XML literals in scala

A really handy feature that has been included in the Scala programming language is xml literals. The xml literals feature allows you to declare blocks of xml directly into your Scala code. As you’ll see below, you’re not limited to static xml blocks and you’re also given the full higher-order function architecture to navigate and process your xml data.

Definition and creation

You can create an xml literal very simply inside of your Scala code:

val people = 
	<people>
		<person firstName="John" 
				lastName="Smith" 
				age="25" 
				gender="M" />
		<person firstName="Mary" 
				lastName="Brown" 
				age="23" 
				gender="F" />
		<person firstName="Jan" 
				lastName="Green" 
				age="31" 
				gender="F" />
		<person firstName="Peter" 
				lastName="Jones" 
				age="23" 
				gender="M" />
	</people>

Scala then creates a variable of type Elem for us.

Xml literals can also be constructed or generated from variable sources

val values = <values>{(1 to 10).map(x => <value number={x.toString} />)}</values>

Take note that the value of x needs to be converted to a string in order to be used in an xml literal.

Another form of generation can be accomplished with a for comprehension:

val names = 
	<names>
	{for (name <- List("Sam", "Peter", "Bill")) yield <name>{name}</name>}
	</names>

Working with literals

Once you have defined your xml literal, you can start to interact with the data just like any other Seq typed structure.

val peopleCount = (people \ "person").length
val menNodes = (people \ "person").filter(x => (x \ "@gender").text == "M")
val mensNames = menNodes.map(_ \ "@firstName")

println(s"Number of people: $peopleCount")
println(s"Mens names: $mensNames")

The usage of map and filter certainly provide a very familiar environment to query your xml data packets.

Transform with RewriteRule

The scala.xml.transform package include a class called RewriteRule. Using this class, you can transform (or re-write) parts of your xml document.

Taking the sample person data at the top of this post, we can write a transform to remove all of the men out of the set:

val removeMen = new RewriteRule {
	override def transform(n: Node): NodeSeq = n match {
		case e: Elem if (e \ "@gender").text == "M" => NodeSeq.Empty
		case n => n
	}
}

We test if the gender attribute contains an “M”, and if so we empty out the node. To apply this transform to the source data, we use the RuleTransformer class.

val noMen = new RuleTransformer(removeMen).transform(people)

Another rule we can write, would be to remove any person who was over the age of 30:

val removeOver30s = new RewriteRule {
	override def transform(n: Node): NodeSeq = n match {
		case e: Elem if e.label == "person" && (e \ "@age").text.toInt > 30 => NodeSeq.Empty
		case n => n
	}
}

Pretty much the same. The only extra complexity is ensuring that we have an age attribute and getting it casted to an integer for us to perform arithmetic testing.

The RuleTransformer class accommodates if we want to use these two transforms in conjunction with each other.

val noMenAndOver30s = new RuleTransformer(removeMen, removeOver30s).transform(people)

Working with lambdas in python

Python provides a simple way to define anonymous functions through the use of the lambda keyword. Today’s post will be a brief introduction to using lambdas in python along with some of the supported higher-order functions.

Declaration

For today’s useless example, I’m going to create a “greeter” function. This function will take in a name and give you back a greeting. This would be defined using a python function like so:

def greet(name):
	return "Hello, %s." % (name,)

Invoking this function gives you endless greetings:

>>> greet("Joe")
'Hello, Joe.'
>>> greet("Paul")
'Hello, Paul.'
>>> greet("Sally")
'Hello, Sally.'

We can transform this function into a lambda with a simple re-structure:

greeter = lambda name: "Hello %s" % (name,)

Just to show you a more complex definition (i.e. one that uses more than one parameter), I’ve prepared a lambda that will execute the quadratic formula.

from math import sqrt

quadratic = lambda a, b, c: ((-b + sqrt((b * b) - (4 * a * c))) / (2 * a), (-b - sqrt((b * b) - (4 * a * c))) / (2 * a))

This is invoked just like any other function:

>>> quadratic(100, 45, 2)
(-0.05, -0.4)
>>> quadratic(100, 41, 2)
(-0.0565917792034417, -0.3534082207965583)
>>> quadratic(100, 41, 4)
(-0.16, -0.25)

Higher order functions

Now that we’re able to define some anonymous functions, they really come into their own when used in conjunction with higher-order functions. The primary functions here are filter, map and reduce.

