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Writing a New Data Struct

ICU4X is a heavily data-driven library. Most new features or components will require pulling in data from an external source.

This tutorial aims to help ICU4X contributors add new data to the data pipeline. It is recommended that readers review data_pipeline.md for additional theory behind the design decisions in the data provider.

Lifecycle of ICU4X Data

It is important to understand the phases of life of ICU4X data as it makes its way from the data source, like CLDR, to the data struct used at runtime. The following flowchart shows the phases and how they connect:

Lifecycle of Data in ICU4X

The following steps take place at build time:

  1. Source data file is obtained from an external source. Examples could include the CLDR JSON release or the Unicode Character Database.
  2. The source data is parsed and transformed into a runtime data struct. This step can be expensive, because it is normally run as an offline build step.
  3. The runtime data struct is stored in a way so that a provider can use it: a postcard blob, JSON directory tree, Rust module, etc.

These steps are performed by the icu_datagen, but clients can also write their own data generation logic.

When deserializing from the blob store, it is a design principle of ICU4X that no heap allocations will be required. We have many utilities and abstractions to help make this safe and easy.

Code Layout

With a mental model of the lifecycle of data in ICU4X, we can discuss where to find the code that performs each step.

Data Structs

The data struct definitions should live in the crate that uses them. By convention, the top-level module provider should contain the struct definitions. For example:

  • icu::decimal::provider::DecimalSymbolsV1
  • icu::locale_canonicalizer::provider::LikelySubtagsV1
  • icu::uniset::provider::PropertyCodePointSetV1

In general, data structs should be annotated with #[icu_provider::data_struct], and they should support at least Debug, PartialEq, Clone, Default, and Serde Serialize and Deserialize.

As explained in data_pipeline.md, the data struct should support zero-copy deserialization. The #[icu_provider::data_struct] annotation will enforce this for you. See more information in style_guide.md, as well as the example below in this tutorial.

Additionally, data structs should keep internal invariants to a minimum. For more information, see data_safety.md.

Data Download

The first step to introduce data into the ICU4X pipeline is to download it from an external source. This corresponds to step 1 above.

When clients use ICU4X, this is generally a manual step, although we may provide tooling to assist with it. For the purpose of ICU4X test data, the tool download-repo-sources should automatically download data from the external source and save it in the ICU4X tree. download-repo-sources should not do anything other than downloading the raw source data.

To download test data into the ICU4X source tree, run:

$ cargo make download-repo-sources

Source Data Providers

"Source data providers" read from a source data file, deserialize it, and transform it to an ICU4X data struct. This corresponds to steps 2 and 3 above.

To add a new source data provider, implement the following traits on DatagenProvider for your data marker(s):

  • DataProvider<M> for one or more data markers M; this impl is the main step where data transformation takes place
  • IterableDataProviderInternal<M>, which automatically results in a cached impl of IterableDataProvider<M>

Source data providers are often complex to write. Rules of thumb:

  • Optimize for readability and maintainability. The source data providers are not used in production, so performance is not a driving concern; however, we want the transformer to be fast enough to make a good developer experience.
  • If the data source is similar to an existing data source (e.g., importing new data from CLDR JSON), try to share code with existing data providers for that source.
  • If the data source is novel, feel free to add a new module under icu_datagen::transform.

Data Exporters and Runtime Data Providers

"Data exporters" read from one or more ICU4X data structs and dump them to storage. This corresponds to step 4 above.

Examples of data exporters include:

"Runtime data providers" are ones that read serialized ICU4X data structs and deserialize them for use at runtime. These are the providers where performance is the key driving factor.

Examples of runtime data providers include:

Most ICU4X contributors will not need to touch the data exporters or runtime data providers. New implementations are only necessary when adding a new ICU4X data struct storage mechanism.

Data Generation Tool (icu4x-datagen)

The data generation tool, i.e., icu4x-datagen, ties together the source data providers with a data exporter.

When adding new data structs, it is necessary to make icu4x-datagen aware of your source data provider. To do this, edit provider/datagen/src/registry.rs and add your data provider to the macro

registry!(
    // ...
    FooV1Marker,
)

as well as to the list of keys

use std::borrow::Cow;
use icu_provider::prelude::*;

#[derive(Debug, PartialEq, Clone)]
#[icu_provider::data_struct(marker(FooV1Marker, "foo/bar@1"))]
pub struct FooV1<'data> {
  message: Cow<'data, str>,
}

When finished, run from the top level:

$ cargo make testdata

If everything is hooked together properly, JSON files for your new data struct should appear under provider/datagen/tests/data/json.

