How can I open a javascript object in Perl?

Question

I have a JavaScript Object file with 5.5mb of longitude latitude data and I would like to open it in Perl so I can apply a crude detail reducing algorithm that would then save a new object file with the reduced dataset. For reducing detail I use a for loop that only takes every 20th long/lat pair.

I can do this in javascript but this requires that I copy/paste each coordinate set and run my JavasSript on it one at a time.

I then thought perhaps I could take each set of coordinates and put them in to a SQL db but that seems like a crude way to do it. And moves a lot of data around.

I settled on Perl being one of the better options, to do it all on the server.

I can open the file with:

#!/usr/bin/perl

# open file
 open(FILE, "reduced_object_latlng.js") or die("Unable to open file");

# read file into an array
@data = <FILE>;

# close file 
close(FILE);

# print file contents
foreach $line (@data)
{
    print $line;
}

The object follows this design:

var paths = {
    mayo: {
        name: 'Mayo',
        colour: 'green',
        coordinates: '-9.854892,53.76898 -9.853634,53.769338 -9.85282,53.769387 -9.851981,53.769561 -9.850952,53.769508 -9.850129,53.769371 -9.849136,53.769171 **data**' 
    },
    galway: {
        name: 'Galway',
        colour: 'purple',
        coordinates: '**data**;
    }
}; //etc.

To illustrate how I reduce the above data my javascript version loads from a file with one var coords = "*data*"

coords = coords.split(" ");
var path = [];
    var output="";
    document.getElementById("map_canvas").innerHTML = "";
for (var i = 0; i < coords.length; i++) {
        if (i%20==0)
        {
            var coord = coords[i].split(",");
            output += coord[0]+","+coord[1]+" ";
        }
}
document.getElementById("map_canvas").innerHTML = output;

I have read some suggesting I convert it to JSON, I'm not sure if I need to do that. And instead of writing a pure text handler is there a way to load the file as an object?

I was stuck for time so I did it this way:

var outputobject = 'var paths = {';
    for (property in copypaths) {
        outputobject += property + ': { ';
        outputobject += "name: '" + copypaths[property].name+"',";
        outputobject += "colour: '"+ copypaths[property].colour+"',";

        var reducedoutput="";
        var coord = copypaths[property].coordinates.split(" ");
        for (var i = 0; i < coord.length; i++) {
            if (i%20==0)
            {
                var coords = coord[i].split(",");
                reducedoutput += coords[0]+","+coords[1]+" ";
            }
        }   
        outputobject += "coordinates: '"+ reducedoutput+"'},";
    }
    outputobject += "};";
    document.getElementById("reduced").innerHTML = outputobject;

it still involves copy/paste and deleting the last ,. Thank you @Oleg V. Volkov, when I have more time later in the week I'll look at the method you laid out.

Oleg V. Volkov · Accepted Answer · 2012-08-06 13:49:54Z

3

Just strip leading JavaScript so you have almost proper JSON with bare keys and use JSON/JSON::PP instance with allow_barekey set to true value to decode resulting string.

answered Aug 6, 2012 at 13:49

Oleg V. Volkov

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2 Comments

chris loughnane Over a year ago

Just to be sure I understand correctly. When you say strip the leading javascript: are you saying I should make a new file in JSON that just contains the coordinate data? I'd just end up with a whole lot of numbers wouldn't I, that would need me to manually copy paste.

Oleg V. Volkov Over a year ago

@Chris, I mean strip var paths = part - everything until opening {. You don't need to write another file, just process content you've read into memory with regexp or like.

Borodin · Accepted Answer · 2012-08-06 15:55:58Z

1

Using JSON is your best bet. It has options that allow you to decode less stringent JSON syntax, and you will need

allow_singlequote to allow single-quoted as well as double-quoted strings
allow_barekey to allow all-alphanumeric hash keys to have no quotes at all
decode_prefix to ignore junk after the end of the data
relaxed for good luck

The program below decodes the JSON to a Perl structure, extracts the coordinates string for the mayo entry, and prints the values out in pairs.

Note that I have removed the semicolon and added a quote at the end of coordinates: '**data**; as I assume this is a mistake rather than actual JavaScript data

use strict;
use warnings;

use JSON -support_by_pp;

my $json = JSON->new->relaxed->allow_singlequote->allow_barekey;

my $data = do {
  local $/;
  <DATA>;
};

my ($hash) = $json->decode_prefix($data =~ /(\{.*)/s);

my @coords = $hash->{mayo}{coordinates} =~ /[-0-9.]+/g;

printf "%f %f\n", splice @coords, 0, 2 while @coords;

__DATA__
var paths = {
    mayo: {
        name: 'Mayo',
        colour: 'green',
        coordinates: '-9.854892,53.76898 -9.853634,53.769338 -9.85282,53.769387 -9.851981,53.769561 -9.850952,53.769508 -9.850129,53.769371 -9.849136,53.769171 **data**' 
    },
    galway: {
        name: 'Galway',
        colour: 'purple',
        coordinates: '**data**'
    }
}; //etc.

output

-9.854892 53.768980
-9.853634 53.769338
-9.852820 53.769387
-9.851981 53.769561
-9.850952 53.769508
-9.850129 53.769371
-9.849136 53.769171

answered Aug 6, 2012 at 15:55

Borodin

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3 Comments

chris loughnane Over a year ago

thank you @Borodin, I was editing my question when you added your answer. **data**; was a typo :P

Borodin Over a year ago

Reading your changes to the question it looks like you are building the JSON-like hash yourself as a string. So where does the data come from? Perl will run on the server machine while JavaScript code is client-side so Perl can't really process JavaScript's output. How do you see this working?

chris loughnane Over a year ago

My data set comes from a google fusion table and contains KML for regions. I converted this data into the JavaScript Object. After I had it all running I saw that the dataset was prohibitively large at 5.5MB so I experimented with reducing the size. The deleted long/lat pairs only become a problem when zoomed on the map. I did all this so I could use the map data to create polygons that can handle events. I don't know any way of getting interactive KML on the map with events other that clicking. I have never used Perl before and thought this would be a nice start.

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