datafusion_functions_aggregate/

count.rs

// 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 ahash::RandomState;
use arrow::{
    array::{Array, ArrayRef, AsArray, BooleanArray, Int64Array, PrimitiveArray},
    buffer::BooleanBuffer,
    compute,
    datatypes::{
        DataType, Date32Type, Date64Type, Decimal128Type, Decimal256Type, Field,
        FieldRef, Float16Type, Float32Type, Float64Type, Int16Type, Int32Type, Int64Type,
        Int8Type, Time32MillisecondType, Time32SecondType, Time64MicrosecondType,
        Time64NanosecondType, TimeUnit, TimestampMicrosecondType,
        TimestampMillisecondType, TimestampNanosecondType, TimestampSecondType,
        UInt16Type, UInt32Type, UInt64Type, UInt8Type,
    },
};
use datafusion_common::{
    downcast_value, internal_err, not_impl_err, stats::Precision,
    utils::expr::COUNT_STAR_EXPANSION, HashMap, Result, ScalarValue,
};
use datafusion_expr::{
    expr::WindowFunction,
    function::{AccumulatorArgs, StateFieldsArgs},
    utils::format_state_name,
    Accumulator, AggregateUDFImpl, Documentation, EmitTo, Expr, GroupsAccumulator,
    ReversedUDAF, SetMonotonicity, Signature, StatisticsArgs, TypeSignature, Volatility,
    WindowFunctionDefinition,
};
use datafusion_functions_aggregate_common::aggregate::{
    count_distinct::BytesDistinctCountAccumulator,
    count_distinct::BytesViewDistinctCountAccumulator,
    count_distinct::DictionaryCountAccumulator,
    count_distinct::FloatDistinctCountAccumulator,
    count_distinct::PrimitiveDistinctCountAccumulator,
    groups_accumulator::accumulate::accumulate_indices,
};
use datafusion_macros::user_doc;
use datafusion_physical_expr::expressions;
use datafusion_physical_expr_common::binary_map::OutputType;
use std::{
    collections::HashSet,
    fmt::Debug,
    mem::{size_of, size_of_val},
    ops::BitAnd,
    sync::Arc,
};

make_udaf_expr_and_func!(
    Count,
    count,
    expr,
    "Count the number of non-null values in the column",
    count_udaf
);

pub fn count_distinct(expr: Expr) -> Expr {
    Expr::AggregateFunction(datafusion_expr::expr::AggregateFunction::new_udf(
        count_udaf(),
        vec![expr],
        true,
        None,
        vec![],
        None,
    ))
}

/// Creates aggregation to count all rows.
///
/// In SQL this is `SELECT COUNT(*) ... `
///
/// The expression is equivalent to `COUNT(*)`, `COUNT()`, `COUNT(1)`, and is
/// aliased to a column named `"count(*)"` for backward compatibility.
///
/// Example
/// ```
/// # use datafusion_functions_aggregate::count::count_all;
/// # use datafusion_expr::col;
/// // create `count(*)` expression
/// let expr = count_all();
/// assert_eq!(expr.schema_name().to_string(), "count(*)");
/// // if you need to refer to this column, use the `schema_name` function
/// let expr = col(expr.schema_name().to_string());
/// ```
pub fn count_all() -> Expr {
    count(Expr::Literal(COUNT_STAR_EXPANSION, None)).alias("count(*)")
}

/// Creates window aggregation to count all rows.
