# CorpusFetch Method Slice (Zero-Prose Python Stdlib Reference)
# Canonical Residual: https://corpusfetch.com/c/what-communication-invariants-prevent-message-amplification-storms-in-gossip-protocols.md
# Remnant Warrant:    https://corpusfetch.com/c/what-communication-invariants-prevent-message-amplification-storms-in-gossip-protocols.warrant.json
# Markdown SHA-256:   0ab60fa39a08fd947954f65f2cfed2b5f9efe0b4c44c8d300b299c9a9235661a
# Code AST SHA-256:   67fad175befbcb6e2a759f039eec5bca3cd18bd2f4c5c0048e522fb91efc4d95
# Verified Classes:   GossipVerdict, GossipMessage, PeerState, GossipManifest, GossipAmplificationGuard
# Defensive Guards:   0 fail-closed invariants (zero external deps)

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

class GossipVerdict(Enum):
    GOSSIP_ALLOWED = auto()
    TTL_EXPIRED = auto()
    DUPLICATE_SUPPRESSED = auto()
    BUDGET_EXHAUSTED = auto()

@dataclass(frozen=True)
class GossipMessage:
    message_id: str
    current_ttl_hops: int
    priority: str

@dataclass(frozen=True)
class PeerState:
    seen_message_ids: frozenset[str]
    fanout_budget_remaining: int
    max_ttl_hops: int

@dataclass(frozen=True)
class GossipManifest:
    verdict: GossipVerdict
    directive: str
    manifest_digest: str

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

    @classmethod
    def evaluate_message(
        cls, msg: GossipMessage, state: PeerState
    ) -> GossipManifest:
        
        # Gate 1: TTL
        if msg.current_ttl_hops >= state.max_ttl_hops:
            return GossipManifest(
                verdict=GossipVerdict.TTL_EXPIRED,
                directive=f"DROP: Message {msg.message_id} exceeded {state.max_ttl_hops} hops.",
                manifest_digest=cls._hash(f"TTL:{msg.message_id}")
            )

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

        # Gate 3: Budget
        if state.fanout_budget_remaining <= 0:
            # Critical priority bypasses budget
            if msg.priority == "CRITICAL":
                digest = cls._hash(json.dumps({
                    "message": msg.message_id,
                    "priority": "CRITICAL",
                    "bypass": "BUDGET"
                }, sort_keys=True))
                return GossipManifest(
                    verdict=GossipVerdict.GOSSIP_ALLOWED,
                    directive="PROCEED: Critical priority bypasses fanout budget.",
                    manifest_digest=digest
                )
            return GossipManifest(
                verdict=GossipVerdict.BUDGET_EXHAUSTED,
                directive=f"DROP: Fanout budget exhausted for message {msg.message_id}.",
                manifest_digest=cls._hash(f"BUDGET:{msg.message_id}")
            )

        # Gate 4: Allowed
        digest = cls._hash(json.dumps({
            "message": msg.message_id,
            "ttl": str(msg.current_ttl_hops),
            "budget": str(state.fanout_budget_remaining),
            "priority": msg.priority
        }, sort_keys=True))
        
        return GossipManifest(
            verdict=GossipVerdict.GOSSIP_ALLOWED,
            directive=f"PROCEED: Gossip allowed with {state.fanout_budget_remaining} budget remaining.",
            manifest_digest=digest
        )