Example

The following example shows all the pieces that make up the data pipeline for DecimalSymbolsV1.

Data Struct

components/decimal/src/provider.rs

use std::borrow::Cow;
use icu_provider::prelude::*;
use icu::decimal::provider::{ AffixesV1, GroupingSizesV1 };

/// Symbols and metadata required for formatting a [`FixedDecimal`](crate::FixedDecimal).
#[icu_provider::data_struct(marker(DecimalSymbolsV1Marker, "decimal/symbols@1", extension_key = "nu" ))]
#[derive(Debug, PartialEq, Clone)]
#[cfg_attr(
    feature = "datagen",
    derive(serde::Serialize, databake::Bake),
    databake(path = icu_decimal::provider),
)]
#[cfg_attr(feature = "serde", derive(serde::Deserialize))]
pub struct DecimalSymbolsV1<'data> {
    /// Prefix and suffix to apply when a negative sign is needed.
    #[cfg_attr(feature = "serde", serde(borrow))]
    pub minus_sign_affixes: AffixesV1<'data>,

    /// Prefix and suffix to apply when a plus sign is needed.
    #[cfg_attr(feature = "serde", serde(borrow))]
    pub plus_sign_affixes: AffixesV1<'data>,

    /// Character used to separate the integer and fraction parts of the number.
    #[cfg_attr(feature = "serde", serde(borrow))]
    pub decimal_separator: Cow<'data, str>,

    /// Character used to separate groups in the integer part of the number.
    #[cfg_attr(feature = "serde", serde(borrow))]
    pub grouping_separator: Cow<'data, str>,

    /// Settings used to determine where to place groups in the integer part of the number.
    pub grouping_sizes: GroupingSizesV1,

    /// Digit characters for the current numbering system. In most systems, these digits are
    /// contiguous, but in some systems, such as *hanidec*, they are not contiguous.
    pub digits: [char; 10],
}

The above example is an abridged definition for DecimalSymbolsV1. Note how the lifetime parameter 'data is passed down into all fields that may need to borrow data.

CLDR JSON Deserialize

provider/datagen/src/transform/cldr/cldr_serde/numbers.rs

use icu::locid::LanguageIdentifier;
use itertools::Itertools;
use serde::de::{Deserializer, Error, MapAccess, Unexpected, Visitor};
use serde::Deserialize;
use std::collections::HashMap;
use tinystr::TinyStr8;

#[derive(PartialEq, Debug, Deserialize)]
pub struct Numbers {
    #[serde(rename = "defaultNumberingSystem")]
    pub default_numbering_system: TinyStr8,
    #[serde(rename = "minimumGroupingDigits")]
    #[serde(deserialize_with = "serde_aux::prelude::deserialize_number_from_string")]
    pub minimum_grouping_digits: u8,
}

#[derive(PartialEq, Debug, Deserialize)]
pub struct LangNumbers {
    pub numbers: Numbers,
}

#[derive(PartialEq, Debug, Deserialize)]
pub struct LangData(pub HashMap<LanguageIdentifier, LangNumbers>);

#[derive(PartialEq, Debug, Deserialize)]
pub struct Resource {
    pub main: LangData,
}

The above example is an abridged definition of the Serde structure corresponding to CLDR JSON. Since this Serde definition is not used at runtime, it does not need to be zero-copy.

Transformer

provider/core/src/data_provider.rs

provider/core/src/datagen/iter.rs

impl DataProvider<FooV1Marker> for DatagenProvider {
    fn load(
        &self,
        req: DataRequest,
    ) -> Result<DataResponse<FooV1Marker>, DataError> {
        // Use the data inside self.source and emit it as an ICU4X data struct.
        // This is the core transform operation. This step could take a lot of
        // work, such as pre-parsing patterns, re-organizing the data, etc.
        // This method will be called once per option returned by supported_locales.
    }
}

impl IterableDataProviderInternal<FooV1Marker> for FooProvider {
    fn supported_locales_impl(
        &self,
    ) -> Result<HashSet<DataLocale>, DataError> {
        // This should list all supported locales.
    }
}

The above example is an abridged snippet of code illustrating the most important boilerplate for implementing and ICU4X data transform.