///
/// In SQL this is `SELECT COUNT(*) OVER (..) ... `
///
/// The expression is equivalent to `COUNT(*)`, `COUNT()`, `COUNT(1)`
///
/// Example
/// ```
/// # use datafusion_functions_aggregate::count::count_all_window;
/// # use datafusion_expr::col;
/// // create `count(*)` OVER ... window function expression
/// let expr = count_all_window();
/// assert_eq!(
///   expr.schema_name().to_string(),
///   "count(Int64(1)) ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING"
/// );
/// // if you need to refer to this column, use the `schema_name` function
/// let expr = col(expr.schema_name().to_string());
/// ```
pub fn count_all_window() -> Expr {
    Expr::from(WindowFunction::new(
        WindowFunctionDefinition::AggregateUDF(count_udaf()),
        vec![Expr::Literal(COUNT_STAR_EXPANSION, None)],
    ))
}

#[user_doc(
    doc_section(label = "General Functions"),
    description = "Returns the number of non-null values in the specified column. To include null values in the total count, use `count(*)`.",
    syntax_example = "count(expression)",
    sql_example = r#"```sql
> SELECT count(column_name) FROM table_name;
+-----------------------+
| count(column_name)     |
+-----------------------+
| 100                   |
+-----------------------+

> SELECT count(*) FROM table_name;
+------------------+
| count(*)         |
+------------------+
| 120              |
+------------------+
```"#,
    standard_argument(name = "expression",)
)]
#[derive(PartialEq, Eq, Hash)]
pub struct Count {
    signature: Signature,
}

impl Debug for Count {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        f.debug_struct("Count")
            .field("name", &self.name())
            .field("signature", &self.signature)
            .finish()
    }
}

impl Default for Count {
    fn default() -> Self {
        Self::new()
    }
}

impl Count {
    pub fn new() -> Self {
        Self {
            signature: Signature::one_of(
                vec![TypeSignature::VariadicAny, TypeSignature::Nullary],
                Volatility::Immutable,
            ),
        }
    }
}
fn get_count_accumulator(data_type: &DataType) -> Box<dyn Accumulator> {
    match data_type {
        // try and use a specialized accumulator if possible, otherwise fall back to generic accumulator
        DataType::Int8 => Box::new(PrimitiveDistinctCountAccumulator::<Int8Type>::new(
            data_type,
        )),
        DataType::Int16 => Box::new(PrimitiveDistinctCountAccumulator::<Int16Type>::new(
            data_type,
        )),
        DataType::Int32 => Box::new(PrimitiveDistinctCountAccumulator::<Int32Type>::new(
            data_type,
        )),
        DataType::Int64 => Box::new(PrimitiveDistinctCountAccumulator::<Int64Type>::new(
            data_type,
        )),
        DataType::UInt8 => Box::new(PrimitiveDistinctCountAccumulator::<UInt8Type>::new(
            data_type,
        )),
        DataType::UInt16 => Box::new(
            PrimitiveDistinctCountAccumulator::<UInt16Type>::new(data_type),
        ),
        DataType::UInt32 => Box::new(
            PrimitiveDistinctCountAccumulator::<UInt32Type>::new(data_type),
        ),
        DataType::UInt64 => Box::new(
            PrimitiveDistinctCountAccumulator::<UInt64Type>::new(data_type),
        ),
        DataType::Decimal128(_, _) => Box::new(PrimitiveDistinctCountAccumulator::<
            Decimal128Type,
        >::new(data_type)),
        DataType::Decimal256(_, _) => Box::new(PrimitiveDistinctCountAccumulator::<
            Decimal256Type,
        >::new(data_type)),

        DataType::Date32 => Box::new(
            PrimitiveDistinctCountAccumulator::<Date32Type>::new(data_type),
        ),
        DataType::Date64 => Box::new(
            PrimitiveDistinctCountAccumulator::<Date64Type>::new(data_type),
        ),
        DataType::Time32(TimeUnit::Millisecond) => Box::new(
            PrimitiveDistinctCountAccumulator::<Time32MillisecondType>::new(data_type),
        ),
        DataType::Time32(TimeUnit::Second) => Box::new(
            PrimitiveDistinctCountAccumulator::<Time32SecondType>::new(data_type),
        ),
        DataType::Time64(TimeUnit::Microsecond) => Box::new(
            PrimitiveDistinctCountAccumulator::<Time64MicrosecondType>::new(data_type),
        ),
        DataType::Time64(TimeUnit::Nanosecond) => Box::new(
            PrimitiveDistinctCountAccumulator::<Time64NanosecondType>::new(data_type),
        ),
        DataType::Timestamp(TimeUnit::Microsecond, _) => Box::new(
            PrimitiveDistinctCountAccumulator::<TimestampMicrosecondType>::new(data_type),
        ),
        DataType::Timestamp(TimeUnit::Millisecond, _) => Box::new(
            PrimitiveDistinctCountAccumulator::<TimestampMillisecondType>::new(data_type),
        ),
        DataType::Timestamp(TimeUnit::Nanosecond, _) => Box::new(
            PrimitiveDistinctCountAccumulator::<TimestampNanosecondType>::new(data_type),
        ),
        DataType::Timestamp(TimeUnit::Second, _) => Box::new(
            PrimitiveDistinctCountAccumulator::<TimestampSecondType>::new(data_type),
        ),

        DataType::Float16 => {
            Box::new(FloatDistinctCountAccumulator::<Float16Type>::new())
        }
        DataType::Float32 => {
            Box::new(FloatDistinctCountAccumulator::<Float32Type>::new())
