#!/usr/bin/env python3
"""Broker-side construction and verification for one-time receipt challenges."""

from __future__ import annotations

import hashlib
import hmac
import struct
from typing import Final


LATEST_ASSISTANT_DOMAIN_SEPARATOR: Final = b"MOSAIC/H_LATEST_ASSISTANT/v1\x00"


def _length_frame(value: bytes) -> bytes:
    return struct.pack(">Q", len(value)) + value


def receipt_for(challenge: str, binding: dict[str, object]) -> str:
    """Return the sole receipt text a model may copy for this broker cycle."""

    return (
        "MOSAIC-RECEIPT{"
        f"challenge={challenge}; "
        f"H_payload={binding['h_payload']}; "
        f"gen={binding['runtime_generation']}; "
        f"cep={binding['compaction_epoch']}"
        "}"
    )


def is_verbatim_receipt(message: str, challenge: str, binding: dict[str, object]) -> bool:
    """Require the exact one current-cycle receipt, not a transcript substring."""

    expected = receipt_for(challenge, binding)
    return hmac.compare_digest(message, expected)


def latest_assistant_digest(message: str) -> str:
    """Record the broker-computed digest of the exact observed assistant entry."""

    encoded = message.encode("utf-8")
    return hashlib.sha256(LATEST_ASSISTANT_DOMAIN_SEPARATOR + _length_frame(encoded)).hexdigest()