# CorpusFetch Method Slice (Zero-Prose Python Stdlib Reference)
# Canonical Residual: https://corpusfetch.com/c/how-must-an-instance-prevent-its-own-prior-reasoning-from-becoming-contaminating-evidence.md
# Remnant Warrant:    https://corpusfetch.com/c/how-must-an-instance-prevent-its-own-prior-reasoning-from-becoming-contaminating-evidence.warrant.json
# Markdown SHA-256:   c60b564f1abec5300175dee55a5301fe8145d6a3b90adb8714d35f8a5f6b6d5c
# Code AST SHA-256:   505a48fb1409fc3b5fe0be81f9fc9104fbc84bc3be2b2f3a0a9649dac69caa9a
# Verified Classes:   EvidenceClass, SelfReferenceVerdict, TraceSegment, TraceReview, SelfReferencePolicy, SelfReferenceManifest, SelfReasoningContaminationGuard
# Defensive Guards:   0 fail-closed invariants (zero external deps)

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

class EvidenceClass(Enum):
    OBSERVATION = auto()
    INFERENCE = auto()
    HYPOTHESIS = auto()

class SelfReferenceVerdict(Enum):
    NO_SELF_REFERENCE = auto()
    LEGITIMATE_OBSERVATION_REUSE = auto()
    CIRCULAR_CONFIRMATION_DETECTED = auto()
    INFERENCE_AS_EVIDENCE_REJECTED = auto()

@dataclass(frozen=True)
class TraceSegment:
    segment_id: str
    evidence_class: EvidenceClass
    derived_from_conclusion: bool
    has_external_verification: bool

@dataclass(frozen=True)
class TraceReview:
    review_id: str
    current_conclusion_id: str
    segments_used_as_evidence: tuple[TraceSegment, ...]

@dataclass(frozen=True)
class SelfReferencePolicy:
    require_observation_class_for_evidence: bool

@dataclass(frozen=True)
class SelfReferenceManifest:
    verdict: SelfReferenceVerdict
    directive: str
    manifest_digest: str

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

    @classmethod
    def evaluate_review(
        cls, review: TraceReview, policy: SelfReferencePolicy
    ) -> SelfReferenceManifest:
        
        if not review.segments_used_as_evidence:
            digest = cls._hash(json.dumps({
                "review": review.review_id,
                "segments": "NONE"
            }, sort_keys=True))
            return SelfReferenceManifest(
                verdict=SelfReferenceVerdict.NO_SELF_REFERENCE,
                directive=f"PROCEED: No prior trace used as evidence.",
                manifest_digest=digest
            )

        # Gate 1: Observation class requirement
        if policy.require_observation_class_for_evidence:
            non_observation = [
                s.segment_id for s in review.segments_used_as_evidence
                if s.evidence_class != EvidenceClass.OBSERVATION
            ]
            if non_observation:
                return SelfReferenceManifest(
                    verdict=SelfReferenceVerdict.INFERENCE_AS_EVIDENCE_REJECTED,
                    directive=f"HALT: {len(non_observation)} segments are inference or hypothesis, not observation.",
                    manifest_digest=cls._hash(f"INFERENCE_AS_EVIDENCE:{len(non_observation)}")
                )

        # Gate 2: Circular confirmation check
        circular = [
            s.segment_id for s in review.segments_used_as_evidence
            if s.derived_from_conclusion
        ]
        if circular:
            return SelfReferenceManifest(
                verdict=SelfReferenceVerdict.CIRCULAR_CONFIRMATION_DETECTED,
                directive=f"HALT: {len(circular)} segments derived from current conclusion, circular.",
                manifest_digest=cls._hash(f"CIRCULAR:{len(circular)}")
            )

        # Gate 3: External verification
        unverified_observations = [
            s.segment_id for s in review.segments_used_as_evidence
            if not s.has_external_verification
        ]
        if unverified_observations and policy.require_observation_class_for_evidence:
            return SelfReferenceManifest(
                verdict=SelfReferenceVerdict.INFERENCE_AS_EVIDENCE_REJECTED,
                directive=f"HALT: {len(unverified_observations)} observations lack external verification.",
                manifest_digest=cls._hash(f"UNVERIFIED_OBS:{len(unverified_observations)}")
            )

        # Gate 4: Legitimate
        digest = cls._hash(json.dumps({
            "review": review.review_id,
            "segments": tuple(s.segment_id for s in review.segments_used_as_evidence),
            "conclusion": review.current_conclusion_id
        }, sort_keys=True))
        
        return SelfReferenceManifest(
            verdict=SelfReferenceVerdict.LEGITIMATE_OBSERVATION_REUSE,
            directive=f"PROCEED: {len(review.segments_used_as_evidence)} observations independently support conclusion.",
            manifest_digest=digest
        )