## Visualizing group structure with colored addition/multiplication tables

When working with finite fields, if the number of elements is a prime power $p^m$ with m > 1, you can represent the elements as polynomials with degree m-1 and do the field addition and multiplication modulo a irreducible polynomial with degree m.

The field GF(5) is composed by the numbers 0 to 4. We don’t need to represent its elements as polynomials since m=1. Addition is done modulo 5 and multiplication also modulo 5. So 2 + 3 = 0; 4 * 2 = 3; and so on. This is the addition table for GF(5):

The rows, top down, represent 0 to 4. The columns, right to left, represent 0 to 4. Each square is the result of the addition of the respective numbers in the row / column it belongs to. Black is 0, purple is 1, red is 2, orange is 3, yellow is 4.

## Access violation errors with callbacks in ctypes

I’ve just spent a few hours trying to solve this bug, so I’m publishing this so maybe it will help someone with this issue…

Assume that you’re working with a DLL/.so library through ctypes in Python, and this library allows you to set a callback for some other function. In my case, I was working with unrar.dll. The code was something among these lines:

 UNRARCALLBACK = ctypes.WINFUNCTYPE(ctypes.c_int, ctypes.c_uint, ctypes.c_long, ctypes.c_long, ctypes.c_long)   #in a class... RARSetCallback(self.handle, UNRARCALLBACK(self.callback_fn), 0) RARProcessFile(self.handle, RAR_TEST, None, None)

The first lines constructs the function prototype, the second sets the callback in a function of the DLL file, and the third calls a function in the DLL which will call the callback.

Can you spot the error?

The code worked fine in Python 2.5, but then I changed to 2.6 and it stopped working. I got a “WindowsError: exception: access violation reading…” (or writing) exception in the third call.

The reason, which is obvious in hindsight, is cleared explained in the docs:

Make sure you keep references to CFUNCTYPE objects as long as they are used from C code. ctypes doesn’t, and if you don’t, they may be garbage collected, crashing your program when a callback is made.

(Though it’s not explicit, it applies to WINFUNCTYPE objects too)

The WINFUNCTYPE object created in the second line no longer exists in the third line, so when the callback was called, it no longer pointed to a valid address. The solution is simple — just keep a reference to the object:

 UNRARCALLBACK = ctypes.WINFUNCTYPE(ctypes.c_int, ctypes.c_uint, ctypes.c_long, ctypes.c_long, ctypes.c_long)   #inside a class... self.callback_ref = UNRARCALLBACK(self.callback_fn) RARSetCallback(self.rarFile.RAR._handle, self.callback_ref, 0) RARProcessFile(self.rarFile.RAR._handle, RAR_TEST, None, None)

The only mystery left is why the old code worked on 2.5!

## Quivi 1.0 released

I’ve just released the new version of my image viewer and manga / comic reader, Quivi (only for Windows right now, but it will be ported to Linux and maybe to Mac). I’ve rewritten it from scratch, check my last post for more info.

• Prefetching of the next image
• Wallpaper dialog
• Start directory option

But that is only the beginning; now it will be much more easy for me to work on Quivi.

There are many comic readers around; like CDisplay (outdated, but very popular), Comical and Comix. They have much more features than Quivi, and I must admit that I often thought on giving up on Quivi. But I just couldn’t.

I wrote it mainly for myself; when I started there was only CDisplay. The first version was written with Visual Basic (go on, laugh). Then I changed to C++. And now I changed to Python, and I hope this is the last language change 🙂 It’s a hobby project that I grew to love. It suits my needs, but I’d really like to suit the needs of other people too!

It is scary when you realize that people are using your software. I feel responsible; wherever I find someone criticizing it, I feel guilty for not attending those people needs. Yes, that’s stupid, but it’s how I am. At the same time, though, it’s awesome to know that there are people who like it and find it useful.

If you have any suggestion, comment, complain about Quivi, please send it to me!

## Why I rewrote Quivi from scratch

Joel Spolsky, popular software engineering, said in this blog:

(…) They [Netscape] did it by making the single worst strategic mistake that any software company can make:

They decided to rewrite the code from scratch.

I agree, mostly. Many projects market that they’ve been “rewritten from scratch” as if it was something marvelous, and most of time, it’s just a sign that the new version probably has more bugs than the previous. But, wait, I just rewrote Quivi (a image viewer and comic / manga reader) from scratch! Well, why?

Well, I thought I really had to justify this. So here are the reasons:

• It is a small project. Rewriting it certainly isn’t as hard as rewriting a browser!
• I couldn’t stand C++ anymore (the programming language I used before). Of course, this isn’t C++ fault per se. It has its uses — the right tool for the right job, and so on. But to write an desktop application? It’s overkill. If you manage to pull it of, hey, kudos to you. But I had no motivation to work on it anymore.
• I love Python (the programming language I use now), and of course, I’m not the only one. Programming is fun in Python, so much that had the motivation to rewrite Quivi from scratch in the first place! And of course, it will be much more easier for me to keep working on Quivi.
• I could change GUI libraries. SmartWin is a nice library, and uses templates in very interesting ways; when programming Quivi I ended up involved with its development too. But it has its bugs, and because it uses templates extensively, it’s awful slow to compile an application that uses it. I’ve changed to wxPython, which is a very mature GUI library – and cross platform to boot.

