// Copyright 2023 Greptime Team
//
// Licensed 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.

//! Wrapper for making aggregate functions out of state/merge functions of original aggregate functions.
//!
//! i.e. for a aggregate function `foo`, we will have a state function `foo_state` and a merge function `foo_merge`.
//!
//! `foo_state`'s input args is the same as `foo`'s, and its output is a state object.
//! Note that `foo_state` might have multiple output columns, so it's a struct array
//! that each output column is a struct field.
//! `foo_merge`'s input arg is the same as `foo_state`'s output, and its output is the same as `foo`'s input.
//!

use std::hash::{Hash, Hasher};
use std::sync::Arc;

use arrow::array::{ArrayRef, BooleanArray, StructArray};
use arrow_schema::{FieldRef, Fields};
use common_telemetry::debug;
use datafusion::functions_aggregate::all_default_aggregate_functions;
use datafusion::functions_aggregate::count::Count;
use datafusion::functions_aggregate::min_max::{Max, Min};
use datafusion::optimizer::AnalyzerRule;
use datafusion::optimizer::analyzer::type_coercion::TypeCoercion;
use datafusion::physical_planner::create_aggregate_expr_and_maybe_filter;
use datafusion_common::{Column, ScalarValue};
use datafusion_expr::expr::{AggregateFunction, AggregateFunctionParams};
use datafusion_expr::function::StateFieldsArgs;
use datafusion_expr::{
    Accumulator, Aggregate, AggregateUDF, AggregateUDFImpl, EmitTo, Expr, ExprSchemable,
    GroupsAccumulator, LogicalPlan, Signature,
};
use datafusion_physical_expr::aggregate::AggregateFunctionExpr;
use datatypes::arrow::datatypes::{DataType, Field};

use crate::aggrs::aggr_wrapper::fix_order::FixStateUdafOrderingAnalyzer;
use crate::function_registry::{FUNCTION_REGISTRY, FunctionRegistry};

pub mod fix_order;
#[cfg(test)]
mod tests;

/// Returns the name of the state function for the given aggregate function name.
/// The state function is used to compute the state of the aggregate function.
/// The state function's name is in the format `__<aggr_name>_state
pub fn aggr_state_func_name(aggr_name: &str) -> String {
    format!("__{}_state", aggr_name)
}

/// Returns the name of the merge function for the given aggregate function name.
/// The merge function is used to merge the states of the state functions.
/// The merge function's name is in the format `__<aggr_name>_merge
pub fn aggr_merge_func_name(aggr_name: &str) -> String {
    format!("__{}_merge", aggr_name)
}

/// Check if the given aggregate expression is steppable.
/// As in if it can be split into multiple steps:
/// i.e. on datanode first call `state(input)` then
/// on frontend call `calc(merge(state))` to get the final result.
pub fn is_all_aggr_exprs_steppable(aggr_exprs: &[Expr]) -> bool {
    aggr_exprs.iter().all(|expr| {
        if let Some(aggr_func) = get_aggr_func(expr) {
            if aggr_func.params.distinct {
                // Distinct aggregate functions are not steppable(yet).
                // TODO(discord9): support distinct aggregate functions.
                return false;
            }

            // whether the corresponding state function exists in the registry
            FUNCTION_REGISTRY.is_aggr_func_exist(&aggr_state_func_name(aggr_func.func.name()))
        } else {
            false
        }
    })
}

pub fn get_aggr_func(expr: &Expr) -> Option<&datafusion_expr::expr::AggregateFunction> {
    let mut expr_ref = expr;
    while let Expr::Alias(alias) = expr_ref {
        expr_ref = &alias.expr;
    }
    if let Expr::AggregateFunction(aggr_func) = expr_ref {
        Some(aggr_func)
    } else {
        None
    }
}

/// A wrapper to make an aggregate function out of the state and merge functions of the original aggregate function.
/// It contains the original aggregate function, the state functions, and the merge function.
///
/// Notice state functions may have multiple output columns, so it's return type is always a struct array, and the merge function is used to merge the states of the state functions.
#[derive(Debug, Clone)]
pub struct StateMergeHelper;

