Pico Lisp to JSON with JavaScript

On many occasions it’s preferable to convert PicoLisp on the client side instead of on the server, here’s an attempt.

However, there are two problems with the below approach:

1) The first huge regexp that checks for a paired list will break if any key or value has an escaped quote in it. I’ve scoured the Internet until my eyes bled but I couldn’t find an example of JavaScript regex that would handle backslashes. I also found out that the JS regexp capabilities are not 100% on par with for instance PERL’s, it might be what what I want is impossible to achieve with regexes alone.

2) The second regexp that checks for a single pair/obj will not work if the key has an escaped quote in it, this is however a less serious problem than 1) as it’s extremely unlikely to happen.

Update: I’ve gotten some help from Mateusz Jan Przybylski on the PicoLisp mailing list. It seems we can handle quotes in the values of objects now. I still couldn’t get it to work though with the standalone, single pair/object so #2 above still holds.

Update 2:: After some discussion on the mailing list again we’ve agreed to handle (”:key” (1 2 3)) as {”key”: [1, 2, 3]}. This current version is more flexible than what you’ve seen here before, it will now handle nested associative lists. It is however not very robust, I can think of a lot of strange edge scenarios that will make it break. Anyway, it’s good enough for me and my current usage scenarios.

function countSubsChrs(str, start_chr, pos, dir){
    var cur_chr = start_chr;
    var count   = 0;
    while(cur_chr == start_chr){
      cur_chr   = str.charAt(pos);
      pos       += dir;
      count     += 1;
    }
    return count;
}

function isEscaped(str, pos){
    var result = countSubsChrs(str, '\\', pos, -1) % 2;
    return result == 0 ? false : true;
}

function getListEnd(str, cur_pos, dir){
    var lvl = 1;
    var pos = cur_pos + dir;
    var dq  = 0;
    while(lvl != 0 && pos != str.length){
        var chr = str.charAt(pos);
        switch(chr){
            case '"':
                if(!isEscaped(str, pos)) dq += 1;
                break;
            case '(':
                if(dq % 2 == 0) lvl += dir;
                break;
            case ')':
                if(dq % 2 == 0) lvl -= dir;
                break;
            default: break;
        }
        if(pos < 1) break;
        pos += dir;
    } 
    return pos; 
}

function isPaired(str){
    var is_p = true;
    if(str.match(/^\(.+\)$/)){  
        for(var i = 0; i < str.length; i++){
            if(str.charAt(i) == '('){
                var rest = str.slice(i+1);
                if(!rest.match(/^"\:[^"]*"\s\(/) && !rest.match(/^("[^"]*"|\d+)\s\.\s[^\s]/)){
                    is_p = false;
                    break;
                }
                i = getListEnd(str, i, 1) - 1;
            }
        }
    }else
        is_p = false;
    return is_p;
}

function hasKey(str, pos){
    var back_str = str.slice(getListEnd(str, pos, -1) + 2, pos);
    return back_str.match(/^("\:[^"]*")\s/);
}

function picoToJson(str){
    str = str.slice(1, -1);
    
    if(isPaired(str))
        var mode = 'pairs';
    else if(str.match(/^"\:[^"]*"\s/))
        var mode ='obj';
    else
        var mode = str.match(/^("[^"]*"|\d+)\s\.\s[^\s]/) ? 'obj' : 'arr';
    
    var rstr = mode == 'arr' ? "[" : "{";
    var is_str = false;
    var chr = "";
    for(var i = 0; i < str.length; i++){
        chr = str.charAt(i);
        if(is_str){
            if(str.charAt(i) == ':' && (str.charAt(i-2) == '(' || str.charAt(i-1) == '"') && (mode == 'obj' || mode == 'pairs'))
                rstr += '';
            else
                rstr += chr;
            if(chr == '"' && str.charAt(i - 1) != '\\')
                is_str = false;    
        }else{
            switch(chr){
                case '(':
                    if(mode != 'pairs'){
                        var end_pos = getListEnd(str, i, 1);
                        rstr += picoToJson( str.slice(i, end_pos));
                        i = end_pos - 1;
                    }else{
                        if(i != 0 && str.charAt(i-2) != ')'){
                            if(hasKey(str, i)){
                                var end_pos = getListEnd(str, i, 1);
                                rstr += picoToJson( str.slice(i, end_pos) );
                                i = end_pos - 1;
                            }
                        }
                    }   
                    break;
                case 'N':
                    if(str.charAt(i+1) == 'I' && str.charAt(i+2) == 'L'){
                        rstr += 'false';
                        i += 2;
                    }
                    break;
                case 'T':
                        rstr += 'true';
                    break;
                case ')':
                    if(i < str.length - 1)
                        rstr += ',';
                    break;
                case '"':
                    if(str.charAt(i - 1) != '\\')
                        is_str = true;
                    rstr += chr;
                    break;
                case ' ':
                    if(str.charAt(i + 1) == '(' && (mode == 'obj' || mode == 'pairs')){
                        if(mode == 'obj')
                            rstr += ": ";
                        else if(mode == 'pairs' && str.charAt(i-1) != ')')
                            rstr += ": ";
                        else
                            rstr += " ";
                    }else if(str.charAt(i + 1) == '.' && (mode == 'obj' || mode == 'pairs')){
                        rstr += ":";
                        i++;
                    }else if(mode == 'arr')
                        rstr += ', ';
                    else
                        rstr += chr;
                    break;
                default:
                    rstr += chr;
                    break;
            }
        }
    }
    rstr += mode == 'arr' ? "]" : "}";
    return rstr;
}

function evalPico(str){
    return eval('('+picoToJson(str.replace(/\n/gm, ''))+')');
}

var str = '("first element" ("obj" . "yes") 2 (":links" (1 2 3)) ((1 . 2) (":links" (4 5 6))) "final element")';
var json = picoToJson(str);
alert(json);
var result = eval('('+json+')');
alert(result[3].links.pop());

A lot of the code above has been converted from the Ruby Pico Editor code.

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