We can filter a list of numbers to only include the even numbers.

filter(lambda x: x%2 == 0, range(1, 10))

Of course it’s the lambda x: x%2 == 0 performing the even-number test for us.

We can reduce a list of numbers to produce the accumulation of all of those values:

reduce(lambda x, y: x + y, range(1, 10))

Finally, we can transform a list or map a function over a list of numbers and turn them into their inverses:

map(lambda x: 1.0/x, range(1, 10))

Virtual buffer for liquid crystal displays

Today’s post will be about a double buffer implementation that I’m working on for the Liquid Crystal display module with the Arduino micro-controller.

What I’m using?

To write this article, I’m using the following hardware:

• Freetronics Eleven
• Freetronics 16x2 LCD Shield

For all of the software development that I’ve done here, I’m using the standard Arduino IDE available from their site.

Using the module “normally”

Having a look at the documentation for the Liquid Crystal library, there’s already a comprehensive implementation of filling, scrolling and blinking functions available.

The idea of this article isn’t to re-implement this library, it’s to build on it to add functionality.

The usual setup of the library would looks something similar to this:

#include <LiquidCrystal.h>

LiquidCrystal lcd( 8, 9, 4, 5, 6, 7 );

void setup() {
  lcd.begin(16, 2);
}

This initializes our LCD module, ready for us to start writing to it:

lcd.print("Hello, world!");

Nothing really special here, and rather than trying to piece together the “Hello, World” example from this code here, you’d be better off checking out the samples section for this module.

What’s the idea?

Given that we have a display of 16 characters in width by two characters in height, we’re rather limited in what we can do. What I want to do is expand the 16x2 interface to have a “virtual canvas” to work with of any arbitrary size that you can just use the 16x2 LCD as a window into the canvas.

To begin with, we’ll create some constants to assert our assumptions!

#define LCD_WIDTH      16
#define LCD_HEIGHT     2
#define LCD_SIZE       (LCD_WIDTH * LCD_HEIGHT)

These are pretty straight forward. All of the constants come in handy. Bounds checking is where they really shine, but we also specifically use LCD_HEIGHT as a pitch-type variable that helps us calculate how far into a flat-memory array we need to be to start writing. More on this later.

What do we want to do?

I think initially it’d be nice to:

• Define a virtual area (that’s larger than our physical display)
• Move the “window” to any point in the virtual area
• Render the view at the window co-ordinates to the physical display
• Print to arbitrary points of the buffer

I’ve created a class, LCDDoubleBuffer that should wrap all of this functionality up. I’m going to start going through the code from here on.

Define the virtual area

Give a LiquidCrystal reference and buffer dimensions, we can start to initialize our double buffer:

LCDDoubleBuffer(LiquidCrystal *lcd, const int width, const int height) {
	this->_width = width;
	this->_height = height;
	this->_size = width * height;
	this->_lcd = lcd;

	this->_win_x = 0;
	this->_win_y = 0;

	this->_buffer = new char[this->_size];
}

_width, _height and _size all manage the internal data structure for us so that we remember how big our virtual area is.

_lcd is our link back to the “real-world” for us to perform some physical side-effects on our display.

_win_x and _win_y are to co-ordinates to the top left-hand corner of the window that we’ll display on the LCD.

Finally, _buffer is our virtual canvas. Be careful here! Looking at the memory specs, you only get a whopping 2k SRAM to play with. With a char taking up 1 byte, you can see that you’ll quickly go over this if you’re not careful.

Working with the buffer

Well, it’s pretty simple. These are all string operations, and they can be reduced even further than this to say that they’re just memory operations.

To clear the buffer out, we use memset.

void clear() {
	memset(this->_buffer, ' ', sizeof(char) * this->_size);
}

To write a string at a given location, we use memcpy. Be careful here! The code that I present below doesn’t perform any bounds checking on where we are in the buffer. This is buffer under-and-overrun city, right here.

void print(const int x, const int y, const char *s) {
	char *o = this->_buffer + (x + (y * this->_width));
	memcpy(o, s, strlen(s));
}

o which is our offset pointer into the buffer is adjusted by calculating the linear address from x and y which is simply:

linear = x + (y * width)

Rendering the window

This is pretty brute-force. We enumerate over every valid cell defined for our LCD and write out the corresponding character.