/// A struct to hold the two aggregate plans, one for the state function(lower) and one for the merge function(upper).
#[allow(unused)]
#[derive(Debug, Clone)]
pub struct StepAggrPlan {
    /// Upper merge plan, which is the aggregate plan that merges the states of the state function.
    pub upper_merge: LogicalPlan,
    /// Lower state plan, which is the aggregate plan that computes the state of the aggregate function.
    pub lower_state: LogicalPlan,
}

impl StateMergeHelper {
    /// Register all the `state` function of supported aggregate functions.
    /// Note that can't register `merge` function here, as it needs to be created from the original aggregate function with given input types.
    pub fn register(registry: &FunctionRegistry) {
        let all_default = all_default_aggregate_functions();
        let greptime_custom_aggr_functions = registry.aggregate_functions();

        // if our custom aggregate function have the same name as the default aggregate function, we will override it.
        let supported = all_default
            .into_iter()
            .chain(greptime_custom_aggr_functions.into_iter().map(Arc::new))
            .collect::<Vec<_>>();
        debug!(
            "Registering state functions for supported: {:?}",
            supported.iter().map(|f| f.name()).collect::<Vec<_>>()
        );

        let state_func = supported.into_iter().filter_map(|f| {
            StateWrapper::new((*f).clone())
                .inspect_err(
                    |e| common_telemetry::error!(e; "Failed to register state function for {:?}", f),
                )
                .ok()
                .map(AggregateUDF::new_from_impl)
        });

        for func in state_func {
            registry.register_aggr(func);
        }
    }

    /// Split an aggregate plan into two aggregate plans, one for the state function and one for the merge function.
    ///
    pub fn split_aggr_node(aggr_plan: Aggregate) -> datafusion_common::Result<StepAggrPlan> {
        let aggr = {
            // certain aggr func need type coercion to work correctly, so we need to analyze the plan first.
            let aggr_plan = TypeCoercion::new().analyze(
                LogicalPlan::Aggregate(aggr_plan).clone(),
                &Default::default(),
            )?;
            if let LogicalPlan::Aggregate(aggr) = aggr_plan {
                aggr
            } else {
                return Err(datafusion_common::DataFusionError::Internal(format!(
                    "Failed to coerce expressions in aggregate plan, expected Aggregate, got: {:?}",
                    aggr_plan
                )));
            }
        };
        let mut lower_aggr_exprs = vec![];
        let mut upper_aggr_exprs = vec![];

        // group exprs for upper plan should refer to the output group expr as column from lower plan
        // to avoid re-compute group exprs again.
        let upper_group_exprs = aggr
            .group_expr
            .iter()
            .map(|c| c.qualified_name())
            .map(|(r, c)| Expr::Column(Column::new(r, c)))
            .collect();

        for aggr_expr in aggr.aggr_expr.iter() {
            let Some(aggr_func) = get_aggr_func(aggr_expr) else {
                return Err(datafusion_common::DataFusionError::NotImplemented(format!(
                    "Unsupported aggregate expression for step aggr optimize: {:?}",
                    aggr_expr
                )));
            };

            let original_input_fields = aggr_func
                .params
                .args
                .iter()
                .map(|e| e.to_field(&aggr.input.schema()).map(|(_, field)| field))
                .collect::<Result<Vec<_>, _>>()?;

            // first create the state function from the original aggregate function.
            let state_func = StateWrapper::new((*aggr_func.func).clone())?;

            let expr = AggregateFunction {
                func: Arc::new(state_func.into()),
                params: aggr_func.params.clone(),
            };
            let expr = Expr::AggregateFunction(expr);
            let lower_state_output_col_name = expr.schema_name().to_string();

            lower_aggr_exprs.push(expr);

            // then create the merge function using the physical expression of the original aggregate function
            let (original_phy_expr, _filter, _ordering) = create_aggregate_expr_and_maybe_filter(
                aggr_expr,
                aggr.input.schema(),
                aggr.input.schema().as_arrow(),
                &Default::default(),
            )?;

            let merge_func = MergeWrapper::new(
                (*aggr_func.func).clone(),
                original_phy_expr,
                original_input_fields,
            )?;
            let arg = Expr::Column(Column::new_unqualified(lower_state_output_col_name));
            let expr = AggregateFunction {
                func: Arc::new(merge_func.into()),
                // notice filter/order_by is not supported in the merge function, as it's not meaningful to have them in the merge phase.
                // do notice this order by is only removed in the outer logical plan, the physical plan still have order by and hence
                // can create correct accumulator with order by.
                params: AggregateFunctionParams {
                    args: vec![arg],
                    distinct: aggr_func.params.distinct,
                    filter: None,
                    order_by: vec![],
                    null_treatment: aggr_func.params.null_treatment,
                },
            };

