from __future__ import annotations

from typing import Dict, Any, Optional
from .contract import SafetyContract, safe_eval


class PolicyEngine:
    """Minimal runtime policy engine for on-board guards.

    Given a SafetyContract, a proposed plan, and the current state, decide whether to allow,
    veto, or modify the plan. This is intentionally small for MVP while being deterministic.
    """

    def __init__(self, contract: SafetyContract):
        self.contract = contract

    def evaluate(self, plan: Dict[str, Any], state: Dict[str, Any]) -> Dict[str, Any]:
        # 1) Pre-conditions must hold
        pre_ok = self.contract.evaluate_pre(state | {"action": plan.get("action")})
        if not pre_ok:
            return {"allowed": False, "reason": "pre_condition_failed"}

        # 2) Budget checks
        costs: Dict[str, float] = plan.get("costs", {})
        for key, limit in (self.contract.budgets or {}).items():
            if key in costs and costs[key] > limit:
                return {"allowed": False, "reason": f"budget_exceeded: {key}"}

        # 3) Collision/warning rules
        for expr in self.contract.collision_rules:
            if expr:
                if safe_eval(expr, {**state, **plan}):
                    return {"allowed": False, "reason": "collision_rule_violation"}

        return {"allowed": True}