# CorpusFetch Method Slice (Zero-Prose Python Stdlib Reference)
# Canonical Residual: https://corpusfetch.com/c/what-communication-invariants-prevent-fan-out-storms-when-a-broadcast-is-amplified-through-a-mesh.md
# Remnant Warrant:    https://corpusfetch.com/c/what-communication-invariants-prevent-fan-out-storms-when-a-broadcast-is-amplified-through-a-mesh.warrant.json
# Markdown SHA-256:   ce978c855305af337e3c55fa612e18351160c425db527b0b9204141f2524df9f
# Code AST SHA-256:   1340f53c3d13be6f48f303799b82b1e181d27b89e4f4896c9ac3e453a301c928
# Verified Classes:   FanoutVerdict, BroadcastMessage, MeshNodeState, FanoutPolicy, FanoutManifest, MeshFanoutGuard
# Defensive Guards:   0 fail-closed invariants (zero external deps)

from dataclasses import dataclass
from enum import Enum, auto
import hashlib
import json

class FanoutVerdict(Enum):
    POINT_TO_POINT = auto()
    REBROADCAST_ALLOWED = auto()
    TTL_EXPIRED = auto()
    DUPLICATE_SUPPRESSED = auto()
    QUOTA_EXHAUSTED = auto()

@dataclass(frozen=True)
class BroadcastMessage:
    message_id: str
    ttl_remaining: int
    priority: str

@dataclass(frozen=True)
class MeshNodeState:
    seen_message_ids: frozenset[str]
    fanout_quota_remaining: int
    max_ttl: int

@dataclass(frozen=True)
class FanoutPolicy:
    allow_critical_bypass: bool

@dataclass(frozen=True)
class FanoutManifest:
    verdict: FanoutVerdict
    directive: str
    manifest_digest: str

class MeshFanoutGuard:
    @staticmethod
    def _hash(data: str) -> str:
        return hashlib.sha256(data.encode("utf-8")).hexdigest()

    @classmethod
    def evaluate_broadcast(
        cls, msg: BroadcastMessage, state: MeshNodeState, policy: FanoutPolicy
    ) -> FanoutManifest:
        
        # Gate 1: TTL
        if msg.ttl_remaining <= 0:
            return FanoutManifest(
                verdict=FanoutVerdict.TTL_EXPIRED,
                directive=f"DROP: Message {msg.message_id} TTL expired.",
                manifest_digest=cls._hash(f"TTL:{msg.message_id}")
            )

        # Gate 2: Duplicate
        if msg.message_id in state.seen_message_ids:
            return FanoutManifest(
                verdict=FanoutVerdict.DUPLICATE_SUPPRESSED,
                directive=f"DROP: Message {msg.message_id} already seen.",
                manifest_digest=cls._hash(f"DUP:{msg.message_id}")
            )

        # Gate 3: Quota
        if state.fanout_quota_remaining <= 0:
            if policy.allow_critical_bypass and msg.priority == "CRITICAL":
                digest = cls._hash(json.dumps({
                    "message": msg.message_id,
                    "priority": "CRITICAL",
                    "bypass": "QUOTA"
                }, sort_keys=True))
                return FanoutManifest(
                    verdict=FanoutVerdict.REBROADCAST_ALLOWED,
                    directive=f"PROCEED: Critical bypass for {msg.message_id}.",
                    manifest_digest=digest
                )
            return FanoutManifest(
                verdict=FanoutVerdict.QUOTA_EXHAUSTED,
                directive=f"DROP: Quota exhausted for {msg.message_id}.",
                manifest_digest=cls._hash(f"QUOTA:{msg.message_id}")
            )

        # Gate 4: Allowed
        digest = cls._hash(json.dumps({
            "message": msg.message_id,
            "ttl": str(msg.ttl_remaining),
            "quota": str(state.fanout_quota_remaining),
            "priority": msg.priority
        }, sort_keys=True))
        
        return FanoutManifest(
            verdict=FanoutVerdict.REBROADCAST_ALLOWED,
            directive=f"PROCEED: Re-broadcast {msg.message_id} with quota {state.fanout_quota_remaining}.",
            manifest_digest=digest
        )