unicode-org · sffc · Dec 11, 2025 · Dec 1, 2025 · Dec 1, 2025 · Dec 1, 2025
@@ -33,6 +33,7 @@ bincode = { workspace = true }
 icu_benchmark_macros = { path = "../../tools/benchmark/macros" }
 litemap = { path = "../../utils/litemap" }
 postcard = { workspace = true, features = ["alloc"] }
+itertools = { workspace = true }
 rand = { workspace = true }
 rand_pcg = { workspace = true }
 rmp-serde = { workspace = true }
@@ -50,12 +51,14 @@ bench = false  # This option is required for Benchmark CI
 
 [features]
 default = []
-alloc = []
+alloc = ["zerovec?/alloc"]
 databake = ["dep:databake", "zerovec?/databake"]
+dense = ["dep:zerovec"]
 litemap = ["dep:litemap", "alloc"]
 serde = ["dep:serde_core", "dep:litemap", "alloc", "litemap/serde", "zerovec?/serde"]
 yoke = ["dep:yoke"]
 zerofrom = ["dep:zerofrom"]
+zerovec = ["dep:zerovec"]
 
 [[bench]]
 name = "overview"
@@ -70,6 +73,10 @@ required-features = ["alloc", "litemap"]
 name = "builder_test"
 required-features = ["alloc", "litemap"]
 
