Archive for September, 2008

Android's locking pattern

As you might already know, Android uses an innovative approach to lock your phone and prevent accidental dialing. Play the short video above to see how it works.

For the rest of this discussion, I will use the following convention to designate a locking pattern:

0 1 2
3 4 5
6 7 8

A pattern can be pretty much anything you want with the following constraints:

  • It needs to contain at least four dots (and obviously, no more than nine).
  • Once a dot is lit, it can’t be used again.

  • You can use one or several “knight moves”, such as in [0 5 4 2]:


    In this example, the knight move is between 0 and 5.

  • You cannot go over an unlit dot without lighting it. For example, the pattern [0 2 1 4] is illegal, because moving your finger between 0 and 2 will light 1.

  • Once a dot is lit, you can use it to reach another unlit dot. For example, both [0 4 3 5] and [0 4 5 3] are legal:



    This example is a bit ambiguous but you get the idea: I trace [0 4 5] and then I slide over 4 again to reach 3.

With these constraints in place, I wondered how many combinations were actually possible, so I wrote a short program to simulate it. It turns out there are quite a few: 389,112. Here is a breakdown:

4 dots: 1624 solutions
5 dots: 7152 solutions
6 dots: 26016 solutions
7 dots: 72912 solutions
8 dots: 140704 solutions
9 dots: 140704 solutions
Total:  389112

If you’re not convinced, here is a full list of all the four dot patterns (the page only works on Firefox and it uses a big canvas object, it might take a few seconds to load).

Still, I’m not sure my calculations are right, so I’d like you, dear reader, to confirm my numbers, and optionally explain how you coded your solution…

Best software blogs

I just came across this list of the Top 100 blogs for development managers and I’m very flattered to find myself in position 14. I’m in good company and I’m happy to see I’m already a regular readers of the top blogs, among which Joel Spolsky, Jeff Atwood’s Coding Horror and my good buddy, Steve Yegge.

Good list overall, I’ll add some of these to my RSS reader.

Shows with replayability value

I recently asked myself what show I’d be willing to watch for a second
time. I watch a decent amount of series, and there are quite a few
good ones, but I realized that very few have replayability value.

After a quick survey of these past years, I came up with Buffy,
Voyager, Babylon 5 and Angel. Going a little bit further back, I thought I might
be interested in watching Twin Peaks again.

How about you?