perf: improve field indexing in JSON StructArrayDecoder (1.7x speed up) (apache#9086)

# Which issue does this PR close?

<!--
We generally require a GitHub issue to be filed for all bug fixes and
enhancements and this helps us generate change logs for our releases.
You can link an issue to this PR using the GitHub syntax.
-->

- Closes #NNN.

# Rationale for this change

Optimize JSON struct decoding on wide objects by reducing per-row
allocations and repeated field lookups.

<!--
Why are you proposing this change? If this is already explained clearly
in the issue then this section is not needed.
Explaining clearly why changes are proposed helps reviewers understand
your changes and offer better suggestions for fixes.
-->

# What changes are included in this PR?

Reuse a flat child-position buffer in `StructArrayDecoder` and add an
optional field-name index for object mode.
Skip building the field-name index for list mode; add
overflow/allocation checks.

```
decode_wide_object_i64_json
                        time:   [11.828 ms 11.865 ms 11.905 ms]
                        change: [−67.828% −67.378% −67.008%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 3 outliers among 100 measurements (3.00%)
  2 (2.00%) high mild
  1 (1.00%) high severe

decode_wide_object_i64_serialize
                        time:   [7.6923 ms 7.7402 ms 7.7906 ms]
                        change: [−75.652% −75.483% −75.331%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) high mild

```

<!--
There is no need to duplicate the description in the issue here but it
is sometimes worth providing a summary of the individual changes in this
PR.
-->

# Are these changes tested?
Yes
<!--
We typically require tests for all PRs in order to:
1. Prevent the code from being accidentally broken by subsequent changes
2. Serve as another way to document the expected behavior of the code

If tests are not included in your PR, please explain why (for example,
are they covered by existing tests)?
-->

# Are there any user-facing changes?
No
<!--
If there are user-facing changes then we may require documentation to be
updated before approving the PR.

If there are any breaking changes to public APIs, please call them out.
-->

Author: Weijun-H

arrow-json/src/reader/struct_array.rs

@@ -21,13 +21,61 @@ use arrow_array::builder::BooleanBufferBuilder;
 use arrow_buffer::buffer::NullBuffer;
 use arrow_data::{ArrayData, ArrayDataBuilder};
 use arrow_schema::{ArrowError, DataType, Fields};
+use std::collections::HashMap;
+
+/// Reusable buffer for tape positions, indexed by (field_idx, row_idx).
+/// A value of 0 indicates the field is absent for that row.
+struct FieldTapePositions {
+    data: Vec<u32>,
+    row_count: usize,
+}
+
+impl FieldTapePositions {
+    fn new() -> Self {
+        Self {
+            data: Vec::new(),
+            row_count: 0,
+        }
+    }
+
+    fn resize(&mut self, field_count: usize, row_count: usize) -> Result<(), ArrowError> {
+        let total_len = field_count.checked_mul(row_count).ok_or_else(|| {
+            ArrowError::JsonError(format!(
+                "FieldTapePositions buffer size overflow for rows={row_count} fields={field_count}"
+            ))
+        })?;
+        self.data.clear();
+        self.data.resize(total_len, 0);
+        self.row_count = row_count;
+        Ok(())
+    }
+
+    fn try_set(&mut self, field_idx: usize, row_idx: usize, pos: u32) -> Option<()> {
+        let idx = field_idx
+            .checked_mul(self.row_count)?
+            .checked_add(row_idx)?;
+        *self.data.get_mut(idx)? = pos;
+        Some(())
+    }
+
+    fn set(&mut self, field_idx: usize, row_idx: usize, pos: u32) {
+        self.data[field_idx * self.row_count + row_idx] = pos;
+    }
+
+    fn field_positions(&self, field_idx: usize) -> &[u32] {
+        let start = field_idx * self.row_count;
+        &self.data[start..start + self.row_count]
+    }
+}
 
 pub struct StructArrayDecoder {
     data_type: DataType,
     decoders: Vec<Box<dyn ArrayDecoder>>,
     strict_mode: bool,
     is_nullable: bool,
     struct_mode: StructMode,
+    field_name_to_index: Option<HashMap<String, usize>>,
+    field_tape_positions: FieldTapePositions,
 }
 