        }
        DataType::Float64 => {
            Box::new(FloatDistinctCountAccumulator::<Float64Type>::new())
        }

        DataType::Utf8 => {
            Box::new(BytesDistinctCountAccumulator::<i32>::new(OutputType::Utf8))
        }
        DataType::Utf8View => {
            Box::new(BytesViewDistinctCountAccumulator::new(OutputType::Utf8View))
        }
        DataType::LargeUtf8 => {
            Box::new(BytesDistinctCountAccumulator::<i64>::new(OutputType::Utf8))
        }
        DataType::Binary => Box::new(BytesDistinctCountAccumulator::<i32>::new(
            OutputType::Binary,
        )),
        DataType::BinaryView => Box::new(BytesViewDistinctCountAccumulator::new(
            OutputType::BinaryView,
        )),
        DataType::LargeBinary => Box::new(BytesDistinctCountAccumulator::<i64>::new(
            OutputType::Binary,
        )),

        // Use the generic accumulator based on `ScalarValue` for all other types
        _ => Box::new(DistinctCountAccumulator {
            values: HashSet::default(),
            state_data_type: data_type.clone(),
        }),
    }
}

impl AggregateUDFImpl for Count {
    fn as_any(&self) -> &dyn std::any::Any {
        self
    }

    fn name(&self) -> &str {
        "count"
    }

    fn signature(&self) -> &Signature {
        &self.signature
    }

    fn return_type(&self, _arg_types: &[DataType]) -> Result<DataType> {
        Ok(DataType::Int64)
    }

    fn is_nullable(&self) -> bool {
        false
    }

    fn state_fields(&self, args: StateFieldsArgs) -> Result<Vec<FieldRef>> {
        if args.is_distinct {
            let dtype: DataType = match &args.input_fields[0].data_type() {
                DataType::Dictionary(_, values_type) => (**values_type).clone(),
                &dtype => dtype.clone(),
            };

            Ok(vec![Field::new_list(
                format_state_name(args.name, "count distinct"),
                // See COMMENTS.md to understand why nullable is set to true
                Field::new_list_field(dtype, true),
                false,
            )
            .into()])
        } else {
            Ok(vec![Field::new(
                format_state_name(args.name, "count"),
                DataType::Int64,
                false,
            )
            .into()])
        }
    }

    fn accumulator(&self, acc_args: AccumulatorArgs) -> Result<Box<dyn Accumulator>> {
        if !acc_args.is_distinct {
            return Ok(Box::new(CountAccumulator::new()));
        }

        if acc_args.exprs.len() > 1 {
            return not_impl_err!("COUNT DISTINCT with multiple arguments");
        }

        let data_type = &acc_args.exprs[0].data_type(acc_args.schema)?;

        Ok(match data_type {
            DataType::Dictionary(_, values_type) => {
                let inner = get_count_accumulator(values_type);
                Box::new(DictionaryCountAccumulator::new(inner))
            }
            _ => get_count_accumulator(data_type),
        })
    }

    fn groups_accumulator_supported(&self, args: AccumulatorArgs) -> bool {
        // groups accumulator only supports `COUNT(c1)`, not
        // `COUNT(c1, c2)`, etc
        if args.is_distinct {
            return false;
        }
        args.exprs.len() == 1
    }

    fn create_groups_accumulator(
        &self,
        _args: AccumulatorArgs,
    ) -> Result<Box<dyn GroupsAccumulator>> {
        // instantiate specialized accumulator
        Ok(Box::new(CountGroupsAccumulator::new()))
    }

    fn reverse_expr(&self) -> ReversedUDAF {
        ReversedUDAF::Identical
    }

    fn default_value(&self, _data_type: &DataType) -> Result<ScalarValue> {
        Ok(ScalarValue::Int64(Some(0)))
    }

    fn value_from_stats(&self, statistics_args: &StatisticsArgs) -> Option<ScalarValue> {
        if statistics_args.is_distinct {
            return None;
        }
        if let Precision::Exact(num_rows) = statistics_args.statistics.num_rows {
            if statistics_args.exprs.len() == 1 {
                // TODO optimize with exprs other than Column
                if let Some(col_expr) = statistics_args.exprs[0]
                    .as_any()
                    .downcast_ref::<expressions::Column>()
                {
                    let current_val = &statistics_args.statistics.column_statistics
                        [col_expr.index()]
                    .null_count;
                    if let &Precision::Exact(val) = current_val {
                        return Some(ScalarValue::Int64(Some((num_rows - val) as i64)));
                    }
                } else if let Some(lit_expr) = statistics_args.exprs[0]
                    .as_any()
                    .downcast_ref::<expressions::Literal>()
                {
                    if lit_expr.value() == &COUNT_STAR_EXPANSION {
                        return Some(ScalarValue::Int64(Some(num_rows as i64)));
                    }
                }
            }
        }
        None
    }

    fn documentation(&self) -> Option<&Documentation> {
        self.doc()
    }

    fn set_monotonicity(&self, _data_type: &DataType) -> SetMonotonicity {
        // `COUNT` is monotonically increasing as it always increases or stays
        // the same as new values are seen.
