"""Agent status transitions — BE-AGT-002.

Implements the state machine for Agent runtime status:

    Idle  ──→  Busy       (woken by a Work slot)
    Idle  ──→  OnCall     (woken by an OnCall slot)
    Busy  ──→  Idle       (task finished / no more pending slots)
    OnCall──→  Idle       (task finished / no more pending slots)
    *     ──→  Offline    (heartbeat timeout — no heartbeat for > 2 min)
    *     ──→  Exhausted  (API quota / rate-limit error)
    Exhausted → Idle      (recovery_at reached)

Design reference: NEXT_WAVE_DEV_DIRECTION.md §6.4 (Status transitions)
"""

from __future__ import annotations

from datetime import datetime, timedelta, timezone
from typing import Optional

from sqlalchemy.orm import Session

from app.models.agent import Agent, AgentStatus, ExhaustReason
from app.models.calendar import SlotType

# Heartbeat timeout threshold in seconds (2 minutes per spec §6.4)
HEARTBEAT_TIMEOUT_SECONDS = 120

# Default recovery duration when we can't parse a retry-after header
DEFAULT_RECOVERY_HOURS = 5


# ---------------------------------------------------------------------------
# Transition helpers
# ---------------------------------------------------------------------------

class AgentStatusError(Exception):
    """Raised when a requested status transition is invalid."""


def _assert_current(agent: Agent, *expected: AgentStatus) -> None:
    """Raise if the agent is not in one of the expected statuses."""
    if agent.status not in expected:
        allowed = ", ".join(s.value for s in expected)
        raise AgentStatusError(
            f"Agent '{agent.agent_id}' is {agent.status.value}; "
            f"expected one of [{allowed}]"
        )


# ---------------------------------------------------------------------------
# Public API
# ---------------------------------------------------------------------------

def transition_to_busy(
    db: Session,
    agent: Agent,
    *,
    slot_type: SlotType,
    now: datetime | None = None,
) -> Agent:
    """Idle → Busy or OnCall depending on *slot_type*.

    Parameters
    ----------
    slot_type : SlotType
        The type of the slot that triggered the wakeup.
        ``SlotType.ON_CALL`` → ``AgentStatus.ON_CALL``, everything else
        → ``AgentStatus.BUSY``.
    """
    _assert_current(agent, AgentStatus.IDLE)

    if slot_type == SlotType.ON_CALL:
        agent.status = AgentStatus.ON_CALL
    else:
        agent.status = AgentStatus.BUSY

    if now is None:
        now = datetime.now(timezone.utc)
    agent.last_heartbeat = now

    db.flush()
    return agent


def transition_to_idle(
    db: Session,
    agent: Agent,
    *,
    now: datetime | None = None,
) -> Agent:
    """Busy / OnCall / Exhausted (recovered) → Idle.

    For Exhausted agents this should only be called when ``recovery_at``
    has been reached; the caller is responsible for checking that.
    """
    _assert_current(
        agent,
        AgentStatus.BUSY,
        AgentStatus.ON_CALL,
        AgentStatus.EXHAUSTED,
        AgentStatus.OFFLINE,
    )

    agent.status = AgentStatus.IDLE

    # Clear exhausted metadata if transitioning out of Exhausted
    agent.exhausted_at = None
    agent.recovery_at = None
    agent.exhaust_reason = None

    if now is None:
        now = datetime.now(timezone.utc)
    agent.last_heartbeat = now

    db.flush()
    return agent


def transition_to_offline(
    db: Session,
    agent: Agent,
) -> Agent:
    """Any status → Offline (heartbeat timeout).

    Typically called by a background check that detects
    ``last_heartbeat`` is older than ``HEARTBEAT_TIMEOUT_SECONDS``.
    """
    # Already offline — no-op
    if agent.status == AgentStatus.OFFLINE:
        return agent

    agent.status = AgentStatus.OFFLINE
    db.flush()
    return agent


def transition_to_exhausted(
    db: Session,
    agent: Agent,
    *,
    reason: ExhaustReason,
    recovery_at: datetime | None = None,
    now: datetime | None = None,
) -> Agent:
    """Any active status → Exhausted (API quota error).

    Parameters
    ----------
    reason : ExhaustReason
        ``RATE_LIMIT`` or ``BILLING``.
    recovery_at : datetime, optional
        Parsed from retry-after / reset headers.  If *None*, defaults to
        ``now + DEFAULT_RECOVERY_HOURS``.
    """
    if now is None:
        now = datetime.now(timezone.utc)

    agent.status = AgentStatus.EXHAUSTED
    agent.exhausted_at = now
    agent.exhaust_reason = reason

    if recovery_at is not None:
        agent.recovery_at = recovery_at
    else:
        agent.recovery_at = now + timedelta(hours=DEFAULT_RECOVERY_HOURS)

    db.flush()
    return agent


# ---------------------------------------------------------------------------
# Heartbeat-driven checks
# ---------------------------------------------------------------------------

def check_heartbeat_timeout(
    db: Session,
    agent: Agent,
    *,
    now: datetime | None = None,
) -> bool:
    """Mark agent Offline if heartbeat has timed out.

    Returns ``True`` if the agent was transitioned to Offline.
    """
    if agent.status == AgentStatus.OFFLINE:
        return False

    if now is None:
        now = datetime.now(timezone.utc)

    if agent.last_heartbeat is None:
        # Never sent a heartbeat — treat as offline
        transition_to_offline(db, agent)
        return True

    elapsed = (now - agent.last_heartbeat).total_seconds()
    if elapsed > HEARTBEAT_TIMEOUT_SECONDS:
        transition_to_offline(db, agent)
        return True

    return False


def check_exhausted_recovery(
    db: Session,
    agent: Agent,
    *,
    now: datetime | None = None,
) -> bool:
    """Recover an Exhausted agent if ``recovery_at`` has been reached.

    Returns ``True`` if the agent was transitioned back to Idle.
    """
    if agent.status != AgentStatus.EXHAUSTED:
        return False

    if now is None:
        now = datetime.now(timezone.utc)

    if agent.recovery_at is not None and now >= agent.recovery_at:
        transition_to_idle(db, agent, now=now)
        return True

    return False


def record_heartbeat(
    db: Session,
    agent: Agent,
    *,
    now: datetime | None = None,
) -> Agent:
    """Update ``last_heartbeat`` timestamp.

    If the agent was Offline and a heartbeat arrives, transition back to
    Idle (the agent has come back online).
    """
    if now is None:
        now = datetime.now(timezone.utc)

    agent.last_heartbeat = now

    if agent.status == AgentStatus.OFFLINE:
        agent.status = AgentStatus.IDLE
        # Clear any stale exhausted metadata
        agent.exhausted_at = None
        agent.recovery_at = None
        agent.exhaust_reason = None

    db.flush()
    return agent