from __future__ import annotations

import collections
from typing import TYPE_CHECKING, Any, ClassVar, Iterator, Mapping, Sequence

import pyarrow as pa
import pyarrow.compute as pc

from narwhals._arrow.utils import cast_to_comparable_string_types, extract_py_scalar
from narwhals._compliant import EagerGroupBy
from narwhals._expression_parsing import evaluate_output_names_and_aliases
from narwhals._utils import generate_temporary_column_name

if TYPE_CHECKING:
    from narwhals._arrow.dataframe import ArrowDataFrame
    from narwhals._arrow.expr import ArrowExpr
    from narwhals._arrow.typing import (  # type: ignore[attr-defined]
        AggregateOptions,
        Aggregation,
        Incomplete,
    )
    from narwhals._compliant.group_by import NarwhalsAggregation
    from narwhals.typing import UniqueKeepStrategy


class ArrowGroupBy(EagerGroupBy["ArrowDataFrame", "ArrowExpr", "Aggregation"]):
    _REMAP_AGGS: ClassVar[Mapping[NarwhalsAggregation, Aggregation]] = {
        "sum": "sum",
        "mean": "mean",
        "median": "approximate_median",
        "max": "max",
        "min": "min",
        "std": "stddev",
        "var": "variance",
        "len": "count",
        "n_unique": "count_distinct",
        "count": "count",
    }
    _REMAP_UNIQUE: ClassVar[Mapping[UniqueKeepStrategy, Aggregation]] = {
        "any": "min",
        "first": "min",
        "last": "max",
    }

    def __init__(
        self,
        df: ArrowDataFrame,
        keys: Sequence[ArrowExpr] | Sequence[str],
        /,
        *,
        drop_null_keys: bool,
    ) -> None:
        self._df = df
        frame, self._keys, self._output_key_names = self._parse_keys(df, keys=keys)
        self._compliant_frame = frame.drop_nulls(self._keys) if drop_null_keys else frame
        self._grouped = pa.TableGroupBy(self.compliant.native, self._keys)
        self._drop_null_keys = drop_null_keys

    def agg(self, *exprs: ArrowExpr) -> ArrowDataFrame:
        self._ensure_all_simple(exprs)
        aggs: list[tuple[str, Aggregation, AggregateOptions | None]] = []
        expected_pyarrow_column_names: list[str] = self._keys.copy()
        new_column_names: list[str] = self._keys.copy()
        exclude = (*self._keys, *self._output_key_names)

        for expr in exprs:
            output_names, aliases = evaluate_output_names_and_aliases(
                expr, self.compliant, exclude
            )

            if expr._depth == 0:
                # e.g. `agg(nw.len())`
                if expr._function_name != "len":  # pragma: no cover
                    msg = "Safety assertion failed, please report a bug to https://github.com/narwhals-dev/narwhals/issues"
                    raise AssertionError(msg)

                new_column_names.append(aliases[0])
                expected_pyarrow_column_names.append(f"{self._keys[0]}_count")
                aggs.append((self._keys[0], "count", pc.CountOptions(mode="all")))
                continue

            function_name = self._leaf_name(expr)
            if function_name in {"std", "var"}:
                assert "ddof" in expr._scalar_kwargs  # noqa: S101
                option: Any = pc.VarianceOptions(ddof=expr._scalar_kwargs["ddof"])
            elif function_name in {"len", "n_unique"}:
                option = pc.CountOptions(mode="all")
            elif function_name == "count":
                option = pc.CountOptions(mode="only_valid")
            else:
                option = None

            function_name = self._remap_expr_name(function_name)
            new_column_names.extend(aliases)
            expected_pyarrow_column_names.extend(
                [f"{output_name}_{function_name}" for output_name in output_names]
            )
            aggs.extend(
                [(output_name, function_name, option) for output_name in output_names]
            )

        result_simple = self._grouped.aggregate(aggs)

        # Rename columns, being very careful
        expected_old_names_indices: dict[str, list[int]] = collections.defaultdict(list)
        for idx, item in enumerate(expected_pyarrow_column_names):
            expected_old_names_indices[item].append(idx)
        if not (
            set(result_simple.column_names) == set(expected_pyarrow_column_names)
            and len(result_simple.column_names) == len(expected_pyarrow_column_names)
        ):  # pragma: no cover
            msg = (
                f"Safety assertion failed, expected {expected_pyarrow_column_names} "
                f"got {result_simple.column_names}, "
                "please report a bug at https://github.com/narwhals-dev/narwhals/issues"
            )
            raise AssertionError(msg)
        index_map: list[int] = [
            expected_old_names_indices[item].pop(0) for item in result_simple.column_names
        ]
        new_column_names = [new_column_names[i] for i in index_map]
        result_simple = result_simple.rename_columns(new_column_names)
        if self.compliant._backend_version < (12, 0, 0):
            columns = result_simple.column_names
            result_simple = result_simple.select(
                [*self._keys, *[col for col in columns if col not in self._keys]]
            )

        return self.compliant._with_native(result_simple).rename(
            dict(zip(self._keys, self._output_key_names))
        )

    def __iter__(self) -> Iterator[tuple[Any, ArrowDataFrame]]:
        col_token = generate_temporary_column_name(
            n_bytes=8, columns=self.compliant.columns
        )
        null_token: str = "__null_token_value__"  # noqa: S105

        table = self.compliant.native
        it, separator_scalar = cast_to_comparable_string_types(
            *(table[key] for key in self._keys), separator=""
        )
        # NOTE: stubs indicate `separator` must also be a `ChunkedArray`
        # Reality: `str` is fine
        concat_str: Incomplete = pc.binary_join_element_wise
        key_values = concat_str(
            *it, separator_scalar, null_handling="replace", null_replacement=null_token
        )
        table = table.add_column(i=0, field_=col_token, column=key_values)

        for v in pc.unique(key_values):
            t = self.compliant._with_native(
                table.filter(pc.equal(table[col_token], v)).drop([col_token])
            )
            row = t.simple_select(*self._keys).row(0)
            yield (
                tuple(extract_py_scalar(el) for el in row),
                t.simple_select(*self._df.columns),
            )