Thursday, April 30, 2015

Mac OS / Linux : Finding all the files that meet some criteria and returning an escaped list of the paths, sorted by length

This is another post "for the next poor bastard"

One of the side effects of running AnimEigo is that I end up with a lot of video on hard disc -- terabytes and terabytes of it. This is amusing when you consider my first hard drive cost $5000, stored 20MB, and shook the table when the disc heads moved.

When projects are finished, we archive all the video and project files just in case we need them later, and rather than just copying the files onto an archive disc (easy and fast), since much of the original source materials are uncompressed audio and video, compressing these files before archiving means we can save a few bucks and free up some hard drives for reuse and replicate the files on multiple drives as insurance.

The Mac finder has a built-in feature that lets you compress files and folders, but it occurred to me recently to check to see if there were better options.

After doing a little research, I settled on using pbzip2, the multicore implementation of bzip2, which seems to do a good job of compressing uncompressed video files -- often down to 20-25% of the original size. If you're using a Mac, the easiest way to install it is by using the fink package manager.

As pbzip2 is a command-line tool, you invoke it using the Terminal app, by typing something like this:

pbzip2 -v "path to the first file you want to compress" "path to the next file" ...

and pbzip happily goes off and (slowly) compresses the file for you. All fine and good, and you can just type the pbzip2 -v part and then drag files in from a folder window to enter the paths.

However, because I'm lazy and thus willing to spend many hours automating things to save myself a few seconds of drudgery, I started playing around in the default Bash shell that Terminal provides; it had been a while since I'd done more than trivial things in it and a refresher couldn't hurt.

The basic philosophy of Unix shell tools is "lots of little tools that do a small number of things well that you can hook up to do something complicated". You send the output of one tool into the next tool using a pipe, represented by the | character. Here's the command sequence I came up with:

find . \( -iname '*.aiff' -or -iname '*.aif' -or -iname '*.wav' -or \( -size +500000 -iname '*.mov' -not -iname '*ProRes*' -not -iname '*H264*' \) \) -print0 2> /dev/null | xargs -0 du -s | sort -n | cut -f 2 | while read line; do printf "%q " "$line" ; done ; echo

Here's what it does. The first part invokes the find command; this finds any file that ends in .aiff, .aif, .wav or .mov, with the added restriction that .mov files need to be at least 5GB long and have a name that doesn't include the strings ProRes or H264; this eliminates most if not all of the compressed video files. The -print0 command says to separate the output file paths with a nul character instead of a linefeed (needed so the next tool doesn't get confused by spaces in filenames), and the 2> /dev/null redirects any error messages to the great bit bucket in the sky.

Each path gets processed by xargs, which is a tool that lets you run other tools on each line. The -0 means use nul as the line delimiter, and it runs du -s (disk usage) on the file paths.

The output of that is a set of lines, each containing the length of the file in disk blocks plus the path, separated by a tab. This gets piped into the sort tool, which is told to sort the lines by their numeric value by using the -n flag; I want them in this order so pbzip2 can compress the smallest files first, freeing up space for the larger ones; often an archive drive will be almost full when I start to compress it.

Next the cut tool is used to extract the second field, which gets us back our list of paths, now sorted smallest to largest.

Finally, I need to put all these paths on a single line, separated by spaces, and properly escaped (spaces changed to "\ ", for example). There is a printf (print formatted) tool for this, but the "%q" formatting code that does the escaping is not implemented in the MacOS version of printf (bitch moan bitch moan). However, printf is also implemented as a built-in command in the Bash shell, and that version does implement "%q", so a little inline shell script will do what I need - it reads each line, prints it out escaped with a space after it, and then echos a blank line. The final result is a single long line containing all the file paths, which admittedly looks like crap but I can just copy it, type in pbzip2 -v (or any other compression command) and paste it in. Actually, given how pbzip2 spawns multiple threads and can chew up a lot of your cpu resources, you probably want to do something like nice -5 pbzip2 -v to make it a bit more polite.

This won't work if the filename has really weird characters in it, like carriage returns, but that isn't a problem for me.

