andygrove commented on code in PR #4798:
URL: https://github.com/apache/datafusion-comet/pull/4798#discussion_r4261918095


##########
native/spark-expr/src/agg_funcs/pivot_first.rs:
##########
@@ -0,0 +1,1179 @@
+// 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.
+
+//! Spark's `PivotFirst` aggregate. Used only by the second phase of the 
optimized pivot plan
+//! generated by `PivotTransformer`. For each group, `PivotFirst` maintains 
one slot per pivot
+//! value; on each input row it looks the pivot column up in the pivot list, 
and writes the value
+//! column into that slot when a match is found and the value is non-null. 
Rows with unmatched
+//! pivot values are ignored; matched rows with a null value column leave the 
slot unchanged
+//! (matches Spark).
+//!
+//! State layout is one column per pivot slot, matching Spark's 
`aggBufferAttributes` (which
+//! declares `indexSize` `AttributeReference`s, one per pivot value). This 
keeps the shuffle
+//! schema between Partial and Final consistent with what Spark catalyst 
declared; otherwise
+//! the shuffle exchange rejects the batch. Spark sizes that buffer from its 
index of the pivot
+//! values, so the serde falls back when the list holds duplicates, and the 
native slot count is
+//! always the list length. The result reassembles the slots into a 
`ListArray` matching
+//! `PivotFirst.dataType = ArrayType(value_type)`.
+//!
+//! The optimized pivot plan groups by every column of the pivoted relation 
other than the pivot
+//! and aggregated columns, so the aggregate runs grouped, through 
[`PivotFirstGroupsAccumulator`],
+//! unless the relation has no other column. That case uses 
[`PivotFirstAccumulator`].
+
+use crate::float_semantics::ieee_equality_key;
+use arrow::array::{
+    Array, ArrayRef, ArrowPrimitiveType, AsArray, BooleanArray, 
BooleanBufferBuilder, ListArray,
+    PrimitiveArray,
+};
+use arrow::buffer::{BooleanBuffer, NullBuffer, OffsetBuffer, ScalarBuffer};
+use arrow::compute::cast;
+use arrow::datatypes::{
+    DataType, Date32Type, Decimal128Type, Field, FieldRef, Float32Type, 
Float64Type, Int16Type,
+    Int32Type, Int64Type, Int8Type, TimeUnit, TimestampMicrosecondType,
+};
+use datafusion::common::utils::SingleRowListArrayBuilder;
+use datafusion::common::{internal_err, not_impl_err, HashMap, Result, 
ScalarValue};
+use datafusion::logical_expr::function::{AccumulatorArgs, StateFieldsArgs};
+use datafusion::logical_expr::Volatility::Immutable;
+use datafusion::logical_expr::{
+    Accumulator, AggregateUDFImpl, EmitTo, GroupsAccumulator, Signature,
+};
+use datafusion::physical_expr::expressions::format_state_name;
+use std::mem::size_of_val;
+use std::sync::Arc;
+
+/// UDAF implementation of Spark's `PivotFirst`.
+///
+/// `pivot_values` is a fixed, plan-time list of the pivot column values that 
occupy each
+/// output slot, and `pivot_index` maps an input row's pivot column value to 
its slot. The index
+/// is wrapped in `Arc` so that every accumulator shares it rather than 
rebuilding it.
+#[derive(Debug)]
+pub struct SparkPivotFirst {
+    signature: Signature,
+    value_type: DataType,
+    // Kept for `PartialEq`/`Hash` (identity of the aggregate for plan 
comparison) and for the
+    // slot count `state_fields` needs.
+    pivot_values: Vec<ScalarValue>,
+    pivot_index: Arc<PivotIndex>,
+}
+
+impl PartialEq for SparkPivotFirst {
+    fn eq(&self, other: &Self) -> bool {
+        self.value_type == other.value_type && self.pivot_values == 
other.pivot_values
+    }
+}
+
+impl Eq for SparkPivotFirst {}
+
+impl std::hash::Hash for SparkPivotFirst {
+    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+        self.value_type.hash(state);
+        self.pivot_values.hash(state);
+    }
+}
+
+impl SparkPivotFirst {
+    /// `pivot_values` are the pivot list in output order. The serde only 
sends a non-empty list
+    /// without duplicates, of a pivot column type that [`PivotIndex`] 
supports.
