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//! [`AggregateUDF`]: User Defined Aggregate Functions

use crate::{Accumulator, Expr};
use crate::{
    AccumulatorFactoryFunction, ReturnTypeFunction, Signature, StateTypeFunction,
};
use arrow::datatypes::DataType;
use datafusion_common::Result;
use std::fmt::{self, Debug, Formatter};
use std::sync::Arc;

/// Logical representation of a user-defined [aggregate function] (UDAF).
///
/// An aggregate function combines the values from multiple input rows
/// into a single output "aggregate" (summary) row. It is different
/// from a scalar function because it is stateful across batches. User
/// defined aggregate functions can be used as normal SQL aggregate
/// functions (`GROUP BY` clause) as well as window functions (`OVER`
/// clause).
///
/// `AggregateUDF` provides DataFusion the information needed to plan
/// and call aggregate functions, including name, type information,
/// and a factory function to create [`Accumulator`], which peform the
/// actual aggregation.
///
/// For more information, please see [the examples].
///
/// [the examples]: https://github.com/apache/arrow-datafusion/tree/main/datafusion-examples#single-process
/// [aggregate function]: https://en.wikipedia.org/wiki/Aggregate_function
/// [`Accumulator`]: crate::Accumulator
#[derive(Clone)]
pub struct AggregateUDF {
    /// name
    name: String,
    /// Signature (input arguments)
    signature: Signature,
    /// Return type
    return_type: ReturnTypeFunction,
    /// actual implementation
    accumulator: AccumulatorFactoryFunction,
    /// the accumulator's state's description as a function of the return type
    state_type: StateTypeFunction,
}

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

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

impl Eq for AggregateUDF {}

impl std::hash::Hash for AggregateUDF {
    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
        self.name.hash(state);
        self.signature.hash(state);
    }
}

impl AggregateUDF {
    /// Create a new AggregateUDF
    pub fn new(
        name: &str,
        signature: &Signature,
        return_type: &ReturnTypeFunction,
        accumulator: &AccumulatorFactoryFunction,
        state_type: &StateTypeFunction,
    ) -> Self {
        Self {
            name: name.to_owned(),
            signature: signature.clone(),
            return_type: return_type.clone(),
            accumulator: accumulator.clone(),
            state_type: state_type.clone(),
        }
    }

    /// creates an [`Expr`] that calls the aggregate function.
    ///
    /// This utility allows using the UDAF without requiring access to
    /// the registry, such as with the DataFrame API.
    pub fn call(&self, args: Vec<Expr>) -> Expr {
        Expr::AggregateFunction(crate::expr::AggregateFunction::new_udf(
            Arc::new(self.clone()),
            args,
            false,
            None,
            None,
        ))
    }

    /// Returns this function's name
    pub fn name(&self) -> &str {
        &self.name
    }

    /// Returns this function's signature (what input types are accepted)
    pub fn signature(&self) -> &Signature {
        &self.signature
    }

    /// Return the type of the function given its input types
    pub fn return_type(&self, args: &[DataType]) -> Result<DataType> {
        // Old API returns an Arc of the datatype for some reason
        let res = (self.return_type)(args)?;
        Ok(res.as_ref().clone())
    }

    /// Return an accumualator the given aggregate, given
    /// its return datatype.
    pub fn accumulator(&self, return_type: &DataType) -> Result<Box<dyn Accumulator>> {
        (self.accumulator)(return_type)
    }

    /// Return the type of the intermediate state used by this aggregator, given
    /// its return datatype. Supports multi-phase aggregations
    pub fn state_type(&self, return_type: &DataType) -> Result<Vec<DataType>> {
        // old API returns an Arc for some reason, try and unwrap it here
        let res = (self.state_type)(return_type)?;
        Ok(Arc::try_unwrap(res).unwrap_or_else(|res| res.as_ref().clone()))
    }
}