Optimize appending max level comparison in DefinitionLevelDecoder

Author: jhorstmann

parquet/src/arrow/array_reader/struct_array.rs

@@ -158,8 +158,9 @@ impl ArrayReader for StructArrayReader {
                     }
                 }
                 None => {
-                    for def_level in def_levels {
-                        bitmap_builder.append(*def_level >= self.struct_def_level)
+                    // Safety: slice iterator has a trusted length
+                    unsafe {
+                        bitmap_builder.extend_trusted_len(def_levels.iter().map(|level| *level >= self.struct_def_level))
                     }
                 }
             }

parquet/src/arrow/record_reader/definition_levels.rs

@@ -160,9 +160,10 @@ impl DefinitionLevelDecoder for DefinitionLevelBufferDecoder {
                 let start = levels.len();
                 let (values_read, levels_read) = decoder.read_def_levels(levels, num_levels)?;
 
-                nulls.reserve(levels_read);
-                for i in &levels[start..] {
-                    nulls.append(i == max_level);
+                // Safety: slice iterator has a trusted length
+                unsafe {
+                    nulls
+                        .extend_trusted_len(levels[start..].iter().map(|level| level == max_level));
                 }
 
                 Ok((values_read, levels_read))