+    pub fn try_new(value_type: DataType, pivot_values: Vec<ScalarValue>) -> 
Result<Self> {
+        let pivot_index = Arc::new(PivotIndex::try_new(&pivot_values)?);
+        Ok(Self {
+            signature: Signature::user_defined(Immutable),
+            value_type,
+            pivot_values,
+            pivot_index,
+        })
+    }
+}
+
+impl AggregateUDFImpl for SparkPivotFirst {
+    fn name(&self) -> &str {
+        "pivot_first"
+    }
+
+    fn signature(&self) -> &Signature {
+        &self.signature
+    }
+
+    fn return_type(&self, _arg_types: &[DataType]) -> Result<DataType> {
+        Ok(DataType::List(Arc::new(Field::new_list_field(
+            self.value_type.clone(),
+            true,
+        ))))
+    }
+
+    fn state_fields(&self, args: StateFieldsArgs) -> Result<Vec<FieldRef>> {
+        // One field per pivot slot, matching Spark's aggBufferAttributes so 
the shuffle
+        // exchange sees the same schema catalyst declared. 
`format_state_name` is the same
+        // helper other aggregates in this crate use (see `avg.rs`, 
`stddev.rs`).
+        Ok((0..self.pivot_values.len())
+            .map(|i| {
+                Arc::new(Field::new(
+                    format_state_name(args.name, &i.to_string()),
+                    self.value_type.clone(),
+                    true,
+                ))
+            })
+            .collect())
+    }
+
+    fn accumulator(&self, _acc_args: AccumulatorArgs) -> Result<Box<dyn 
Accumulator>> {
+        Ok(Box::new(PivotFirstAccumulator::new(
+            self.value_type.clone(),
+            Arc::clone(&self.pivot_index),
+            self.pivot_values.len(),
+        )))
+    }
+
+    fn groups_accumulator_supported(&self, _args: AccumulatorArgs) -> bool {
+        // Every value type in Spark's `PivotFirst.supportsDataType`.
+        matches!(
+            self.value_type,
+            DataType::Boolean
+                | DataType::Int8
+                | DataType::Int16
+                | DataType::Int32
+                | DataType::Int64
+                | DataType::Float32
+                | DataType::Float64
+                | DataType::Decimal128(_, _)
+        )
+    }
+
+    fn create_groups_accumulator(
+        &self,
+        _args: AccumulatorArgs,
+    ) -> Result<Box<dyn GroupsAccumulator>> {
+        let index = Arc::clone(&self.pivot_index);
+        let num_slots = self.pivot_values.len();
+        let value_type = self.value_type.clone();
+        Ok(match &self.value_type {
+            // Booleans are stored as 0/1 bytes, so that one primitive 
implementation covers
+            // every value type.
+            DataType::Boolean | DataType::Int8 => Box::new(
+                PivotFirstGroupsAccumulator::<Int8Type>::new(value_type, 
index, num_slots),
+            ),
+            DataType::Int16 => 
Box::new(PivotFirstGroupsAccumulator::<Int16Type>::new(
+                value_type, index, num_slots,
+            )),
+            DataType::Int32 => 
Box::new(PivotFirstGroupsAccumulator::<Int32Type>::new(
+                value_type, index, num_slots,
+            )),
+            DataType::Int64 => 
Box::new(PivotFirstGroupsAccumulator::<Int64Type>::new(
+                value_type, index, num_slots,
+            )),
+            DataType::Float32 => 
Box::new(PivotFirstGroupsAccumulator::<Float32Type>::new(
+                value_type, index, num_slots,
+            )),
+            DataType::Float64 => 
Box::new(PivotFirstGroupsAccumulator::<Float64Type>::new(
+                value_type, index, num_slots,
+            )),
+            DataType::Decimal128(_, _) => Box::new(
+                PivotFirstGroupsAccumulator::<Decimal128Type>::new(value_type, 
index, num_slots),
+            ),
+            other => return internal_err!("PivotFirst does not support value 
type {other}"),
+        })
+    }
+}
+
+/// Marks a row whose pivot column value is in no slot.
