# CorpusFetch Method Slice (Zero-Prose Python Stdlib Reference)
# Canonical Residual: https://corpusfetch.com/c/what-discovery-invariants-prevent-semantic-drift-from-silently-altering-vector-search-relevance.md
# Remnant Warrant:    https://corpusfetch.com/c/what-discovery-invariants-prevent-semantic-drift-from-silently-altering-vector-search-relevance.warrant.json
# Markdown SHA-256:   2780e6a7045f770b512138b13c7c333aa994c3ecdabe5e06935aaf53c2a08a27
# Code AST SHA-256:   92dbc4fcb774ab5240dcb00cb53259edc4ca6649cb11f0cc76831c8e936b1ba5
# Verified Classes:   DriftVerdict, SemanticState, DriftPolicy, DriftManifest, SemanticDriftGuard
# Defensive Guards:   0 fail-closed invariants (zero external deps)

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

class DriftVerdict(Enum):
    SEMANTIC_ALIGNMENT_OK = auto()
    QUERY_DRIFT_DETECTED = auto()
    MODEL_MISMATCH = auto()
    RELEVANCE_DEGRADED = auto()

@dataclass(frozen=True)
class SemanticState:
    centroid_distance: float
    acceptable_centroid_distance: float
    embedding_model_matches: bool
    relevance_degradation_ratio: float

@dataclass(frozen=True)
class DriftPolicy:
    max_acceptable_relevance_degradation: float

@dataclass(frozen=True)
class DriftManifest:
    verdict: DriftVerdict
    directive: str
    manifest_digest: str

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

    @classmethod
    def evaluate_drift(
        cls, state: SemanticState, policy: DriftPolicy
    ) -> DriftManifest:
        
        # Gate 1: Model mismatch
        if not state.embedding_model_matches:
            return DriftManifest(
                verdict=DriftVerdict.MODEL_MISMATCH,
                directive="HALT: Embedding model mismatch. Queries and corpus use different models.",
                manifest_digest=cls._hash("MODEL_MISMATCH")
            )

        # Gate 2: Centroid drift
        if state.centroid_distance > state.acceptable_centroid_distance:
            return DriftManifest(
                verdict=DriftVerdict.QUERY_DRIFT_DETECTED,
                directive=f"HALT: Query centroid drifted {state.centroid_distance:.3f} > {state.acceptable_centroid_distance:.3f}.",
                manifest_digest=cls._hash(f"CENTROID:{state.centroid_distance:.3f}")
            )

        # Gate 3: Relevance degradation
        if state.relevance_degradation_ratio > policy.max_acceptable_relevance_degradation:
            return DriftManifest(
                verdict=DriftVerdict.RELEVANCE_DEGRADED,
                directive=f"HALT: Relevance degraded by {state.relevance_degradation_ratio:.2%}.",
                manifest_digest=cls._hash(f"RELEVANCE:{state.relevance_degradation_ratio:.3f}")
            )

        # Gate 4: Aligned
        digest = cls._hash(json.dumps({
            "centroid": f"{state.centroid_distance:.3f}",
            "relevance": f"{state.relevance_degradation_ratio:.3f}"
        }, sort_keys=True))
        
        return DriftManifest(
            verdict=DriftVerdict.SEMANTIC_ALIGNMENT_OK,
            directive="PROCEED: Semantic alignment verified.",
            manifest_digest=digest
        )