Implement PackedPatternsV1 with packing and unpacking

components/datetime/src/provider/mod.rs

@@ -17,6 +17,7 @@
 
 pub mod calendar;
 pub(crate) mod date_time;
+pub mod packed_pattern;
 pub mod time_zones;
 
 /// Module for new DateSymbols design

components/datetime/src/provider/packed_pattern.rs

@@ -0,0 +1,108 @@
+// This file is part of ICU4X. For terms of use, please see the file
+// called LICENSE at the top level of the ICU4X source tree
+// (online at: https://github.com/unicode-org/icu4x/blob/main/LICENSE ).
+
+//! Data structures for packing of datetime patterns.
+
+use crate::helpers::size_test;
+use icu_plurals::PluralElements;
+use zerovec::VarZeroVec;
+
+use crate::pattern::runtime::{Pattern, PatternULE};
+
+#[derive(Debug)]
+pub struct LengthPluralElements<'a> {
+    pub long: PluralElements<&'a Pattern<'a>>,
+    pub medium: PluralElements<&'a Pattern<'a>>,
+    pub short: PluralElements<&'a Pattern<'a>>,
+}
+
+/// A builder for a [`PackedSkeletonDataV2`].
+#[derive(Debug)]
+pub struct PackedSkeletonDataBuilder<'a> {
+    pub standard: LengthPluralElements<'a>,
+    /// Patterns for variant 0. If `None`, falls back to standard.
+    pub variant0: Option<LengthPluralElements<'a>>,
+    /// Patterns for variant 1. If `None`, falls back to variant 0.
+    pub variant1: Option<LengthPluralElements<'a>>,
+}
+
+size_test!(PackedSkeletonDataV2, packed_skeleton_data_size, 32);
+
+/// Main data struct for packed datetime patterns.
+/// 
+/// ## Variants
+/// 
+/// This supports a set of "standard" patterns plus up to two "variants".
+/// The variants are currently used by year formatting:
+/// 
+/// - Standard: Year, which could be partial precision (2-digit Gregorain)
+/// - Variant 0: Full Year, which is always full precision
+/// - Variant 1: Year With Era
+/// 
+/// Variants should be used when the pattern could depend on the value being
+/// formatted. For example, with [`EraDisplay::Auto`], any of these three
+/// patterns could be selected based on the year value.
+/// 
+/// ## Representation
+/// 
+/// Currently, there are at most 9 patterns that need to be stored together,
+/// named according to this table:
+/// 
+/// |        | Standard | Variant 0 | Variant 1 |
+/// |--------|----------|-----------|-----------|
+/// | Long   | La       | Lb        | Lc        |
+/// | Medium | Ma       | Mb        | Mc        |
+/// | Short  | Sa       | Sb        | Sc        |
+/// 
+/// The header byte encodes which pattern in the patterns array corresponds to
+/// a particular cell in the table. It contains the following information:
+/// 
+/// - Bits 0-1: "LMS" value of the standard column
+/// - Bit 2: "Q" value: 1 for directly-indexed variants; 0 for per-cell offsets
+/// - Bits 3-20: Packed offset into patterns table for each variant cell
+/// - Bits 21-31: unused/reserved
+/// 
+/// The LMS value determines which pattern index is used for the first column:
+/// 
+/// | LMS Value | Long Index | Medium Index | Short Index |
+/// |-----------|------------|--------------|-------------|
+/// | 0         | 0          | 0            | 0           |
+/// | 1         | 0          | 1            | 1           |
+/// | 2         | 0          | 0            | 1           |
+/// | 3         | 0          | 1            | 2           |
+/// 
+/// If bit 2 is 1 (Q=1), it means there is one pattern per variant index,
+/// with the index offset by the short index `S` from the table above.
+/// However, this requires storing multiple, possibly duplicate, patterns in
+/// the packed structure. The more common case is Q=0 and then to store
+/// per-cell offsets in chunks of 3 bits per cell:
+/// 
+/// - Chunk = 0: Inherit according to the table below
+/// - Chunk = 1-7: Use pattern index Chunk - 1
+/// 
+/// This is summarized below:
+/// 
+/// | Cell in Table | Q=1 Pattern Index | Q=0 Header Bits | Inheritance |
+/// |---------------|-------------------|-----------------|-------------|
+/// | Lb            | S + 1             | 3-5             | La          |
+/// | Mb            | S + 2             | 6-8             | Ma          |
+/// | Sb            | S + 3             | 9-11            | Sa          |
+/// | Lc            | S + 4             | 12-14           | Lb          |
+/// | Mc            | S + 5             | 15-17           | Mb          |
+/// | Sc            | S + 6             | 18-20           | Mc          |
+/// 
+/// [`EraDisplay::Auto`]: crate::neo_skeleton::EraDisplay::Auto
+#[doc = packed_skeleton_data_size!()]
+#[derive(Debug, PartialEq, Clone)]
+#[cfg_attr(feature = "datagen", derive(serde::Serialize, databake::Bake))]
+#[cfg_attr(feature = "datagen", databake(path = icu_datetime::provider::neo))]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize))]
+pub struct PackedSkeletonDataV2<'data> {
+    /// An encoding of which standard/variant cell corresponds to which entry
+    /// in the patterns table. See class docs.
+    pub header: u32,
+    /// The list of patterns.
+    #[cfg_attr(feature = "serde", serde(borrow))]
+    pub patterns: VarZeroVec<'data, PatternULE>,
+}