table: keep dense lookup for wide messages

.changes/unreleased/changed-20261002-205900-table-wide-dense.yaml

@@ -0,0 +1,4 @@
+kind: Changed
+body: |-
+    **Table-codec messages with 255 or more fields keep the dense lookup for low field numbers.** Fields below 64 now use the same direct lookup as smaller messages instead of falling back to binary search. Encoded bytes and decode semantics are unchanged.
+time: 2026-10-02T20:59:00+02:00

buffa-codegen/src/table_codec.rs

@@ -142,13 +142,13 @@ pub(crate) fn generate_table_impl(
 }
 
 /// `dense[n]` is one plus the index of the field numbered `n`, or `0`, for
-/// the numbers up to the highest field number below 64. Empty, so that the
-/// table searches instead, for a message with 255 fields or more, because
-/// `Table::new` accepts a dense array only when every index fits below 255.
+/// the numbers up to the highest field number below 64.
+///
+/// Entries are sorted by field number, and protobuf field numbers start at
+/// one, so a field numbered below 64 is necessarily among the first 63
+/// entries. Its one-based index therefore fits in `u8` regardless of the
+/// total number of fields.
 fn dense_lookup(fields: &[TableField<'_>]) -> Vec<u8> {
-    if fields.len() >= 255 {
-        return Vec::new();
-    }
     let top = fields
         .iter()
         .map(|f| f.number)
@@ -158,7 +158,7 @@ fn dense_lookup(fields: &[TableField<'_>]) -> Vec<u8> {
     let mut dense = vec![0u8; top + 1];
     for (i, f) in fields.iter().enumerate() {
         if let Some(slot) = dense.get_mut(f.number as usize) {
-            // At most 254 fields, so this fits.
+            // A field numbered below 64 is among the first 63 sorted entries.
             *slot = (i + 1) as u8;
         }
     }

buffa-codegen/src/tests/table_codec.rs

@@ -478,6 +478,25 @@ fn field_numbers_index_the_dense_array_and_missing_ones_are_zero() {
     assert!(plain.contains("dense=&[0u8,1u8,0u8,2u8]"), "{plain}");
 }
 
+#[test]
+fn wide_messages_keep_dense_lookup_for_low_field_numbers() {
+    let mut file = schema();
+    file.message_type[0].field = (1..=300)
+        .map(|n| scalar(&format!("f{n}"), n, Type::TYPE_INT32))
+        .collect();
+    let (files, _) = generate_with_diagnostics(
+        &[file],
+        &["t.proto".to_string()],
+        &table_config(CodecStrategy::Table),
+    )
+    .unwrap();
+    let code = squashed(&joined(&files));
+    let plain = code.split("static__BUFFA_TABLE_Plain").nth(1).unwrap();
+    let table = plain.split("impl::buffa::Message").next().unwrap();
+    assert!(table.contains("dense=&[0u8,1u8,2u8,3u8"), "{table}");
+    assert!(table.contains(",63u8]"), "{table}");
+}
+
 #[test]
 fn the_warning_texts_say_what_to_do() {
     let rule = CodeGenWarning::CodecStrategyRuleMatchedNothing {

buffa-test/build.rs

@@ -90,8 +90,7 @@ fn read_proto(file: &str) -> String {
 }
 
 /// A message with 300 fields, of which 280 are messages, so that a table needs
-/// more than 255 entries (which turns off its dense array) and more than 255
-/// field descriptors.
+/// more than 255 entries and more than 255 field descriptors.
 fn wide_proto() -> String {
     let mut proto = String::from(
         "syntax = \"proto3\";\npackage wide;\nmessage Leaf { int32 x = 1; }\nmessage Wide {\n",

buffa/src/table/mod.rs

@@ -452,8 +452,8 @@ impl<M> Table<M> {
     ///
     /// `abi` is the [`ABI`] the caller was generated for. `entries` are the
     /// fields sorted by number; `dense` is the lookup array described on
-    /// [`MessageTable`], which must be empty if there are 255 entries or more;
-    /// `aux` holds the descriptors that entries index by their `aux` value;
+    /// [`MessageTable`]; `aux` holds the descriptors that entries index by
+    /// their `aux` value;
     /// `unknown` is the offset of the message's `UnknownFields`, if it keeps
     /// any.
     ///
@@ -530,10 +530,6 @@ impl<M> Table<M> {
             }
             i += 1;
         }
-        assert!(
-            dense.is_empty() || entries.len() < 255,
-            "buffa table: the dense lookup needs fewer than 255 entries"
-        );
         // Every dense slot names the entry with its number, and every entry
         // below the dense range has a slot, so the two agree.
         let mut named = 0;