# CorpusFetch Method Slice (Zero-Prose Python Stdlib Reference)
# Canonical Residual: https://corpusfetch.com/c/what-communication-invariants-prevent-message-replay-in-a-decentralized-agent-swarm.md
# Remnant Warrant:    https://corpusfetch.com/c/what-communication-invariants-prevent-message-replay-in-a-decentralized-agent-swarm.warrant.json
# Markdown SHA-256:   4ca8f3c9be0f99711fb1e257df7486be95b2f8339c0a3f3c9b186ab09927cd9a
# Code AST SHA-256:   621fff4a2ab5e017d03a84d2ac14c83f31b12fb49562b235d3b25a4f621872e0
# Verified Classes:   ReplayVerdict, SwarmMessage, ReceiverState, ReplayManifest, DecentralizedReplayGuard
# Defensive Guards:   0 fail-closed invariants (zero external deps)

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

class ReplayVerdict(Enum):
    FRESH_MESSAGE = auto()
    REPLAY_DETECTED = auto()
    STALE_MESSAGE = auto()
    EXPIRED_MESSAGE = auto()

@dataclass(frozen=True)
class SwarmMessage:
    message_id: str
    sender_vector_clock_hash: str
    timestamp_epoch: float
    signature: str

@dataclass(frozen=True)
class ReceiverState:
    seen_message_hashes: frozenset[str]
    current_vector_clock_hash: str
    current_epoch: float
    freshness_window_sec: float

@dataclass(frozen=True)
class ReplayManifest:
    verdict: ReplayVerdict
    directive: str
    manifest_digest: str

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

    @classmethod
    def evaluate_message(
        cls, msg: SwarmMessage, state: ReceiverState
    ) -> ReplayManifest:
        
        # Gate 1: Deduplication
        if msg.message_id in state.seen_message_hashes:
            return ReplayManifest(
                verdict=ReplayVerdict.REPLAY_DETECTED,
                directive=f"HALT: Message {msg.message_id} already seen. Replay attack.",
                manifest_digest=cls._hash(f"REPLAY:{msg.message_id}")
            )

        # Gate 2: Freshness window
        age = state.current_epoch - msg.timestamp_epoch
        if age > state.freshness_window_sec:
            return ReplayManifest(
                verdict=ReplayVerdict.EXPIRED_MESSAGE,
                directive=f"HALT: Message is {age:.0f}s old, exceeding {state.freshness_window_sec:.0f}s window.",
                manifest_digest=cls._hash(f"EXPIRED:{msg.message_id}")
            )

        # Gate 3: Vector clock
        if msg.sender_vector_clock_hash == state.current_vector_clock_hash:
            # Same clock = likely stale or replayed
            return ReplayManifest(
                verdict=ReplayVerdict.STALE_MESSAGE,
                directive=f"HALT: Message vector clock matches current. Likely stale.",
                manifest_digest=cls._hash(f"STALE:{msg.message_id}")
            )

        # Gate 4: Fresh
        digest = cls._hash(json.dumps({
            "message": msg.message_id,
            "timestamp": f"{msg.timestamp_epoch:.0f}",
            "clock": msg.sender_vector_clock_hash
        }, sort_keys=True))
        
        return ReplayManifest(
            verdict=ReplayVerdict.FRESH_MESSAGE,
            directive="PROCEED: Message is fresh and not a replay.",
            manifest_digest=digest
        )