        SetMonotonicity::Increasing
    }

    fn create_sliding_accumulator(
        &self,
        args: AccumulatorArgs,
    ) -> Result<Box<dyn Accumulator>> {
        if args.is_distinct {
            let acc =
                SlidingDistinctCountAccumulator::try_new(args.return_field.data_type())?;
            Ok(Box::new(acc))
        } else {
            let acc = CountAccumulator::new();
            Ok(Box::new(acc))
        }
    }
}

// DistinctCountAccumulator does not support retract_batch and sliding window
// this is a specialized accumulator for distinct count that supports retract_batch
// and sliding window.
#[derive(Debug)]
pub struct SlidingDistinctCountAccumulator {
    counts: HashMap<ScalarValue, usize, RandomState>,
    data_type: DataType,
}

impl SlidingDistinctCountAccumulator {
    pub fn try_new(data_type: &DataType) -> Result<Self> {
        Ok(Self {
            counts: HashMap::default(),
            data_type: data_type.clone(),
        })
    }
}

impl Accumulator for SlidingDistinctCountAccumulator {
    fn state(&mut self) -> Result<Vec<ScalarValue>> {
        let keys = self.counts.keys().cloned().collect::<Vec<_>>();
        Ok(vec![ScalarValue::List(ScalarValue::new_list_nullable(
            keys.as_slice(),
            &self.data_type,
        ))])
    }

    fn update_batch(&mut self, values: &[ArrayRef]) -> Result<()> {
        let arr = &values[0];
        for i in 0..arr.len() {
            let v = ScalarValue::try_from_array(arr, i)?;
            if !v.is_null() {
                *self.counts.entry(v).or_default() += 1;
            }
        }
        Ok(())
    }

    fn retract_batch(&mut self, values: &[ArrayRef]) -> Result<()> {
        let arr = &values[0];
        for i in 0..arr.len() {
            let v = ScalarValue::try_from_array(arr, i)?;
            if !v.is_null() {
                if let Some(cnt) = self.counts.get_mut(&v) {
                    *cnt -= 1;
                    if *cnt == 0 {
                        self.counts.remove(&v);
                    }
                }
            }
        }
        Ok(())
    }

    fn merge_batch(&mut self, states: &[ArrayRef]) -> Result<()> {
        let list_arr = states[0].as_list::<i32>();
        for inner in list_arr.iter().flatten() {
            for j in 0..inner.len() {
                let v = ScalarValue::try_from_array(&*inner, j)?;
                *self.counts.entry(v).or_default() += 1;
            }
        }
        Ok(())
    }

    fn evaluate(&mut self) -> Result<ScalarValue> {
        Ok(ScalarValue::Int64(Some(self.counts.len() as i64)))
    }

    fn supports_retract_batch(&self) -> bool {
        true
    }

    fn size(&self) -> usize {
        size_of_val(self)
    }
}

#[derive(Debug)]
struct CountAccumulator {
    count: i64,
}

impl CountAccumulator {
    /// new count accumulator
    pub fn new() -> Self {
        Self { count: 0 }
    }
}

impl Accumulator for CountAccumulator {
    fn state(&mut self) -> Result<Vec<ScalarValue>> {
        Ok(vec![ScalarValue::Int64(Some(self.count))])
    }

    fn update_batch(&mut self, values: &[ArrayRef]) -> Result<()> {
        let array = &values[0];
        self.count += (array.len() - null_count_for_multiple_cols(values)) as i64;
        Ok(())
    }

    fn retract_batch(&mut self, values: &[ArrayRef]) -> Result<()> {
        let array = &values[0];
        self.count -= (array.len() - null_count_for_multiple_cols(values)) as i64;
        Ok(())
    }

    fn merge_batch(&mut self, states: &[ArrayRef]) -> Result<()> {
        let counts = downcast_value!(states[0], Int64Array);
        let delta = &compute::sum(counts);
        if let Some(d) = delta {
            self.count += *d;
        }
        Ok(())
    }

    fn evaluate(&mut self) -> Result<ScalarValue> {
        Ok(ScalarValue::Int64(Some(self.count)))
    }

    fn supports_retract_batch(&self) -> bool {
        true
    }

    fn size(&self) -> usize {
        size_of_val(self)
    }
}

/// An accumulator to compute the counts of [`PrimitiveArray<T>`].
