Category Archives: geek

Scala Question Regarding readLine

I have stumbled upon a weirdness regarding the behavior of readLine in Scala (at least with my current basic understanding of the language).

The following snippet is supposed to read three times a line from standard in. But it reads 3 times the same line (the first).

(1 to 3) foreach {
  val line = Console.readLine
  i => println(line)
}

while the following snippet (very similar) behaves as expected (reading the three first line from stdin):

(1 to 3) foreach {
  i => println(Console.readLine)
}

Any idea why?

Java Quiz of the Day

Java quiz of the day (easy): what gives

Object[] a = new Integer[]{};
List<Object> b = new ArrayList<Integer>();

Response: we get an exception on the second line:

Type mismatch: cannot convert from ArrayList<Integer> to List<Object>

Arrays and Generics are not consistent. While for arrays, an array of some type is also an array of one of its parent types, the same is not true for Generics. If it was, we could construct weird things where we would get at most a runtime exception when trying to access items of a collection. See the following example:

class A{
	void foo(List<Integer> list) {
		List<Object> otherList = list; // (A)
		otherList.add("hi"); // (B)
	}
}

List<Integer> list = new ArrayList<Integer>();
list.add(5);
new A().foo(list);
for (Integer i : list) {
	System.out.println(i.intValue()); // (C)
}

Here line (A) is not a legal assignment. If it was, then we could insert into our originally List a String on line (B). Then back from our method foo, we would get a runtime exception when processing the list on line (C). This problem was fixed for Generics, while similar problems exist for arrays.

Java Quiz of the Day II

What gives:

Integer a = 100, b = 100;
Integer c = 200, d = 200;
System.out.println( (a == b) == (c == d) ); // A

Response: counter intuitively, the program prints ‘false’ out. Integer in Java is a non-native class, where ‘==’ means ‘reference equality’ (two variables are reference equal if they are pointing to the same object). From there we would expect the expression of line (A) to be reduced to ‘false == false’, but it is slightly more complicated: the Integer class has the particularity that all Integer (like Short) in the range [-128, 127] are shared. This means the two variables ‘a’ and ‘b’ are actually pointing to the same shared object. Hence the expression of line (A) is in fact reduced to ‘true == false’.

Java Quiz of the Day

What gives

Double a = null;
Double b = a == null ? a : 1.0;

Response: as strange as it can seem at first look, b is not equal to null after the second statement, but the code produces a NullPointerException.

The reason for that is the type of the second expression (double) inside the ternery operator. The first expression ‘a’ of type Double is unboxed to double (calling doubleValue() on ‘a’ which is null), causing the NPE.

You can find a discussion about that here.

Sparse Files – Commands Overview

I was just checking which commands are supporting sparse files. I post hereafter a short overview of what I found out (working on Linux).

  • Create a sparse file of 20 GiB:
    dd if=/dev/zero of=foo bs=1 count=1 seek=20G

  • Check that a file is sparse:
    ls -alsh
    Compare the first versus second size column (the first one is the space taken on disk).

  • Copy a sparse file:
    cp foo bar
    Copy already detects and handles correctly sparse files.

  • Make a non-sparse file sparse (works only if it contains blocks filled with zeros):
    cp --sparse=always foo bar

  • Expand a sparse file to a non-sparse one:
    cp --sparse=never foo bar

  • Copy remotely a sparse file: scp does not support sparse files, it will expand them to non-sparse ones. Rsync does support them. Just use the -S or --sparse option. Example:
    rsync -vS foo root@someserver:/some/path/

How to Know if a File on Linux is Sparse?

A sparse file is a file which does not take more space on disk than needed. Such a file is usually used to store a partition image on disk, for instance with a virtualization solution like Xen.

It’s super easy, to know if a file is sparse or not. Just use the ‘s’ option of ls.

ls -alsh

will yield:

4.0K drwxr-xr-x 9 root root 4.0K 2010-03-23 18:22 .
4.0K drwxr-xr-x 23 root root 4.0K 2009-01-09 19:47 ..
12G -rw-r----- 1 root root 24G 2007-01-10 19:55 dompat.data
3.7G -rw-r--r-- 1 root root 7.1G 2009-01-06 21:09 dompat-hardy.sys
501M -rw-r----- 1 root root 501M 2007-01-07 16:40 dompat.swap

Where the first size column is the effective space taken on disk while the second size column is the max space of that file. We see that dompat.data is sparse, since its max size is 24 GB while it takes only 12 GB on disk.

