Thursday evening, I found a link to Can you crack it? posted by a friend on Facebook. He also added
the link stage 2 of 3, which is what you find when you solve the first code.
I failed to solve the first code on my own, not realizing that it could
be machine code. What you have to do in this second stage, is to write
a virtual machine in JavaScript. Inside the
comments there is a description of the virtual machine, but when I tried
to implement it, it turned out to be not so accurate. I had a hard time
with respect to the jump ("jmp" and "jmpe") instructions. First I discovered
that "r2" in "r2:r1" did not mean the contents of register indicated by
operand2 but the value of operand2. But it took me a long time to realize
that register cs should be set to this value as well, which is nowhere
mentioned in the description. But the letters cs stands for "code segment" and
the register was not mentioned in any other way in the description, so I could
have guessed it. I did think about it, when I was reading the blog
Can you crack it? (nop, I tried though). When I looked at some other
implementations, I found this idea implemented. All of the implementations I
look at where in other languages. I wrote a version in C++ as well when my
JavaScript implementation would loop and could not give me a clue about what
was going on.
The desciption is not clear about whether register fl is accessible
like the other registers. It is not used as such, but why would you specify
that it should contain a value of 0xff in the compare operation, when it
is impossible to read out that value, because you only have a conditional
jump instruction on it being zero.
This morning, I improved my JavaScript
implementation (and my C++ version
as well). The main improvement being the "ip" register a 8-bit
value and using the getMem function in the implementation of getMemIP
function using register cs as the segment value, which seems very natural.
If JavaScript is enabled, and you have a relatively
up-to-date JavaScript support, the result should show below. I added some
code to display the string that starts at memory location 448 and contains
the answer to the second stage. If the value behind "debug" in the script
is replaced a non-zero value, the script will display some debug information
about the instructions being executed and it gives a memory dump at the
end of the execution.
Today, a colleague encoutered a situation where some sorting with
quicksort did not
work. As a solution, he used a different sorting algorithm. Usually,
when sorting with quicksort does not work, there is a problem with the
compare operator not being transitive. And indeed, this appeared to be the case. The sorting
involved a partial relationship and in case two elements where not
involved they where compared on name. The question was, how to use
quicksort in case you have a partial transitive relation. Two colleagues
came up with an alternative. One said that if the compare function would
always return false when the two elements where not related, that it
would work. I was not immediately convinced of this fact and he asked
him if he could give a formal proof. He replied with asking me to give
a counter example. I could not immediately think of one, but replied
that it was not my task to do this. (As I joke, I told him that I
believed that NP!=P and if he could give me a counter example of that.)
My greatest objection against his compare function is that although it
might work for quicksort, it would probably not work with any of the
other uses in the Standard Template Library that we are using. Another colleague
came up with the idea to sort on the maximum relation depth, which
is defined as the length of the longest chain of elements with a given
element that pairwise match the relation and where the last element is
this element. It is easy to see that if two elements are in the relation
the maximum relations depth of the second is always at least one higher
than the first. Elements with the same maximum relation depth can be
compared on name as before. Calculating the maximum relation depth is
not always cheap, but this compare function does meet all the requirements
for a regular compare function. I am still thinking about the optimal
sorting algorith when the partial relation is not transitive by definition
and when you do not want to calculate the transitive relation of the
relation first.
Around ten o'clock in the morning, there were big flakes of snow falling from the sky and the snow stayed on about half of
the ground. But soon the flakes became smaller and also wetter and
the snow did not stay long because the temperatures were well above
the freezing point.
This afternoon, I went to bookshop Broekhuis.
I looked at the exhibition of Rudy Klomp and
Ans van Berkum. I found the drawings of Van Berkum as interesting as the
painting by Klomp. At 13:46, I bought
Moleskine Daily Diary/Planner 2012 (ISBN 9788862937306) for € 15.95.
At home, I discovered that again, I bought the soft-cover edition and not the
hard-cover as I wanted. I guess that I picked the hard-cover edition with the
address book insert, but then noticed that it was a little thicker than the
edition with "Adhesive Labels" and did not realize that it was a soft-cover
edition. I gusee, I stick with this version, because the one from 2011 still
looks okay. I use these diaries to record what I
did during the day.
This morning, there was a broken layer of snow on
the ground. Not all the ground was covered but by judging the layer of snow
on the cars, it seems that about one cm (half an inch) of snow fell
during the night. Most of it melted before noon.
This evening, I opened andy
and removed some parts because I want to dispose of it soon. I removed
the hard disk, a Connor CFS635A, with 635MB unformated size. I also
took out the two 8 Mbyte memory modules and the network card from the
Multu Player Gamers Network Kit, which I installed on March 10, 2001.
I finished readingNotes on a Scandal by Zoë Heller, which I started reading
on November 22. I bought the book through a second hand website after I had
seen the film adaptation. Of course, there are some differences between the
plots of the film and the book. The most notable, I feel, is that Sheba stays
with Barbara at the end in the book, while in the film she leaves her. It seems
that the manipulative character of Barbara is far more explicit than in the
book, where it seems that Barbara is not actively trying to manipulate situations.
Nevertheless, I found the book interesting to read.
On December 16, Timo Ewalds presented his master thesis Playing and solving Havannah, which is about his program for playing
the abstract game Havannah. He also solved
Havannah for board size four (and smaller). He made the source of his program
Castro open source on github.
This months interesting links
