"""CRC helpers recovered from ``communication_center/utils/crc16.dart`` / ``crc32.dart``.

See ``docs/RE_DART_FINDINGS.md`` §6.

* CRC16 = **CRC-16/MODBUS** (init ``0xFFFF``, reflected poly ``0xA001`` — the
  disassembly shows the literal constant ``40961``).
* The app's ``Crc16::calculate`` returns the CRC with **hi/lo bytes swapped**,
  rendered as a **4 lowercase hex digits** string.
* CRC32 (zlib/IEEE) is used for the JSON envelope integrity check.
"""

from __future__ import annotations

import zlib

CRC16_INIT = 0xFFFF
CRC16_POLY = 0xA001  # reflected 0x8005 (CRC-16/MODBUS)


def crc16_modbus(data: bytes) -> int:
    """CRC-16/MODBUS over *data* (init 0xFFFF, reflected poly 0xA001)."""
    crc = CRC16_INIT
    for byte in data:
        crc ^= byte
        for _ in range(8):
            if crc & 1:
                crc = (crc >> 1) ^ CRC16_POLY
            else:
                crc >>= 1
    return crc & 0xFFFF


def crc16_hex_string(data: bytes) -> str:
    """The app's ``Crc16::calculate`` equivalent.

    CRC-16/MODBUS, hi/lo bytes swapped, 4 lowercase hex digits, e.g.
    ``crc16_modbus(b"123456789") == 0x4B37`` -> swapped ``0x374B`` -> ``"374b"``.
    """
    crc = crc16_modbus(data)
    swapped = ((crc & 0xFF) << 8) | (crc >> 8)
    return f"{swapped:04x}"


def crc32(data: bytes) -> int:
    """CRC-32 (IEEE/zlib) — envelope integrity check."""
    return zlib.crc32(data) & 0xFFFFFFFF


def int16_to_le_bytes(value: int) -> bytes:
    """``utils.int16ToBytes`` — little-endian 16-bit."""
    return (value & 0xFFFF).to_bytes(2, "little")


def le_bytes_to_int16(data: bytes) -> int:
    """``utils.bytesToInt16`` — little-endian 16-bit."""
    return int.from_bytes(data[:2], "little")