+[[test]]
+name = "dense_test"
+required-features = ["alloc", "dense"]
+
 [[test]]
 name = "locale_aux_test"
 required-features = ["alloc", "litemap"]
@@ -0,0 +1,186 @@
+// 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 ).
+
+use crate::dense::DenseType;
+use crate::dense::ZeroAsciiDenseSparse2dTrieOwned;
+use crate::ZeroTrieBuildError;
+use crate::ZeroTrieSimpleAscii;
+use alloc::collections::BTreeMap;
+use alloc::collections::BTreeSet;
+use alloc::string::String;
+use alloc::vec::Vec;
+use zerovec::ZeroVec;
+
+#[derive(PartialEq, Eq, PartialOrd, Ord)]
+struct Row<'a> {
+    prefix: &'a str,
+    row_value_offset: usize,
+    offsets: Vec<DenseType>,
+}
+
+#[derive(Default)]
+pub(crate) struct DenseSparse2dAsciiWithFixedDelimiterBuilder<'a> {
+    primary: Vec<(&'a str, &'a str, usize)>,
+    suffixes: BTreeSet<&'a str>,
+    dense: Vec<Row<'a>>,
+    delimiter: u8,
+}
+
+impl<'a> DenseSparse2dAsciiWithFixedDelimiterBuilder<'a> {
+    /// Add a prefix and all values associated with the prefix to the builder.
+    pub(crate) fn add_prefix(
+        &mut self,
+        prefix: &'a str,
+        values: &BTreeMap<&'a str, usize>,
+    ) -> Result<(), ZeroTrieBuildError> {
+        // TODO: Is there a more Rusty way to compute min and max together?
+        let mut min = usize::MAX;
+        let mut max = 0;
+        for value in values.values() {
+            min = core::cmp::min(min, *value);
+            max = core::cmp::max(max, *value);
+        }
+        // >= because DenseType::MAX is the sentinel
+        if max - min >= usize::from(DenseType::MAX) {
+            // How to implement this when we need it:
+            // 1. Select the row offset that gets the greatest number of values into the dense matrix.
+            // 2. Put all out-of-range values into the primary trie, as we do with sparse rows.
+            todo!("values in row are not in a sufficiently compact range");
+        }
+        let sentinel = min + usize::from(DenseType::MAX);
+        // Partition the entries based on whether they can be encoded as dense
+        let (dense_or_sparse, sparse_only) = values
+            .iter()
+            .map(|(suffix, value)| (*suffix, *value))
+            .partition::<BTreeMap<&'a str, usize>, _>(|(suffix, _value)| {
+                // TODO: Also filter for suffixes that are out of range of the dense row offset
+                self.suffixes.contains(suffix)
+            });
+        // Check whether the sparse trie is smaller than the dense row
+        let sub_trie = dense_or_sparse
+            .iter()
+            .map(|(suffix, value)| (*suffix, *value))
+            .collect::<ZeroTrieSimpleAscii<Vec<u8>>>();
+        if sub_trie.byte_len() > self.suffixes.len() * core::mem::size_of::<DenseType>() {
+            // Create a dense prefix
+            let row_value_offset = min;
+            let offsets = self
+                .suffixes
+                .iter()
+                .map(|suffix| {
+                    let value = sub_trie.get(suffix).unwrap_or(sentinel);
+                    let Ok(offset) = DenseType::try_from(value - row_value_offset) else {
+                        unreachable!("this should have been handled earlier");
+                    };
+                    offset
+                })
+                .collect::<Vec<DenseType>>();
+            self.dense.push(Row {
+                prefix,
+                row_value_offset,
+                offsets,
+            });
+            for (suffix, value) in sparse_only.iter() {
+                self.primary.push((prefix, *suffix, *value));
+            }
+            Ok(())
+        } else {
+            // Create a sparse prefix
+            for (suffix, value) in values.iter() {
+                self.primary.push((prefix, *suffix, *value));
+            }
+            Ok(())
+        }
+    }
+
+    /// Assemble the intermediate structures into the final layout.
+    pub(crate) fn build(mut self) -> Result<ZeroAsciiDenseSparse2dTrieOwned, ZeroTrieBuildError> {
+        self.dense.sort();
+        let Ok(suffix_count) = DenseType::try_from(self.suffixes.len()) else {
+            return Err(ZeroTrieBuildError::CapacityExceeded);
+        };
+        let delimiter = self.delimiter as char;
+        let mut primary_contents = BTreeMap::new();
+        for (prefix, suffix, value) in self.primary.iter() {
+            if prefix.contains(delimiter) || suffix.contains(delimiter) {
+                debug_assert!(false, "handled earlier");
+                return Err(ZeroTrieBuildError::IllegalDelimiter);
+            }
+            let mut delimited = String::with_capacity(prefix.len() + suffix.len() + 1);
+            delimited.push_str(prefix);
+            delimited.push(delimiter);
+            delimited.push_str(suffix);
+            primary_contents.insert(delimited, *value);
+        }
+        let mut dense = Vec::<DenseType>::with_capacity(self.dense.len() * self.suffixes.len());
+        for (
+            row_index,
+            Row {
+                prefix,
+                row_value_offset,
+                offsets,
+            },
+        ) in self.dense.iter().enumerate()
+        {
+            primary_contents.insert(String::from(*prefix), row_index);
+            let mut prefix_with_delim = String::with_capacity(prefix.len() + 1);
+            prefix_with_delim.push_str(prefix);
+            prefix_with_delim.push(delimiter);
+            primary_contents.insert(prefix_with_delim, *row_value_offset);
+            dense.extend(offsets);
+        }
+        let suffixes = self
+            .suffixes
+            .iter()
+            .enumerate()
+            .map(|(column_index, suffix)| (*suffix, column_index))
+            .collect::<BTreeMap<&str, usize>>();
+        Ok(ZeroAsciiDenseSparse2dTrieOwned {
+            primary: ZeroTrieSimpleAscii::try_from_btree_map_str(&primary_contents)?,
+            suffixes: ZeroTrieSimpleAscii::try_from_btree_map_str(&suffixes)?,
+            dense: ZeroVec::from_slice_or_alloc(dense.as_slice()).into_owned(),
+            suffix_count,
+            delimiter: self.delimiter,
+        })
+    }
+}
+
+impl ZeroAsciiDenseSparse2dTrieOwned {
+    /// Builds one of these from a two-dimensional BTreeMap and a delimiter.
+    ///
+    /// Keep in mind the recommendations for optimal data size described in
+    /// the [class docs].
+    ///
+    /// [class docs]: ZeroAsciiDenseSparse2dTrieOwned
+    pub fn try_from_btree_map_str(
+        entries: &BTreeMap<&str, BTreeMap<&str, usize>>,
+        delimiter: u8,
+    ) -> Result<Self, ZeroTrieBuildError> {
+        let mut builder = DenseSparse2dAsciiWithFixedDelimiterBuilder {
+            delimiter,
+            ..Default::default()
+        };
+        // TODO: Prune low-frequency suffixes.
+        // For now, build with all suffixes.
+        builder.suffixes = entries
+            .values()
+            .flat_map(|inner| inner.keys())
+            .copied()
+            .map(|s| {
+                if s.contains(delimiter as char) {
+                    Err(ZeroTrieBuildError::IllegalDelimiter)
+                } else {
+                    Ok(s)
+                }
+            })
+            .collect::<Result<_, ZeroTrieBuildError>>()?;
+        for (prefix, values) in entries.iter() {
+            if prefix.contains(delimiter as char) {
+                return Err(ZeroTrieBuildError::IllegalDelimiter);
+            }
+            builder.add_prefix(prefix, values)?;
+        }
+        builder.build()
+    }
+}
@@ -151,6 +151,8 @@
 
 mod branch_meta;
 pub(crate) mod bytestr;
+#[cfg(all(feature = "alloc", feature = "dense"))]
+pub(crate) mod dense;
 pub(crate) mod konst;
 #[cfg(feature = "litemap")]
 mod litemap;