Parsing and Converting¶

Tachi faces an interesting problem with its score importing code. We could rewrite our entire score importing process for every possible import method, or we could try to factor out the common parts of this process.

The latter is what we've gone for, but it isn't without its own traps.

We face the first obvious problem when it comes to actually "getting" the scores from a source and turning them into our format.

Our source needs to be dynamically set, sometimes we might be drawing scores from a file, sometimes from a HTTP request body, sometimes from another API!

Then, we need to convert the data that document gave us into a format the rest of our code to understand.

To solve this problem, Tachi uses two processes, Parsing and Converting.

Outline¶

As a rough outline, the parser converts data from any source into an iterable.

The iterable is then traversed, and the Converter is called for every element of the iterable.

The converter function turns the element into something the rest of the Tachi import code can work with.

Parsing¶

Parsing is the job of drawing the data from the source into an iterable (The usage of iterable and NOT array will be explained later).

A parser function is intended to be created outside of score-import-main, which means it can be really whatever it wants to be, and draw data from whatever source it needs to.

Warning

Parser functions return more than just an iterable, but in the interest of introducing complexity slowly we're going to pretend that they only return an iterable for now.

Implementation¶

As an example, let's say we had a CSV being sent to us as a file.

We could write a parser for that as follows:

// note: awful example code, this is NOT how to parse a CSV!
function ParserFunction(request) {
    let csv = request.file;

    return csv.toString().split("\n");
}

This takes the provided file, splits it on newlines, and returns the array. This meets all our criteria, as it returns an iterable, but we still have problems.

Firstly, the calling signature for this involves a request. Score Import Main doesn't know of any HTTP requests, as it might not even be triggered by one.

We could solve this by passing a request object, and also any other context a parser could possibly need, but that quickly breaks down when we start needing to fetch from APIs.

Instead, we define a ParserFunction is defined to be a function that takes one argument, the Import Logger.

This means that we actually have to use some closures in order to call a parser.

function CreateParser(request) {
    return (logger) => {
        return request.file.toString().split("\n");
    }
}

const ParserFunction = CreateParser(request);

Using closures, we can dynamically create the parser function and enclose any context it needs to use inside of it.

This approach allows ScoreImportMain to continue knowing nothing about where the data is coming from, and avoids passing lots of initial context, bloating up the score import function.

The iterable returned by the parser should be in the most immediately sensible format.

Info

Parser Functions may be asynchronous.

Iterable Vs. Array¶

Above, we specifically mentioned that the Parser function returns an iterable. There's a subtle difference here that allows for significant performance gains when getting data from a paginated API.

Let's write an example where we retrieve data from an API.

function CreateParser() {
    return async (logger) => {
        let page = 1;

        let array = [];
        while (true) {
            // pretend this returns some scores

            let res = await fetch("example.com?page=" + page).then(r => r.json());

            array.push(...res.scores);

            if (!res.moreData) {
                break;
            }

            page++;
        }

        return array;
    }
}

It's not bad, but it has some performance issues if the user has a lot of scores, by creating a massive array in memory and then passing it.

We can improve this code significantly using generators.

function CreateParser() {
    return async function* (logger) {
        let page = 1;

        while (true) {
            let res = await fetch("example.com?page=" + page).then(r => r.json());

            for (const score of res.scores) {
                yield score;
            }

            if (!res.moreData) {
                break;
            }

            page++;
        }
    }
}

With generators, this code can now yield each score from the data returned, instead of having to bubble it up in one large array.

Other Returns¶

We mentioned above that parsers return more than just an iterable.

In practice, a Parser returns four things in an object. Context, the Game this import is for, the Iterable discussed above and a Class Handler.

`context`¶

Some import formats require context to be parsed properly.

The most obvious example is the IIDX E-Amusement CSV format, which does not declare whether the scores are from single or double play inside the file.

To properly interpret a score of, say, 1000 on 5.1.1. ANOTHER, we need to have some context about the score. That is, we need data that isn't part of the 'score'.

To solve this, parsers return a context object which contains context for the converter function.

For IIDX E-Amusement, the parser function could interpret the playtype from user input, and then return it as context.

`game`¶

Sometimes, the game an import is for is not statically known from the Import Type. This means that the parser will have to return the game this import is for rather than inferring it.

An example of a format doing this would be the BATCH MANUAL format, which declares what game the import is for in a header.

`classHandler`¶

Sometimes, an import may result in the changing of a users' classes. This may also sometimes be dependent on data that is part of the import.

An example of this would be the Fervidex Static import type, which passes a set of the users scores, and also passes their Dans.

Since dans are part of the user's IIDX classes, we need to take the information from that request and update classes accordingly.

The classHandler returned from the parser should be a callable function that will update the user's classes dependent on this data.

Info

If no class information is dependent on the content known by the parser function, then classHandler can be set to null, and it will not be called.

Final Example¶

With all these other things to return, we can write a simple example of a parser as follows:

function CreateIIDXFileParser(request) {
    return (logger) => {
        let csv = request.file;

        let playtype = request.body.playtype;

        if (playtype !== "SP" && playtype !== "DP") {
            // todo: throw error
        }

        return {
            iterable: csv.toString().split("\n"),
            game: "iidx",
            context: {
                playtype,
            },
            classHandler: null,
        }
    }
}

Converters¶

let's say we have a parser for SDVX that returns:

[{
    score: 9000000,
    lamp: "HARD CLEAR",
    timestamp: 1623760037
},
// <more similar elements here...>
]

Our converter has to take every element here and convert it into the form the rest of Tachi can work with.

Unlike Parser Functions, Converter Functions are static, that means that they cannot be dynamically constructed with closures or similar.

Arguments¶

Converters are called with these four arguments:

Property	Type	Description
`data`	ParserElement	The given element in the parser's iterable.
`context`	ParserContext	The context the parser returned.
`importType`	ImportType	The import type this import is for.
`logger`	Logger	The import logger.

Returns¶

Converter functions are expected to return three values on success.

dryScore

A dry score is a 'partial' real Tachi Score format with certain properties left blank (because they can be filled elsewhere).

In short, the Dry Score is all the properties that can only be derived from the parser's passed data.

Other properties, like calculated data are filled out in a score "hydration" process (hence the name Dry Score).

chart

The chart this score is for.

song

The song this score is for.

Throws¶

Converters are also expected to fail. For this, we have a very specific throw format that should be thrown whenever a specific error has occured.

These are called Failures, and a full list of them can be found here.

If a non-failure is thrown, it is logged as an error and the score is ignored (because crashing the entire import from an unexpected error is a bad idea.)