"""Pure state transitions for the daily playtime budget.

Separated from persistence: nothing here touches the disk or the clock
beyond what it is handed, which is what makes the budget arithmetic
straightforward to test.
"""

from __future__ import annotations

from dataclasses import replace
import logging
from typing import TYPE_CHECKING

from steam_backlog_enforcer._playtime_state import (
    _DELTA_CLAMP_FACTOR,
    PlaytimeRules,
    PlaytimeState,
)

if TYPE_CHECKING:
    from datetime import datetime

logger = logging.getLogger(__name__)


def accumulate(
    state: PlaytimeState,
    *,
    now: datetime,
    qualifying: set[int],
    interval: float,
    credited_key: str = "",
) -> PlaytimeState:
    """Add this tick's elapsed gaming time to *state*.

    Pure. Every argument is mandatory, including the clock — a defaulted ``now``
    would add a branch that exists only to be covered.

    The delta is clamped to ``_DELTA_CLAMP_FACTOR`` intervals so that a suspend,
    a skipped tick or a forward NTP step cannot inflate the count, and floored
    at zero so a backward step cannot refund time.

    Args:
        state: Current accounting state.
        now: Timezone-aware local timestamp for this tick.
        qualifying: PIDs currently counting against the budget.
        interval: Nominal seconds between ticks.
        credited_key: Attribution key to charge, or ``""`` when the tick cannot
            be pinned to one game. An unattributable tick still bills
            ``seconds``; the shortfall is rendered as Unattributed rather than
            charged to a guess, which is why ``per_game`` only ever sums to at
            most ``seconds``.

    Returns:
        The updated state.
    """
    return bill(
        state,
        now=now,
        billed=bool(qualifying),
        interval=interval,
        credited_key=credited_key,
    )


def bill(
    state: PlaytimeState,
    *,
    now: datetime,
    billed: bool,
    interval: float,
    credited_key: str = "",
) -> PlaytimeState:
    """:func:`accumulate` with the "anything qualifies?" answer given directly.

    The leisure budget (Phase 2) decides billing from the shadow verdict, not
    from game PIDs; everything else -- the clamp, the first tick, a backward
    step, the attribution -- is the same arithmetic.

    Args:
        state: Current accounting state.
        now: Timezone-aware local timestamp for this tick.
        billed: Whether this tick counts against the budget.
        interval: Nominal seconds between ticks.
        credited_key: Attribution key to charge, or ``""``.

    Returns:
        The updated state.
    """
    stamp = now.timestamp()
    if state.last_tick_at <= 0.0:
        return replace(state, last_tick_at=stamp)

    delta = stamp - state.last_tick_at
    if delta <= 0.0 or not billed:
        return replace(state, last_tick_at=stamp)

    credited = min(delta, interval * _DELTA_CLAMP_FACTOR)
    per_game = dict(state.per_game)
    if credited_key:
        per_game[credited_key] = per_game.get(credited_key, 0.0) + credited
    return replace(
        state,
        last_tick_at=stamp,
        seconds=state.seconds + credited,
        per_game=per_game,
        last_credited_key=credited_key or state.last_credited_key,
    )


def roll_over(state: PlaytimeState, *, day_key: str) -> PlaytimeState:
    """Reset *state* if the gaming day has changed.

    Pure. ``last_tick_at`` carries across the boundary so the first tick of the
    new day measures a sane delta rather than restarting from zero. Nothing else
    does: the outgoing day's unspent time is dropped.

    Args:
        state: Current accounting state.
        day_key: The gaming day *now* falls in.

    Returns:
        Either *state* unchanged, or a fresh record for *day_key*.
    """
    if state.day_key == day_key:
        return state
    return PlaytimeState(day_key=day_key, last_tick_at=state.last_tick_at)


def pending_warning(state: PlaytimeState, rules: PlaytimeRules) -> int | None:
    """Return the largest un-fired warning threshold *state* has crossed.

    Pure. Returning only the largest matters when a tick is skipped: crossing
    two thresholds at once should warn once, not twice.

    Args:
        state: Current accounting state.
        rules: Policy for this tick.

    Returns:
        The threshold in seconds-remaining, or ``None`` if none is due.
    """
    remaining = rules.budget_seconds - state.seconds
    for threshold in rules.warn_at:
        if remaining <= threshold and threshold not in state.warned_seconds:
            return threshold
    return None