+const NO_SLOT: u32 = u32::MAX;
+
+/// Maps a pivot column value to its output slot the way Spark's `PivotFirst` 
matches it.
+///
+/// For the pivot column types the serde admits, Spark looks the value up in a 
Scala
+/// `HashMap[Any, Int]` built from the pivot list. Matching therefore goes 
through Scala's `==`
+/// on the boxed Catalyst value, which is plain value equality except for 
floats, where it is
+/// IEEE 754 equality: `-0.0` and `0.0` are one key, and NaN matches nothing, 
not even a NaN in
+/// the pivot list. A null pivot column value matches a null in the pivot 
list, which is what
+/// Spark's `pivotIndex.getOrElse(null, -1)` does.
+#[derive(Debug)]
+struct PivotIndex {
+    /// The Arrow type of the pivot values. A pivot column with another 
layout, such as a
+    /// dictionary-encoded string column, is cast to it before the lookup.
+    key_type: DataType,
+    keys: PivotKeys,
+    null_slot: Option<u32>,
+}
+
+/// The slot of each non-null pivot value.
+#[derive(Debug)]
+enum PivotKeys {
+    /// Slots of `false` and `true`.
+    Boolean([Option<u32>; 2]),
+    /// Integers, dates, timestamps and decimals, keyed on their value as an 
`i128`.
+    Integer(HashMap<i128, u32>),
+    /// Floats, keyed on the bits of their [`ieee_equality_key`] as an `f64`. 
A NaN in the pivot
+    /// list has no key, so its slot is never written.
+    Float(HashMap<u64, u32>),
+    Utf8(HashMap<Box<str>, u32>),
+}
+
+impl PivotIndex {
+    fn try_new(pivot_values: &[ScalarValue]) -> Result<Self> {
+        let Some(first) = pivot_values.first() else {
+            return internal_err!("PivotFirst requires at least one pivot 
value");
+        };
+        let key_type = first.data_type();
+        let mut keys = match &key_type {
+            DataType::Boolean => PivotKeys::Boolean([None, None]),
+            DataType::Int8
+            | DataType::Int16
+            | DataType::Int32
+            | DataType::Int64
+            | DataType::Date32
+            | DataType::Timestamp(TimeUnit::Microsecond, _)
+            | DataType::Decimal128(_, _) => PivotKeys::Integer(HashMap::new()),
+            DataType::Float32 | DataType::Float64 => 
PivotKeys::Float(HashMap::new()),
+            DataType::Utf8 => PivotKeys::Utf8(HashMap::new()),
+            other => return internal_err!("PivotFirst does not support pivot 
column type {other}"),
+        };
+        if pivot_values.len() >= NO_SLOT as usize {
+            return internal_err!("PivotFirst supports fewer than {NO_SLOT} 
pivot values");
+        }
+        let mut null_slot = None;
+        for (slot, value) in pivot_values.iter().enumerate() {
+            let slot = slot as u32;
+            // `Some(true)` for a new key, `Some(false)` for a repeated one, 
`None` for a NaN.
+            let inserted = match (&mut keys, value) {
+                (_, value) if value.is_null() => 
Some(null_slot.replace(slot).is_none()),
+                (PivotKeys::Boolean(slots), ScalarValue::Boolean(Some(b))) => {
+                    Some(slots[*b as usize].replace(slot).is_none())
+                }
+                (PivotKeys::Integer(map), value) => match integer_key(value) {
+                    Some(key) => Some(map.insert(key, slot).is_none()),
+                    None => return mismatched_pivot_value(value, &key_type),
+                },
+                (PivotKeys::Float(map), ScalarValue::Float32(Some(v))) => {
+                    float_key(*v).map(|key| map.insert(key, slot).is_none())
+                }
+                (PivotKeys::Float(map), ScalarValue::Float64(Some(v))) => {
+                    float_key(*v).map(|key| map.insert(key, slot).is_none())
+                }
+                (PivotKeys::Utf8(map), ScalarValue::Utf8(Some(s))) => {
+                    Some(map.insert(s.as_str().into(), slot).is_none())
+                }
+                (_, value) => return mismatched_pivot_value(value, &key_type),
+            };
+            if inserted == Some(false) {
+                // Spark keeps a repeated value's last position and sizes its 
buffer from the
+                // distinct values, so the slot layout here would no longer 
match its schema.
