"""Utilities for union (sum type) disambiguation."""
from collections import OrderedDict
from functools import reduce
from operator import or_
from typing import (  # noqa: F401, imported for Mypy.
    Callable,
    Dict,
    Mapping,
    Optional,
    Type,
)

from attr import fields, NOTHING

from cattr._compat import get_origin


def create_uniq_field_dis_func(*classes: Type) -> Callable:
    """Given attr classes, generate a disambiguation function.

    The function is based on unique fields."""
    if len(classes) < 2:
        raise ValueError("At least two classes required.")
    cls_and_attrs = [
        (cl, set(at.name for at in fields(get_origin(cl) or cl)))
        for cl in classes
    ]
    if len([attrs for _, attrs in cls_and_attrs if len(attrs) == 0]) > 1:
        raise ValueError("At least two classes have no attributes.")
    # TODO: Deal with a single class having no required attrs.
    # For each class, attempt to generate a single unique required field.
    uniq_attrs_dict = OrderedDict()  # type: Dict[str, Type]
    cls_and_attrs.sort(key=lambda c_a: -len(c_a[1]))

    fallback = None  # If none match, try this.

    for i, (cl, cl_reqs) in enumerate(cls_and_attrs):
        other_classes = cls_and_attrs[i + 1 :]
        if other_classes:
            other_reqs = reduce(or_, (c_a[1] for c_a in other_classes))
            uniq = cl_reqs - other_reqs
            if not uniq:
                m = "{} has no usable unique attributes.".format(cl)
                raise ValueError(m)
            # We need a unique attribute with no default.
            cl_fields = fields(get_origin(cl) or cl)
            for attr_name in uniq:
                if getattr(cl_fields, attr_name).default is NOTHING:
                    break
            else:
                raise ValueError(f"{cl} has no usable non-default attributes.")
            uniq_attrs_dict[attr_name] = cl
        else:
            fallback = cl

    def dis_func(data):
        # type: (Mapping) -> Optional[Type]
        if not isinstance(data, Mapping):
            raise ValueError("Only input mappings are supported.")
        for k, v in uniq_attrs_dict.items():
            if k in data:
                return v
        return fallback

    return dis_func
