Teach length::Bag how to switch hour cycles

components/datetime/Cargo.toml

@@ -58,6 +58,7 @@ std = ["icu_provider/std", "icu_locid/std"]
 default = ["provider_serde"]
 bench = []
 provider_serde = ["serde", "litemap/serde"]
+provider_transform_internals = ["std"]
 
 [[bench]]
 name = "datetime"

components/datetime/src/options/length.rs

@@ -82,7 +82,6 @@ use serde::{Deserialize, Serialize};
 pub struct Bag {
     pub date: Option<Date>,
     pub time: Option<Time>,
-    #[cfg_attr(feature = "serde", serde(skip_serializing, skip_deserializing))]
     pub preferences: Option<preferences::Bag>,
 }
 

components/datetime/src/options/preferences.rs

@@ -51,7 +51,7 @@ pub struct Bag {
 #[derive(Debug, Clone, Copy, PartialEq)]
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
 pub enum HourCycle {
-    /// Hour is formatted to be in range 1-24
+    /// Hour is formatted to be in range 1-24 where midnight is 24:00.
     ///
     /// # Examples
     ///
@@ -63,7 +63,7 @@ pub enum HourCycle {
     /// ```
     #[cfg_attr(feature = "serde", serde(rename = "h24"))]
     H24,
-    /// Hour is formatted to be in range 0-23
+    /// Hour is formatted to be in range 0-23 where midnight is 00:00.
     ///
     /// # Examples
     ///
@@ -75,7 +75,7 @@ pub enum HourCycle {
     /// ```
     #[cfg_attr(feature = "serde", serde(rename = "h23"))]
     H23,
-    /// Hour is formatted to be in range 1-12
+    /// Hour is formatted to be in range 1-12 where midnight is 12:00.
     ///
     /// # Examples
     ///
@@ -87,7 +87,7 @@ pub enum HourCycle {
     /// ```
     #[cfg_attr(feature = "serde", serde(rename = "h12"))]
     H12,
-    /// Hour is formatted to be in range 0-11
+    /// Hour is formatted to be in range 0-11 where midnight is 00:00.
     ///
     /// # Examples
     ///

components/datetime/src/pattern/mod.rs

@@ -4,6 +4,7 @@
 
 mod error;
 mod parser;
+pub mod transform_hour_cycle;
 
 use crate::fields::{self, Field, FieldLength, FieldSymbol};
 #[cfg(feature = "provider_serde")]
@@ -292,3 +293,25 @@ impl Serialize for Pattern {
         }
     }
 }
+
+/// Used to represent either H11/H12, or H23/H24. Skeletons only store these
+/// hour cycles as H12 or H23.
+#[derive(Debug, PartialEq, Clone, Copy)]
+#[cfg_attr(
+    feature = "provider_serde",
+    derive(serde::Serialize, serde::Deserialize)
+)]
+pub enum CoarseHourCycle {
+    /// Can either be fields::Hour::H11 or fields::Hour::H12
+    H11H12,
+    /// Can either be fields::Hour::H23 or fields::Hour::H24
+    H23H24,
+}
+
+/// Default is required for serialization. H23H24 is the more locale-agnostic choice, as it's
+/// less likely to have a day period in it.
+impl Default for CoarseHourCycle {
+    fn default() -> Self {
+        CoarseHourCycle::H23H24
+    }
+}

