options).unwrap();

// "pollo" > "polvo" in traditional Spanish
assert_eq!(collator_es.compare("pollo", "polvo"), Ordering::Greater);

let mut options = CollatorOptions::default();
options.strength = Some(Strength::Primary);
let collator_en = Collator::try_new(locale!("en").into(), options).unwrap();

// "pollo" < "polvo" according to English rules
assert_eq!(collator_en.compare("pollo", "polvo"), Ordering::Less);
```

### Examples of `CollatorOptions`

The [`CollatorOptions`] struct configures specific custom behavior for the `Collator`.  See docs
for [`CollatorOptions`] for more details.  Some basic descriptions and examples are below.

### Strength

The collation strength indicates how many levels to compare. The primary
level considers base letters, i.e. 'a' and 'b' are unequal but 'E' and 'é'
are equal, with higher levels dealing with distinctions such as accents
and case.

If an lower level isn't equal, the lower level is decisive.
If the comparison result is equal on one level,
but the collator's strength input value is higher than that,
then the collator comparison iteratively proceeds to the next higher level.

Note that lowering the strength value given to the collator means that more user-perceptible
 differences will compare as equal. This may make sense when sorting more complex structures
where the string to be compared is just one field, and ties between strings
that differ only in case, accent, or similar are resolved by comparing some
secondary field in the larger structure to be sorted.

Therefore, if the sort is just a string sort without some other field for
resolving ties, lowering the strength means that factors that don't make
sense to the user (such as the order of items prior to sorting with a stable
sort algorithm or the internal details of a sorting algorithm that doesn't
provide the stability property) affect the relative order of strings that
do have user-perceptible differences particularly in accents or case.

Lowering the strength is less of a perfomance optimization than it may seem
directly from the above description. As described above, in the case
of identical strings to be compared, the algorithm has to work though all
the levels, from primary up to the provided strength value given to collator, without an early exit. However, this
collator implements an identical prefix optimization, which examines the
code units of the strings to be compared to skip the identical prefix before
starting the actual collation algorithm. When the strings to be compared
are identical on the byte level, they are found to be equal without the
actual collation algorithm running at all! Therefore, the strength setting
only has an effect (whether order effect or performance effect) for
comparisons where the strings to be compared are not equal on the byte level
but are equal on the primary level/strength. The common cases are that
a comparison is decided on the primary level or the strings are byte
equal, which narrows the performance effect of lowering the strength
setting.

```rust
use core::cmp::Ordering;
use icu::collator::{options::*, *};

// Primary Level

let mut options_l1 = CollatorOptions::default();
options_l1.strength = Some(Strength::Primary);
let collator_l1 =
    Collator::try_new(Default::default(), options_l1).unwrap();

assert_eq!(collator_l1.compare("a", "b"), Ordering::Less); // primary
assert_eq!(collator_l1.compare("as", "às"), Ordering::Equal); // secondary
assert_eq!(collator_l1.compare("às", "at"), Ordering::Less);
assert_eq!(collator_l1.compare("ao", "Ao"), Ordering::Equal); // tertiary
assert_eq!(collator_l1.compare("Ao", "aò"), Ordering::Equal);
assert_eq!(collator_l1.compare("A", "Ⓐ"), Ordering::Equal);

// Secondary Level

let mut options_l2 = CollatorOptions::default();
options_l2.strength = Some(Strength::Secondary);
let collator_l2 =
    Collator::try_new(Default::default(), options_l2).unwrap();

assert_eq!(collator_l2.compare("a", "b"), Ordering::Less); // primary
assert_eq!(collator_l2.compare("as", "às"), Ordering::Less); // secondary
assert_eq!(collator_l2.compare("às", "at"), Ordering::Less);
assert_eq!(collator_l2.compare("ao", "Ao"), Ordering::Equal); // tertiary
assert_eq!(collator_l2.compare("Ao", "aò"), Ordering::Less);
assert_eq!(collator_l2.compare("A", "Ⓐ"), Ordering::Equal);

