# CorpusFetch Method Slice (Zero-Prose Python Stdlib Reference)
# Canonical Residual: https://corpusfetch.com/c/what-data-retention-decay-policies-must-be-cryptographically-sealed-for-vector-indices.md
# Remnant Warrant:    https://corpusfetch.com/c/what-data-retention-decay-policies-must-be-cryptographically-sealed-for-vector-indices.warrant.json
# Markdown SHA-256:   79bba09af2b13a91840b37f8a2883ea88a3c3e046f62108ccde542569b35e384
# Code AST SHA-256:   1b30f2e7996c0719505e597e3968465400fb503e678a8573da9f6abab56d1f1a
# Verified Classes:   DecayVerdict, VectorRetentionState, DecayPolicy, DecayManifest, VectorRetentionGuard
# Defensive Guards:   0 fail-closed invariants (zero external deps)

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

class DecayVerdict(Enum):
    POLICY_COMPLIANT = auto()
    DECAY_OVERDUE = auto()
    STALE_VECTOR_VIOLATION = auto()

@dataclass(frozen=True)
class VectorRetentionState:
    policy_hash: str
    last_decay_epoch: float
    current_epoch: float
    decay_interval_sec: float
    max_age_sec: float
    oldest_vector_age_sec: float

@dataclass(frozen=True)
class DecayPolicy:
    policy_hash: str
    decay_interval_sec: float
    max_age_sec: float

@dataclass(frozen=True)
class DecayManifest:
    verdict: DecayVerdict
    directive: str
    manifest_digest: str

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

    @classmethod
    def evaluate_retention(
        cls, state: VectorRetentionState, policy: DecayPolicy
    ) -> DecayManifest:
        
        # Gate 1: Policy match
        if state.policy_hash != policy.policy_hash:
            return DecayManifest(
                verdict=DecayVerdict.DECAY_OVERDUE,
                directive="HALT: Retention policy hash mismatch. Reconcile policy before decay.",
                manifest_digest=cls._hash("POLICY_MISMATCH")
            )

        # Gate 2: Max age violation
        if state.oldest_vector_age_sec > policy.max_age_sec:
            return DecayManifest(
                verdict=DecayVerdict.STALE_VECTOR_VIOLATION,
                directive=f"HALT: Oldest vector is {state.oldest_vector_age_sec:.0f}s old, exceeding max {policy.max_age_sec:.0f}s. Purge immediately.",
                manifest_digest=cls._hash(f"MAX_AGE:{state.oldest_vector_age_sec:.0f}")
            )

        # Gate 3: Decay interval check
        elapsed = state.current_epoch - state.last_decay_epoch
        if elapsed > policy.decay_interval_sec:
            return DecayManifest(
                verdict=DecayVerdict.DECAY_OVERDUE,
                directive=f"ACTION: Decay interval exceeded ({elapsed:.0f}s > {policy.decay_interval_sec:.0f}s). Execute scheduled decay.",
                manifest_digest=cls._hash(f"OVERDUE:{elapsed:.0f}")
            )

        # Gate 4: Compliant
        digest = cls._hash(json.dumps({
            "policy": state.policy_hash,
            "elapsed": f"{elapsed:.0f}",
            "oldest": f"{state.oldest_vector_age_sec:.0f}"
        }, sort_keys=True))
        
        return DecayManifest(
            verdict=DecayVerdict.POLICY_COMPLIANT,
            directive="PROCEED: Vector retention within policy bounds.",
            manifest_digest=digest
        )