            // alias to the original aggregate expr's schema name, so parent plan can refer to it
            // correctly.
            let expr = Expr::AggregateFunction(expr).alias(aggr_expr.schema_name().to_string());
            upper_aggr_exprs.push(expr);
        }

        let mut lower = aggr.clone();
        lower.aggr_expr = lower_aggr_exprs;
        let lower_plan = LogicalPlan::Aggregate(lower);

        // update aggregate's output schema
        let lower_plan = lower_plan.recompute_schema()?;

        // should only affect two udaf `first_value/last_value`
        // which only them have meaningful order by field
        let fixed_lower_plan =
            FixStateUdafOrderingAnalyzer.analyze(lower_plan, &Default::default())?;

        let upper = Aggregate::try_new(
            Arc::new(fixed_lower_plan.clone()),
            upper_group_exprs,
            upper_aggr_exprs.clone(),
        )?;
        let aggr_plan = LogicalPlan::Aggregate(aggr);

        // upper schema's output schema should be the same as the original aggregate plan's output schema
        let upper_check = upper;
        let upper_plan = LogicalPlan::Aggregate(upper_check).recompute_schema()?;
        if *upper_plan.schema() != *aggr_plan.schema() {
            return Err(datafusion_common::DataFusionError::Internal(format!(
                "Upper aggregate plan's schema is not the same as the original aggregate plan's schema: \n[transformed]:{}\n[original]:{}",
                upper_plan.schema(),
                aggr_plan.schema()
            )));
        }

        Ok(StepAggrPlan {
            lower_state: fixed_lower_plan,
            upper_merge: upper_plan,
        })
    }
}

/// Wrapper to make an aggregate function out of a state function.
#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub struct StateWrapper {
    inner: AggregateUDF,
    name: String,
    /// Default to empty, might get fixed by analyzer later
    ordering: Vec<FieldRef>,
    /// Default to false, might get fixed by analyzer later
    distinct: bool,
}

impl StateWrapper {
    /// `state_index`: The index of the state in the output of the state function.
    pub fn new(inner: AggregateUDF) -> datafusion_common::Result<Self> {
        let name = aggr_state_func_name(inner.name());
        Ok(Self {
            inner,
            name,
            ordering: vec![],
            distinct: false,
        })
    }

    pub fn inner(&self) -> &AggregateUDF {
        &self.inner
    }

    /// Deduce the return type of the original aggregate function
    /// based on the accumulator arguments.
    ///
    pub fn deduce_aggr_return_type(
        &self,
        acc_args: &datafusion_expr::function::AccumulatorArgs,
    ) -> datafusion_common::Result<FieldRef> {
        let input_fields = acc_args
            .exprs
            .iter()
            .map(|e| e.return_field(acc_args.schema))
            .collect::<Result<Vec<_>, _>>()?;
        self.inner.return_field(&input_fields).inspect_err(|e| {
            common_telemetry::error!(
                "StateWrapper: {:#?}\nacc_args:{:?}\nerror:{:?}",
                &self,
                &acc_args,
                e
            );
        })
    }

    fn fix_inner_acc_args<'b>(
        &self,
        mut acc_args: datafusion_expr::function::AccumulatorArgs<'b>,
    ) -> datafusion_common::Result<datafusion_expr::function::AccumulatorArgs<'b>> {
        acc_args.return_field = self.deduce_aggr_return_type(&acc_args)?;
        Ok(acc_args)
    }
}

impl AggregateUDFImpl for StateWrapper {
    fn accumulator<'a, 'b>(
        &'a self,
        acc_args: datafusion_expr::function::AccumulatorArgs<'b>,
    ) -> datafusion_common::Result<Box<dyn Accumulator>> {
        // fix and recover proper acc args for the original aggregate function.
        let state_type = acc_args.return_type().clone();
        let inner = self.inner.accumulator(self.fix_inner_acc_args(acc_args)?)?;

