# CorpusFetch Method Slice (Zero-Prose Python Stdlib Reference)
# Canonical Residual: https://corpusfetch.com/c/how-must-an-instance-negotiate-backpressure-when-receiver-capabilities-are-unknown.md
# Remnant Warrant:    https://corpusfetch.com/c/how-must-an-instance-negotiate-backpressure-when-receiver-capabilities-are-unknown.warrant.json
# Markdown SHA-256:   15946c86f56bef95091cd7fb4ab6f1543cfa40a85de2b1391ad6f5f1b5c9b4fb
# Code AST SHA-256:   a123a2d02510087cb9ee6683d320d24a05eed185989c08856a8ee09315f0a49e
# Verified Classes:   BackpressureVerdict, ReceiverCapability, SenderIntent, BackpressurePolicy, BackpressureManifest, UnknownCapacityBackpressureGuard
# Defensive Guards:   0 fail-closed invariants (zero external deps)

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

class BackpressureVerdict(Enum):
    CAPABILITIES_KNOWN = auto()
    NEGOTIATION_ACTIVE = auto()
    DYNAMIC_BACKPRESSURE = auto()
    FLOOD_RISK = auto()

@dataclass(frozen=True)
class ReceiverCapability:
    receiver_id: str
    capability_known: bool
    buffer_size_bytes: int | None
    max_rate_per_sec: float | None
    negotiation_protocol: str | None
    capability_probe_available: bool

@dataclass(frozen=True)
class SenderIntent:
    message_rate_per_sec: float
    message_size_bytes: int

@dataclass(frozen=True)
class BackpressurePolicy:
    min_safe_rate_assumption: float

@dataclass(frozen=True)
class BackpressureManifest:
    verdict: BackpressureVerdict
    effective_rate: float
    directive: str
    manifest_digest: str

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

    @classmethod
    def evaluate_negotiation(
        cls, cap: ReceiverCapability, intent: SenderIntent, policy: BackpressurePolicy
    ) -> BackpressureManifest:
        
        # Gate 1: Known capabilities
        if cap.capability_known and cap.buffer_size_bytes is not None and cap.max_rate_per_sec is not None:
            effective_rate = min(intent.message_rate_per_sec, cap.max_rate_per_sec)
            digest = cls._hash(json.dumps({
                "receiver": cap.receiver_id,
                "rate": f"{effective_rate:.2f}",
                "buffer": str(cap.buffer_size_bytes)
            }, sort_keys=True))
            return BackpressureManifest(
                verdict=BackpressureVerdict.CAPABILITIES_KNOWN,
                effective_rate=effective_rate,
                directive=f"PROCEED: Known capabilities, rate {effective_rate:.2f}/sec.",
                manifest_digest=digest
            )

        # Gate 2: Negotiation protocol
        if cap.negotiation_protocol:
            digest = cls._hash(json.dumps({
                "receiver": cap.receiver_id,
                "protocol": cap.negotiation_protocol
            }, sort_keys=True))
            return BackpressureManifest(
                verdict=BackpressureVerdict.NEGOTIATION_ACTIVE,
                effective_rate=policy.min_safe_rate_assumption,
                directive=f"ACTION: Negotiate via {cap.negotiation_protocol} before sending.",
                manifest_digest=digest
            )

        # Gate 3: Dynamic backpressure via probe
        if cap.capability_probe_available:
            digest = cls._hash(json.dumps({
                "receiver": cap.receiver_id,
                "probe": "AVAILABLE",
                "initial_rate": f"{policy.min_safe_rate_assumption:.2f}"
            }, sort_keys=True))
            return BackpressureManifest(
                verdict=BackpressureVerdict.DYNAMIC_BACKPRESSURE,
                effective_rate=policy.min_safe_rate_assumption,
                directive=f"PROCEED_WITH_PROBE: Start at {policy.min_safe_rate_assumption:.2f}/sec, ramp based on acks.",
                manifest_digest=digest
            )

        # Gate 4: Flood risk
        return BackpressureManifest(
            verdict=BackpressureVerdict.FLOOD_RISK,
            effective_rate=0.0,
            directive=f"HALT: No capability knowledge, no negotiation, no probe. Flood risk for {cap.receiver_id}.",
            manifest_digest=cls._hash(f"FLOOD_RISK:{cap.receiver_id}")
        )