"""DTN custody header and simple custody policy helpers.

This module provides a minimal CustodyHeader dataclass used by DTN custody
holders to make decisions about storing/transferring PlanDelta payloads.
It also exposes a small helper that decides whether to accept custody based
on priority, remaining lifetime, and a node reputation score.

The implementation is intentionally small and deterministic for unit testing.
"""
from dataclasses import dataclass, field
from typing import List, Optional
import time


@dataclass
class CustodyHeader:
    delta_id: str
    created_at: float = field(default_factory=time.time)
    lifetime_secs: int = 3600  # how long the custody is expected to be valid
    custody_chain: List[str] = field(default_factory=list)
    priority_key: int = 0  # combined urgency/impact key (0..127)

    def is_expired(self, now: Optional[float] = None) -> bool:
        if now is None:
            now = time.time()
        return now > (self.created_at + self.lifetime_secs)

    def add_holder(self, node_id: str):
        # append to custody chain (no dedupe to keep deterministic append-only)
        self.custody_chain.append(node_id)


def should_accept_custody(header: CustodyHeader, node_reputation: float, min_reputation: float = 0.2,
                          now: Optional[float] = None) -> bool:
    """Decide whether a node should accept custody for a delta.

    Decision factors (simple heuristic):
    - If the header is already expired, refuse custody.
    - Prefer accepting higher priority_key values.
    - Gate acceptance by a minimum reputation threshold.

    Returns True if the node should accept custody.
    """
    if header.is_expired(now=now):
        return False

    # Enforce a minimum reputation gate: nodes below min_reputation refuse custody
    if node_reputation < min_reputation:
        return False

    # decode urgency and impact from priority_key (urgency<<4)|impact
    urgency = (header.priority_key >> 4) & 0x07
    impact = header.priority_key & 0x0F

    # compute a simple desirability score
    score = urgency * 2 + impact + (node_reputation * 4)

    # threshold depends on min_reputation; higher min_reputation raises bar
    threshold = 5.0 + (1.0 - min_reputation) * 3.0
    return score >= threshold