use metadata lenght as the max_id for field id

Author: klion26

parquet-variant/src/builder.rs

@@ -1204,7 +1204,11 @@ impl<'a> ObjectBuilder<'a> {
             let field_b_name = metadata_builder.field_name(field_b_id as usize);
             field_a_name.cmp(field_b_name)
         });
-        let max_id = self.fields.iter().map(|(i, _)| *i).max().unwrap_or(0);
+
+        // the length of the metadata's field names is a very cheap to compute the upper bound.
+        // it will almost always be a tight upper bound as well -- it would take a pretty
+        // carefully  crafted object to use only the early field ids of a large dictionary.
+        let max_id = metadata_builder.field_names.len();
         let id_size = int_size(max_id as usize);
 
         let parent_buffer = self.parent_state.buffer();