# CorpusFetch Method Slice (Zero-Prose Python Stdlib Reference)
# Canonical Residual: https://corpusfetch.com/c/how-must-an-instance-account-for-semantic-drift-when-translating-instructions-between-languages.md
# Remnant Warrant:    https://corpusfetch.com/c/how-must-an-instance-account-for-semantic-drift-when-translating-instructions-between-languages.warrant.json
# Markdown SHA-256:   db005935da44b5e8c17055f981d792e2388cfd263ee439b0ee7dba28c0e6484e
# Code AST SHA-256:   a9b9e9eb91bbab7b3965c0218021836b8c12a817f0e6ca2cae7c2dfeecc5ba63
# Verified Classes:   TranslationVerdict, TranslationState, TranslationPolicy, TranslationManifest, TranslationDriftGuard
# Defensive Guards:   0 fail-closed invariants (zero external deps)

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

class TranslationVerdict(Enum):
    NO_TRANSLATION = auto()
    FIDELITY_VERIFIED = auto()
    CRITICAL_FIELD_DRIFT = auto()
    LOW_FIDELITY = auto()

@dataclass(frozen=True)
class TranslationState:
    source_hash: str
    translated_hash: str
    semantic_fidelity_score: float
    critical_fields_preserved: bool
    back_translation_matches: bool

@dataclass(frozen=True)
class TranslationPolicy:
    min_fidelity_score: float
    require_back_translation: bool

@dataclass(frozen=True)
class TranslationManifest:
    verdict: TranslationVerdict
    directive: str
    manifest_digest: str

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

    @classmethod
    def evaluate_translation(
        cls, state: TranslationState, policy: TranslationPolicy
    ) -> TranslationManifest:
        
        # Gate 1: Same language
        if state.source_hash == state.translated_hash:
            digest = cls._hash(json.dumps({
                "status": "NO_TRANSLATION"
            }, sort_keys=True))
            return TranslationManifest(
                verdict=TranslationVerdict.NO_TRANSLATION,
                directive="PROCEED: No translation involved.",
                manifest_digest=digest
            )

        # Gate 2: Critical fields
        if not state.critical_fields_preserved:
            return TranslationManifest(
                verdict=TranslationVerdict.CRITICAL_FIELD_DRIFT,
                directive="HALT: Critical fields (numbers, modals, negations) not preserved. Re-translate or escalate.",
                manifest_digest=cls._hash(f"CRITICAL_DRIFT:{state.translated_hash}")
            )

        # Gate 3: Fidelity score
        if state.semantic_fidelity_score < policy.min_fidelity_score:
            return TranslationManifest(
                verdict=TranslationVerdict.LOW_FIDELITY,
                directive=f"HALT: Fidelity {state.semantic_fidelity_score:.3f} < {policy.min_fidelity_score:.3f}. Re-translate.",
                manifest_digest=cls._hash(f"LOW_FIDELITY:{state.semantic_fidelity_score:.3f}")
            )

        # Gate 4: Back translation
        if policy.require_back_translation and not state.back_translation_matches:
            return TranslationManifest(
                verdict=TranslationVerdict.LOW_FIDELITY,
                directive="HALT: Back-translation does not match source. Escalate.",
                manifest_digest=cls._hash("BACK_MISMATCH")
            )

        # Gate 5: Verified
        digest = cls._hash(json.dumps({
            "source": state.source_hash,
            "translated": state.translated_hash,
            "fidelity": f"{state.semantic_fidelity_score:.3f}"
        }, sort_keys=True))
        
        return TranslationManifest(
            verdict=TranslationVerdict.FIDELITY_VERIFIED,
            directive=f"PROCEED: Translation fidelity {state.semantic_fidelity_score:.3f} verified.",
            manifest_digest=digest
        )