        Ok(Box::new(StateAccum::new(inner, state_type)?))
    }

    fn groups_accumulator_supported(
        &self,
        acc_args: datafusion_expr::function::AccumulatorArgs,
    ) -> bool {
        self.fix_inner_acc_args(acc_args)
            .map(|args| self.inner.inner().groups_accumulator_supported(args))
            .unwrap_or(false)
    }

    fn create_groups_accumulator(
        &self,
        acc_args: datafusion_expr::function::AccumulatorArgs,
    ) -> datafusion_common::Result<Box<dyn GroupsAccumulator>> {
        let state_type = acc_args.return_type().clone();
        let inner = self
            .inner
            .inner()
            .create_groups_accumulator(self.fix_inner_acc_args(acc_args)?)?;
        Ok(Box::new(StateGroupsAccum::new(inner, state_type)?))
    }

    fn as_any(&self) -> &dyn std::any::Any {
        self
    }
    fn name(&self) -> &str {
        self.name.as_str()
    }

    fn is_nullable(&self) -> bool {
        self.inner.is_nullable()
    }

    /// Return state_fields as the output struct type.
    ///
    fn return_type(&self, arg_types: &[DataType]) -> datafusion_common::Result<DataType> {
        let input_fields = &arg_types
            .iter()
            .map(|x| Arc::new(Field::new("x", x.clone(), false)))
            .collect::<Vec<_>>();

        let state_fields_args = StateFieldsArgs {
            name: self.inner().name(),
            input_fields,
            return_field: self.inner.return_field(input_fields)?,
            // those args are also needed as they are vital to construct the state fields correctly.
            ordering_fields: &self.ordering,
            is_distinct: self.distinct,
        };
        let state_fields = self.inner.state_fields(state_fields_args)?;

        let state_fields = state_fields
            .into_iter()
            .map(|f| {
                let mut f = f.as_ref().clone();
                // since state can be null when no input rows, so make all fields nullable
                f.set_nullable(true);
                Arc::new(f)
            })
            .collect::<Vec<_>>();

        let struct_field = DataType::Struct(state_fields.into());
        Ok(struct_field)
    }

    /// The state function's output fields are the same as the original aggregate function's state fields.
    fn state_fields(
        &self,
        args: datafusion_expr::function::StateFieldsArgs,
    ) -> datafusion_common::Result<Vec<FieldRef>> {
        let state_fields_args = StateFieldsArgs {
            name: args.name,
            input_fields: args.input_fields,
            return_field: self.inner.return_field(args.input_fields)?,
            ordering_fields: args.ordering_fields,
            is_distinct: args.is_distinct,
        };
        self.inner.state_fields(state_fields_args)
    }

    /// The state function's signature is the same as the original aggregate function's signature,
    fn signature(&self) -> &Signature {
        self.inner.signature()
    }

    /// Coerce types also do nothing, as optimizer should be able to already make struct types
    fn coerce_types(&self, arg_types: &[DataType]) -> datafusion_common::Result<Vec<DataType>> {
        self.inner.coerce_types(arg_types)
    }

    fn value_from_stats(
        &self,
        statistics_args: &datafusion_expr::StatisticsArgs,
    ) -> Option<ScalarValue> {
        let inner = self.inner().inner().as_any();
        // only count/min/max need special handling here, for getting result from statistics
        // the result of count/min/max is also the result of count_state so can return directly
        let can_use_stat = inner.is::<Count>() || inner.is::<Max>() || inner.is::<Min>();
        if !can_use_stat {
            return None;
        }

        // fix return type by extract the first field's data type from the struct type
        let state_type = if let DataType::Struct(fields) = &statistics_args.return_type {
            if fields.is_empty() {
                return None;
            }
            fields[0].data_type().clone()
        } else {
            return None;
        };

        let fixed_args = datafusion_expr::StatisticsArgs {
            statistics: statistics_args.statistics,
            return_type: &state_type,
            is_distinct: statistics_args.is_distinct,
            exprs: statistics_args.exprs,
        };

        let ret = self.inner().value_from_stats(&fixed_args)?;