/// Stores values as native types, and does overflow checking
///
/// Unlike most other accumulators, COUNT never produces NULLs. If no
/// non-null values are seen in any group the output is 0. Thus, this
/// accumulator has no additional null or seen filter tracking.
#[derive(Debug)]
struct CountGroupsAccumulator {
    /// Count per group.
    ///
    /// Note this is an i64 and not a u64 (or usize) because the
    /// output type of count is `DataType::Int64`. Thus by using `i64`
    /// for the counts, the output [`Int64Array`] can be created
    /// without copy.
    counts: Vec<i64>,
}

impl CountGroupsAccumulator {
    pub fn new() -> Self {
        Self { counts: vec![] }
    }
}

impl GroupsAccumulator for CountGroupsAccumulator {
    fn update_batch(
        &mut self,
        values: &[ArrayRef],
        group_indices: &[usize],
        opt_filter: Option<&BooleanArray>,
        total_num_groups: usize,
    ) -> Result<()> {
        assert_eq!(values.len(), 1, "single argument to update_batch");
        let values = &values[0];

        // Add one to each group's counter for each non null, non
        // filtered value
        self.counts.resize(total_num_groups, 0);
        accumulate_indices(
            group_indices,
            values.logical_nulls().as_ref(),
            opt_filter,
            |group_index| {
                self.counts[group_index] += 1;
            },
        );

        Ok(())
    }

    fn merge_batch(
        &mut self,
        values: &[ArrayRef],
        group_indices: &[usize],
        // Since aggregate filter should be applied in partial stage, in final stage there should be no filter
        _opt_filter: Option<&BooleanArray>,
        total_num_groups: usize,
    ) -> Result<()> {
        assert_eq!(values.len(), 1, "one argument to merge_batch");
        // first batch is counts, second is partial sums
        let partial_counts = values[0].as_primitive::<Int64Type>();

        // intermediate counts are always created as non null
        assert_eq!(partial_counts.null_count(), 0);
        let partial_counts = partial_counts.values();

        // Adds the counts with the partial counts
        self.counts.resize(total_num_groups, 0);
        group_indices.iter().zip(partial_counts.iter()).for_each(
            |(&group_index, partial_count)| {
                self.counts[group_index] += partial_count;
            },
        );

        Ok(())
    }

    fn evaluate(&mut self, emit_to: EmitTo) -> Result<ArrayRef> {
        let counts = emit_to.take_needed(&mut self.counts);

        // Count is always non null (null inputs just don't contribute to the overall values)
        let nulls = None;
        let array = PrimitiveArray::<Int64Type>::new(counts.into(), nulls);

        Ok(Arc::new(array))
    }

    // return arrays for counts
    fn state(&mut self, emit_to: EmitTo) -> Result<Vec<ArrayRef>> {
        let counts = emit_to.take_needed(&mut self.counts);
        let counts: PrimitiveArray<Int64Type> = Int64Array::from(counts); // zero copy, no nulls
        Ok(vec![Arc::new(counts) as ArrayRef])
    }

    /// Converts an input batch directly to a state batch
    ///
    /// The state of `COUNT` is always a single Int64Array:
    /// * `1` (for non-null, non filtered values)
    /// * `0` (for null values)
    fn convert_to_state(
        &self,
        values: &[ArrayRef],
        opt_filter: Option<&BooleanArray>,
    ) -> Result<Vec<ArrayRef>> {
        let values = &values[0];

        let state_array = match (values.logical_nulls(), opt_filter) {
            (None, None) => {
                // In case there is no nulls in input and no filter, returning array of 1
                Arc::new(Int64Array::from_value(1, values.len()))
            }
            (Some(nulls), None) => {
                // If there are any nulls in input values -- casting `nulls` (true for values, false for nulls)
                // of input array to Int64
                let nulls = BooleanArray::new(nulls.into_inner(), None);
                compute::cast(&nulls, &DataType::Int64)?
            }
            (None, Some(filter)) => {
                // If there is only filter
                // - applying filter null mask to filter values by bitand filter values and nulls buffers
                //   (using buffers guarantees absence of nulls in result)
                // - casting result of bitand to Int64 array
                let (filter_values, filter_nulls) = filter.clone().into_parts();

                let state_buf = match filter_nulls {
                    Some(filter_nulls) => &filter_values & filter_nulls.inner(),
                    None => filter_values,
                };

                let boolean_state = BooleanArray::new(state_buf, None);
                compute::cast(&boolean_state, &DataType::Int64)?