components/datetime/src/pattern/transform_hour_cycle.rs

@@ -0,0 +1,88 @@
+// 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 ).
+
+//! This hour cycle module represents various utilities for working with hour cycles in order
+//! to apply a user preference.
+
+#![doc(hidden)]
+#![cfg(feature = "provider_transform_internals")]
+
+use crate::fields;
+use crate::pattern::{CoarseHourCycle, Pattern, PatternItem};
+use crate::provider;
+use crate::skeleton;
+
+/// Figure out the coarse hour cycle given a pattern, which is useful for generating the provider
+/// patterns for `length::Bag`.
+pub fn determine_coarse_hour_cycle(pattern: &Pattern) -> Option<CoarseHourCycle> {
+    for item in pattern.items() {
+        if let PatternItem::Field(fields::Field {
+            symbol: fields::FieldSymbol::Hour(pattern_hour),
+            length: _,
+        }) = item
+        {
+            return Some(match pattern_hour {
+                fields::Hour::H11 | fields::Hour::H12 => CoarseHourCycle::H11H12,
+                fields::Hour::H23 | fields::Hour::H24 => CoarseHourCycle::H23H24,
+            });
+        }
+    }
+
+    None
+}
+
+/// Invoke the pattern matching machinery to transform the hour cycle of a pattern. This provides
+/// a safe mapping from a h11/h12 to h23/h24 for transforms.
+#[doc(hidden)]
+#[cfg(feature = "provider_transform_internals")]
+pub fn apply_coarse_hour_cycle(
+    datetime: &provider::gregory::patterns::DateTimeFormatsV1,
+    pattern_str: &str,
+    mut pattern: Pattern,
+    coarse_hour_cycle: CoarseHourCycle,
+) -> Option<String> {
+    for item in pattern.items_mut() {
+        if let PatternItem::Field(fields::Field { symbol, length: _ }) = item {
+            if let fields::FieldSymbol::Hour(pattern_hour) = symbol {
+                if match coarse_hour_cycle {
+                    CoarseHourCycle::H11H12 => match pattern_hour {
+                        fields::Hour::H11 | fields::Hour::H12 => true,
+                        fields::Hour::H23 | fields::Hour::H24 => false,
+                    },
+                    CoarseHourCycle::H23H24 => match pattern_hour {
+                        fields::Hour::H11 | fields::Hour::H12 => false,
+                        fields::Hour::H23 | fields::Hour::H24 => true,
+                    },
+                } {
+                    // The preference hour cycle matches the pattern, bail out early and
+                    // return the current pattern.
+                    return Some(pattern_str.into());
+                } else {
+                    // Mutate the pattern with the new symbol, so that it can be matched against.
+                    *symbol = fields::FieldSymbol::Hour(match coarse_hour_cycle {
+                        CoarseHourCycle::H11H12 => fields::Hour::H12,
+                        CoarseHourCycle::H23H24 => fields::Hour::H23,
+                    });
+                    break;
+                }
+            }
+        }
+    }
+
+    let skeleton = skeleton::Skeleton::from(&pattern);
+
+    match skeleton::create_best_pattern_for_fields(
+        &datetime.skeletons,
+        &datetime.length_patterns,
+        &skeleton.as_slice(),
+        &None,
+        // Prefer using the matched pattern directly, rather than mutating it to match the
+        // requested fields.
+        true,
+    ) {
+        skeleton::BestSkeleton::AllFieldsMatch(pattern)
+        | skeleton::BestSkeleton::MissingOrExtraFields(pattern) => Some(format!("{}", pattern)),
+        skeleton::BestSkeleton::NoMatch => None,
+    }
+}

components/datetime/src/provider/gregory.rs

@@ -2,6 +2,7 @@
 // called LICENSE at the top level of the ICU4X source tree
 // (online at: https://github.com/unicode-org/icu4x/blob/main/LICENSE ).
 
+use crate::pattern;
 use alloc::borrow::Cow;
 use icu_provider::yoke::{self, *};
 
@@ -30,7 +31,18 @@ pub struct DateSymbolsV1 {
 pub struct DatePatternsV1 {
     pub date: patterns::LengthPatternsV1,
 
-    pub time: patterns::LengthPatternsV1,
+    /// These patterns are common uses of time formatting, broken down by the length of the
+    /// pattern. Users can override the hour cycle with a preference, so there are two
+    /// pattern groups stored here. Note that the pattern will contain either h11 or h12.
+    pub time_h11_h12: patterns::LengthPatternsV1,
+
+    /// These patterns are common uses of time formatting, broken down by the length of the
+    /// pattern. Users can override the hour cycle with a preference, so there are two
+    /// pattern groups stored here. Note that the pattern will contain either h23 or h24.
+    pub time_h23_h24: patterns::LengthPatternsV1,
+
+    /// By default a locale will prefer one hour cycle type over another.
+    pub preferred_hour_cycle: pattern::CoarseHourCycle,
 
