Binary Parsing - Null Terminated ASCII with Max bytes

[jgreene]

I am attempting to parse a field that is a potentially Null Terminated ASCII string with a max of 64 bytes. If there is no null terminator at the 64th byte then I would like to terminate the string automatically.

This is what I have for a Null Terminated ASCII string and it appears to work fine. I just need to find a way to stop parsing past 64 bytes if there is no null terminator. Ideally in an efficient manner.

static Parser<byte, IEnumerable<byte>> NullTerminatedStringBytes = SingleByte.Until(Token(b => b == 0x00).Labelled("Null Terminated"));

        
public static Parser<byte, string> NullTerminatedString = Map((nullString) =>
{
    var buffer = nullString.ToArray();
    var result = Encoding.ASCII.GetString(buffer);
    return result;
}, NullTerminatedStringBytes);

Thoughts?

[benjamin-hodgson]

I don't think there's a combinator which'll do this for-break type of loop out of the box. You can code it up yourself using recursion:

Parser<byte, IEnumerable<byte>> NullTerminated()
    => NullTerminated(64);

Parser<byte, IEnumerable<byte>> NullTerminated(int count)
{
    if (count == 0)
    {
        return Return(new[] { });
    }
    return Byte.Then(b =>
        b == 0
            ? Return(new[] { })
            : NullTerminated(count - 1).Select(bs => new[]{ b }.Concat(bs))
    );
}

ofc that won't be very efficient cos you're building parsers at runtime and storing the output bytes in a linked list shaped structure. (Could perhaps tweak this by eg putting the bytes in a List or caching the recursive NullTerminated calls but that's messy.)

---

One way to design an API for this might be some sort of "bounded Many", which runs a parser in a loop until it fails without consuming input or some maximum number of repetitions is reached. Then your code would look like this:

Parser<byte, IEnumerable<byte>> NullTerminated
    => Token(b => b != 0).ManyUpTo(64);  // name TBD

I'd be happy to accept a PR implementing this ManyUpTo combinator.