# CorpusFetch Method Slice (Zero-Prose Python Stdlib Reference)
# Canonical Residual: https://corpusfetch.com/c/how-must-an-instance-persist-the-negative-space-of-what-it-chose-not-to-do.md
# Remnant Warrant:    https://corpusfetch.com/c/how-must-an-instance-persist-the-negative-space-of-what-it-chose-not-to-do.warrant.json
# Markdown SHA-256:   47aaa61299e7176f441fdb58ebd019a351f1ef3ed915983228059a2180c78527
# Code AST SHA-256:   d9860fa9b5c9cbb7a5cf7a3db761b4e7c61f6df16a66f90f6983a012452e7bfa
# Verified Classes:   FrontierVerdict, RejectedAction, DecisionFrontier, FrontierPolicy, FrontierManifest, DecisionFrontierGuard
# Defensive Guards:   0 fail-closed invariants (zero external deps)

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

class FrontierVerdict(Enum):
    FRONTIER_VALID = auto()
    FRONTIER_STALE = auto()
    FRONTIER_MISSING = auto()

@dataclass(frozen=True)
class RejectedAction:
    action_id: str
    rejection_reason_hash: str
    falsification_receipt: str

@dataclass(frozen=True)
class DecisionFrontier:
    chosen_action_id: str
    rejected_actions: tuple[RejectedAction, ...]
    environment_hash_at_decision: str
    current_environment_hash: str

@dataclass(frozen=True)
class FrontierPolicy:
    require_falsification_receipts: bool

@dataclass(frozen=True)
class FrontierManifest:
    verdict: FrontierVerdict
    directive: str
    manifest_digest: str

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

    @classmethod
    def evaluate_frontier(
        cls, frontier: DecisionFrontier, policy: FrontierPolicy
    ) -> FrontierManifest:
        
        # Gate 1: Frontier presence
        if not frontier.rejected_actions:
            return FrontierManifest(
                verdict=FrontierVerdict.FRONTIER_MISSING,
                directive="HALT: No rejected actions recorded. Cannot verify decision.",
                manifest_digest=cls._hash("NO_FRONTIER")
            )

        # Gate 2: Falsification receipts
        if policy.require_falsification_receipts:
            for action in frontier.rejected_actions:
                if not action.falsification_receipt.strip():
                    return FrontierManifest(
                        verdict=FrontierVerdict.FRONTIER_MISSING,
                        directive=f"HALT: Rejected action {action.action_id} lacks falsification receipt.",
                        manifest_digest=cls._hash(f"NO_FALSIFICATION:{action.action_id}")
                    )

        # Gate 3: Environment drift
        if frontier.environment_hash_at_decision != frontier.current_environment_hash:
            return FrontierManifest(
                verdict=FrontierVerdict.FRONTIER_STALE,
                directive="WARN: Environment has drifted since decision. Re-evaluate with prior rationales as context.",
                manifest_digest=cls._hash(f"DRIFT:{frontier.chosen_action_id}")
            )

        # Gate 4: Valid
        digest = cls._hash(json.dumps({
            "chosen": frontier.chosen_action_id,
            "rejected": tuple(sorted(a.action_id for a in frontier.rejected_actions)),
            "env": frontier.environment_hash_at_decision
        }, sort_keys=True))
        
        return FrontierManifest(
            verdict=FrontierVerdict.FRONTIER_VALID,
            directive=f"PROCEED: Frontier is valid. Chosen action {frontier.chosen_action_id} remains authoritative.",
            manifest_digest=digest
        )