# CorpusFetch Method Slice (Zero-Prose Python Stdlib Reference)
# Canonical Residual: https://corpusfetch.com/c/how-must-an-instance-handle-partial-failure-during-cross-region-replication.md
# Remnant Warrant:    https://corpusfetch.com/c/how-must-an-instance-handle-partial-failure-during-cross-region-replication.warrant.json
# Markdown SHA-256:   1fde90213bbd2ff414273340549212a89a6dfd07b187b93a1864f0998438ea34
# Code AST SHA-256:   12a46b16d1e62a1ca4a9a4e548bce86d67458635edfab7b45e7f05ff45af6436
# Verified Classes:   ReplicationVerdict, TargetAck, ReplicationState, ReplicationPolicy, ReplicationManifest, CrossRegionReplicationGuard
# Defensive Guards:   0 fail-closed invariants (zero external deps)

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

class ReplicationVerdict(Enum):
    FULL_CONVERGENCE = auto()
    PARTIAL_FAILURE_ISOLATED = auto()
    PARTIAL_FAILURE_UNISOLATED = auto()

@dataclass(frozen=True)
class TargetAck:
    region_id: str
    acknowledged: bool
    ack_epoch: float | None

@dataclass(frozen=True)
class ReplicationState:
    source_commit_epoch: float
    targets: tuple[TargetAck, ...]
    reads_routed_away_from_failed: bool

@dataclass(frozen=True)
class ReplicationPolicy:
    require_read_isolation: bool

@dataclass(frozen=True)
class ReplicationManifest:
    verdict: ReplicationVerdict
    failed_regions: tuple[str, ...]
    directive: str
    manifest_digest: str

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

    @classmethod
    def evaluate_replication(
        cls, state: ReplicationState, policy: ReplicationPolicy
    ) -> ReplicationManifest:
        
        failed = tuple(
            t.region_id for t in state.targets if not t.acknowledged
        )

        # Gate 1: Full convergence
        if not failed:
            digest = cls._hash(json.dumps({
                "source_epoch": f"{state.source_commit_epoch:.0f}",
                "targets": tuple(sorted(t.region_id for t in state.targets))
            }, sort_keys=True))
            return ReplicationManifest(
                verdict=ReplicationVerdict.FULL_CONVERGENCE,
                failed_regions=tuple(),
                directive="PROCEED: All regions converged.",
                manifest_digest=digest
            )

        # Gate 2: Partial failure with isolation
        if state.reads_routed_away_from_failed:
            digest = cls._hash(json.dumps({
                "source_epoch": f"{state.source_commit_epoch:.0f}",
                "failed": tuple(sorted(failed)),
                "isolation": "ACTIVE"
            }, sort_keys=True))
            return ReplicationManifest(
                verdict=ReplicationVerdict.PARTIAL_FAILURE_ISOLATED,
                failed_regions=failed,
                directive=f"PROCEED_WITH_CAUTION: Failed regions {sorted(failed)} excluded from reads. Replication pending.",
                manifest_digest=digest
            )

        # Gate 3: Partial failure without isolation
        if policy.require_read_isolation:
            return ReplicationManifest(
                verdict=ReplicationVerdict.PARTIAL_FAILURE_UNISOLATED,
                failed_regions=failed,
                directive=f"HALT: Failed regions {sorted(failed)} not isolated from reads. Risk of stale reads.",
                manifest_digest=cls._hash(f"UNISOLATED:{len(failed)}")
            )

        # Gate 4: Accepted without isolation (policy allows)
        digest = cls._hash(json.dumps({
            "source_epoch": f"{state.source_commit_epoch:.0f}",
            "failed": tuple(sorted(failed)),
            "isolation": "NONE",
            "policy": "LENIENT"
        }, sort_keys=True))
        
        return ReplicationManifest(
            verdict=ReplicationVerdict.PARTIAL_FAILURE_ISOLATED,
            failed_regions=failed,
            directive=f"PROCEED_WITH_RISK: Policy allows reads from failed regions {sorted(failed)}.",
            manifest_digest=digest
        )