void render() {
	int buf_y = this->_win_y;
	for (int y = 0; y < LCD_HEIGHT; y ++) {
	  
		int buf_x = this->_win_x;

		for (int x = 0; x < LCD_WIDTH; x ++) {

			this->_lcd->setCursor(x, y);

			if (buf_y >= 0 && buf_y < this->_height &&
			    buf_x >= 0 && buf_x < this->_width) {
				int ofs = buf_x + (buf_y * this->_width);
				this->_lcd->print(this->_buffer[ofs]);
			} else {
				this->_lcd->print(" ");
			}

			buf_x ++;
		}

		buf_y ++;
	}

}

That big-fat-if-statement in the middle is protecting us from colouring outside the lines. When we are outside the bounds, we’ll write a clearing character (in this case a whitespace).

Actually using the code

We’ve got a double-buffer defined, ready to go. To test it out, I’ve decided to spice-up the “Hello, World” example and get it scrolling on the display using cosine values so it should ease-out and ease-in; back and forth.

Don’t get too excited though. It’s not brilliant.

// create the LCD reference and double buffer
LiquidCrystal lcd( 8, 9, 4, 5, 6, 7 );
LCDDoubleBuffer dbuf(&lcd, 32, 32);

void setup() {
	Serial.begin(9600);

	lcd.begin(16, 2);

	// put "hello world" in the top corner
	dbuf.clear();  
	dbuf.print(0, 0, "Hello, world!");
}

float degs = 0.0f;

void loop() {
	// calculate and flatten to an int
	float a = -3 + (cos(degs) * 5);
	int x = (int)a;

	// move in position and render
	dbuf.moveTo(x, 0);
	dbuf.render();

	delay(50);

	// move along the cos curve
	degs += 0.1f;

	// 360 degrees == 0 degrees
	if (degs > 360.0f) {
		degs = 0.0f;
	}  
}

And there you have it. The full source of this demo is available here as a gist in my GitHub repository.

Textfile processing with bash

Today’s post will be a bash on liner that allows you to process text files, line-by-line.

while read line; do wget $line; done < urls.txt

In this case, urls.txt is a list of urls that we want to process using wget. $line is the value that we’re currently processing (the line of text).

This is a pretty heavy handed way of downloading a handful of URLs. You could much easier do it with xargs which I’d mentioned in a previous post. Anyway, the while way gives you a bit more room to breathe in your loops.

Developing with Tomcat user instances

In today’s post, I’ll take you through installing Apache Tomcat for the purposes of development.

Installing a user instance

To get started, install tomcat8 as you normally would:

$ sudo apt-get install tomcat8

For the purposes of development, it makes sense to have your own instance of tomcat which is away from the system installation. The tomcat8-user package allows you to do just that.

$ sudo apt-get install tomcat8-user

After this install is complete, you can create yourself a local tomcat instance that you can blow up without hurting the system’s version. You do this with the tomcat8-instance-create command:

$ tomcat7-instance-create -p 10080 -c 10005 tomcat

The switches -p puts this instance listening for application requests on port 10080 and the -c switch puts the control port on 10005.

After you’ve done this, you’ll be notified by the console.

You are about to create a Tomcat instance in directory 'tomcat'
* New Tomcat instance created in tomcat
* You might want to edit default configuration in tomcat/conf
* Run tomcat/bin/startup.sh to start your Tomcat instance

The directory that has been setup for you now looks like this:

.
├── bin
│   ├── setenv.sh
│   ├── shutdown.sh
│   └── startup.sh
├── conf
│   ├── catalina.properties
│   ├── context.xml
│   ├── logging.properties
│   ├── server.xml
│   ├── tomcat-users.xml
│   └── web.xml
├── logs
├── temp
├── webapps
└── work

Integrating with Eclipse

To get Eclipse to play nicely with your user-local version of tomcat, you’ll still need to add a few components. This tip is largly based off of the information in this stack overflow question.

$ ln -s /usr/share/tomcat8/lib
$ cp /etc/tomcat8/policy.d/03catalina.policy conf/catalina.policy
$ ln -s /usr/share/tomcat8/bin/bootstrap.jar bin/bootstrap.jar
$ ln -s /usr/share/tomcat8/bin/tomcat-juli.jar bin/tomcat-juli.jar
$ mkdir -p common/classes;
$ mkdir -p server/classes;
$ mkdir -p shared/classes;

You can now add your local user instance of tomcat to Eclipse.