        // wrap the result into struct scalar value
        let fields = if let DataType::Struct(fields) = &statistics_args.return_type {
            fields
        } else {
            return None;
        };

        let array = ret.to_array().ok()?;

        let struct_array = StructArray::new(fields.clone(), vec![array], None);
        let ret = ScalarValue::Struct(Arc::new(struct_array));
        Some(ret)
    }
}

/// The wrapper's input is the same as the original aggregate function's input,
/// and the output is the state function's output.
#[derive(Debug)]
pub struct StateAccum {
    inner: Box<dyn Accumulator>,
    state_fields: Fields,
}

pub struct StateGroupsAccum {
    inner: Box<dyn GroupsAccumulator>,
    state_fields: Fields,
}

impl StateGroupsAccum {
    fn new(
        inner: Box<dyn GroupsAccumulator>,
        state_type: DataType,
    ) -> datafusion_common::Result<Self> {
        let DataType::Struct(fields) = state_type else {
            return Err(datafusion_common::DataFusionError::Internal(format!(
                "Expected a struct type for state, got: {:?}",
                state_type
            )));
        };
        Ok(Self {
            inner,
            state_fields: fields,
        })
    }

    fn wrap_state_arrays(&self, arrays: Vec<ArrayRef>) -> datafusion_common::Result<ArrayRef> {
        let array_type = arrays
            .iter()
            .map(|array| array.data_type().clone())
            .collect::<Vec<_>>();
        let expected_type = self
            .state_fields
            .iter()
            .map(|field| field.data_type().clone())
            .collect::<Vec<_>>();
        if array_type != expected_type {
            debug!(
                "State mismatch, expected: {}, got: {} for expected fields: {:?} and given array types: {:?}",
                self.state_fields.len(),
                arrays.len(),
                self.state_fields,
                array_type,
            );
            let guess_schema = arrays
                .iter()
                .enumerate()
                .map(|(index, array)| {
                    Field::new(
                        format!("col_{index}[mismatch_state]").as_str(),
                        array.data_type().clone(),
                        true,
                    )
                })
                .collect::<Fields>();
            let array = StructArray::try_new(guess_schema, arrays, None)?;
            return Ok(Arc::new(array));
        }

        Ok(Arc::new(StructArray::try_new(
            self.state_fields.clone(),
            arrays,
            None,
        )?))
    }
}

impl GroupsAccumulator for StateGroupsAccum {
    fn update_batch(
        &mut self,
        values: &[ArrayRef],
        group_indices: &[usize],
        opt_filter: Option<&BooleanArray>,
        total_num_groups: usize,
    ) -> datafusion_common::Result<()> {
        self.inner
            .update_batch(values, group_indices, opt_filter, total_num_groups)
    }

    fn merge_batch(
        &mut self,
        values: &[ArrayRef],
        group_indices: &[usize],
        opt_filter: Option<&BooleanArray>,
        total_num_groups: usize,
    ) -> datafusion_common::Result<()> {
        self.inner
            .merge_batch(values, group_indices, opt_filter, total_num_groups)
    }

    fn evaluate(&mut self, emit_to: EmitTo) -> datafusion_common::Result<ArrayRef> {
        let state = self.inner.state(emit_to)?;
        self.wrap_state_arrays(state)
    }

    fn state(&mut self, emit_to: EmitTo) -> datafusion_common::Result<Vec<ArrayRef>> {
        self.inner.state(emit_to)
    }

    fn convert_to_state(
        &self,
        values: &[ArrayRef],
        opt_filter: Option<&BooleanArray>,
    ) -> datafusion_common::Result<Vec<ArrayRef>> {
        self.inner.convert_to_state(values, opt_filter)
    }

    fn supports_convert_to_state(&self) -> bool {
        self.inner.supports_convert_to_state()
    }

    fn size(&self) -> usize {
        self.inner.size()
    }
}

impl StateAccum {
    pub fn new(
        inner: Box<dyn Accumulator>,
        state_type: DataType,
    ) -> datafusion_common::Result<Self> {
        let DataType::Struct(fields) = state_type else {
            return Err(datafusion_common::DataFusionError::Internal(format!(
                "Expected a struct type for state, got: {:?}",
                state_type
            )));
        };
        Ok(Self {
            inner,
            state_fields: fields,
        })
    }
}

