"""The half-hour slot, in one place (MNT-12).

Availability, coach bookings and matches are all built from the same rule:
*a slot given a start and no end lasts thirty minutes*. It was written five
times, in four modules, and the copies had drifted in the way that matters —
not in the arithmetic, but in what each did when the input was wrong:

    matches.py            flashed an error and redirected
    matches.py, later     set start_time to None and reported success
    team_matches.py       `except ValueError: pass`, silently
    availability.py       `continue`, dropping the slot without a word

Waves E and G closed the match side by putting a marshmallow schema at the
form boundary. This module is the other half: the constant and the two
functions the remaining callers need, so that the availability routes can
use the same schema treatment without each one re-deciding what a slot is.

`DEFAULT_SLOT_MINUTES` is deliberately not configurable. It is a product
decision written into the UI — the availability grid draws half-hour cells —
and a setting would let the two disagree.
"""

from datetime import date as date_cls
from datetime import datetime, timedelta

#: How long a slot lasts when only its start is given.
DEFAULT_SLOT_MINUTES = 30

#: Monday-first, matching `datetime.weekday()` and the availability grid.
DAY_NAMES = ('Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday')


def slot_end(start_time, minutes=DEFAULT_SLOT_MINUTES, on=None):
    """The end of a slot starting at `start_time`.

    Args:
        start_time: A `datetime.time`.
        minutes: Slot length; defaults to DEFAULT_SLOT_MINUTES.
        on: The date the slot falls on. Only matters for a slot that would
            cross midnight, where the arbitrary date the old helpers used
            (`datetime.today()`) made the result depend on when the code ran.

    Returns:
        datetime.time: The end of the slot, wrapping past midnight like the
        previous implementations did.
    """
    anchor = on or date_cls(2000, 1, 1)
    return (datetime.combine(anchor, start_time) + timedelta(minutes=minutes)).time()


def day_name(day_of_week):
    """Name of a weekday index, or a readable fallback.

    Every caller indexed a module-level list directly, so an out-of-range
    day — which nothing prevented before the schemas — was an IndexError
    inside a JSON route, i.e. a 500 with an HTML body.
    """
    if 0 <= day_of_week < len(DAY_NAMES):
        return DAY_NAMES[day_of_week]
    return f'Day {day_of_week}'