            }
            (Some(nulls), Some(filter)) => {
                // For both input nulls and filter
                // - applying filter null mask to filter values by bitand filter values and nulls buffers
                //   (using buffers guarantees absence of nulls in result)
                // - applying values null mask to filter buffer by another bitand on filter result and
                //   nulls from input values
                // - casting result to Int64 array
                let (filter_values, filter_nulls) = filter.clone().into_parts();

                let filter_buf = match filter_nulls {
                    Some(filter_nulls) => &filter_values & filter_nulls.inner(),
                    None => filter_values,
                };
                let state_buf = &filter_buf & nulls.inner();

                let boolean_state = BooleanArray::new(state_buf, None);
                compute::cast(&boolean_state, &DataType::Int64)?
            }
        };

        Ok(vec![state_array])
    }

    fn supports_convert_to_state(&self) -> bool {
        true
    }

    fn size(&self) -> usize {
        self.counts.capacity() * size_of::<usize>()
    }
}

/// count null values for multiple columns
/// for each row if one column value is null, then null_count + 1
fn null_count_for_multiple_cols(values: &[ArrayRef]) -> usize {
    if values.len() > 1 {
        let result_bool_buf: Option<BooleanBuffer> = values
            .iter()
            .map(|a| a.logical_nulls())
            .fold(None, |acc, b| match (acc, b) {
                (Some(acc), Some(b)) => Some(acc.bitand(b.inner())),
                (Some(acc), None) => Some(acc),
                (None, Some(b)) => Some(b.into_inner()),
                _ => None,
            });
        result_bool_buf.map_or(0, |b| values[0].len() - b.count_set_bits())
    } else {
        values[0]
            .logical_nulls()
            .map_or(0, |nulls| nulls.null_count())
    }
}

/// General purpose distinct accumulator that works for any DataType by using
/// [`ScalarValue`].
///
/// It stores intermediate results as a `ListArray`
///
/// Note that many types have specialized accumulators that are (much)
/// more efficient such as [`PrimitiveDistinctCountAccumulator`] and
/// [`BytesDistinctCountAccumulator`]
#[derive(Debug)]
struct DistinctCountAccumulator {
    values: HashSet<ScalarValue, RandomState>,
    state_data_type: DataType,
}

impl DistinctCountAccumulator {
    // calculating the size for fixed length values, taking first batch size *
    // number of batches This method is faster than .full_size(), however it is
    // not suitable for variable length values like strings or complex types
    fn fixed_size(&self) -> usize {
        size_of_val(self)
            + (size_of::<ScalarValue>() * self.values.capacity())
            + self
                .values
                .iter()
                .next()
                .map(|vals| ScalarValue::size(vals) - size_of_val(vals))
                .unwrap_or(0)
            + size_of::<DataType>()
    }

    // calculates the size as accurately as possible. Note that calling this
    // method is expensive
    fn full_size(&self) -> usize {
        size_of_val(self)
            + (size_of::<ScalarValue>() * self.values.capacity())
            + self
                .values
                .iter()
                .map(|vals| ScalarValue::size(vals) - size_of_val(vals))
                .sum::<usize>()
            + size_of::<DataType>()
    }
}

impl Accumulator for DistinctCountAccumulator {
    /// Returns the distinct values seen so far as (one element) ListArray.
    fn state(&mut self) -> Result<Vec<ScalarValue>> {
        let scalars = self.values.iter().cloned().collect::<Vec<_>>();
        let arr =
            ScalarValue::new_list_nullable(scalars.as_slice(), &self.state_data_type);
        Ok(vec![ScalarValue::List(arr)])
    }

    fn update_batch(&mut self, values: &[ArrayRef]) -> Result<()> {
        if values.is_empty() {
            return Ok(());
        }

        let arr = &values[0];
        if arr.data_type() == &DataType::Null {
            return Ok(());
        }

        (0..arr.len()).try_for_each(|index| {
            let scalar = ScalarValue::try_from_array(arr, index)?;
            if !scalar.is_null() {
                self.values.insert(scalar);
            }
            Ok(())
        })
    }

    /// Merges multiple sets of distinct values into the current set.
    ///
    /// The input to this function is a `ListArray` with **multiple** rows,
    /// where each row contains the values from a partial aggregate's phase (e.g.