+                return internal_err!(
+                    "PivotFirst pivot value {value} is repeated; the serde 
should have fallen back"
+                );
+            }
+        }
+        Ok(Self {
+            key_type,
+            keys,
+            null_slot,
+        })
+    }
+
+    /// The slot of every row of `pivots`, or [`NO_SLOT`] for a row whose 
value is not in the
+    /// pivot list.
+    fn slots(&self, pivots: &ArrayRef) -> Result<Vec<u32>> {
+        // A timestamp's value does not depend on its time zone label, so only 
the unit has to
+        // agree for the column to be read as it is.
+        let same_layout = match (pivots.data_type(), &self.key_type) {
+            (DataType::Timestamp(a, _), DataType::Timestamp(b, _)) => a == b,
+            (a, b) => a == b,
+        };
+        let pivots = if same_layout {
+            Arc::clone(pivots)
+        } else {
+            cast(pivots, &self.key_type)?
+        };
+        let null_slot = self.null_slot.unwrap_or(NO_SLOT);
+        Ok(match &self.keys {
+            PivotKeys::Boolean(slots) => {
+                lookup(pivots.as_boolean().iter(), null_slot, |b| slots[b as 
usize])
+            }
+            PivotKeys::Integer(map) => match pivots.data_type() {
+                DataType::Int8 => integer_slots::<Int8Type>(&pivots, 
null_slot, map),
+                DataType::Int16 => integer_slots::<Int16Type>(&pivots, 
null_slot, map),
+                DataType::Int32 => integer_slots::<Int32Type>(&pivots, 
null_slot, map),
+                DataType::Int64 => integer_slots::<Int64Type>(&pivots, 
null_slot, map),
+                DataType::Date32 => integer_slots::<Date32Type>(&pivots, 
null_slot, map),
+                DataType::Timestamp(_, _) => {
+                    integer_slots::<TimestampMicrosecondType>(&pivots, 
null_slot, map)
+                }
+                DataType::Decimal128(_, _) => {
+                    integer_slots::<Decimal128Type>(&pivots, null_slot, map)
+                }
+                other => return internal_err!("PivotFirst cannot read pivot 
column type {other}"),
+            },
+            PivotKeys::Float(map) => match pivots.data_type() {
+                DataType::Float32 => float_slots::<Float32Type>(&pivots, 
null_slot, map),
+                DataType::Float64 => float_slots::<Float64Type>(&pivots, 
null_slot, map),
+                other => return internal_err!("PivotFirst cannot read pivot 
column type {other}"),
+            },
+            PivotKeys::Utf8(map) => lookup(pivots.as_string::<i32>().iter(), 
null_slot, |s| {
+                map.get(s).copied()
+            }),
+        })
+    }
+}
+
+fn mismatched_pivot_value<T>(value: &ScalarValue, key_type: &DataType) -> 
Result<T> {
+    internal_err!("PivotFirst pivot value {value:?} does not have the pivot 
column type {key_type}")
+}
+
+fn integer_key(value: &ScalarValue) -> Option<i128> {
+    Some(match value {
+        ScalarValue::Int8(Some(v)) => i128::from(*v),
+        ScalarValue::Int16(Some(v)) => i128::from(*v),
+        ScalarValue::Int32(Some(v)) | ScalarValue::Date32(Some(v)) => 
i128::from(*v),
+        ScalarValue::Int64(Some(v)) | 
ScalarValue::TimestampMicrosecond(Some(v), _) => {
+            i128::from(*v)
+        }
+        ScalarValue::Decimal128(Some(v), _, _) => *v,
+        _ => return None,
+    })
+}
+
+fn float_key(v: impl Into<f64>) -> Option<u64> {
+    ieee_equality_key(v.into()).map(f64::to_bits)