 impl StructArrayDecoder {
@@ -38,119 +86,140 @@ impl StructArrayDecoder {
         is_nullable: bool,
         struct_mode: StructMode,
     ) -> Result<Self, ArrowError> {
-        let decoders = struct_fields(&data_type)
-            .iter()
-            .map(|f| {
-                // If this struct nullable, need to permit nullability in child array
-                // StructArrayDecoder::decode verifies that if the child is not nullable
-                // it doesn't contain any nulls not masked by its parent
-                let nullable = f.is_nullable() || is_nullable;
-                make_decoder(
-                    f.data_type().clone(),
-                    coerce_primitive,
-                    strict_mode,
-                    nullable,
-                    struct_mode,
-                )
-            })
-            .collect::<Result<Vec<_>, ArrowError>>()?;
+        let (decoders, field_name_to_index) = {
+            let fields = struct_fields(&data_type);
+            let decoders = fields
+                .iter()
+                .map(|f| {
+                    // If this struct nullable, need to permit nullability in child array
+                    // StructArrayDecoder::decode verifies that if the child is not nullable
+                    // it doesn't contain any nulls not masked by its parent
+                    let nullable = f.is_nullable() || is_nullable;
+                    make_decoder(
+                        f.data_type().clone(),
+                        coerce_primitive,
+                        strict_mode,
+                        nullable,
+                        struct_mode,
+                    )
+                })
+                .collect::<Result<Vec<_>, ArrowError>>()?;
+            let field_name_to_index = if struct_mode == StructMode::ObjectOnly {
+                build_field_index(fields)
+            } else {
+                None
+            };
+            (decoders, field_name_to_index)
+        };
 
         Ok(Self {
             data_type,
             decoders,
             strict_mode,
             is_nullable,
             struct_mode,
+            field_name_to_index,
+            field_tape_positions: FieldTapePositions::new(),
         })
     }
 }
 
 impl ArrayDecoder for StructArrayDecoder {
     fn decode(&mut self, tape: &Tape<'_>, pos: &[u32]) -> Result<ArrayData, ArrowError> {
         let fields = struct_fields(&self.data_type);
-        let mut child_pos: Vec<_> = (0..fields.len()).map(|_| vec![0; pos.len()]).collect();
-
+        let row_count = pos.len();
+        let field_count = fields.len();
+        self.field_tape_positions.resize(field_count, row_count)?;
         let mut nulls = self
             .is_nullable
             .then(|| BooleanBufferBuilder::new(pos.len()));
 
-        // We avoid having the match on self.struct_mode inside the hot loop for performance
-        // TODO: Investigate how to extract duplicated logic.
-        match self.struct_mode {
-            StructMode::ObjectOnly => {
-                for (row, p) in pos.iter().enumerate() {
-                    let end_idx = match (tape.get(*p), nulls.as_mut()) {
-                        (TapeElement::StartObject(end_idx), None) => end_idx,
-                        (TapeElement::StartObject(end_idx), Some(nulls)) => {
-                            nulls.append(true);
-                            end_idx
-                        }
-                        (TapeElement::Null, Some(nulls)) => {
-                            nulls.append(false);
-                            continue;
-                        }
-                        (_, _) => return Err(tape.error(*p, "{")),
-                    };
-
-                    let mut cur_idx = *p + 1;
-                    while cur_idx < end_idx {
-                        // Read field name
-                        let field_name = match tape.get(cur_idx) {
-                            TapeElement::String(s) => tape.get_string(s),
-                            _ => return Err(tape.error(cur_idx, "field name")),
+        {
+            // We avoid having the match on self.struct_mode inside the hot loop for performance
+            // TODO: Investigate how to extract duplicated logic.
+            match self.struct_mode {
+                StructMode::ObjectOnly => {
+                    for (row, p) in pos.iter().enumerate() {
+                        let end_idx = match (tape.get(*p), nulls.as_mut()) {
+                            (TapeElement::StartObject(end_idx), None) => end_idx,
+                            (TapeElement::StartObject(end_idx), Some(nulls)) => {
+                                nulls.append(true);
+                                end_idx
+                            }
+                            (TapeElement::Null, Some(nulls)) => {
+                                nulls.append(false);
+                                continue;
+                            }
+                            (_, _) => return Err(tape.error(*p, "{")),
                         };
 
