import datafusion as df
import pyarrow
import pyarrow.compute as pc
from typing import List

class AverageAccumulator(df.Accumulator):
    def __init__(self):
        self._sum = 0
        self._count = 0

    def update(self, values: pyarrow.Array) -> None:
        self._sum += pc.sum(values).as_py()
        self._count += len(values)

    def merge(self, other) -> None:
        self._sum += other._sum
        self._count += other._count

    def state(self) -> pyarrow.Array:
        return pyarrow.array([self._sum, self._count])

    def evaluate(self) -> pyarrow.Scalar:
        return pyarrow.scalar(self._sum / self._count)

average_udaf = df.udaf(
    AverageAccumulator,
    pyarrow.float64(),
    pyarrow.float64(),
    [pyarrow.float64()],
    'stable'
)

ctx = df.SessionContext()

ctx.register_udaf(average_udaf)