+}
+
+fn lookup<T>(
+    values: impl Iterator<Item = Option<T>>,
+    null_slot: u32,
+    slot_of: impl Fn(T) -> Option<u32>,
+) -> Vec<u32> {
+    values
+        .map(|v| match v {
+            Some(v) => slot_of(v).unwrap_or(NO_SLOT),
+            None => null_slot,
+        })
+        .collect()
+}
+
+fn integer_slots<T>(pivots: &ArrayRef, null_slot: u32, map: &HashMap<i128, 
u32>) -> Vec<u32>
+where
+    T: ArrowPrimitiveType,
+    T::Native: Into<i128>,
+{
+    lookup(pivots.as_primitive::<T>().iter(), null_slot, |v| {
+        map.get(&v.into()).copied()
+    })
+}
+
+fn float_slots<T>(pivots: &ArrayRef, null_slot: u32, map: &HashMap<u64, u32>) 
-> Vec<u32>
+where
+    T: ArrowPrimitiveType,
+    T::Native: Into<f64>,
+{
+    lookup(pivots.as_primitive::<T>().iter(), null_slot, |v| {
+        float_key(v).and_then(|key| map.get(&key).copied())
+    })
+}
+
+/// Grouped `PivotFirst` state. Each group owns `num_slots` cells, stored 
group-major in one flat
+/// buffer, so that a group's cells are contiguous and `evaluate` hands the 
buffer to the output
+/// list as it is.
+#[derive(Debug)]
+struct PivotFirstGroupsAccumulator<T: ArrowPrimitiveType> {
+    /// The value type Spark declared. `T` stores it, as `Int8` for `Boolean`.
+    value_type: DataType,
+    pivot_index: Arc<PivotIndex>,
+    num_slots: usize,
+    /// `values[group * num_slots + slot]`. Only the cells set in `is_set` 
hold a value.
+    values: Vec<T::Native>,
+    /// Which cells have been written. A cell nothing has written to is null, 
as in Spark.
+    is_set: BooleanBufferBuilder,
+}
+
+impl<T: ArrowPrimitiveType> PivotFirstGroupsAccumulator<T> {
+    fn new(value_type: DataType, pivot_index: Arc<PivotIndex>, num_slots: 
usize) -> Self {
+        Self {
+            value_type,
+            pivot_index,
+            num_slots,
+            values: Vec::new(),
+            is_set: BooleanBufferBuilder::new(0),
+        }
+    }
+
+    fn resize(&mut self, total_num_groups: usize) {
+        let num_cells = total_num_groups * self.num_slots;
+        if num_cells > self.values.len() {
+            self.values.resize(num_cells, T::Native::default());
+            self.is_set.append_n(num_cells - self.is_set.len(), false);
+        }
+    }
+
+    /// Writes `values[row]` into `cell` for each `(row, cell)` pair, skipping 
a null value, which
+    /// Spark never writes. A later row overwrites an earlier one, as Spark's 
`update` and `merge`
+    /// do.
+    fn write(
+        &mut self,
+        values: &ArrayRef,
+        writes: impl Iterator<Item = (usize, usize)>,
+    ) -> Result<()> {
+        let values = if self.value_type == DataType::Boolean {
+            cast(values, &DataType::Int8)?
+        } else {
+            Arc::clone(values)
+        };
+        let Some(values) = values.as_primitive_opt::<T>() else {
+            return internal_err!(
+                "PivotFirst expected {} values, got {}",
+                self.value_type,
+                values.data_type()
+            );
+        };
+        for (row, cell) in writes {
+            if values.is_valid(row) {
+                self.values[cell] = values.value(row);
+                self.is_set.set_bit(cell, true);
+            }
+        }
+        Ok(())
+    }
+
+    /// Removes the cells of the groups that `emit_to` covers, returning their 
values and which of
+    /// them are set. The remaining groups move to the front.