    /// the result of calling `Self::state` on multiple accumulators).
    fn merge_batch(&mut self, states: &[ArrayRef]) -> Result<()> {
        if states.is_empty() {
            return Ok(());
        }
        assert_eq!(states.len(), 1, "array_agg states must be singleton!");
        let array = &states[0];
        let list_array = array.as_list::<i32>();
        for inner_array in list_array.iter() {
            let Some(inner_array) = inner_array else {
                return internal_err!(
                    "Intermediate results of COUNT DISTINCT should always be non null"
                );
            };
            self.update_batch(&[inner_array])?;
        }
        Ok(())
    }

    fn evaluate(&mut self) -> Result<ScalarValue> {
        Ok(ScalarValue::Int64(Some(self.values.len() as i64)))
    }

    fn size(&self) -> usize {
        match &self.state_data_type {
            DataType::Boolean | DataType::Null => self.fixed_size(),
            d if d.is_primitive() => self.fixed_size(),
            _ => self.full_size(),
        }
    }
}

#[cfg(test)]
mod tests {

    use super::*;
    use arrow::{
        array::{DictionaryArray, Int32Array, NullArray, StringArray},
        datatypes::{DataType, Field, Int32Type, Schema},
    };
    use datafusion_expr::function::AccumulatorArgs;
    use datafusion_physical_expr::expressions::Column;
    use std::sync::Arc;
    /// Helper function to create a dictionary array with non-null keys but some null values
    /// Returns a dictionary array where:
    /// - keys are [0, 1, 2, 0, 1] (all non-null)
    /// - values are ["a", null, "c"]
    /// - so the keys reference: "a", null, "c", "a", null
    fn create_dictionary_with_null_values() -> Result<DictionaryArray<Int32Type>> {
        let values = StringArray::from(vec![Some("a"), None, Some("c")]);
        let keys = Int32Array::from(vec![0, 1, 2, 0, 1]); // references "a", null, "c", "a", null
        Ok(DictionaryArray::<Int32Type>::try_new(
            keys,
            Arc::new(values),
        )?)
    }

    #[test]
    fn count_accumulator_nulls() -> Result<()> {
        let mut accumulator = CountAccumulator::new();
        accumulator.update_batch(&[Arc::new(NullArray::new(10))])?;
        assert_eq!(accumulator.evaluate()?, ScalarValue::Int64(Some(0)));
        Ok(())
    }

    #[test]
    fn test_nested_dictionary() -> Result<()> {
        let schema = Arc::new(Schema::new(vec![Field::new(
            "dict_col",
            DataType::Dictionary(
                Box::new(DataType::Int32),
                Box::new(DataType::Dictionary(
                    Box::new(DataType::Int32),
                    Box::new(DataType::Utf8),
                )),
            ),
            true,
        )]));

        // Using Count UDAF's accumulator
        let count = Count::new();
        let expr = Arc::new(Column::new("dict_col", 0));
        let args = AccumulatorArgs {
            schema: &schema,
            exprs: &[expr],
            is_distinct: true,
            name: "count",
            ignore_nulls: false,
            is_reversed: false,
            return_field: Arc::new(Field::new_list_field(DataType::Int64, true)),
            order_bys: &[],
        };

        let inner_dict =
            DictionaryArray::<Int32Type>::from_iter(["a", "b", "c", "d", "a", "b"]);

        let keys = Int32Array::from(vec![0, 1, 2, 0, 3, 1]);
        let dict_of_dict =
            DictionaryArray::<Int32Type>::try_new(keys, Arc::new(inner_dict))?;

        let mut acc = count.accumulator(args)?;
        acc.update_batch(&[Arc::new(dict_of_dict)])?;
        assert_eq!(acc.evaluate()?, ScalarValue::Int64(Some(4)));

        Ok(())
    }

    #[test]
    fn count_distinct_accumulator_dictionary_with_null_values() -> Result<()> {
        let dict_array = create_dictionary_with_null_values()?;

        // The expected behavior is that count_distinct should count only non-null values
        // which in this case are "a" and "c" (appearing as 0 and 2 in keys)
        let mut accumulator = DistinctCountAccumulator {
            values: HashSet::default(),
            state_data_type: dict_array.data_type().clone(),
        };

        accumulator.update_batch(&[Arc::new(dict_array)])?;

        // Should have 2 distinct non-null values ("a" and "c")
        assert_eq!(accumulator.evaluate()?, ScalarValue::Int64(Some(2)));
