Thread: Help with with Lisp Reader Technique

Hi I am developing a game that uses Lisp based text files to store data. Although I can't seem to figure out how to right the code. Here is an example text file that I would like to load. The number of level sets will increase as users make their own with the built in level editor, so I need a way of dynamicly loading all of them.

Code:

;; TuxBlocks Level Sets
(numofsets 3
  (levelset 1
    (name "Classic")
    (location "classic/classic.tbls")
    (levels 11))
  (levelset 2
    (name "Confusion")
    (location "confusion/confusion.tbls")
    (levels 0))
  (levelset 3
    (name "Insanity")
    (location "insanity/insanity.tbls")
    (levels 1)))

;; EOF ;;

Could somebody please help understand how to use the lisp reader technique?

So your question is, "How do I write a LISP parser"?

Yes.

Well, that's pretty complicated, especially for someone who can't read binary files.

I don't know anything about it. There might be a library like libxml2 that lets you parse LISP files.

Just out of curiosity, why are you using LISP?

Well I'm not using LISP itself. It's more of a LISP based syntax. My reasoning is because it provides an easy to write data storage file. Have a look at the supertux source code. It uses the same syntax that I am using for every non-C textfile that it loads, and it seems to me to be one of the best ways of storing huge amounts of data.

You could use XML for that kind of thing.

If you really want your own parser . . . I guess I'd use a recursive function that would look for '('s, ')'s, tokens and '='s.

I haven't tried this yet but I think I figured out what I need to do. If anybody is for sure on what to do then please say so. I'll report back as soon as I'm done with this idea. If it works then I will post the code as well.

The problem with downloading a free parser is it is usually something you have to install wich would mean putting it as the requirements. So far my game only requires SDL and SDL_image support and nothing else. I hope to keep it this way because the game itself is way to simple of a game to be having a lot of requirements.

It would be a bit of work, but reinventing the wheel is always great academically. You could probably do a keyword driven parsing rather than having to read a whole book.
Basically:

Code:

Token input = GetToken();

if (input == TOK_OPAREN)
     push input;
else if (input == TOK_CLPAREN)
{
    stackVal = pop;
    if (stackVal != TOK_OPAREN)
        Error("expected open parenthese\n");
}
else
{
    switch(input)
    {
    case TOK_NUMOFSETS:
        if ((input = GetToken()) != TOK_NUMBER)
            Error("expected a number\n");
        else
            numOfSets = input.value;
    break;

    case other tokens ... etc.

    default:
        Error("Unknown keyword");
    }
}

Where GetToken() is something like:

Code:

string = GetCharFromFile();

if (string == '(')
    return(TOK_OPAREN);
else if (string == ')')
    return(TOK_CLPAREN);
else
{
    while (not a space and not a ')' and not a '(')
        string += GetCharFromFile();

    compare string to your stuff and return the correct token
}

Not the most efficient but simple and fairly easy to implement.

edit:
That input.value thing, make Token a struct and when you recognize a number just convert the string with atoi and put the result in input.value

This seems like an interesting concept. Of course it'll require a few tweaks here and there to get it to work. But I think this should work, at least for what I need it to do. Unfortunately I am to busy with homework to do it at the current moment and it might take me a little bit to get it all coded and tested. So I'll let you know if it worked or not. In the mean time if anybody has any more suggestions or notices flaws with this please post it.

Originally Posted by tuxinator

The problem with downloading a free parser is it is usually something you have to install wich would mean putting it as the requirements. So far my game only requires SDL and SDL_image support and nothing else. I hope to keep it this way because the game itself is way to simple of a game to be having a lot of requirements.

Many libraries just require you to link to them, and even if it does require a DLL or something you could distribute the DLL along with SDL.DLL and SDL_Image.DLL and no one would know the difference.

Ok I've got it to work now. Except I can't figure out how to copy the temporary value of the object into a permanent location without the temporary destination being a pointer and without getting a segmentation fault error. Basicly I need to copy the value over to a variable that I will use later on. I have tried using "strcpy" but it gives me a segmentation fault error. By the way the type of the two strings that I am working with is "char *string". Everything I've found so far only works with type "char string[MAX_LENGTH]".

Originally Posted by valis
It would be a bit of work, but reinventing the wheel is always great academically.

This is exactly why I'm writing my own parser. Is to learn how to do it, not just to get a task done.