[GitHub] [arrow-rs] alamb commented on a diff in pull request #2123: Faster parquet DictEncoder (~20%)

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

alamb commented on code in PR #2123:
URL: https://github.com/apache/arrow-rs/pull/2123#discussion_r927522428


##########
parquet/Cargo.toml:
##########
@@ -30,6 +30,7 @@ edition = "2021"
 rust-version = "1.57"
 
 [dependencies]
+ahash = "0.7"

Review Comment:
   These seem to be new dependencies (if optional features are not enabled)



##########
parquet/src/encodings/encoding/dict_encoder.rs:
##########
@@ -0,0 +1,199 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+// ----------------------------------------------------------------------
+// Dictionary encoding
+
+use crate::basic::{Encoding, Type};
+use crate::data_type::{AsBytes, DataType};
+use crate::encodings::encoding::{Encoder, PlainEncoder};
+use crate::encodings::rle::RleEncoder;
+use crate::errors::{ParquetError, Result};
+use crate::schema::types::ColumnDescPtr;
+use crate::util::bit_util::num_required_bits;
+use crate::util::memory::ByteBufferPtr;
+use hashbrown::hash_map::RawEntryMut;
+use hashbrown::HashMap;
+use std::hash::{BuildHasher, Hash, Hasher};
+use std::io::Write;
+use crate::data_type::private::ParquetValueType;
+
+/// Dictionary encoder.
+/// The dictionary encoding builds a dictionary of values encountered in a given column.
+/// The dictionary page is written first, before the data pages of the column chunk.
+///
+/// Dictionary page format: the entries in the dictionary - in dictionary order -
+/// using the plain encoding.
+///
+/// Data page format: the bit width used to encode the entry ids stored as 1 byte
+/// (max bit width = 32), followed by the values encoded using RLE/Bit packed described
+/// above (with the given bit width).
+pub struct DictEncoder<T: DataType> {
+    /// Descriptor for the column to be encoded.
+    desc: ColumnDescPtr,
+
+    state: ahash::RandomState,
+
+    /// Used to provide a lookup from value to unique value

Review Comment:
   Given the replication of this pattern (maybe now in three places?) perhaps we can factor it into its own structure, mostly for readability as the use of `HashMap` to implement a `HashSet` takes some thought to totally grok



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