Let me end with a big shoutout to all the contributors to the many postings on stackoverflow that helped me find the right tools and combinations.

PS: I later stumbled upon this excellent comparison of various compression tools which includes an efficiency/time tradeoff chart. Of course, depending on what you are compressing, your mileage may vary!

Friday, February 20, 2015

The anagram of Samarang is Anagrams

A few days ago I randomly stumbled upon the "check if two strings are anagrams" interview question, and it piqued my interest.

While it is immediately obvious there is an O(n-logn) algorithm for this -- sort the character arrays and compare them -- after a few moments I flashed on the O(n) algorithm for comparing two strings and determining if they are anagrams of each other.

This got me thinking, and I remembered the fun I had about 15 years ago assembling a huge list of english words and checking to see if the .com domain names were available. A little quick googling found some interesting word lists and I was off to the races.

I ended up implementing the O(n) comparison algorithm along with a data structure that chops up the word list by word length and then hashes the words using a hashing algorithm that causes potential anagrams to hash to the same bucket. However, checking all the words in each hash bucket is O(n^2)

Thus, it is typically much slower than the simpler nlogn algorithm in Python, because the lengths of the strings and the typical hash bucket are small, the python list.sort() function is highly optimized, and once you do have the strings and bucket sorted, the search for anagrams is O(n).

And O(n^2-logn) < O(n^2-n)

The moral of this story is, in order to figure out the best algorithm, you need to consider the whole problem. I fell in love with the O(n) comparison, but didn't notice for a while that changing the problem from "check if two strings are anagrams" to "find all the anagrams in a word list" made a big difference.

Still, it was a fun evening of coding, and my python is getting marginally less horrible.

You can find the python code, word lists and sample output here.

PS: Samarang is a place in Indonesia

Wednesday, January 7, 2015

Troubleshooting a Brother DCPL2540DW Wifi Configuration

This is one of those posts I'm mostly doing for the benefit of "The Next Poor Bastard". My Mom got a new DCPL2540DW printer the other day and it took a little headbanging to get it talking to the other devices in the house.

It's a fairly nice printer: built in scanner/copier, two-sided printing, WiFi with autoconfiguration, etc. Initial setup was easy, including one-touch wifi configuration.

Just one problem: you could see the printer on the network, but attempts to print to it just died -- nothing could talk to it. Connecting a machine to it via USB worked fine, so there was a problem with the configuration.

Condensing down about two hours of troubleshooting:

  • The Wifi status printout page doesn't tell you anything useful, but the Network menus have a status option that does -- stuff like IP address.
  • The Printer was connecting to the WiFi router fine, but instead of getting an address in the 192.168.2.X range like all the other devices, it was showing a 169.X.X.X address -- a self-assigned IP address.
  • My guess is that the WiFi router doesn't like to route packets to 169.X.X.X addresses, but probably accepts packets from them. So all the local devices could see the printer's "hey guys, open for business" announcement, but not do anything with it.
  • OK, several possible ways to fix this: tweak the router so it routes between 192.168.X.X and 169.X.X.X, force the printer to get its IP from the router, or the brute-force solution: give the printer a fixed IP address in the 192.168.2.X range.
  • So brute-force it is: go into the router configuration DHCP settings, and adjust the range of IP addresses it gives out to connected devices. It was, I changed it to .2-249. Then go into the printer's settings, and there's an option for setting the IP address -- set it to

Bing! Printing now works from everywhere; Macs, iPads (Airprint), even the phones.

Some other minor tidbits:

  • The Mac drivers package only installs on Mac OS 10.7 and higher, but Mom had an older 10.6.8 machine. However, you can download individual components (CUPS printer driver and Twain scanner driver) and they do install.
  • With the scanner driver installed, you can use Image Capture to control the printer and scan, even from a 10.6.8 machine.
  • You can print from a 10.5.X machine using the generic PCL 6 CUPS driver, but not over the network; you have to use the USB connection.

I'm probably going to try and get remote email printing working at some point, if so I'll update this post.

Brother Support Pages for Printer