# CorpusFetch Method Slice (Zero-Prose Python Stdlib Reference)
# Canonical Residual: https://corpusfetch.com/c/what-invariants-must-survive-when-only-metadata-survives-a-backup-restore.md
# Remnant Warrant:    https://corpusfetch.com/c/what-invariants-must-survive-when-only-metadata-survives-a-backup-restore.warrant.json
# Markdown SHA-256:   8aa629f1c792b55cdc328a0dfb7f8e778b34533bfca22a4827eab951ef4e1246
# Code AST SHA-256:   829a2ba9f2f9ab764889ca26b01c62ec90f865c016404b3d5bf2f756c21588a1
# Verified Classes:   RestoreVerdict, RestoreState, RestorePolicy, RestoreManifest, MetadataOnlyRestoreGuard
# Defensive Guards:   1 fail-closed invariants (zero external deps)

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

class RestoreVerdict(Enum):
    FULL_RESTORE_VERIFIED = auto()
    METADATA_ONLY = auto()
    INCOHERENT_REFERENCE_GRAPH = auto()

@dataclass(frozen=True)
class RestoreState:
    metadata_artifact_count: int
    verified_payload_artifacts: int
    dangling_references: int
    reconstruction_source_available: bool

@dataclass(frozen=True)
class RestorePolicy:
    min_payload_coverage_ratio: float
    max_acceptable_dangling: int

@dataclass(frozen=True)
class RestoreManifest:
    verdict: RestoreVerdict
    payload_coverage_ratio: float
    directive: str
    manifest_digest: str

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

    @classmethod
    def evaluate_restore(
        cls, state: RestoreState, policy: RestorePolicy
    ) -> RestoreManifest:
        
        if state.metadata_artifact_count <= 0:
            raise ValueError("Restore must include at least one metadata artifact.")

        coverage = state.verified_payload_artifacts / state.metadata_artifact_count

        # Gate 1: Dangling references
        if state.dangling_references > policy.max_acceptable_dangling:
            return RestoreManifest(
                verdict=RestoreVerdict.INCOHERENT_REFERENCE_GRAPH,
                payload_coverage_ratio=coverage,
                directive=f"HALT: {state.dangling_references} dangling refs > {policy.max_acceptable_dangling}. Re-restore from full backup.",
                manifest_digest=cls._hash(f"DANGLING:{state.dangling_references}")
            )

        # Gate 2: Coverage check
        if coverage >= policy.min_payload_coverage_ratio:
            digest = cls._hash(json.dumps({
                "coverage": f"{coverage:.3f}",
                "verified": str(state.verified_payload_artifacts),
                "total": str(state.metadata_artifact_count)
            }, sort_keys=True))
            return RestoreManifest(
                verdict=RestoreVerdict.FULL_RESTORE_VERIFIED,
                payload_coverage_ratio=coverage,
                directive=f"PROCEED: Coverage {coverage:.2%} >= {policy.min_payload_coverage_ratio:.2%}.",
                manifest_digest=digest
            )

        # Gate 3: Metadata-only
        if state.reconstruction_source_available:
            return RestoreManifest(
                verdict=RestoreVerdict.METADATA_ONLY,
                payload_coverage_ratio=coverage,
                directive="ACTION: Metadata-only state. Reconstruct payload from source before serving.",
                manifest_digest=cls._hash(f"RECONSTRUCT:{coverage:.3f}")
            )

        return RestoreManifest(
            verdict=RestoreVerdict.METADATA_ONLY,
            payload_coverage_ratio=coverage,
            directive=f"HALT: Metadata-only ({coverage:.2%} coverage), no reconstruction source. Do not serve reads.",
            manifest_digest=cls._hash(f"PHANTOM:{coverage:.3f}")
        )