namespace ZB.MOM.WW.OtOpcUa.Driver.TwinCAT;

/// <summary>
///     Serves a bit-indexed BOOL tag (<c>MAIN.Flags.3</c>, <c>.ADSSTATE_RUN.0</c>) from its PARENT word.
///     ADS has no symbol for <c>WORD.N</c> — live on a TwinCAT 2.11 target a notification registered on
///     <c>.ADSSTATE_RUN.0</c> failed with <c>DeviceSymbolNotFound</c> — so a native notification registers
///     the parent as raw bytes at its resolved width and reads the bit here, exactly as a read already
///     reads the parent word and extracts the bit.
/// </summary>
internal static class TwinCATBitAccess
{
    /// <summary>The parent word's path: the bit path without its trailing <c>.N</c>.</summary>
    /// <param name="symbolPath">The bit path (<c>MAIN.Flags.3</c>).</param>
    /// <returns>The parent path, or <paramref name="symbolPath"/> when it has no numeric last segment.</returns>
    public static string ParentPath(string symbolPath)
    {
        var lastDot = symbolPath.LastIndexOf('.');
        if (lastDot < 0) return symbolPath;
        return int.TryParse(symbolPath.AsSpan(lastDot + 1), out _) ? symbolPath[..lastDot] : symbolPath;
    }

    /// <summary>Checks that <paramref name="bit"/> addresses a bit of an integer parent word.</summary>
    /// <param name="bit">The bit index.</param>
    /// <param name="parent">The parent symbol's shape.</param>
    /// <returns><c>null</c> when addressable; otherwise the reason.</returns>
    public static string? CheckParent(int bit, TwinCATPlcSymbolShape parent)
    {
        if (parent.Kind != TwinCATPlcKind.Scalar || parent.IsFloat)
            return $"bit {bit} of {parent.TypeName} ({parent.ByteSize} B) — only an integer word has addressable bits";
        return bit >= 0 && bit < parent.ByteSize * 8
            ? null
            : $"bit {bit} is outside the {parent.ByteSize * 8}-bit parent {parent.TypeName}";
    }

    /// <summary>Reads bit <paramref name="bit"/> of a little-endian parent word.</summary>
    /// <param name="parent">The parent word's raw bytes.</param>
    /// <param name="bit">The bit index (0 = least significant bit of the first byte).</param>
    /// <returns>The bit's value.</returns>
    public static bool ReadBit(ReadOnlySpan<byte> parent, int bit) =>
        (parent[bit / 8] & (1 << (bit % 8))) != 0;
}