+    fn take_cells(&mut self, emit_to: EmitTo) -> (Vec<T::Native>, 
BooleanBuffer) {
+        let num_cells = match emit_to {
+            EmitTo::All => self.values.len(),
+            EmitTo::First(n) => n * self.num_slots,
+        };
+        let rest = self.values.split_off(num_cells);
+        let values = std::mem::replace(&mut self.values, rest);
+        let is_set = self.is_set.finish();
+        self.is_set
+            .append_buffer(&is_set.slice(num_cells, is_set.len() - num_cells));
+        (values, is_set.slice(0, num_cells))
+    }
+
+    /// Gives stored cells the declared value type: the precision and scale of 
a decimal, or
+    /// booleans in place of their `Int8` storage.
+    fn to_value_type(&self, cells: PrimitiveArray<T>) -> Result<ArrayRef> {
+        if self.value_type == DataType::Boolean {
+            Ok(cast(&cells, &DataType::Boolean)?)
+        } else {
+            Ok(Arc::new(cells.with_data_type(self.value_type.clone())))
+        }
+    }
+}
+
+impl<T: ArrowPrimitiveType> GroupsAccumulator for 
PivotFirstGroupsAccumulator<T> {
+    fn update_batch(
+        &mut self,
+        values: &[ArrayRef],
+        group_indices: &[usize],
+        opt_filter: Option<&BooleanArray>,
+        total_num_groups: usize,
+    ) -> Result<()> {
+        let [pivots, values] = values else {
+            return internal_err!(
+                "PivotFirst expects 2 inputs (pivot, value); got {}",
+                values.len()
+            );
+        };
+        self.resize(total_num_groups);
+        let slots = self.pivot_index.slots(pivots)?;
+        let num_slots = self.num_slots;
+        let writes = group_indices
+            .iter()
+            .zip(slots)
+            .enumerate()
+            .filter(|&(row, (_, slot))| {
+                slot != NO_SLOT && opt_filter.is_none_or(|f| f.is_valid(row) 
&& f.value(row))
+            })
+            .map(|(row, (&group, slot))| (row, group * num_slots + slot as 
usize));
+        self.write(values, writes)
+    }
+
+    fn merge_batch(
+        &mut self,
+        values: &[ArrayRef],
+        group_indices: &[usize],
+        total_num_groups: usize,
+    ) -> Result<()> {
+        if values.len() != self.num_slots {
+            return internal_err!(
+                "PivotFirst merge expects {} state columns; got {}",
+                self.num_slots,
+                values.len()
+            );
+        }
+        self.resize(total_num_groups);
+        // Each column is one slot, and any non-null cell overwrites the 
group's slot (matches
+        // Spark's `PivotFirst.merge`). A group's state holds only the pivot 
values its rows had,
+        // so the columns are mostly null, and only their valid rows are 
visited.
+        let num_slots = self.num_slots;
+        for (slot, column) in values.iter().enumerate() {
+            let cell = |row: usize| (row, group_indices[row] * num_slots + 
slot);
+            match column.logical_nulls() {
+                Some(nulls) => self.write(column, 
nulls.valid_indices().map(cell))?,
+                None => self.write(column, (0..column.len()).map(cell))?,
+            }
+        }
+        Ok(())
+    }
+
+    fn evaluate(&mut self, emit_to: EmitTo) -> Result<ArrayRef> {
+        let (values, is_set) = self.take_cells(emit_to);
+        let num_groups = values.len() / self.num_slots;
+        let cells = PrimitiveArray::new(values.into(), 
Some(NullBuffer::new(is_set)));
+        let offsets =
+            
OffsetBuffer::<i32>::from_lengths(std::iter::repeat_n(self.num_slots, 
num_groups));
+        let field = Arc::new(Field::new_list_field(self.value_type.clone(), 
true));
+        Ok(Arc::new(ListArray::try_new(
+            field,
+            offsets,
+            self.to_value_type(cells)?,
+            None,
+        )?))