        Ok(())
    }

    #[test]
    fn count_accumulator_dictionary_with_null_values() -> Result<()> {
        let dict_array = create_dictionary_with_null_values()?;

        // The expected behavior is that count should only count non-null values
        let mut accumulator = CountAccumulator::new();

        accumulator.update_batch(&[Arc::new(dict_array)])?;

        // 5 elements in the array, of which 2 reference null values (the two 1s in the keys)
        // So we should count 3 non-null values
        assert_eq!(accumulator.evaluate()?, ScalarValue::Int64(Some(3)));
        Ok(())
    }

    #[test]
    fn count_distinct_accumulator_dictionary_all_null_values() -> Result<()> {
        // Create a dictionary array that only contains null values
        let dict_values = StringArray::from(vec![None, Some("abc")]);
        let dict_indices = Int32Array::from(vec![0; 5]);
        let dict_array =
            DictionaryArray::<Int32Type>::try_new(dict_indices, Arc::new(dict_values))?;

        let mut accumulator = DistinctCountAccumulator {
            values: HashSet::default(),
            state_data_type: dict_array.data_type().clone(),
        };

        accumulator.update_batch(&[Arc::new(dict_array)])?;

        // All referenced values are null so count(distinct) should be 0
        assert_eq!(accumulator.evaluate()?, ScalarValue::Int64(Some(0)));
        Ok(())
    }

    #[test]
    fn sliding_distinct_count_accumulator_basic() -> Result<()> {
        // Basic update_batch + evaluate functionality
        let mut acc = SlidingDistinctCountAccumulator::try_new(&DataType::Int32)?;
        // Create an Int32Array: [1, 2, 2, 3, null]
        let values: ArrayRef = Arc::new(Int32Array::from(vec![
            Some(1),
            Some(2),
            Some(2),
            Some(3),
            None,
        ]));
        acc.update_batch(&[values])?;
        // Expect distinct values {1,2,3} → count = 3
        assert_eq!(acc.evaluate()?, ScalarValue::Int64(Some(3)));
        Ok(())
    }

    #[test]
    fn sliding_distinct_count_accumulator_retract() -> Result<()> {
        // Test that retract_batch properly decrements counts
        let mut acc = SlidingDistinctCountAccumulator::try_new(&DataType::Utf8)?;
        // Initial batch: ["a", "b", "a"]
        let arr1 = Arc::new(StringArray::from(vec![Some("a"), Some("b"), Some("a")]))
            as ArrayRef;
        acc.update_batch(&[arr1])?;
        assert_eq!(acc.evaluate()?, ScalarValue::Int64(Some(2))); // {"a","b"}

        // Retract batch: ["a", null, "b"]
        let arr2 =
            Arc::new(StringArray::from(vec![Some("a"), None, Some("b")])) as ArrayRef;
        acc.retract_batch(&[arr2])?;
        // Before: a→2, b→1; after retract a→1, b→0 → b removed; remaining {"a"}
        assert_eq!(acc.evaluate()?, ScalarValue::Int64(Some(1)));
        Ok(())
    }

    #[test]
    fn sliding_distinct_count_accumulator_merge_states() -> Result<()> {
        // Test merging multiple accumulator states with merge_batch
        let mut acc1 = SlidingDistinctCountAccumulator::try_new(&DataType::Int32)?;
        let mut acc2 = SlidingDistinctCountAccumulator::try_new(&DataType::Int32)?;
        // acc1 sees [1, 2]
        acc1.update_batch(&[Arc::new(Int32Array::from(vec![Some(1), Some(2)]))])?;
        // acc2 sees [2, 3]
        acc2.update_batch(&[Arc::new(Int32Array::from(vec![Some(2), Some(3)]))])?;
        // Extract their states as Vec<ScalarValue>
        let state_sv1 = acc1.state()?;
        let state_sv2 = acc2.state()?;
        // Convert ScalarValue states into Vec<ArrayRef>, propagating errors
        // NOTE we pass `1` because each ScalarValue.to_array produces a 1‑row ListArray
        let state_arr1: Vec<ArrayRef> = state_sv1
            .into_iter()
            .map(|sv| sv.to_array())
            .collect::<Result<_>>()?;
        let state_arr2: Vec<ArrayRef> = state_sv2
            .into_iter()
            .map(|sv| sv.to_array())
            .collect::<Result<_>>()?;
        // Merge both states into a fresh accumulator
        let mut merged = SlidingDistinctCountAccumulator::try_new(&DataType::Int32)?;
        merged.merge_batch(&state_arr1)?;
        merged.merge_batch(&state_arr2)?;
        // Expect distinct {1,2,3} → count = 3
        assert_eq!(merged.evaluate()?, ScalarValue::Int64(Some(3)));
        Ok(())
    }
}