// Tertiary Level

let mut options_l3 = CollatorOptions::default();
options_l3.strength = Some(Strength::Tertiary);
let collator_l3 =
    Collator::try_new(Default::default(), options_l3).unwrap();

assert_eq!(collator_l3.compare("a", "b"), Ordering::Less); // primary
assert_eq!(collator_l3.compare("as", "às"), Ordering::Less); // secondary
assert_eq!(collator_l3.compare("às", "at"), Ordering::Less);
assert_eq!(collator_l3.compare("ao", "Ao"), Ordering::Less); // tertiary
assert_eq!(collator_l3.compare("Ao", "aò"), Ordering::Less);
assert_eq!(collator_l3.compare("A", "Ⓐ"), Ordering::Less);
```

### Alternate Handling

Allows alternate handling for certain customized collation orderings, including the option to
ignore the special handling for the strings of such customizations.  Specifically,
alternate handling is used to control the handling of the so-called **variable** characters in the
Unicode Collation Algorithm: whitespace, punctuation and symbols.

Note that `AlternateHandling::ShiftTrimmed` and `AlternateHandling::Blanked` are
unimplemented. The default is `AlternateHandling::NonIgnorable`, except
for Thai, whose default is `AlternateHandling::Shifted`.

```rust
use core::cmp::Ordering;
use icu::collator::{*, options::*};

// If alternate handling is set to `NonIgnorable`, then differences among
// these characters are of the same importance as differences among letters.

let mut options_3n = CollatorOptions::default();
options_3n.strength = Some(Strength::Tertiary);
options_3n.alternate_handling = Some(AlternateHandling::NonIgnorable);
let collator_3n =
    Collator::try_new(Default::default(), options_3n).unwrap();

assert_eq!(collator_3n.compare("di Silva", "Di Silva"), Ordering::Less);
assert_eq!(collator_3n.compare("Di Silva", "diSilva"), Ordering::Less);
assert_eq!(collator_3n.compare("diSilva", "U.S.A."), Ordering::Less);
assert_eq!(collator_3n.compare("U.S.A.", "USA"), Ordering::Less);

// If alternate handling is set to `Shifted`, then these characters are of only minor
// importance. The Shifted value is often used in combination with Strength
// set to Quaternary.

let mut options_3s = CollatorOptions::default();
options_3s.strength = Some(Strength::Tertiary);
options_3s.alternate_handling = Some(AlternateHandling::Shifted);
let collator_3s =
    Collator::try_new(Default::default(), options_3s).unwrap();

assert_eq!(collator_3s.compare("di Silva", "diSilva"), Ordering::Equal);
assert_eq!(collator_3s.compare("diSilva", "Di Silva"), Ordering::Less);
assert_eq!(collator_3s.compare("Di Silva", "U.S.A."), Ordering::Less);
assert_eq!(collator_3s.compare("U.S.A.", "USA"), Ordering::Equal);

let mut options_4s = CollatorOptions::default();
options_4s.strength = Some(Strength::Quaternary);
options_4s.alternate_handling = Some(AlternateHandling::Shifted);
let collator_4s =
    Collator::try_new(Default::default(), options_4s).unwrap();

assert_eq!(collator_4s.compare("di Silva", "diSilva"), Ordering::Less);
assert_eq!(collator_4s.compare("diSilva", "Di Silva"), Ordering::Less);
assert_eq!(collator_4s.compare("Di Silva", "U.S.A."), Ordering::Less);
assert_eq!(collator_4s.compare("U.S.A.", "USA"), Ordering::Less);
```

### Case Level

Whether to distinguish case in sorting, even for sorting levels higher than tertiary,
without having to use tertiary level just to enable case level differences.

```rust
use core::cmp::Ordering;
use icu::collator::{*, options::*};

// Primary

let mut options = CollatorOptions::default();
options.strength = Some(Strength::Primary);
options.case_level = Some(CaseLevel::Off);
let primary =
  Collator::try_new(Default::default(),
                    options).unwrap();

assert_eq!(primary.compare("ⓓⓔⓐⓛ", "DEAL"), Ordering::Equal);
assert_eq!(primary.compare("dejavu", "dejAvu"), Ordering::Equal);
assert_eq!(primary.compare("dejavu", "déjavu"), Ordering::Equal);

