kindid / tiny_rdp_js

A Tiny Recursive Descent Parser in Javascript

tiny_rdp_js

A Tiny Recursive Descent Parser in Javascript

More to come, I've just added this out of interest more than anything.

TODO

1. Allow an array to be passed into parser.is() as this will make writing rules far more compact
2. Allow matching of whole strings not just characters
3. Build an actual tree instead of just spans of characters
4. Allow a rule recording to be turned on/off as sometimes you don't care about intermediate nodes in the tree (e.g. value, you only care about number or name)

function Parser(input) {
    this.input = input
    this.index = 0
    this.stack = []
}

module.exports.Parser = Parser;

Parser.prototype.is = function(t, t1) {
    if (t instanceof Function) {
        let depth = this.stack.length
        this.stack.push({ name:t, start:this.index, end:-1 })
        if (t(this) !== true) {
            this.stack.pop()
            return false
        } else {
            this.stack[depth].end = this.index
            return true
        }
    } else {
        let cc = this.input[this.index]
        let q = false
        if (t1 === undefined) q = (cc === t);
        else q = (cc >= t && cc <= t1)
        if (q) this.index++;
        return q;
    }
}

Above is the guts of the whole thing.

Example

This parser shows a simple list with integers and names where a list can contain lists of lists and so on.

function name_char(parser) {
    return(parser.is('a', 'z') || parser.is('_'))
}

function digit_char(parser) {
    return(parser.is('0', '9'))
}

function first_number_char(parser) {
    return(parser.is('0', '9') || parser.is('-') || parser.is('+'))
}

function separator(parser) {
    return parser.is(' ');
}

function name(parser) {
    if (name_char(parser)) {
        while (name_char(parser)) ;
        return true;
    }
}

function value(parser) {
    if (parser.is(name)) return true
    else if (parser.is(number)) return true
    else if (parser.is(list)) return true
}

function list(parser) {
    if (parser.is('[')) {
        while(parser.is(value)) {
            if(!parser.is(separator)) break;
        }
        if (parser.is(']')) {
            return true;
        }
    }
}

function number(parser) {
    if (first_number_char(parser)) {
        while (digit_char(parser)) ;
        return true;
    }
}

Invokation

Firstly require the parser then create a parser object then try and parse using the is function.

It will return true or false.

let P = new Parser(text);
let q = P.parse(my_parsing_function)

(C) kuiash.com 2017