GNOME_COMPILE_WARNINGS(maximum) – Syntax Error in configure

I’m still encountering migration issues from Gentoo to Ubuntu. Apperantly, Gentoo is much more user friendly than Ubuntu when it comes to compiling packages. In Gentoo you’ve got almost all the major dependencies you need. In Ubuntu, on the other hand, you need to hunt them down. It’s much easier with the main ones, as they are listed. But there are some small ones which are harder to track. I came across the following error while trying to compile gitg, a GUI for Git, today:

./configure: line 14447: syntax error near unexpected token `maximum'
./configure: line 14447: `GNOME_COMPILE_WARNINGS(maximum)'

After not so short investigation I found out I was missing gnome-common

sudo apt-get install gnome-common

Why can’t be one distribution which is user-friendly like Ubuntu and in the same time developer-friendly like Gentoo?

Solving Sudoku using Python and Prolog

Two weeks ago, I add came up with an interesting algorithm for solving Hidato which basically involves decomposing the board the grid (can be square, hexagonal or any other shape), into classes of pieces and then arranging them (maybe I’ll write a detailed post on it in the future). So while pondering whether it would be interesting enough to go forward and actually implementing the algorithm compared to the work it would require, I started thinking what will be the simplest way to solve such puzzles, as opposed to efficient.

At first I’ve looked at general purpose constraint solvers, and decided to tackle Sudoku instead as it’s a bit simple to define in terms of constraints. I considered several libraries but in the end I’ve settled on plainly using Prolog. I chose Prolog because as a logic programming language, constraints are its bread and butter. I although kind of liked it as I haven’t done anything in Prolog for quite a few years.

Describing Sudoku in terms of constraints is extremely simple. You need to state that every cell is in a given range and that all rows, columns and sub-grid contain different integers. As mangling with lists in prolog isn’t fun, I’ve wrote a python program that outputs all the prolog statements with hardcoded references to the variables which build-up the board. It’s ugly but dead simple. The script gets the dimensions of the sub-grid.
Continue reading Solving Sudoku using Python and Prolog

GCC Usage quirks

This is more of a note to myself, as it’s an error I keep bumping into. When compiling manually using gcc (or g++) you should pass all the libraries you’re compiling against at the end of the argument list. E.g.:

g++ -Wall -lboost_thread my_file.cpp

will result in error like:

undefined reference to `boost::thread::join()'
undefined reference to `boost::thread::~thread()'

while

g++ -Wall vortex5_brute.cpp -lboost_thread

works fine.

Number Exercises Separately in LyX

Say you’ve got a document with a bunch of exercises and few lemmas. You may want the exercises numbered separately from the numbering of the lemmas and theorem, unlike LyX’s default behavior. This can be achieved by redefining xca, the environment LyX uses for exercises. Add the following to your LaTeX preamble:

\let\xca\@undefined
\theoremstyle{plain}
\newtheorem{xca}{\protect\exercisename}

LyX will still display the incorrect numbering, but the output will be correct nonetheless. The first line, undefines the LyX’s definition of xca, then we set the style to match the old one and we redefine xca, this time without a reference to the theorems’ counter.

Expectation Symbol in LaTeX

After looking for a builtin expectation symbol in LaTeX, and coming up with none, I’ve defined one. Just add:

% Expectation symbol
\DeclareMathOperator*{\E}{\mathbb{E}}

to your LaTeX preamble and you’re done. You’ll also need to add \usepackage{amsmath} or in LyX to tick “Use AMS math package” under Document->Settings->Math Options.

Using the starred version of \DeclareMathOperator makes sure subscripts goes beneath the symbol in display mode.

Modified Variant Whitespace Template

Variant Whitespace is a nice minimalistic template by Andreas Viklund.

Andreas chose to put the sidebar above the content, which I prefer not to do. Furthermore as the sidebar was a “float” that came before the content, it caused additional inconveniences. E.g. if you had an element with clear: both it would be pushed bellow the sidebar. I’ve patched it a bit in order to fix those issues. You can find my modified version here: variant-whitespace.tar.gz

Author: (no author) not defined in authors file

I came across the following error message

Author: (no author) not defined in authors file

when I tried to import an SVN repository (Open Yahtzee‘s) to Git using git svn and I specified an authors file (using -A). Indeed, the first commit to the svn (which was done using cvs2svn) had no username for the commiter. Apperantly the workaround is to add the following line to your author file.

