alamb commented on code in PR #9021:
URL: https://github.com/apache/arrow-rs/pull/9021#discussion_r2729495830
##########
arrow-json/src/reader/mod.rs:
##########
@@ -373,6 +386,95 @@ impl<R: BufRead> RecordBatchReader for Reader<R> {
}
}
+/// A trait to create custom decoders for specific data types.
+///
+/// This allows overriding the default decoders for specific data types,
+/// or adding new decoders for custom data types.
+///
+/// # Examples
+///
+/// ```
+/// use arrow_json::{ArrayDecoder, DecoderFactory, TapeElement, Tape,
ReaderBuilder, StructMode};
+/// use arrow_schema::ArrowError;
+/// use arrow_schema::{DataType, Field, Fields, Schema};
+/// use arrow_array::cast::AsArray;
+/// use arrow_array::Array;
+/// use arrow_array::builder::StringBuilder;
+/// use arrow_data::ArrayData;
+/// use std::sync::Arc;
+///
+/// struct IncorrectStringAsNullDecoder {}
+///
+/// impl ArrayDecoder for IncorrectStringAsNullDecoder {
+/// fn decode(&mut self, tape: &Tape<'_>, pos: &[u32]) ->
Result<ArrayData, ArrowError> {
+/// let mut builder = StringBuilder::new();
+/// for p in pos {
+/// match tape.get(*p) {
+/// TapeElement::String(idx) => {
+/// builder.append_value(tape.get_string(idx));
+/// }
+/// _ => builder.append_null(),
+/// }
+/// }
+/// Ok(builder.finish().into_data())
+/// }
+/// }
+///
+/// #[derive(Debug)]
+/// struct IncorrectStringAsNullDecoderFactory;
+///
+/// impl DecoderFactory for IncorrectStringAsNullDecoderFactory {
+/// fn make_default_decoder<'a>(
+/// &self,
+/// _field: Option<FieldRef>,
+/// data_type: DataType,
+/// _coerce_primitive: bool,
+/// _strict_mode: bool,
+/// _is_nullable: bool,
+/// _struct_mode: StructMode,
+/// ) -> Result<Option<Box<dyn ArrayDecoder>>, ArrowError> {
+/// match data_type {
+/// DataType::Utf8 =>
Ok(Some(Box::new(IncorrectStringAsNullDecoder {}))),
+/// _ => Ok(None),
+/// }
+/// }
+/// }
+///
+/// let json = r#"
+/// {"a": "a"}
+/// {"a": 12}
+/// "#;
+/// let batch =
ReaderBuilder::new(Arc::new(Schema::new(Fields::from(vec![Field::new(
+/// "a",
+/// DataType::Utf8,
+/// true,
+/// )]))))
+/// .with_decoder_factory(Arc::new(IncorrectStringAsNullDecoderFactory))
+/// .build(json.as_bytes())
+/// .unwrap()
+/// .next()
+/// .unwrap()
+/// .unwrap();
+///
+/// let values = batch.column(0).as_string::<i32>();
+/// assert_eq!(values.len(), 2);
+/// assert_eq!(values.value(0), "a");
+/// assert!(values.is_null(1));
+/// ```
+pub trait DecoderFactory: std::fmt::Debug + Send + Sync {
Review Comment:
I think the key to making this easier to work with would be to register a
decoder by path rather than by type -- so like you would register a custom
decoder for a particular type you could register it by path
So like if you are decoding a top level schema, you would register it by
field name or schema position:
```
{
foo: Int64,
bar: Float64,
baz: <custom decoder>
}
```
We would have to figure out nested types somehow though 🤔
##########
parquet-variant-compute/src/decoder.rs:
##########
@@ -0,0 +1,295 @@
+// 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.
+
+use crate::{VariantArrayBuilder, VariantType};
+use arrow_array::{Array, StructArray};
+use arrow_data::ArrayData;
+use arrow_json::{DecoderFactory, StructMode};
+use arrow_schema::extension::ExtensionType;
+use arrow_schema::{ArrowError, DataType, FieldRef};
+use parquet_variant::{ObjectFieldBuilder, Variant, VariantBuilderExt};
+
+use arrow_json::reader::ArrayDecoder;
+use arrow_json::reader::{Tape, TapeElement};
+
+/// An [`ArrayDecoder`] implementation that decodes JSON values into a Variant
array.
+///
+/// This decoder converts JSON tape elements (parsed JSON tokens) into Parquet
Variant
+/// format, preserving the full structure of arbitrary JSON including nested
objects,
+/// arrays, and primitive types.
+///
+/// This decoder is typically used indirectly via
[`VariantArrayDecoderFactory`] when
+/// reading JSON data into Variant columns.