Of course, there are some downsides with the change. The whole software package is much bigger due to the dependencies (the installer jumped from 900K to 5MB!). And the program is a little bit slower, mainly when starting up, and uses more memory (9MB to 30MB with no images loaded). But I think it was a good enough trade-off.

Rewriting from scratch must be considered carefully, and in this case, I think it was a good idea. Quivi is a hobby project, and I guess the main point of it is to have fun writing it, and to make users happy. I hope I can do both with this new version (which will be released soon).

## "Access is denied" on "My Pictures" in Windows Vista

Just for the record, in case anyone gets stuck with this issue.

The “C:/Users/username/Documents” folder in Vista has three hidden junctions named “My Pictures”, “My Music” and “My Videos” which point to the folders “Pictures”, “Music” and “Video” inside the “C:/Users/username” folder. But they have the “List folder / read data” permission denied for all users as default. So any attempt to open them will result in a “Access denied” error.

It’s possible to “fix” this, but I don’t recommend it. Just open the pointed folders.

The question is, of course, why the permission is set, and why there are those hidden junctions in the first place?

I’m only guessing here, but it makes sense. The junctions are there for compatibility. If a program was written with a reference to a file inside the “(My Documents Folder)/My Pictures” path (assuming the “My Pictures” folder would always be inside the My Documents folder), it would break on Vista. With the junction, you can write to or read any files inside this (now non existent) folder.

(That is why there is also a junction “C:\Documents and Settings” pointing to “C:\Users”)

And why is the list folder permission denied? At first I thought it was there to prevent an infinite loop on programs who list the directory tree recursively, but those junctions don’t point to parent directories…

And the last question is, of course, why they changed those folders locations in the first place? Well, I must admit it’s much better this way (spaces inside paths are a pain).

## Closure gotcha (with Python)

Do you know what a closure is? Basically, it’s a function created at runtime that references variables defined in a outer scope. For example:

def make_number_printer(n):
def number_printer():
print n
return number_printer

printer = make_number_printer(5)
printer()

The function make_number_printer receives a number and returns a function which, when called, prints that same number. It’s not the most useful function in the world, but it does show how closures work. In this case, the number_printer function is a closure, because it references the variable n which is in an outer scope (of the make_number_printer function).

My intention here, though, it’s not to explain what closures are but to show a property of them which may cause some confusion (and made me spent quite some time hunting for a bug caused by it). What does this code prints when run?

printer_lst = []
for i in xrange(10):
def number_printer():
print i
printer_lst.append(number_printer)

for printer in printer_lst:
printer()

It loops between 0 and 10 creating a number_printer function and appending it to a list. Then it loops through the functions of the list, calling them. You would expect it to print the numbers 0 to 9, right?

Wrong! It prints the number 9 ten times.

The problem is that most people (including me, before learning this) think that closures work by evaluating the variables in the outer scope and storing their values to use when necessary. But actually they keep an reference to those variables, and if their contents change, the closure will use the new value. Since our variable i stores the value 9 after the closures are created, that is the value they will print. This happens even if the variable i goes out of scope after the closures are created (e.g., if the first for were inside a function).

So how to solve this? You can move the creation of the closure to another function, like this:

def make_number_printer(n):
def number_printer():
print n
return number_printer

printer_lst = []
for i in xrange(10):
printer_lst.append(make_number_printer(i))

for printer in printer_lst:
printer()

Here the closure will reference the variable n, which has different instances for each different closure created. Another alternative is to use a somewhat contrived Python “feature” which is the fact that default parameter values are evaluated when the function is defined and not when they are called:

printer_lst = []
for i in xrange(10):
def number_printer(x=i):
print x
printer_lst.append(number_printer)

for printer in printer_lst:
printer()

When def number_printer(x=i): is run, the variable i is evaluated and its value is saved in the function definition; so, each time the function is defined (i.e., the closure is created), the current value of i is “frozen”.

If somebody is thinking, “but I’ll never run into this situation”, here is a little more real example (which actually happened to me when I was coding a Flash game, which uses ActionScript). Basically it’s the same code above and has the same issue:

class Button:
#This is a dummy Button class; suppose
#it's part of a GUI library and for some reason
#you can't subclass it

def __init__(self):
self.listener = None

def set_click_listener(self, fn):
self.listener = fn

def on_click(self):
self.listener()

#Create 10 buttons...
buttons = [Button() for i in xrange(10)]
#And suppose the buttons are added to the GUI after

#Set the listeners to the click event.
#The number of buttons may change in the future
#and all of them have the same code, the
#only difference is the button index. So it's better
#to do this within a loop.
for i in xrange(10):
def on_click():
#Suppose there is something more useful here
print i
buttons[i].set_click_listener(on_click)

#Simulate a click in each button
for j in xrange(10):
buttons[j].on_click()