friendlymatthew commented on code in PR #7833:
URL: https://github.com/apache/arrow-rs/pull/7833#discussion_r2183779196


##########
parquet-variant/src/builder.rs:
##########
@@ -237,18 +237,40 @@ impl ValueBuffer {
 struct MetadataBuilder {
     // Field names -- field_ids are assigned in insert order
     field_names: IndexSet<String>,
+
+    // flag that checks if field names by insertion order are also 
lexicographically sorted
+    is_sorted: bool,
 }
 
 impl MetadataBuilder {
     /// Upsert field name to dictionary, return its ID
     fn upsert_field_name(&mut self, field_name: &str) -> u32 {
-        let (id, _) = self.field_names.insert_full(field_name.to_string());
+        let (id, new_entry) = 
self.field_names.insert_full(field_name.to_string());
+
+        if new_entry {
+            let n = self.num_field_names();
+
+            if n == 1 {
+                self.is_sorted = true;
+            } else {
+                self.is_sorted &= self.field_names[n - 2] < self.field_names[n 
- 1];
+            }
+        }
 
         id as u32
     }
 
+    /// Returns the number of field names stored in the metadata builder.
+    /// Note: this method should be the only place to call 
`self.field_names.len()`
+    ///
+    /// # Panics
+    ///
+    /// If the number of field names exceeds the maximum allowed value for 
`u32`.
     fn num_field_names(&self) -> usize {
-        self.field_names.len()
+        let n = self.field_names.len();
+        assert!(n < u32::MAX as usize);
+
+        n

Review Comment:
   I added an assert here to make sure we fit in a `u32`. This is unrelated to 
this PR but addresses 
https://github.com/apache/arrow-rs/pull/7833/files#r2180370340.



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