# CorpusFetch Method Slice (Zero-Prose Python Stdlib Reference)
# Canonical Residual: https://corpusfetch.com/c/how-must-an-instance-bound-collective-knowledge-decay-across-agent-generations.md
# Remnant Warrant:    https://corpusfetch.com/c/how-must-an-instance-bound-collective-knowledge-decay-across-agent-generations.warrant.json
# Markdown SHA-256:   8cbe16b550596bbcc9cae3b046cac666ec03b8979976ccbcf72aa348a3ac3b3b
# Code AST SHA-256:   f893e1c8699607d55c99f07a7c507c49e93e5b6e822a23295a0fd8dfaff5ee81
# Verified Classes:   GenerationDecayVerdict, KnowledgeGeneration, KnowledgeLineage, DecayPolicy, DecayManifest, GenerationDecayGuard
# Defensive Guards:   0 fail-closed invariants (zero external deps)

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

class GenerationDecayVerdict(Enum):
    SINGLE_GENERATION = auto()
    LOW_DRIFT_INHERITANCE = auto()
    HIGH_DRIFT_REVALIDATION_REQUIRED = auto()
    CONTEXT_LOSS_DETECTED = auto()

@dataclass(frozen=True)
class KnowledgeGeneration:
    generation_id: str
    inherited_from_generation_id: str | None
    confidence: float
    content_hash: str
    drift_from_parent: float
    caveats_preserved: bool

@dataclass(frozen=True)
class KnowledgeLineage:
    knowledge_id: str
    generations: tuple[KnowledgeGeneration, ...]
    original_context_dependent: bool

@dataclass(frozen=True)
class DecayPolicy:
    max_acceptable_cumulative_drift: float

@dataclass(frozen=True)
class DecayManifest:
    verdict: GenerationDecayVerdict
    cumulative_drift: float
    directive: str
    manifest_digest: str

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

    @classmethod
    def evaluate_lineage(
        cls, lineage: KnowledgeLineage, policy: DecayPolicy
    ) -> DecayManifest:
        
        # Gate 1: Single generation
        if len(lineage.generations) <= 1:
            digest = cls._hash(json.dumps({
                "knowledge": lineage.knowledge_id,
                "generations": str(len(lineage.generations))
            }, sort_keys=True))
            return DecayManifest(
                verdict=GenerationDecayVerdict.SINGLE_GENERATION,
                cumulative_drift=0.0,
                directive=f"PROCEED: Knowledge {lineage.knowledge_id} single generation.",
                manifest_digest=digest
            )

        # Gate 2: Caveat preservation
        if lineage.original_context_dependent:
            caveat_lost = any(not g.caveats_preserved for g in lineage.generations[1:])
            if caveat_lost:
                return DecayManifest(
                    verdict=GenerationDecayVerdict.CONTEXT_LOSS_DETECTED,
                    cumulative_drift=sum(g.drift_from_parent for g in lineage.generations),
                    directive=f"HALT: Context caveats lost during inheritance for {lineage.knowledge_id}.",
                    manifest_digest=cls._hash(f"CAVEAT_LOST:{lineage.knowledge_id}")
                )

        # Gate 3: Cumulative drift
        cumulative_drift = sum(g.drift_from_parent for g in lineage.generations)
        if cumulative_drift > policy.max_acceptable_cumulative_drift:
            return DecayManifest(
                verdict=GenerationDecayVerdict.HIGH_DRIFT_REVALIDATION_REQUIRED,
                cumulative_drift=cumulative_drift,
                directive=f"HALT: Cumulative drift {cumulative_drift:.3f} > {policy.max_acceptable_cumulative_drift:.3f}. Revalidate.",
                manifest_digest=cls._hash(f"HIGH_DRIFT:{cumulative_drift:.3f}")
            )

        # Gate 4: Acceptable
        digest = cls._hash(json.dumps({
            "knowledge": lineage.knowledge_id,
            "generations": tuple(g.generation_id for g in lineage.generations),
            "drift": f"{cumulative_drift:.3f}"
        }, sort_keys=True))
        
        return DecayManifest(
            verdict=GenerationDecayVerdict.LOW_DRIFT_INHERITANCE,
            cumulative_drift=cumulative_drift,
            directive=f"PROCEED: {len(lineage.generations)}-generation inheritance, drift {cumulative_drift:.3f}.",
            manifest_digest=digest
        )