# CorpusFetch Method Slice (Zero-Prose Python Stdlib Reference)
# Canonical Residual: https://corpusfetch.com/c/what-invariants-preserve-contract-integrity-when-the-delegating-principal-changes.md
# Remnant Warrant:    https://corpusfetch.com/c/what-invariants-preserve-contract-integrity-when-the-delegating-principal-changes.warrant.json
# Markdown SHA-256:   5ff3f38d4f6c116a44a1f4fb76313fd70d0692d332e5fa5e6fb11c6236d41a80
# Code AST SHA-256:   e9963f1e950dd8846c15a2b1065db699f83666f1adac98671e29deddfa41f085
# Verified Classes:   ContractContinuityVerdict, ContractTransition, ContinuityPolicy, ContinuityManifest, PrincipalTransitionGuard
# Defensive Guards:   0 fail-closed invariants (zero external deps)

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

class ContractContinuityVerdict(Enum):
    NO_TRANSITION = auto()
    CONTINUED_ACCEPTED = auto()
    HALTED_REJECTED = auto()
    CONTINUED_PARTIAL = auto()

@dataclass(frozen=True)
class ContractTransition:
    contract_id: str
    original_principal: str
    new_principal: str
    new_principal_acceptance: bool
    partial_acceptance_receipt: str | None
    transition_instrument_exists: bool

@dataclass(frozen=True)
class ContinuityPolicy:
    require_explicit_acceptance: bool

@dataclass(frozen=True)
class ContinuityManifest:
    verdict: ContractContinuityVerdict
    directive: str
    manifest_digest: str

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

    @classmethod
    def evaluate_transition(
        cls, transition: ContractTransition, policy: ContinuityPolicy
    ) -> ContinuityManifest:
        
        # Gate 1: No transition
        if transition.original_principal == transition.new_principal:
            digest = cls._hash(json.dumps({
                "contract": transition.contract_id,
                "transition": False
            }, sort_keys=True))
            return ContinuityManifest(
                verdict=ContractContinuityVerdict.NO_TRANSITION,
                directive=f"PROCEED: Contract {transition.contract_id} no principal change.",
                manifest_digest=digest
            )

        # Gate 2: Full acceptance
        if transition.new_principal_acceptance:
            digest = cls._hash(json.dumps({
                "contract": transition.contract_id,
                "accepted": True
            }, sort_keys=True))
            return ContinuityManifest(
                verdict=ContractContinuityVerdict.CONTINUED_ACCEPTED,
                directive=f"PROCEED: Contract {transition.contract_id} accepted by new principal.",
                manifest_digest=digest
            )

        # Gate 3: Partial acceptance
        if transition.partial_acceptance_receipt:
            digest = cls._hash(json.dumps({
                "contract": transition.contract_id,
                "partial": transition.partial_acceptance_receipt
            }, sort_keys=True))
            return ContinuityManifest(
                verdict=ContractContinuityVerdict.CONTINUED_PARTIAL,
                directive=f"PROCEED_WITH_SCOPE: Contract {transition.contract_id} partially accepted.",
                manifest_digest=digest
            )

        # Gate 4: Transition instrument
        if transition.transition_instrument_exists and not policy.require_explicit_acceptance:
            digest = cls._hash(json.dumps({
                "contract": transition.contract_id,
                "instrument": True
            }, sort_keys=True))
            return ContinuityManifest(
                verdict=ContractContinuityVerdict.CONTINUED_ACCEPTED,
                directive=f"PROCEED: Contract {transition.contract_id} inherited via transition instrument.",
                manifest_digest=digest
            )

        # Gate 5: Rejected
        return ContinuityManifest(
            verdict=ContractContinuityVerdict.HALTED_REJECTED,
            directive=f"HALT: Contract {transition.contract_id} not accepted by new principal.",
            manifest_digest=cls._hash(f"REJECTED:{transition.contract_id}")
        )