+    }
+
+    fn state(&mut self, emit_to: EmitTo) -> Result<Vec<ArrayRef>> {
+        let (values, is_set) = self.take_cells(emit_to);
+        let num_slots = self.num_slots;
+        let num_groups = values.len() / num_slots;
+        (0..num_slots)
+            .map(|slot| {
+                let column: ScalarBuffer<T::Native> = (0..num_groups)
+                    .map(|group| values[group * num_slots + slot])
+                    .collect();
+                let nulls = BooleanBuffer::collect_bool(num_groups, |group| {
+                    is_set.value(group * num_slots + slot)
+                });
+                self.to_value_type(PrimitiveArray::new(column, 
Some(NullBuffer::new(nulls))))
+            })
+            .collect()
+    }
+
+    fn convert_to_state(
+        &self,
+        _values: &[ArrayRef],
+        _opt_filter: Option<&BooleanArray>,
+    ) -> Result<Vec<ArrayRef>> {
+        not_impl_err!("Input batch conversion to state not implemented")
+    }
+
+    fn size(&self) -> usize {
+        size_of_val(self)
+            + self.values.capacity() * size_of::<T::Native>()
+            + self.is_set.capacity() / 8
+    }
+}
+
+/// Ungrouped state: `slots[i]` holds the latest non-null value assigned to 
pivot slot `i`, or
+/// `None` when nothing has written to that slot yet.
+#[derive(Debug)]
+struct PivotFirstAccumulator {

Review Comment:
   Added both in 24ae599fc6. The empty one filters with `WHERE 1 = 0` rather 
than `WHERE false`. A literal `false` lets the optimizer replace the scan with 
an empty `LocalTableScan`, which Comet doesn't convert by default, so that 
query ran entirely in Spark. The fixtures run without constant folding, so `1 = 
0` keeps the scan, the filter and both `PivotFirst` aggregates native, and the 
row of nulls comes from the native final aggregate. Since cafc8d1f65 the 
ungrouped accumulator is the grouped one holding a single group, so these run 
the same code as the grouped cases.



##########
spark/src/main/scala/org/apache/comet/serde/aggregates.scala:
##########
@@ -1434,6 +1434,96 @@ object CometMode extends 
CometAggregateExpressionSerde[Mode] with CometTypeShim
   }
 }
 
+object CometPivotFirst extends CometAggregateExpressionSerde[PivotFirst] with 
CometTypeShim {
+
+  // Delegate to Spark's own PivotFirst.supportsDataType so the two lists 
cannot drift if a
+  // future Spark version adds a value type to the fast-path gate.
+  private def unsupportedValueTypeReason(dt: DataType): String =
+    s"Unsupported value data type: $dt"
+
+  private def unsupportedPivotTypeReason(dt: DataType): String =
+    s"Unsupported pivot column data type: $dt"
+
+  private val emptyPivotValuesReason = "Pivot values list is empty"
+
+  private val duplicatePivotValuesReason = "Pivot values list contains 
duplicates"
+
+  override def getUnsupportedReasons(): Seq[String] = Seq(
+    "Value data type outside PivotFirst.supportsDataType " +
+      "(Boolean, Byte, Short, Int, Long, Float, Double, Decimal)",
+    "Pivot column of a type other than Boolean, Byte, Short, Int, Long, Float, 
Double, " +
+      "Decimal, String (UTF8_BINARY collation), Date, Timestamp or 
TimestampNTZ, which " +
+      "includes the struct that a pivot on more than one column builds",
+    emptyPivotValuesReason,
+    s"$duplicatePivotValuesReason, including `0.0` next to `-0.0`")
+
+  // The pivot column types for which Spark finds a value's slot in a 
`HashMap` keyed by plain
+  // value equality, which the native lookup reproduces. Spark keys arrays and 
structs in a
+  // `TreeMap` by their interpreted ordering. It keys binary values by 
reference identity before
+  // 4.2 and by ordering from 4.2, which orders strings with a non-UTF8_BINARY 
collation too.
