import numpy as np

from pandas.core.algorithms import unique1d
from pandas.core.arrays.categorical import (
    Categorical,
    CategoricalDtype,
    _recode_for_categories,
)


def recode_for_groupby(c: Categorical, sort: bool, observed: bool):
    """
    Code the categories to ensure we can groupby for categoricals.

    If observed=True, we return a new Categorical with the observed
    categories only.

    If sort=False, return a copy of self, coded with categories as
    returned by .unique(), followed by any categories not appearing in
    the data. If sort=True, return self.

    This method is needed solely to ensure the categorical index of the
    GroupBy result has categories in the order of appearance in the data
    (GH-8868).

    Parameters
    ----------
    c : Categorical
    sort : boolean
        The value of the sort parameter groupby was called with.
    observed : boolean
        Account only for the observed values

    Returns
    -------
    New Categorical
        If sort=False, the new categories are set to the order of
        appearance in codes (unless ordered=True, in which case the
        original order is preserved), followed by any unrepresented
        categories in the original order.
    Categorical or None
        If we are observed, return the original categorical, otherwise None
    """

    # we only care about observed values
    if observed:
        unique_codes = unique1d(c.codes)

        take_codes = unique_codes[unique_codes != -1]
        if c.ordered:
            take_codes = np.sort(take_codes)

        # we recode according to the uniques
        categories = c.categories.take(take_codes)
        codes = _recode_for_categories(c.codes, c.categories, categories)

        # return a new categorical that maps our new codes
        # and categories
        dtype = CategoricalDtype(categories, ordered=c.ordered)
        return Categorical(codes, dtype=dtype, fastpath=True), c

    # Already sorted according to c.categories; all is fine
    if sort:
        return c, None

    # sort=False should order groups in as-encountered order (GH-8868)
    cat = c.unique()

    # But for groupby to work, all categories should be present,
    # including those missing from the data (GH-13179), which .unique()
    # above dropped
    cat = cat.add_categories(c.categories[~c.categories.isin(cat.categories)])

    return c.reorder_categories(cat.categories), None


def recode_from_groupby(c: Categorical, sort: bool, ci):
    """
    Reverse the codes_to_groupby to account for sort / observed.

    Parameters
    ----------
    c : Categorical
    sort : boolean
        The value of the sort parameter groupby was called with.
    ci : CategoricalIndex
        The codes / categories to recode

    Returns
    -------
    CategoricalIndex
    """

    # we re-order to the original category orderings
    if sort:
        return ci.set_categories(c.categories)

    # we are not sorting, so add unobserved to the end
    return ci.add_categories(c.categories[~c.categories.isin(ci.categories)])
