from __future__ import annotations

import re
from collections.abc import Iterable
from functools import cache
from typing import TypeVar

_T = TypeVar('_T')

_undefined = object()  # unique object for when None is not a good default


def first(iterable, default=_undefined):
    """Return the first item of *iterable*, or *default* if *iterable* is
    empty.

        >>> first([0, 1, 2, 3])
        0
        >>> first([], 'some default')
        'some default'

    If *default* is not provided and there are no items in the iterable,
    raise ``ValueError``.

    :func:`first` is useful when you have a generator of expensive-to-retrieve
    values and want any arbitrary one. It is marginally shorter than
    ``next(iter(iterable), default)``.

    """
    # NOTE: https://more-itertools.readthedocs.io/en/stable/_modules/more_itertools/more.html#first
    try:
        return next(iter(iterable))
    except StopIteration as e:
        # I'm on the edge about raising ValueError instead of StopIteration. At
        # the moment, ValueError wins, because the caller could conceivably
        # want to do something different with flow control when I raise the
        # exception, and it's weird to explicitly catch StopIteration.
        if default is _undefined:
            raise ValueError(
                'first() was called on an empty iterable, and no '
                'default value was provided.',
            ) from e
        return default


def match_to_find(m: re.Match):
    if m is None:
        return None
    g = m.groups(default=m.string[0:0])
    if len(g) == 1:
        return g[0]
    else:
        return g or m.group()


def findalliter(pattern, string, pos=None, endpos=None, flags=0):
    """
    like finditer(), but with return values like findall()

    implementation taken from cpython/Modules/_sre.c/findall()
    """
    r = (
        pattern
        if isinstance(pattern, re.Pattern)
        else re.compile(pattern, flags=flags)
    )
    if endpos is not None:
        iterator = r.finditer(string, pos=pos, endpos=endpos)
    elif pos is not None:
        iterator = r.finditer(string, pos=pos)
    else:
        iterator = r.finditer(string)
    for m in iterator:
        yield match_to_find(m)


def findfirst(pattern, string, pos=None, endpos=None, flags=0, default=_undefined):
    """
    Avoids using the inefficient findall(...)[0], or first(findall(...))
    """
    return first(
        findalliter(pattern, string, pos=pos, endpos=endpos, flags=flags),
        default=default,
    )


def topsort(nodes: Iterable[_T], order: Iterable[tuple[_T, _T]]) -> list[_T]:
    # https://en.wikipedia.org/wiki/Topological_sorting

    order = set(order)
    result: list[_T] = []  # Empty list that will contain the sorted elements

    def with_incoming() -> set[_T]:
        return {m for (_, m) in order}

    # Set of all nodes with no incoming edges
    pending = list(set(nodes) - with_incoming())
    while pending:
        n = pending.pop()
        result.append(n)

        # nodes m with an edge from n to m
        outgoing = {m for (x, m) in order if x == n}
        order -= {(n, m) for m in outgoing}
        pending.extend(outgoing - with_incoming())

    if order:
        raise ValueError('There are cycles in the topological order')

    return list(reversed(result))  # a topologically sorted list


@cache
def cached_re_compile(regex: re.Pattern | str | bytes) -> re.Pattern | None:
    if isinstance(regex, re.Pattern):
        return regex
    return re.compile(regex) if isinstance(regex, (str | bytes)) else None