+  private def isSupportedPivotType(dt: DataType): Boolean = dt match {
+    case BooleanType | ByteType | ShortType | IntegerType | LongType => true
+    case FloatType | DoubleType => true
+    case _: DecimalType => true
+    case DateType | TimestampType | TimestampNTZType => true
+    case st: StringType => !isStringCollationType(st)

Review Comment:
   Added `PivotFirst_collation.sql` in 24ae599fc6, with `MinSparkVersion: 4.0` 
and the UTF8_LCASE shape from SPARK-56306. On 4.1 the query falls back before 
it reaches `PivotFirst` anyway, since the native scan declines the collated 
schema and Comet shuffle declines a collated hash key. What the fixture pins is 
the gate's own reason, which nothing else records. With the collation check 
removed, `expect_fallback` fails on the missing reason. It passes locally on 
4.0.4, 4.1.3 and 4.2.0.



##########
spark/src/main/scala/org/apache/comet/serde/aggregates.scala:
##########
@@ -1434,6 +1434,96 @@ object CometMode extends 
CometAggregateExpressionSerde[Mode] with CometTypeShim
   }
 }
 
+object CometPivotFirst extends CometAggregateExpressionSerde[PivotFirst] with 
CometTypeShim {
+
+  // Delegate to Spark's own PivotFirst.supportsDataType so the two lists 
cannot drift if a

Review Comment:
   Reworded in cafc8d1f65, and the check changed with it. Now that the 
ungrouped accumulator is built on the grouped one, native code has nothing that 
could store a value type it doesn't know. With `PivotFirst.supportsDataType` as 
the check, a type that a later Spark version adds would get through to an 
internal error instead of a fallback. So `getSupportLevel` calls 
`isSupportedValueType`, which lists the same types Spark accepts on 3.4.3 
through 4.2.0. The comment now sits on that list and names the three places a 
new type needs: the list itself, `getUnsupportedReasons`, and 
`SparkPivotFirst::groups_accumulator`. `groups_accumulator_supported` no longer 
lists types.



##########
native/spark-expr/src/agg_funcs/pivot_first.rs:
##########
@@ -0,0 +1,1179 @@
+// 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.
+
+//! Spark's `PivotFirst` aggregate. Used only by the second phase of the 
optimized pivot plan
+//! generated by `PivotTransformer`. For each group, `PivotFirst` maintains 
one slot per pivot

Review Comment:
   Fixed in all three places. The module doc (cafc8d1f65) and the 
`PivotFirst.sql` header (24ae599fc6) now name `ResolvePivot`, and the 
`CometAggregateSuite` comment went away with the tests.



##########
spark/src/test/resources/sql-tests/expressions/aggregate/PivotFirst.sql:
##########
@@ -0,0 +1,525 @@
+-- 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.
+
+-- PivotFirst is Spark's internal aggregate emitted by the two-phase optimized 
pivot plan
+-- (PivotTransformer). It runs whenever every aggregate output type is in
+-- PivotFirst.supportsDataType (Boolean, Byte, Short, Int, Long, Float, 
Double, Decimal).
+-- These SQL tests exercise that path via SQL PIVOT clauses, one per supported 
value type and
+-- per supported pivot column type, plus a handful of edge cases (unmatched 
pivot values,
+-- all-null groups, null pivot column). The pivot column types and pivot lists 
whose matching
+-- Comet does not reproduce come last and must fall back.
+
+-- ============================================================
+-- Setup: sales-like table used for most cases
+-- ============================================================
+
+statement
+CREATE TABLE pf_sales (year int, course string, earnings int) USING parquet
+
+statement
+INSERT INTO pf_sales VALUES
+  (2012, 'dotNET', 15000),
+  (2012, 'Java',   20000),
+  (2013, 'dotNET', 48000),
+  (2013, 'Java',   30000)
+
+-- ============================================================
+-- Int values: canonical optimized-pivot query. Extra pivot values
+-- (1..10) push the aggregate over the threshold that turns on the

Review Comment:
   Dropped in 24ae599fc6.



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