[GitHub] [arrow-rs] tustvold commented on a change in pull request #1041: Generify ColumnReaderImpl and RecordReader (#1040)

[GitBox <gi...@apache.org>]

tustvold commented on a change in pull request #1041:
URL: https://github.com/apache/arrow-rs/pull/1041#discussion_r777120783



##########
File path: parquet/src/column/reader/decoder.rs
##########
@@ -0,0 +1,215 @@
+use std::collections::HashMap;
+use std::ops::Range;
+
+use crate::basic::Encoding;
+use crate::data_type::DataType;
+use crate::decoding::{get_decoder, Decoder, DictDecoder, PlainDecoder};
+use crate::encodings::rle::RleDecoder;
+use crate::errors::{ParquetError, Result};
+use crate::memory::ByteBufferPtr;
+use crate::schema::types::ColumnDescPtr;
+use crate::util::bit_util::BitReader;
+
+/// A type that can have level data written to it by a [`ColumnLevelDecoder`]
+pub trait LevelsWriter {
+    fn capacity(&self) -> usize;
+
+    fn get(&self, idx: usize) -> i16;
+
+    fn count_nulls(&self, range: Range<usize>, max_level: i16) -> usize;
+}
+
+impl LevelsWriter for [i16] {
+    fn capacity(&self) -> usize {
+        self.len()
+    }
+
+    fn get(&self, idx: usize) -> i16 {
+        self[idx]
+    }
+
+    fn count_nulls(&self, range: Range<usize>, max_level: i16) -> usize {
+        self[range].iter().filter(|i| **i != max_level).count()
+    }
+}
+
+/// A type that can have value data written to it by a [`ColumnValueDecoder`]
+pub trait ValuesWriter {
+    fn capacity(&self) -> usize;
+}
+
+impl<T> ValuesWriter for [T] {
+    fn capacity(&self) -> usize {
+        self.len()
+    }
+}
+
+/// Decodes level data to a [`LevelsWriter`]
+pub trait ColumnLevelDecoder {
+    type Writer: LevelsWriter + ?Sized;
+
+    fn create(max_level: i16, encoding: Encoding, data: ByteBufferPtr) -> Self;
+
+    fn read(&mut self, out: &mut Self::Writer, range: Range<usize>) -> Result<usize>;
+}
+
+/// Decodes value data to a [`ValuesWriter`]
+pub trait ColumnValueDecoder {
+    type Writer: ValuesWriter + ?Sized;
+
+    fn create(col: &ColumnDescPtr) -> Self;
+
+    fn set_dict(
+        &mut self,
+        buf: ByteBufferPtr,
+        num_values: u32,
+        encoding: Encoding,
+        is_sorted: bool,
+    ) -> Result<()>;
+
+    fn set_data(
+        &mut self,
+        encoding: Encoding,
+        data: ByteBufferPtr,
+        num_values: usize,
+    ) -> Result<()>;
+
+    fn read(&mut self, out: &mut Self::Writer, range: Range<usize>) -> Result<usize>;
+}
+
+/// An implementation of [`ColumnValueDecoder`] for `[T::T]`
+pub struct ColumnValueDecoderImpl<T: DataType> {
+    descr: ColumnDescPtr,
+
+    current_encoding: Option<Encoding>,
+
+    // Cache of decoders for existing encodings
+    decoders: HashMap<Encoding, Box<dyn Decoder<T>>>,
+}
+
+impl<T: DataType> ColumnValueDecoder for ColumnValueDecoderImpl<T> {
+    type Writer = [T::T];
+
+    fn create(descr: &ColumnDescPtr) -> Self {
+        Self {
+            descr: descr.clone(),
+            current_encoding: None,
+            decoders: Default::default(),
+        }
+    }
+
+    fn set_dict(
+        &mut self,
+        buf: ByteBufferPtr,
+        num_values: u32,
+        mut encoding: Encoding,
+        _is_sorted: bool,
+    ) -> Result<()> {
+        if encoding == Encoding::PLAIN || encoding == Encoding::PLAIN_DICTIONARY {
+            encoding = Encoding::RLE_DICTIONARY
+        }
+
+        if self.decoders.contains_key(&encoding) {
+            return Err(general_err!("Column cannot have more than one dictionary"));
+        }
+
+        if encoding == Encoding::RLE_DICTIONARY {
+            let mut dictionary = PlainDecoder::<T>::new(self.descr.type_length());
+            dictionary.set_data(buf, num_values as usize)?;
+
+            let mut decoder = DictDecoder::new();
+            decoder.set_dict(Box::new(dictionary))?;
+            self.decoders.insert(encoding, Box::new(decoder));
+            Ok(())
+        } else {
+            Err(nyi_err!(
+                "Invalid/Unsupported encoding type for dictionary: {}",
+                encoding
+            ))
+        }
+    }
+
+    fn set_data(
+        &mut self,
+        mut encoding: Encoding,
+        data: ByteBufferPtr,
+        num_values: usize,
+    ) -> Result<()> {
+        if encoding == Encoding::PLAIN_DICTIONARY {
+            encoding = Encoding::RLE_DICTIONARY;
+        }
+
+        let decoder = if encoding == Encoding::RLE_DICTIONARY {
+            self.decoders
+                .get_mut(&encoding)
+                .expect("Decoder for dict should have been set")
+        } else {
+            // Search cache for data page decoder
+            #[allow(clippy::map_entry)]
+            if !self.decoders.contains_key(&encoding) {
+                // Initialize decoder for this page
+                let data_decoder = get_decoder::<T>(self.descr.clone(), encoding)?;
+                self.decoders.insert(encoding, data_decoder);
+            }
+            self.decoders.get_mut(&encoding).unwrap()
+        };
+
+        decoder.set_data(data, num_values)?;
+        self.current_encoding = Some(encoding);
+        Ok(())
+    }
+
+    fn read(&mut self, out: &mut Self::Writer, range: Range<usize>) -> Result<usize> {
+        let encoding = self
+            .current_encoding
+            .expect("current_encoding should be set");
+
+        let current_decoder = self
+            .decoders
+            .get_mut(&encoding)
+            .unwrap_or_else(|| panic!("decoder for encoding {} should be set", encoding));
+
+        current_decoder.get(&mut out[range])
+    }
+}
+
+/// An implementation of [`ColumnLevelDecoder`] for `[i16]`
+pub struct ColumnLevelDecoderImpl {
+    inner: LevelDecoderInner,

Review comment:
       This would require introducing some type representation of the encoding type. This would be a fair bit of additional code/complexity that I don't think would not lead to a meaningful performance uplift. Assuming `ColumnLevelDecoderImpl::read` is called with a reasonable batch size of ~1024, the overheads of a jump table are likely to be irrelevant.




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