tustvold commented on a change in pull request #1041:
URL: https://github.com/apache/arrow-rs/pull/1041#discussion_r777130877
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File path: parquet/src/column/reader/decoder.rs
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@@ -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
Review comment:
I didn't write this logic, just moved it, but my guess is this is a way
to placate the borrow checker. `Decoder::get` requires a mutable reference, and
we wish for decoders, in particular the dictionary decoder, to be usable across
multiple `set_data` calls.
In order to have a current_decoder construct we would either need to perform
a convoluted move dance moving data in and out of the decoder map, or use
`Rc<RefCell>`. This is simpler, if possibly a little less performant. FWIW I'd
wager that the overheads of a hashmap keyed on a low cardinality enumeration
are pretty low.
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