References

  • More information on command which can handle sparse files in this article

GPGMail Compatible with Snow Leopard

I had posted that some times ago on Twitter. There is a new version of the excellent GPGMail plug-in for Mail (OS X). Grab it, if you haven’t already.

GPGMail for OS X 10.6.2

Just put it inside ~/Library/Mail/Bundles and restart Mail. (More information on GPGMail’s web page.)

Edit: This is not an official version of GPGMail. There is still no official version of GPGMail compatible with Snow Leopard.

NFS Automount with Snow Leopard

Before upgrading to Snow Leopard, I was using the NFS automount capability of Mac OS X. It was a handy way for my laptop to connect to my home NAS server (a linux box running Ubuntu).

Unfortunately I had to redo the NFS config after upgrading and I noticed that Directory Utility was not present anymore in Snow Leopard.

Here is what I found out after some research:

  • the NFS automount configuration is now done directly in Disk Utility (under File > NFS Mounts…) and not in Directory Utility like in Leopard (which does not exist anymore).
  • the configuration that worked for me was the following one (for accessing a read-write NFS share):
    Remote NFS URL: nfs://[server]/[path]
    Mount location: [path to local mount folder]
    Advanced Mount Parameters: -i,-s,-w=32768,-r=32768
    
  • for automount to reload its configuration, I had to run the following command:
    sudo automount -vc
  • That’s it. The NFS share should now be accessible.

    References:

  • http://discussions.apple.com/thread.jspa?threadID=2137675
  • Time Machine: Handy but Bitchy!

    I have been spending the last couple of days of my spare time performing a simple operation with my mac, yet slightly more complicated than expected: merge the two partitions of my hard drive into one. This was necessary since I did a mapping, quite usual on Linux, where my system was sitting on the first partition (50 GB) while my user data was on the second one (the rest of 300 GB). But this mapping was not making me happy, since the system partition was always almost full (Mac OS X is not well suited for a multi partition disk drive, I find).

    Although you can resize a live partition with Disk Utility (great feature, even if you booted with it), you cannot move the base position of a partition. You can merely resize it, if more space is available. So I had to back my second partition up for later restoring it. I went for Time Machine, since I was already using it for backing up my laptop.

    Time Machine Caveats

    Unfortunetely I noticed a couple of caveats during the restore operation:

  • When using the Time Machine restore utility shipped with the OS X install DVD, you can only restore the partition where the system is, but not other ones. This was quite of a surpise to me. Although you can choose on which partition you want to restore your system (the target, not the source).
  • Although your data is actually sitting on the Time Machine backup drive, you cannot use it directly (i.e. copy it back using the Finder or a shell). The reason for that is the ACLs that Leopard is putting on each and every file and folder to protect changing the Time Machine backup. In addition the ACL system (which is a parralel access control to the Unix one, which I did not know the existence of beforehand) is deeply flawed: you cannot reliably remove recursively the ACLs from a directory structure (you will still find files scattered within the structure having ACLs) and you even cannot remove the ACL permissions on symbolic links (this seems to be a bug of chmod on Leopard, although ACLs on a symlink do not seem to have any effect)
  • Solution for merging two partitions

    I finally found a solution to restore the second partition or to merge them: I had to fiddle directly with the Time Machine backup, copying manually the files I wanted from the second partition to the first, and did a restore of the first partition using Time Machine restore utility.

    Here are the details of the steps I took:

  • Plug my external drive where I have my Time Machine backup, open a terminal and sudo as root:
    sudo -s
  • Deactivate on that drive the ACL checks, so that I can modify directly the Time Machine backup:
    fsaclctl -p /Volumes/[backup drive] -d
  • Move the folders from the second partition to the first one with
    mv [from] [to]
  • Reactivate the ACL checks on the external drive:
    fsaclctl -p /Volumes/[backup drive] -e
  • Do the restore of the first partition with the Time Machine restore utility (located on the Leopard install DVD).
  • This worked for me. You should now have a merged partition on your drive, the restore utility having removed the ACLs during the operation. Phew! This was not a straight forward action! I still cannot believe that Apple did not take into account that people can have more than 1 partition on their drive (I might still have missed the way to do it, did not find it so far though).