+#[derive(Default)]
+pub struct VariantArrayDecoder;
+
+impl ArrayDecoder for VariantArrayDecoder {
+ fn decode(&mut self, tape: &Tape<'_>, pos: &[u32]) -> Result<ArrayData,
ArrowError> {
+ let mut array_builder = VariantArrayBuilder::new(pos.len());
+ for p in pos {
+ variant_from_tape_element(&mut array_builder, *p, tape)?;
+ }
+ let variant_struct_array = StructArray::from(array_builder.build());
+ Ok(variant_struct_array.into_data())
+ }
+}
+
+/// A [`DecoderFactory`] that creates [`VariantArrayDecoder`] instances for
Variant-typed fields.
+///
+/// This factory integrates with the Arrow JSON reader to automatically decode
JSON values
+/// into Variant arrays when the target field is registered as a
[`VariantType`] extension type.
+///
+/// # Example
+///
+/// ```ignore
+/// use arrow_json::reader::ReaderBuilder;
+/// use arrow_json::StructMode;
+/// use std::sync::Arc;
+///
+/// let builder = ReaderBuilder::new(Arc::new(schema));
+/// let reader = builder
+/// .with_struct_mode(StructMode::ObjectOnly)
+/// .with_decoder_factory(Arc::new(VariantArrayDecoderFactory))
+/// .build(json_input)?;
+/// ```
+#[derive(Default, Debug)]
+#[allow(unused)]
+pub struct VariantArrayDecoderFactory;
+
+impl DecoderFactory for VariantArrayDecoderFactory {
+ fn make_custom_decoder<'a>(
+ &self,
+ field: Option<FieldRef>,
+ _data_type: DataType,
+ _coerce_primitive: bool,
+ _strict_mode: bool,
+ _is_nullable: bool,
+ _struct_mode: StructMode,
+ ) -> Result<Option<Box<dyn ArrayDecoder>>, ArrowError> {
+ if let Some(field) = field
+ && field.extension_type_name() == Some(VariantType::NAME)
+ {
+ return Ok(Some(Box::new(VariantArrayDecoder)));
+ }
+ Ok(None)
+ }
+}
+
+fn variant_from_tape_element(
+ builder: &mut impl VariantBuilderExt,
+ mut p: u32,
+ tape: &Tape,
+) -> Result<u32, ArrowError> {
+ match tape.get(p) {
+ TapeElement::StartObject(end_idx) => {
+ let mut object_builder = builder.try_new_object()?;
+ p += 1;
+ while p < end_idx {
+ // Read field name
+ let field_name = match tape.get(p) {
+ TapeElement::String(s) => tape.get_string(s),
+ _ => return Err(tape.error(p, "field name")),
+ };
+
+ let mut field_builder = ObjectFieldBuilder::new(field_name,
&mut object_builder);
+ p = tape.next(p, "field value")?;
+ p = variant_from_tape_element(&mut field_builder, p, tape)?;
+ }
+ object_builder.finish();
+ }
+ TapeElement::EndObject(_u32) => {
+ return Err(ArrowError::JsonError(
+ "unexpected end of object".to_string(),
+ ));
+ }
+ TapeElement::StartList(end_idx) => {
+ let mut list_builder = builder.try_new_list()?;
+ p += 1;
+ while p < end_idx {
+ p = variant_from_tape_element(&mut list_builder, p, tape)?;
+ }
+ list_builder.finish();
+ }
+ TapeElement::EndList(_u32) => {
+ return Err(ArrowError::JsonError("unexpected end of
list".to_string()));
+ }
+ TapeElement::String(idx) => builder.append_value(tape.get_string(idx)),
+ TapeElement::Number(idx) => {
+ let s = tape.get_string(idx);
+ builder.append_value(parse_number(s)?)
+ }
+ TapeElement::I64(i) => {
+ return Err(ArrowError::JsonError(format!(
+ "I64 tape element not supported: {i}"
+ )));
+ }
+ TapeElement::I32(i) => {
+ return Err(ArrowError::JsonError(format!(
+ "I32 tape element not supported: {i}"
+ )));
+ }
+ TapeElement::F64(f) => {
+ return Err(ArrowError::JsonError(format!(
+ "F64 tape element not supported: {f}"
+ )));
+ }
+ TapeElement::F32(f) => {
+ return Err(ArrowError::JsonError(format!(
+ "F32 tape element not supported: {f}"
+ )));
+ }
+ TapeElement::True => builder.append_value(true),
+ TapeElement::False => builder.append_value(false),
+ TapeElement::Null => builder.append_value(Variant::Null),
+ }
+ p += 1;
+ Ok(p)
+}
+
+fn parse_number<'a, 'b>(s: &'a str) -> Result<Variant<'a, 'b>, ArrowError> {
+ if let Ok(v) = lexical_core::parse(s.as_bytes()) {
Review Comment:
it is supposedly fastert
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