"""Slot state machine: 6 states + 8 allowed transitions (REQ-001, REQ-002).

The transitions match dcy-001 FR-2 and BAT-001 design.
"""
from __future__ import annotations

from enum import Enum


# Shared PostgreSQL advisory lock id for Buy/Sell decision threads and
# operator CLI manual transitions. Single constant so all callers serialize
# through the same lock (REQ-002 NFR-1, dcy-001 R-1).
SLOT_ADVISORY_LOCK_ID: int = 0x5107_4D31  # arbitrary mnemonic constant


class SlotState(str, Enum):
    """6-state slot lifecycle (dcy-001 5.3.5.1)."""

    IDLE = "idle"
    PENDING_BUY = "pending_buy"
    HOLDING = "holding"
    PENDING_SELL = "pending_sell"
    CLOSED = "closed"
    ERROR = "error"


# 8 allowed transitions per dcy-001 FR-2. Any pair not in this set must be
# rejected by SlotService.
# Note: `* -> error` is represented as transitions from every non-error state.
_BASE_TRANSITIONS: frozenset[tuple[SlotState, SlotState]] = frozenset(
    {
        (SlotState.IDLE, SlotState.PENDING_BUY),      # 1. buy decision
        (SlotState.PENDING_BUY, SlotState.HOLDING),   # 2. buy filled
        (SlotState.PENDING_BUY, SlotState.IDLE),      # 3. buy rejected/rollback
        (SlotState.HOLDING, SlotState.PENDING_SELL),  # 4. sell decision
        (SlotState.PENDING_SELL, SlotState.CLOSED),   # 5. sell filled
        (SlotState.PENDING_SELL, SlotState.HOLDING),  # 6. sell rejected/rollback
        (SlotState.CLOSED, SlotState.IDLE),           # 7. recovery (with daily_symbol_entries +1)
    }
)

# 8th transition family: any non-error state may move to error (operator manual
# resolve uses `error -> idle | holding` which is also covered explicitly).
_TO_ERROR_TRANSITIONS: frozenset[tuple[SlotState, SlotState]] = frozenset(
    (s, SlotState.ERROR) for s in SlotState if s is not SlotState.ERROR
)

# Operator manual resolve transitions out of error (REQ-004).
_FROM_ERROR_TRANSITIONS: frozenset[tuple[SlotState, SlotState]] = frozenset(
    {
        (SlotState.ERROR, SlotState.IDLE),
        (SlotState.ERROR, SlotState.HOLDING),
    }
)


ALLOWED_TRANSITIONS: frozenset[tuple[SlotState, SlotState]] = (
    _BASE_TRANSITIONS | _TO_ERROR_TRANSITIONS | _FROM_ERROR_TRANSITIONS
)


def is_allowed_transition(from_state: SlotState, to_state: SlotState) -> bool:
    """Return True iff (from_state, to_state) is one of the 8 transition families."""
    return (from_state, to_state) in ALLOWED_TRANSITIONS