    Links/Info

  • Macosxhint: reconnect Time Machine backup after a drive swap
  • Inspect the ACLs of your file with ls:
    ls -le

    For the record, this will yield the following listing when ACLs are present (pay attention to the lines starting with ’0: ‘ and ’1: ‘:
    drwxrwxr-x@ 138 root admin 4692 Sep 13 2008 Applications
    0: group:everyone deny add_file,delete,add_subdirectory,delete_child,writeattr,writeextattr,chown
    1: group:everyone deny delete
    drwxr-xr-x@ 2 pat staff 68 May 9 22:12 DeveloperSDK3
    0: group:everyone deny add_file,delete,add_subdirectory,delete_child,writeattr,writeextattr,chown
    drwxr-xr-x@ 5 pat staff 170 Oct 23 2008 VirtualBox
    0: group:everyone deny add_file,delete,add_subdirectory,delete_child,writeattr,writeextattr,chown

    You can also do that recursively using the following command:
    ls -lateR > filelist

  • You can remove ACLs on a file with this command (not working properly for a recursive operation with the option -R)
    chmod -a# 0 [file]
    where 0 is the ACL entry to remove.

  • The Geekiest Hello World

    Folks, I think that I have found (one of) the geekiest hello world out there. Implemented in Groovy using the dynamic interception of method calls. Here it goes:

    class Hello {
      Object invokeMethod(String name, Object arguments) {
        System.out.println "hello $name!"
      }
    }
    
    def hello = new Hello()
    hello.john()
    hello.you()
    

    Which produce the following output:

    hello john!
    hello you!
    

    I suppose the same mechanism can be used with Ruby (and some other dynamic languages).

    Self-Reproducing Program in Groovy

    I had this in mind since yesterday: how to write a self-reproducing program in Groovy, or a Quine, a program which prints out its source code. It’s actually a fun exercise.

    Here is my version:

    a = 'c = (char)39; println "a = $c$a$c"; println "$a"'
    c = (char)39; println "a = $c$a$c"; println "$a"

    It looks scary, but it is actually not that terrible. The first line is the definition of a variable a of the form:

    a = '[some string]'
    

    The second line is first the definition of the single quote character assigned to c. We then print out the first line (including the variable definition), where we replace the single quote by our character c, and where $a is the content of a. This part looks like:

    println "a = $c$a$c";
    

    Finally we print out a.

    The only thing that we need now to do, is to replace [some string] in the first line. It happens that if we take the complete second line and assign it to ‘a’ in the first line, we will get the desired output. We see here that if we had not used $c in the first println of the second line instead of the single quote, the string assigned to a in the first line would not be legal, since it would contain inside the string a string delimiter (and if we escape it, we don’t get the same output as the program code).

    That’s it. It was fun.

    Links:

  • A quine on Wikipedia
  • Interesting blog post about a self-reproducing program
  • Annoying ‘screen’ Backspace Problem

    Already a long time that this issue is bugging me. The great ‘screen’ unix command has a problem, at least on ubuntu: the backspace key produces the same effect than delete (first char on the right of the cursor gets deleted). Very annoying. Hopefully the solution is easy. Edit your file ‘~/.bashrc’ and add the following line:

    alias screen='TERM=screen screen'

    Pidgin Status Update from Twitter

    Just figured out how to get Twitter update the pidgin status (e.g. GTalk, Jabber, ICQ, MSN, etc.). For that, you need to install a pidgin plug-in written in Perl. Here is a small howto about it (as the Perl stuff was a bit tricky):

  • Download and install the multi-network Pidgin client
  • Install Perl. On Windows (yeah, I know. It’s at work. ;) ), install Active Perl version 5.8.8.X. Don’t take the last one, the 5.10.0, as Pidgin does not support it yet.
  • Install the Package XML:XPath, either using the CPAN console (install XML:XPath) or using the package manager of Active Perl.
  • Restart Pidgin and check that Perl is enabled. (Help -> About -> at the bottom of the page, you should see ‘Perl: Enabled’).
  • Download the twitter plugin and put it in the plugin folder of Pidgin. (For instance on Windows in ‘C:\Program Files\Pidgin\plugins’)
  • Configure the plug-in and enjoy!
  • Flock for Blogging

    Just trying the Flock browser for posting a blog news. It’s great. It’s about like writing an e-mail, with a good blog integration, where you can define tags, categories and that kind of news-related things. Really cool. Much quicker than the web interface. ;)