// Primary with case level on

options.strength = Some(Strength::Primary);
options.case_level = Some(CaseLevel::On);
let primary_and_case =
  Collator::try_new(Default::default(),
                    options).unwrap();

assert_eq!(primary_and_case.compare("ⓓⓔⓐⓛ", "DEAL"), Ordering::Less);
assert_eq!(primary_and_case.compare("dejavu", "dejAvu"), Ordering::Less);
assert_eq!(primary_and_case.compare("dejavu", "déjavu"), Ordering::Equal);

// Secondary with case level on

options.strength = Some(Strength::Secondary);
options.case_level = Some(CaseLevel::On);
let secondary_and_case =
  Collator::try_new(Default::default(),
                    options).unwrap();

assert_eq!(secondary_and_case.compare("ⓓⓔⓐⓛ", "DEAL"), Ordering::Less);
assert_eq!(secondary_and_case.compare("dejavu", "dejAvu"), Ordering::Less);
assert_eq!(secondary_and_case.compare("dejavu", "déjavu"), Ordering::Less);  // secondary difference

// Tertiary

options.strength = Some(Strength::Tertiary);
options.case_level = Some(CaseLevel::Off);
let tertiary =
  Collator::try_new(Default::default(),
                    options).unwrap();

assert_eq!(tertiary.compare("ⓓⓔⓐⓛ", "DEAL"), Ordering::Less);
assert_eq!(tertiary.compare("dejavu", "dejAvu"), Ordering::Less);
assert_eq!(tertiary.compare("dejavu", "déjavu"), Ordering::Less);
```


### Backward second level

Compare the second level in backward order. The default is `false` (off), except for Canadian
French.

### Examples of `CollatorPreferences`

The [`CollatorPreferences`] struct configures specific custom behavior for the `Collator`, like
[`CollatorOptions`]. However, unlike `CollatorOptions`, this set of preferences can also be set
implicitly by the locale. See docs for [`CollatorPreferences`] for more details.
Some basic descriptions and examples are below.

### Case First

Whether to swap the ordering of uppercase and lowercase.

```rust
use core::cmp::Ordering;
use icu::collator::preferences::*;
use icu::collator::{options::*, *};

// Use the locale's default.

let mut prefs_no_case = CollatorPreferences::default();
prefs_no_case.case_first = Some(CollationCaseFirst::False);
let collator_no_case =
    Collator::try_new(prefs_no_case, Default::default()).unwrap();
assert_eq!(collator_no_case.compare("ab", "AB"), Ordering::Less);

// Lowercase is less

let mut prefs_lower_less = CollatorPreferences::default();
prefs_lower_less.case_first = Some(CollationCaseFirst::Lower);
let collator_lower_less =
    Collator::try_new(prefs_lower_less, Default::default()).unwrap();
assert_eq!(collator_lower_less.compare("ab", "AB"), Ordering::Less);

// Uppercase is less

let mut prefs_upper_greater = CollatorPreferences::default();
prefs_upper_greater.case_first = Some(CollationCaseFirst::Upper);
let collator_upper_greater =
    Collator::try_new(prefs_upper_greater, Default::default()).unwrap();
assert_eq!(collator_upper_greater.compare("AB", "ab"), Ordering::Less);
```

### Numeric

When set to `true` (on), any sequence of decimal
digits is sorted at a primary level according to the
numeric value.

```rust
use core::cmp::Ordering;
use icu::collator::preferences::*;
use icu::collator::{options::*, *};

// Numerical sorting off

let mut prefs_num_off = CollatorPreferences::default();
prefs_num_off.numeric_ordering = Some(CollationNumericOrdering::False);
let collator_num_off =
    Collator::try_new(prefs_num_off, Default::default()).unwrap();
assert_eq!(collator_num_off.compare("a10b", "a2b"), Ordering::Less);

// Numerical sorting on

let mut prefs_num_on = CollatorPreferences::default();
prefs_num_on.numeric_ordering = Some(CollationNumericOrdering::True);
let collator_num_on =
    Collator::try_new(prefs_num_on, Default::default()).unwrap();
assert_eq!(collator_num_on.compare("a10b", "a2b"), Ordering::Greater);
```

[`CollatorOptions`]: options::CollatorOptions

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## More Information

For more information on development, authorship, contributing etc. please visit [`ICU4X home page`](https://github.com/unicode-org/icu4x).
