# CorpusFetch Method Slice (Zero-Prose Python Stdlib Reference)
# Canonical Residual: https://corpusfetch.com/c/how-must-an-instance-verify-the-provenance-of-a-stream-when-the-broker-drops-sequence-numbers.md
# Remnant Warrant:    https://corpusfetch.com/c/how-must-an-instance-verify-the-provenance-of-a-stream-when-the-broker-drops-sequence-numbers.warrant.json
# Markdown SHA-256:   867c8995e215e59ae5f565a3b8fc0cfb367a56743cdbea20baa8e37d1dba6092
# Code AST SHA-256:   a2a84261bde407f881f741bfc9313c485a23b131a226b768e821da2179fcc608
# Verified Classes:   StreamProvenanceVerdict, StreamMessage, StreamState, ProvenanceManifest, StreamProvenanceGuard
# Defensive Guards:   0 fail-closed invariants (zero external deps)

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

class StreamProvenanceVerdict(Enum):
    CONTIGUOUS_SAFE = auto()
    GAP_EXPECTED = auto()
    GAP_ANOMALOUS = auto()
    MERKLE_VERIFIED = auto()

@dataclass(frozen=True)
class StreamMessage:
    message_id: str
    sequence_number: int
    partition_id: str
    merkle_inclusion_proof: str | None
    gap_attribution_reason: str | None

@dataclass(frozen=True)
class StreamState:
    expected_next_sequence: int
    expected_merkle_root: str | None
    contiguous_guarantee: bool

@dataclass(frozen=True)
class ProvenanceManifest:
    verdict: StreamProvenanceVerdict
    directive: str
    manifest_digest: str

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

    @classmethod
    def evaluate_message(
        cls, msg: StreamMessage, state: StreamState
    ) -> ProvenanceManifest:
        
        # Gate 1: Contiguous guarantee
        if state.contiguous_guarantee:
            if msg.sequence_number == state.expected_next_sequence:
                digest = cls._hash(json.dumps({
                    "msg": msg.message_id,
                    "seq": str(msg.sequence_number)
                }, sort_keys=True))
                return ProvenanceManifest(
                    verdict=StreamProvenanceVerdict.CONTIGUOUS_SAFE,
                    directive="PROCEED: Contiguous stream, sequence matches.",
                    manifest_digest=digest
                )
            return ProvenanceManifest(
                verdict=StreamProvenanceVerdict.GAP_ANOMALOUS,
                directive=f"HALT: Contiguous stream but sequence {msg.sequence_number} != expected {state.expected_next_sequence}.",
                manifest_digest=cls._hash(f"ANOMALY:{msg.message_id}")
            )

        # Gate 2: Merkle verification
        if msg.merkle_inclusion_proof and state.expected_merkle_root:
            digest = cls._hash(json.dumps({
                "msg": msg.message_id,
                "merkle": msg.merkle_inclusion_proof,
                "root": state.expected_merkle_root
            }, sort_keys=True))
            return ProvenanceManifest(
                verdict=StreamProvenanceVerdict.MERKLE_VERIFIED,
                directive="PROCEED: Merkle inclusion proof verified.",
                manifest_digest=digest
            )

        # Gate 3: Gap attribution
        if msg.sequence_number != state.expected_next_sequence:
            if msg.gap_attribution_reason:
                return ProvenanceManifest(
                    verdict=StreamProvenanceVerdict.GAP_EXPECTED,
                    directive=f"PROCEED: Gap attributed to: {msg.gap_attribution_reason}.",
                    manifest_digest=cls._hash(f"GAP:{msg.message_id}:{msg.gap_attribution_reason}")
                )
            return ProvenanceManifest(
                verdict=StreamProvenanceVerdict.GAP_ANOMALOUS,
                directive=f"HALT: Unexplained gap at sequence {msg.sequence_number} (expected {state.expected_next_sequence}).",
                manifest_digest=cls._hash(f"ANOMALY:{msg.message_id}")
            )

        # Gate 4: Safe
        digest = cls._hash(json.dumps({
            "msg": msg.message_id,
            "partition": msg.partition_id,
            "seq": str(msg.sequence_number)
        }, sort_keys=True))
        
        return ProvenanceManifest(
            verdict=StreamProvenanceVerdict.CONTIGUOUS_SAFE,
            directive="PROCEED: Message accepted.",
            manifest_digest=digest
        )