CRC16Verifier.cs

using System;
using System.Globalization;
using System.Text;

namespace DSMRParser.CRCHandling;

/// <summary>
/// Implements a CRC-16-IBM CRC.
/// </summary>
/// <remarks>See https://en.wikipedia.org/wiki/Cyclic_redundancy_check</remarks>
public class CRC16Verifier : ICRCVerifier
{
    /// <summary>
    /// Internal lookup table
    /// </summary>
    private readonly ushort[] _lookup = new ushort[256];

    /// <summary>
    /// Initializes a new instance of a <see cref="CRC16Verifier" /> with the given polynomial (defaults to a reversed polynomial 0XA001).
    /// </summary>
    /// <param name="polynomial">The polynomial this </param>
    public CRC16Verifier(ushort polynomial = 0xA001)
    {
        ushort value;
        ushort temp;
        // Prepare lookup table
        for (ushort i = 0; i < _lookup.Length; ++i)
        {
            value = 0;
            temp = i;
            for (byte j = 0; j < 8; ++j)
            {
                value = (((value ^ temp) & 0x0001) != 0) ? (ushort)((value >> 1) ^ polynomial) : (ushort)(value >> 1);
                temp >>= 1;
            }
            _lookup[i] = value;
        }
    }

    /// <summary>
    /// Extracts the CRC-16-IBM of the given Telegram data and verifies it against the calculated CRC-16-IBM.
    /// </summary>
    /// <param name="rawTelegram">The Telegram in raw byte form.</param>
    /// <param name="exception">
    /// When this method returns, contains the <see cref="CRCException"/> containing information on why the CRC failed,
    /// if the CRC failed, or null if the CRC is correct. This parameter is passed uninitialized; any value originally
    /// supplied in result will be overwritten.
    /// </param>
    /// <returns>true if value was converted successfully, false otherwise.</returns>
    public bool TryVerify(Span<byte> rawTelegram, out CRCException? exception)
    {
        // Find where the CRC starts
        var crcstart = rawTelegram.LastIndexOf((byte)'!') + 1;
        // Calculate the CRC
        var calculatedcrc = CalculateCRC(rawTelegram[0..crcstart]);

        exception = null;
        if (crcstart != rawTelegram.Length - 6)
        {
            exception = new CRCException("CRC must be 4 bytes");
            return false;
        }

        // Get the CRC claimed by the telegram
        var crcstring = Encoding.ASCII.GetString(rawTelegram[crcstart..(crcstart + 4)]);
        // Compare CRCs
        if (int.TryParse(crcstring, NumberStyles.HexNumber, CultureInfo.InvariantCulture, out var claimedcrc) && claimedcrc == calculatedcrc)
        {
            return true;
        }

        exception = new CRCException(calculatedcrc, claimedcrc);
        return false;
    }

    /// <summary>
    /// Calculates the CRC-16-IBM of the given Telegram data.
    /// </summary>
    /// <param name="rawTelegram">The Telegram in raw byte form.</param>
    /// <returns>Returns the CRC-16-IBM of the given Telegram.</returns>
    public ushort CalculateCRC(Span<byte> rawTelegram)
    {
        ushort crc = 0;
        for (var i = 0; i < rawTelegram.Length; ++i)
        {
            crc = (ushort)((crc >> 8) ^ _lookup[(byte)(crc ^ rawTelegram[i])]);
        }

        return crc;
    }
}