"""Exact, read-only firmware configuration decoder. Never serializes a write."""
import math
from pathlib import Path
import struct
import xml.etree.ElementTree as ET


def crc32c(data):
    value = 0xffffffff
    for byte in data:
        value ^= byte
        for _ in range(8):
            value = (value >> 1) ^ (0x82f63b78 if value & 1 else 0)
    return value ^ 0xffffffff


def decode(data, kind):
    code, name = {"motor": (14, "mcconf"), "application": (17, "appconf")}[kind]
    xml = ET.parse(Path(__file__).parent / "schemas/5.02" / ("parameters_" + name + ".xml")).getroot()
    params = {p.tag: p for p in xml.find("Params")}
    order = [p.text for p in xml.find("SerOrder")]
    signature = "".join(n + params[n].findtext("type", "0") + params[n].findtext("vTx", "0")
                        + "".join(x.text or "" for x in params[n].findall("enumNames")) for n in order)
    if len(data) < 5 or data[0] != code or int.from_bytes(data[1:5], "big") != crc32c(signature.encode()):
        raise ValueError("Configuration signature differs from firmware 5.02 schema")
    offset, result = 5, {}
    for name in order:
        p = params[name]; kind = int(p.findtext("type")); tx = int(p.findtext("vTx", "0"))
        if kind in (4, 5): fmt = "b"
        elif kind == 6: fmt = "B"
        elif kind == 2: fmt = {1: "B", 2: "b", 3: "H", 4: "h", 5: "I", 6: "i"}[tx]
        elif kind == 1: fmt = {7: "h", 8: "i", 9: "I"}[tx]
        else: raise ValueError("Unsupported configuration type")
        size = struct.calcsize(">" + fmt)
        if offset + size > len(data): raise ValueError("Truncated configuration")
        value = struct.unpack_from(">" + fmt, data, offset)[0]; offset += size
        if kind == 1:
            if tx == 9:
                exponent, fraction = (value >> 23) & 255, value & 0x7fffff
                part = fraction / 16777216.0 + 0.5 if exponent or fraction else 0.0
                value = math.ldexp(-part if value & 0x80000000 else part, exponent - 126)
            else: value /= float(p.findtext("vTxDoubleScale", "1"))
            if not math.isfinite(value): raise ValueError("Non-finite parameter")
        result[name] = value
    if offset != len(data): raise ValueError("Unexpected configuration tail")
    return result