     pub datetime: patterns::DateTimeFormatsV1,
 }

components/datetime/src/provider/helpers.rs

@@ -5,8 +5,8 @@
 use crate::date;
 use crate::error::DateTimeFormatError;
 use crate::fields;
-use crate::options::{components, length, DateTimeFormatOptions};
-use crate::pattern::Pattern;
+use crate::options::{components, length, preferences, DateTimeFormatOptions};
+use crate::pattern::{Pattern, PatternItem};
 use crate::provider;
 use crate::skeleton;
 use alloc::borrow::Cow;
@@ -21,7 +21,11 @@ pub trait DateTimePatterns {
     ) -> Result<Option<Pattern>>;
     fn get_pattern_for_length_bag(&self, length: &length::Bag) -> Result<Option<Pattern>>;
     fn get_pattern_for_date_length(&self, length: length::Date) -> Result<Pattern>;
-    fn get_pattern_for_time_length(&self, length: length::Time) -> Result<Pattern>;
+    fn get_pattern_for_time_length(
+        &self,
+        length: length::Time,
+        preferences: &Option<preferences::Bag>,
+    ) -> Result<Pattern>;
     fn get_pattern_for_datetime_length(
         &self,
         length: length::Date,
@@ -72,6 +76,7 @@ impl DateTimePatterns for provider::gregory::DatePatternsV1 {
                 &self.datetime.length_patterns,
                 &requested_fields,
                 &components.preferences,
+                false, // Prefer the requested fields over the matched pattern.
             ) {
                 skeleton::BestSkeleton::AllFieldsMatch(pattern)
                 | skeleton::BestSkeleton::MissingOrExtraFields(pattern) => Some(pattern),
@@ -83,10 +88,12 @@ impl DateTimePatterns for provider::gregory::DatePatternsV1 {
     fn get_pattern_for_length_bag(&self, length: &length::Bag) -> Result<Option<Pattern>> {
         match (length.date, length.time) {
             (None, None) => Ok(None),
-            (None, Some(time_length)) => self.get_pattern_for_time_length(time_length).map(Some),
+            (None, Some(time_length)) => self
+                .get_pattern_for_time_length(time_length, &length.preferences)
+                .map(Some),
             (Some(date_length), None) => self.get_pattern_for_date_length(date_length).map(Some),
             (Some(date_length), Some(time_length)) => {
-                let time = self.get_pattern_for_time_length(time_length)?;
+                let time = self.get_pattern_for_time_length(time_length, &length.preferences)?;
                 let date = self.get_pattern_for_date_length(date_length)?;
 
                 self.get_pattern_for_datetime_length(date_length, date, time)
@@ -122,15 +129,54 @@ impl DateTimePatterns for provider::gregory::DatePatternsV1 {
         Ok(Pattern::from_bytes_combination(s, date, time)?)
     }
 
-    fn get_pattern_for_time_length(&self, length: length::Time) -> Result<Pattern> {
-        let time = &self.time;
-        let s = match length {
+    /// Look up the proper pre-computed pattern for a given length. If a preference for an hour
+    /// cycle is set, it will look look up a pattern in the time_h11_12 or time_h23_h24 provider
+    /// data, and then manually modify the symbol in the pattern if needed.
+    fn get_pattern_for_time_length(
+        &self,
+        length: length::Time,
+        preferences: &Option<preferences::Bag>,
+    ) -> Result<Pattern> {
+        // Determine the coarse hour cycle patterns to use from either the preference bag,
+        // or the preferred hour cycle for the locale.
+        let time = if let Some(preferences::Bag {
+            hour_cycle: Some(hour_cycle_pref),
+        }) = preferences
+        {
+            match hour_cycle_pref {
+                preferences::HourCycle::H11 | preferences::HourCycle::H12 => &self.time_h11_h12,
+                preferences::HourCycle::H23 | preferences::HourCycle::H24 => &self.time_h23_h24,
+            }
+        } else {
+            match self.preferred_hour_cycle {
+                crate::pattern::CoarseHourCycle::H11H12 => &self.time_h11_h12,
+                crate::pattern::CoarseHourCycle::H23H24 => &self.time_h23_h24,
+            }
+        };
+
+        let mut pattern = Pattern::from_bytes(match length {
             length::Time::Full => &time.full,
             length::Time::Long => &time.long,
             length::Time::Medium => &time.medium,
             length::Time::Short => &time.short,
-        };
-        Ok(Pattern::from_bytes(s)?)
+        })?;
+
+        if let Some(preferences::Bag {
+            hour_cycle: Some(hour_cycle_pref),
+        }) = preferences
+        {
+            // Apply the preference::Bag override and change the pattern from a coarse hour cycle
+            // to the specific hour cycle.
+            for item in pattern.items_mut() {
+                if let PatternItem::Field(fields::Field { symbol, length: _ }) = item {
+                    if let fields::FieldSymbol::Hour(_) = symbol {
+                        *symbol = fields::FieldSymbol::Hour(hour_cycle_pref.field());
+                    }
+                }
+            }
+        }
+
+        Ok(pattern)
     }
 }