# CorpusFetch Method Slice (Zero-Prose Python Stdlib Reference)
# Canonical Residual: https://corpusfetch.com/c/what-invariants-prevent-infinite-verification-regress-in-agent-networks.md
# Remnant Warrant:    https://corpusfetch.com/c/what-invariants-prevent-infinite-verification-regress-in-agent-networks.warrant.json
# Markdown SHA-256:   62f3c486d03d71260d74b3f5f44fa5974f48beab6e71c3b4cf6ef83606bc5876
# Code AST SHA-256:   5362ddc2ebd5a46c1c58547db98d8971676c45eb13fc02cffdc443353f86ad99
# Verified Classes:   RegressVerdict, VerificationHop, VerificationChain, RegressPolicy, RegressManifest, VerificationRegressGuard
# Defensive Guards:   0 fail-closed invariants (zero external deps)

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

class RegressVerdict(Enum):
    SINGLE_HOP = auto()
    GROUNDED_CHAIN = auto()
    QUORUM_GROUNDED = auto()
    UNGROUNDED_REGRESS = auto()

@dataclass(frozen=True)
class VerificationHop:
    hop_id: str
    verifier_id: str
    claims_verified: bool
    grounds_in_evidence: bool

@dataclass(frozen=True)
class VerificationChain:
    chain_id: str
    hops: tuple[VerificationHop, ...]
    foundational_anchor: str | None
    quorum_receipt: str | None
    trust_root: str | None

@dataclass(frozen=True)
class RegressPolicy:
    max_acceptable_depth: int
    trusted_roots: frozenset[str]

@dataclass(frozen=True)
class RegressManifest:
    verdict: RegressVerdict
    effective_depth: int
    directive: str
    manifest_digest: str

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

    @classmethod
    def evaluate_chain(
        cls, chain: VerificationChain, policy: RegressPolicy
    ) -> RegressManifest:
        
        # Gate 1: Single hop
        if len(chain.hops) <= 1:
            digest = cls._hash(json.dumps({
                "chain": chain.chain_id,
                "hops": str(len(chain.hops))
            }, sort_keys=True))
            return RegressManifest(
                verdict=RegressVerdict.SINGLE_HOP,
                effective_depth=len(chain.hops),
                directive=f"PROCEED: Single hop, no regress for {chain.chain_id}.",
                manifest_digest=digest
            )

        # Gate 2: Foundational anchor
        if chain.foundational_anchor:
            digest = cls._hash(json.dumps({
                "chain": chain.chain_id,
                "anchor": chain.foundational_anchor,
                "depth": str(len(chain.hops))
            }, sort_keys=True))
            return RegressManifest(
                verdict=RegressVerdict.GROUNDED_CHAIN,
                effective_depth=len(chain.hops),
                directive=f"PROCEED: Chain grounded at {chain.foundational_anchor}, depth {len(chain.hops)}.",
                manifest_digest=digest
            )

        # Gate 3: Trust root
        if chain.trust_root and chain.trust_root in policy.trusted_roots:
            digest = cls._hash(json.dumps({
                "chain": chain.chain_id,
                "trust_root": chain.trust_root,
                "depth": str(len(chain.hops))
            }, sort_keys=True))
            return RegressManifest(
                verdict=RegressVerdict.GROUNDED_CHAIN,
                effective_depth=len(chain.hops),
                directive=f"PROCEED: Chain grounded at trusted root {chain.trust_root}.",
                manifest_digest=digest
            )

        # Gate 4: Quorum
        if chain.quorum_receipt:
            digest = cls._hash(json.dumps({
                "chain": chain.chain_id,
                "quorum": chain.quorum_receipt,
                "depth": str(len(chain.hops))
            }, sort_keys=True))
            return RegressManifest(
                verdict=RegressVerdict.QUORUM_GROUNDED,
                effective_depth=len(chain.hops),
                directive=f"PROCEED: Chain terminated at independent quorum.",
                manifest_digest=digest
            )

        # Gate 5: Depth check
        if len(chain.hops) > policy.max_acceptable_depth:
            return RegressManifest(
                verdict=RegressVerdict.UNGROUNDED_REGRESS,
                effective_depth=len(chain.hops),
                directive=f"HALT: Depth {len(chain.hops)} > {policy.max_acceptable_depth} with no grounding.",
                manifest_digest=cls._hash(f"UNGROUNDED:{chain.chain_id}")
            )

        # Gate 6: No grounding yet
        return RegressManifest(
            verdict=RegressVerdict.UNGROUNDED_REGRESS,
            effective_depth=len(chain.hops),
            directive=f"HALT: Chain {chain.chain_id} has no foundational grounding at depth {len(chain.hops)}.",
            manifest_digest=cls._hash(f"NO_GROUND:{chain.chain_id}")
        )