impl Accumulator for StateAccum {
    fn evaluate(&mut self) -> datafusion_common::Result<ScalarValue> {
        let state = self.inner.state()?;

        let array = state
            .iter()
            .map(|s| s.to_array())
            .collect::<Result<Vec<_>, _>>()?;
        let array_type = array
            .iter()
            .map(|a| a.data_type().clone())
            .collect::<Vec<_>>();
        let expected_type: Vec<_> = self
            .state_fields
            .iter()
            .map(|f| f.data_type().clone())
            .collect();
        if array_type != expected_type {
            debug!(
                "State mismatch, expected: {}, got: {} for expected fields: {:?} and given array types: {:?}",
                self.state_fields.len(),
                array.len(),
                self.state_fields,
                array_type,
            );
            let guess_schema = array
                .iter()
                .enumerate()
                .map(|(index, array)| {
                    Field::new(
                        format!("col_{index}[mismatch_state]").as_str(),
                        array.data_type().clone(),
                        true,
                    )
                })
                .collect::<Fields>();
            let arr = StructArray::try_new(guess_schema, array, None)?;

            return Ok(ScalarValue::Struct(Arc::new(arr)));
        }

        let struct_array = StructArray::try_new(self.state_fields.clone(), array, None)?;
        Ok(ScalarValue::Struct(Arc::new(struct_array)))
    }

    fn merge_batch(
        &mut self,
        states: &[datatypes::arrow::array::ArrayRef],
    ) -> datafusion_common::Result<()> {
        self.inner.merge_batch(states)
    }

    fn update_batch(
        &mut self,
        values: &[datatypes::arrow::array::ArrayRef],
    ) -> datafusion_common::Result<()> {
        self.inner.update_batch(values)
    }

    fn size(&self) -> usize {
        self.inner.size()
    }

    fn state(&mut self) -> datafusion_common::Result<Vec<ScalarValue>> {
        self.inner.state()
    }
}

/// TODO(discord9): mark this function as non-ser/de able
///
/// This wrapper shouldn't be register as a udaf, as it contain extra data that is not serializable.
/// and changes for different logical plans.
#[derive(Debug, Clone)]
pub struct MergeWrapper {
    inner: AggregateUDF,
    name: String,
    merge_signature: Signature,
    /// The original physical expression of the aggregate function, can't store the original aggregate function directly, as PhysicalExpr didn't implement Any
    original_phy_expr: Arc<AggregateFunctionExpr>,
    return_field: FieldRef,
}
impl MergeWrapper {
    pub fn new(
        inner: AggregateUDF,
        original_phy_expr: Arc<AggregateFunctionExpr>,
        original_input_fields: Vec<FieldRef>,
    ) -> datafusion_common::Result<Self> {
        let name = aggr_merge_func_name(inner.name());
        // the input type is actually struct type, which is the state fields of the original aggregate function.
        let merge_signature = Signature::user_defined(datafusion_expr::Volatility::Immutable);
        let return_field = inner.return_field(&original_input_fields)?.clone();

        Ok(Self {
            inner,
            name,
            merge_signature,
            original_phy_expr,
            return_field,
        })
    }

    pub fn inner(&self) -> &AggregateUDF {
        &self.inner
    }
}

impl AggregateUDFImpl for MergeWrapper {
    fn accumulator<'a, 'b>(
        &'a self,
        acc_args: datafusion_expr::function::AccumulatorArgs<'b>,
    ) -> datafusion_common::Result<Box<dyn Accumulator>> {
        if acc_args.exprs.len() != 1
            || !matches!(
                acc_args.exprs[0].data_type(acc_args.schema)?,
                DataType::Struct(_)
            )
        {
            return Err(datafusion_common::DataFusionError::Internal(format!(
                "Expected one struct type as input, got: {:?}",
                acc_args.schema
            )));
        }
        let input_type = acc_args.exprs[0].data_type(acc_args.schema)?;
        let DataType::Struct(fields) = input_type else {
            return Err(datafusion_common::DataFusionError::Internal(format!(
                "Expected a struct type for input, got: {:?}",
                input_type
            )));
        };