(no author) = no_author <no_author@no_author>

I tried also doing the same without an email address, but it just didn’t work. It seems Git requires that all authors have an email address.

passha – A hashapass variant

I like the idea of hashapass, but I’m unwilling to use an online tool, as I fear that someday it might be compromised. So I wrote down my own variant of hashapass. It uses slightly longer passwords, and sha256 as the hash function.

#! /usr/bin/python

"""
passha.py - Generate passwords from a master password and a parameter.

Based on hashapass (http://hashapass.com)
"""
import hmac
import hashlib

def main(passwd, param):
    hm = hmac.HMAC(passwd, param, hashlib.sha256)
    print hm.digest().encode("base64")[:10]
    
if __name__=="__main__":
    import getpass
    passwd = getpass.getpass()
    param = raw_input("Parameter: ")
    main(passwd, param)

Reinstall grub in Ubuntu

My brother asked me to repair his boot loader, after he accidentally erased his MBR. This can be done easily via LiveCD and the command line.

Boot the system using a LiveCD (I’ve used Ubuntu from USB stick) and do the following:

$ sudo mount /dev/sda /mnt
$ sudo mount --bind /usr/sbin /mnt/usr/sbin
$ sudo mount --bind /usr/lib /mnt/usr/lib
$ sudo mount --bind /dev/ /mnt/dev
$ sudo chroot /mnt

# grub-install /dev/sda

I hope it will be useful for others as well, as the Ubuntu community documentations offers a solution based on Boot-Repair, which seems an overkill for me.

Automated Encrypted Backups to S3 Using Duplicity

This tutorial will hopefully guide you in making automated encrypted backups to Amazon’s S3 using duplicity. It was written as a followup for Using Duplicity and Amazon S3 – Notes and Examples, in order to organize all the necessary information into a simple tutorial.

Will start by creating a simple wrapper to duplicity:

#! /usr/bin/python
import sys
import os

duplicity_bin = '/usr/bin/duplicity'

env = {
    'AWS_ACCESS_KEY_ID':     'PUT YOUR KEY ID HERE',
    'AWS_SECRET_ACCESS_KEY': 'PUT YOUR SECRET ACCESS KEY HERE',
    'PASSPHRASE':            'PUT ENCRYPTION PASSPHRASE',
}
env.update(os.environ)

os.execve(duplicity_bin, sys.argv, env)

save this under dupicity-wrapper.py and chmod 0500 it so only you will be able to read and execute it.

Note: You’ll want to write down the passphrase and store it in a safe location (preferably in two separate locations). So in case you’ll need to restore the backups, you won’t have useless encrypted files.

Now edit your crontab and add a line like the following:

10 1 * * 0 /path/to/duplicity-wrapper.py /path/to/folder/ s3+http://bucket-name/somefolder &>> ~/log/backups.log

This will create a weekly backup for /path/to/folder. The backup will be encrypted with what ever passphrase you’ve given in the duplicity-wrapper.py. The output of the backup process will be saved into ~/log/backups.log.

You should also run

/path/to/duplicity-wrapper.py full /path/to/folder/ s3+http://bucket-name/somefolder

in order to create full backups. You might want to periodically verify your backups:

/path/to/duplicity-wrapper.py collection-status s3+http://bucket-name/somefolder
/path/to/duplicity-wrapper.py verify s3+http://bucket-name/somefolder /path/to/folder/

To check the status of the backups and to verify them.

And last but not least, in case you ever need the backups, you can restore them using:

/path/to/duplicity-wrapper.py restore s3+http://bucket-name/somefolder /path/to/folder/

Security Considerations

As I know, some people will comment on the saving of the encryption passphrase plainly in a file, I will explain my reasoning. I use the above encryption in order to secure my files in case of data leakage from Amazon S3. In order to read my backups, or silently temper with them, some on will have to get the passphrase from my machine. While, this isn’t impossible, I will say it’s unlikely. Furthermore, if someone has access allowing him to read files from my computer, he doesn’t need the backups, he can access the files directly.

I’ve given some thought about making the backups more secure, but it seems you always have to compromise on either automation or incremental backups. But, as I wrote, the current solution seems to me strong enough given the circumstances. Nonetheless, if you’ve got a better solution it would be nice to hear.