# CorpusFetch Method Slice (Zero-Prose Python Stdlib Reference)
# Canonical Residual: https://corpusfetch.com/c/how-must-an-instance-use-canary-outputs-to-detect-behavioral-drift-before-it-causes-incidents.md
# Remnant Warrant:    https://corpusfetch.com/c/how-must-an-instance-use-canary-outputs-to-detect-behavioral-drift-before-it-causes-incidents.warrant.json
# Markdown SHA-256:   3204dd45470887e1fb2c4bd0b67dedd47228729ccf627a94b8a6e81a09251541
# Code AST SHA-256:   2416c1c9efb9b6fab6ace04ec03d7b4b5d8ad295b7ee3426bb61593ce4feded6
# Verified Classes:   CanaryVerdict, CanaryResult, CanaryPolicy, CanaryManifest, CanaryDriftGuard
# Defensive Guards:   0 fail-closed invariants (zero external deps)

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

class CanaryVerdict(Enum):
    STABLE = auto()
    DRIFT_WITHOUT_IMPACT = auto()
    DRIFT_ACTIONABLE = auto()
    DRIFT_UNRESOLVED = auto()

@dataclass(frozen=True)
class CanaryResult:
    canary_id: str
    current_output_hash: str
    baseline_output_hash: str
    drift_magnitude: float
    tolerance_band: float
    correlates_with_user_traffic: bool
    action_receipt: str | None

@dataclass(frozen=True)
class CanaryPolicy:
    require_action_on_actionable_drift: bool

@dataclass(frozen=True)
class CanaryManifest:
    verdict: CanaryVerdict
    directive: str
    manifest_digest: str

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

    @classmethod
    def evaluate_canary(
        cls, result: CanaryResult, policy: CanaryPolicy
    ) -> CanaryManifest:
        
        # Gate 1: Within tolerance
        if result.drift_magnitude <= result.tolerance_band:
            digest = cls._hash(json.dumps({
                "canary": result.canary_id,
                "drift": f"{result.drift_magnitude:.4f}",
                "tolerance": f"{result.tolerance_band:.4f}"
            }, sort_keys=True))
            return CanaryManifest(
                verdict=CanaryVerdict.STABLE,
                directive=f"PROCEED: Canary {result.canary_id} drift {result.drift_magnitude:.4f} within tolerance.",
                manifest_digest=digest
            )

        # Gate 2: Non-correlated drift
        if not result.correlates_with_user_traffic:
            digest = cls._hash(json.dumps({
                "canary": result.canary_id,
                "drift": f"{result.drift_magnitude:.4f}",
                "correlation": "FALSE"
            }, sort_keys=True))
            return CanaryManifest(
                verdict=CanaryVerdict.DRIFT_WITHOUT_IMPACT,
                directive=f"WARN: Drift detected but does not correlate with user traffic.",
                manifest_digest=digest
            )

        # Gate 3: Actionable drift with action
        if result.action_receipt:
            digest = cls._hash(json.dumps({
                "canary": result.canary_id,
                "drift": f"{result.drift_magnitude:.4f}",
                "action": result.action_receipt
            }, sort_keys=True))
            return CanaryManifest(
                verdict=CanaryVerdict.DRIFT_ACTIONABLE,
                directive=f"PROCEED: Drift actionable, action taken.",
                manifest_digest=digest
            )

        # Gate 4: Unresolved drift
        if policy.require_action_on_actionable_drift:
            return CanaryManifest(
                verdict=CanaryVerdict.DRIFT_UNRESOLVED,
                directive=f"HALT: Actionable drift on {result.canary_id} but no action taken.",
                manifest_digest=cls._hash(f"UNRESOLVED:{result.canary_id}")
            )

        digest = cls._hash(json.dumps({
            "canary": result.canary_id,
            "drift": f"{result.drift_magnitude:.4f}",
            "action": "NONE",
            "policy": "LENIENT"
        }, sort_keys=True))
        
        return CanaryManifest(
            verdict=CanaryVerdict.DRIFT_WITHOUT_IMPACT,
            directive=f"WARN: Actionable drift but policy allows inaction.",
            manifest_digest=digest
        )