        let inner_accum = self.original_phy_expr.create_accumulator()?;
        Ok(Box::new(MergeAccum::new(inner_accum, &fields)))
    }

    fn as_any(&self) -> &dyn std::any::Any {
        self
    }
    fn name(&self) -> &str {
        self.name.as_str()
    }

    fn is_nullable(&self) -> bool {
        self.inner.is_nullable()
    }

    /// Notice here the `arg_types` is actually the `state_fields`'s data types,
    /// so return fixed return type instead of using `arg_types` to determine the return type.
    fn return_type(&self, _arg_types: &[DataType]) -> datafusion_common::Result<DataType> {
        // The return type is the same as the original aggregate function's return type.
        Ok(self.return_field.data_type().clone())
    }

    /// Similar to return_type, we just return the fixed return field.
    fn return_field(&self, _arg_fields: &[FieldRef]) -> datafusion_common::Result<FieldRef> {
        Ok(self.return_field.clone())
    }

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

    /// Coerce types also do nothing, as optimizer should be able to already make struct types
    fn coerce_types(&self, arg_types: &[DataType]) -> datafusion_common::Result<Vec<DataType>> {
        // just check if the arg_types are only one and is struct array
        if arg_types.len() != 1 || !matches!(arg_types.first(), Some(DataType::Struct(_))) {
            return Err(datafusion_common::DataFusionError::Internal(format!(
                "Expected one struct type as input, got: {:?}",
                arg_types
            )));
        }
        Ok(arg_types.to_vec())
    }

    /// Just return the original aggregate function's state fields.
    fn state_fields(
        &self,
        _args: datafusion_expr::function::StateFieldsArgs,
    ) -> datafusion_common::Result<Vec<FieldRef>> {
        self.original_phy_expr.state_fields()
    }
}

impl PartialEq for MergeWrapper {
    fn eq(&self, other: &Self) -> bool {
        self.inner == other.inner
    }
}

impl Eq for MergeWrapper {}

impl Hash for MergeWrapper {
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.inner.hash(state);
    }
}

/// The merge accumulator, which modify `update_batch`'s behavior to accept one struct array which
/// include the state fields of original aggregate function, and merge said states into original accumulator
/// the output is the same as original aggregate function
#[derive(Debug)]
pub struct MergeAccum {
    inner: Box<dyn Accumulator>,
    state_fields: Fields,
}

impl MergeAccum {
    pub fn new(inner: Box<dyn Accumulator>, state_fields: &Fields) -> Self {
        Self {
            inner,
            state_fields: state_fields.clone(),
        }
    }
}

impl Accumulator for MergeAccum {
    fn evaluate(&mut self) -> datafusion_common::Result<ScalarValue> {
        self.inner.evaluate()
    }

    fn merge_batch(&mut self, states: &[arrow::array::ArrayRef]) -> datafusion_common::Result<()> {
        self.inner.merge_batch(states)
    }

    fn update_batch(&mut self, values: &[arrow::array::ArrayRef]) -> datafusion_common::Result<()> {
        let value = values.first().ok_or_else(|| {
            datafusion_common::DataFusionError::Internal("No values provided for merge".to_string())
        })?;
        // The input values are states from other accumulators, so we merge them.
        let struct_arr = value
            .as_any()
            .downcast_ref::<StructArray>()
            .ok_or_else(|| {
                datafusion_common::DataFusionError::Internal(format!(
                    "Expected StructArray, got: {:?}",
                    value.data_type()
                ))
            })?;
        let fields = struct_arr.fields();
        if fields != &self.state_fields {
            debug!(
                "State fields mismatch, expected: {:?}, got: {:?}",
                self.state_fields, fields
            );
            // state fields mismatch might be acceptable by datafusion, continue
        }

        // now fields should be the same, so we can merge the batch
        // by pass the columns as order should be the same
        let state_columns = struct_arr.columns();
        self.inner.merge_batch(state_columns)
    }

    fn size(&self) -> usize {
        self.inner.size()
    }

    fn state(&mut self) -> datafusion_common::Result<Vec<ScalarValue>> {
        self.inner.state()
    }
}