-                        // Update child pos if match found
-                        match fields.iter().position(|x| x.name() == field_name) {
-                            Some(field_idx) => child_pos[field_idx][row] = cur_idx + 1,
-                            None => {
-                                if self.strict_mode {
-                                    return Err(ArrowError::JsonError(format!(
-                                        "column '{field_name}' missing from schema",
-                                    )));
+                        let mut cur_idx = *p + 1;
+                        while cur_idx < end_idx {
+                            // Read field name
+                            let field_name = match tape.get(cur_idx) {
+                                TapeElement::String(s) => tape.get_string(s),
+                                _ => return Err(tape.error(cur_idx, "field name")),
+                            };
+
+                            // Update child pos if match found
+                            let field_idx = match &self.field_name_to_index {
+                                Some(map) => map.get(field_name).copied(),
+                                None => fields.iter().position(|x| x.name() == field_name),
+                            };
+                            match field_idx {
+                                Some(field_idx) => {
+                                    self.field_tape_positions.set(field_idx, row, cur_idx + 1);
+                                }
+                                None => {
+                                    if self.strict_mode {
+                                        return Err(ArrowError::JsonError(format!(
+                                            "column '{field_name}' missing from schema",
+                                        )));
+                                    }
                                 }
                             }
+                            // Advance to next field
+                            cur_idx = tape.next(cur_idx + 1, "field value")?;
                         }
-                        // Advance to next field
-                        cur_idx = tape.next(cur_idx + 1, "field value")?;
                     }
                 }
-            }
-            StructMode::ListOnly => {
-                for (row, p) in pos.iter().enumerate() {
-                    let end_idx = match (tape.get(*p), nulls.as_mut()) {
-                        (TapeElement::StartList(end_idx), None) => end_idx,
-                        (TapeElement::StartList(end_idx), Some(nulls)) => {
-                            nulls.append(true);
-                            end_idx
-                        }
-                        (TapeElement::Null, Some(nulls)) => {
-                            nulls.append(false);
-                            continue;
-                        }
-                        (_, _) => return Err(tape.error(*p, "[")),
-                    };
+                StructMode::ListOnly => {
+                    for (row, p) in pos.iter().enumerate() {
+                        let end_idx = match (tape.get(*p), nulls.as_mut()) {
+                            (TapeElement::StartList(end_idx), None) => end_idx,
+                            (TapeElement::StartList(end_idx), Some(nulls)) => {
+                                nulls.append(true);
+                                end_idx
+                            }
+                            (TapeElement::Null, Some(nulls)) => {
+                                nulls.append(false);
+                                continue;
+                            }
+                            (_, _) => return Err(tape.error(*p, "[")),
+                        };
 
-                    let mut cur_idx = *p + 1;
-                    let mut entry_idx = 0;
-                    while cur_idx < end_idx {
-                        if entry_idx >= fields.len() {
+                        let mut cur_idx = *p + 1;
+                        let mut entry_idx = 0;
+                        while cur_idx < end_idx {
+                            self.field_tape_positions
+                                .try_set(entry_idx, row, cur_idx)
+                                .ok_or_else(|| {
+                                    ArrowError::JsonError(format!(
+                                        "found extra columns for {} fields",
+                                        fields.len()
+                                    ))
+                                })?;
+                            entry_idx += 1;
+                            // Advance to next field
+                            cur_idx = tape.next(cur_idx, "field value")?;
+                        }
+                        if entry_idx != fields.len() {
                             return Err(ArrowError::JsonError(format!(
-                                "found extra columns for {} fields",
+                                "found {} columns for {} fields",
+                                entry_idx,
                                 fields.len()
                             )));
                         }
-                        child_pos[entry_idx][row] = cur_idx;
-                        entry_idx += 1;
-                        // Advance to next field
-                        cur_idx = tape.next(cur_idx, "field value")?;
-                    }
-                    if entry_idx != fields.len() {
-                        return Err(ArrowError::JsonError(format!(
-                            "found {} columns for {} fields",
-                            entry_idx,
-                            fields.len()
-                        )));
                     }
                 }
             }
@@ -159,10 +228,11 @@ impl ArrayDecoder for StructArrayDecoder {
         let child_data = self
             .decoders
             .iter_mut()
-            .zip(child_pos)
+            .enumerate()
             .zip(fields)
-            .map(|((d, pos), f)| {
-                d.decode(tape, &pos).map_err(|e| match e {
+            .map(|((field_idx, d), f)| {
+                let pos = self.field_tape_positions.field_positions(field_idx);
+                d.decode(tape, pos).map_err(|e| match e {
                     ArrowError::JsonError(s) => {
                         ArrowError::JsonError(format!("whilst decoding field '{}': {s}", f.name()))
                     }
@@ -205,3 +275,20 @@ fn struct_fields(data_type: &DataType) -> &Fields {
         _ => unreachable!(),
     }
 }
+
+fn build_field_index(fields: &Fields) -> Option<HashMap<String, usize>> {
+    // Heuristic threshold: for small field counts, linear scan avoids HashMap overhead.
+    const FIELD_INDEX_LINEAR_THRESHOLD: usize = 16;
+    if fields.len() < FIELD_INDEX_LINEAR_THRESHOLD {
+        return None;
+    }
+
+    let mut map = HashMap::with_capacity(fields.len());
+    for (idx, field) in fields.iter().enumerate() {
+        let name = field.name();
+        if !map.contains_key(name) {
+            map.insert(name.to_string(), idx);
+        }
+    }
+    Some(map)
+}