# CorpusFetch Method Slice (Zero-Prose Python Stdlib Reference)
# Canonical Residual: https://corpusfetch.com/c/what-invariants-prevent-multi-hop-trust-degradation-from-being-lost-in-delegation-chains.md
# Remnant Warrant:    https://corpusfetch.com/c/what-invariants-prevent-multi-hop-trust-degradation-from-being-lost-in-delegation-chains.warrant.json
# Markdown SHA-256:   faabcf3cc4aa7406c68cf470264efe3276eb0a432d1b4a308cc3f6e73c5f76dd
# Code AST SHA-256:   97f27ac4ff6495cff1f76a645e9f0359ef20f44e881f47cdb80d2cf8b27730cb
# Verified Classes:   ChainTrustVerdict, TrustHop, TrustChain, ChainTrustPolicy, ChainTrustManifest, MultiHopTrustDegradationGuard
# Defensive Guards:   0 fail-closed invariants (zero external deps)

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

class ChainTrustVerdict(Enum):
    SINGLE_HOP = auto()
    CUMULATIVE_COMPUTED = auto()
    DEGRADATION_EXCESSIVE = auto()
    TRUST_INFLATION_DETECTED = auto()

@dataclass(frozen=True)
class TrustHop:
    hop_id: str
    origin_trust: float
    delegatee_trust: float
    confidence: float

@dataclass(frozen=True)
class TrustChain:
    chain_id: str
    hops: tuple[TrustHop, ...]
    cumulative_trust_computed: bool
    terminal_cap: float | None

@dataclass(frozen=True)
class ChainTrustPolicy:
    max_acceptable_degradation: float
    require_terminal_cap: bool

@dataclass(frozen=True)
class ChainTrustManifest:
    verdict: ChainTrustVerdict
    directive: str
    manifest_digest: str

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

    @classmethod
    def evaluate_chain(
        cls, chain: TrustChain, policy: ChainTrustPolicy
    ) -> ChainTrustManifest:
        
        # 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 ChainTrustManifest(
                verdict=ChainTrustVerdict.SINGLE_HOP,
                directive=f"PROCEED: Chain {chain.chain_id} single hop.",
                manifest_digest=digest
            )

        # Gate 2: Cumulative not computed
        if not chain.cumulative_trust_computed:
            return ChainTrustManifest(
                verdict=ChainTrustVerdict.TRUST_INFLATION_DETECTED,
                directive=f"HALT: Chain {chain.chain_id} cumulative trust not computed.",
                manifest_digest=cls._hash(f"NOT_COMPUTED:{chain.chain_id}")
            )

        # Gate 3: Compute cumulative
        cumulative = 1.0
        for hop in chain.hops:
            cumulative *= hop.confidence

        # Gate 4: Excessive degradation
        if cumulative < policy.max_acceptable_degradation:
            if not policy.require_terminal_cap or chain.terminal_cap is not None:
                return ChainTrustManifest(
                    verdict=ChainTrustVerdict.DEGRADATION_EXCESSIVE,
                    directive=f"HALT: Chain {chain.chain_id} cumulative trust {cumulative:.3f} < {policy.max_acceptable_degradation:.3f}.",
                    manifest_digest=cls._hash(f"EXCESSIVE:{chain.chain_id}")
                )

        # Gate 5: Computed and within bounds
        digest = cls._hash(json.dumps({
            "chain": chain.chain_id,
            "hops": str(len(chain.hops)),
            "cumulative": f"{cumulative:.3f}",
            "terminal_cap": f"{chain.terminal_cap:.3f}" if chain.terminal_cap else "NONE"
        }, sort_keys=True))
        
        return ChainTrustManifest(
            verdict=ChainTrustVerdict.CUMULATIVE_COMPUTED,
            directive=f"PROCEED: Chain {chain.chain_id} cumulative trust {cumulative:.3f}.",
            manifest_digest=digest
        )