tustvold commented on code in PR #2500:
URL: https://github.com/apache/arrow-rs/pull/2500#discussion_r949142066


##########
parquet/src/arrow/array_reader/map_array.rs:
##########
@@ -65,131 +85,122 @@ impl ArrayReader for MapArrayReader {
     }
 
     fn read_records(&mut self, batch_size: usize) -> Result<usize> {
-        let key_len = self.key_reader.read_records(batch_size)?;
-        let value_len = self.value_reader.read_records(batch_size)?;
-        // Check that key and value have the same lengths
-        if key_len != value_len {
-            return Err(general_err!(
-                "Map key and value should have the same lengths."
-            ));
-        }
-        Ok(key_len)
+        self.reader.read_records(batch_size)
     }
 
     fn consume_batch(&mut self) -> Result<ArrayRef> {
-        let key_array = self.key_reader.consume_batch()?;
-        let value_array = self.value_reader.consume_batch()?;
-
-        // Check that key and value have the same lengths
-        let key_length = key_array.len();
-        if key_length != value_array.len() {
-            return Err(general_err!(
-                "Map key and value should have the same lengths."
-            ));
-        }
-
-        let def_levels = self
-            .key_reader
-            .get_def_levels()
-            .ok_or_else(|| ArrowError("item_reader def levels are 
None.".to_string()))?;
-        let rep_levels = self
-            .key_reader
-            .get_rep_levels()
-            .ok_or_else(|| ArrowError("item_reader rep levels are 
None.".to_string()))?;
-
-        if !((def_levels.len() == rep_levels.len()) && (rep_levels.len() == 
key_length)) {
-            return Err(ArrowError(
-                "Expected item_reader def_levels and rep_levels to be same 
length as batch".to_string(),
-            ));
-        }
-
-        let entry_data_type = if let ArrowType::Map(field, _) = 
&self.data_type {
-            field.data_type().clone()
-        } else {
-            return Err(ArrowError("Expected a map arrow type".to_string()));
-        };
-
-        let entry_data = ArrayDataBuilder::new(entry_data_type)
-            .len(key_length)
-            .add_child_data(key_array.into_data())
-            .add_child_data(value_array.into_data());
-        let entry_data = unsafe { entry_data.build_unchecked() };
-
-        let entry_len = rep_levels.iter().filter(|level| **level == 0).count();
-
-        // first item in each list has rep_level = 0, subsequent items have 
rep_level = 1
-        let mut offsets: Vec<i32> = Vec::new();
-        let mut cur_offset = 0;
-        def_levels.iter().zip(rep_levels).for_each(|(d, r)| {
-            if *r == 0 || d == &self.map_def_level {
-                offsets.push(cur_offset);
-            }
-            if d > &self.map_def_level {
-                cur_offset += 1;
-            }
-        });
-        offsets.push(cur_offset);
-
-        let num_bytes = bit_util::ceil(offsets.len(), 8);
-        // TODO: A useful optimization is to use the null count to fill with
-        // 0 or null, to reduce individual bits set in a loop.
-        // To favour dense data, set every slot to true, then unset
-        let mut null_buf = 
MutableBuffer::new(num_bytes).with_bitset(num_bytes, true);
-        let null_slice = null_buf.as_slice_mut();
-        let mut list_index = 0;
-        for i in 0..rep_levels.len() {
-            // If the level is lower than empty, then the slot is null.
-            // When a list is non-nullable, its empty level = null level,
-            // so this automatically factors that in.
-            if rep_levels[i] == 0 && def_levels[i] < self.map_def_level {
-                // should be empty list
-                bit_util::unset_bit(null_slice, list_index);
-            }
-            if rep_levels[i] == 0 {
-                list_index += 1;
-            }
-        }
-        let value_offsets = Buffer::from(&offsets.to_byte_slice());
-
-        // Now we can build array data
-        let array_data = ArrayDataBuilder::new(self.data_type.clone())
-            .len(entry_len)
-            .add_buffer(value_offsets)
-            .null_bit_buffer(Some(null_buf.into()))
-            .add_child_data(entry_data);
-
-        let array_data = unsafe { array_data.build_unchecked() };
-
-        Ok(Arc::new(MapArray::from(array_data)))
+        let array = self.reader.consume_batch().unwrap();
+        let data = array.data().clone();
+        let builder = data.into_builder().data_type(self.data_type.clone());
+        Ok(Arc::new(MapArray::from(unsafe {
+            builder.build_unchecked()
+        })))
     }
 
     fn skip_records(&mut self, num_records: usize) -> Result<usize> {
-        let key_skipped = self.key_reader.skip_records(num_records)?;
-        let value_skipped = self.value_reader.skip_records(num_records)?;
-        if key_skipped != value_skipped {
-            return Err(general_err!(
-                "MapArrayReader out of sync, skipped {} keys and {} values",
-                key_skipped,
-                value_skipped
-            ));
-        }
-        Ok(key_skipped)
+        self.reader.skip_records(num_records)
     }
 
     fn get_def_levels(&self) -> Option<&[i16]> {
-        // Children definition levels should describe the same parent 
structure,
-        // so return key_reader only
-        self.key_reader.get_def_levels()
+        self.reader.get_def_levels()
     }
 
     fn get_rep_levels(&self) -> Option<&[i16]> {
-        // Children repetition levels should describe the same parent 
structure,
-        // so return key_reader only
-        self.key_reader.get_rep_levels()
+        self.reader.get_rep_levels()
     }
 }
 
 #[cfg(test)]
 mod tests {
-    //TODO: Add unit tests (#1561)
+    use super::*;
+    use crate::arrow::arrow_reader::ParquetRecordBatchReader;
+    use crate::arrow::ArrowWriter;
+    use arrow::array;
+    use arrow::array::{MapBuilder, PrimitiveBuilder, StringBuilder};
+    use arrow::datatypes::{Field, Int32Type, Schema};
+    use arrow::record_batch::RecordBatch;
+    use bytes::Bytes;
+
+    #[test]

Review Comment:
   Credit to @d-willis for this test



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