malloc's comments

On: Watch what you store on SkyDrive–you may lose your Microsoft life

[malloc]

Ideally, client side encryption might be a good solution for this. From what I've seen, there isn't a public API to work with skydive. Even some open-source efforts are mostly outdated by now. Maybe that could be something worth looking?

On: A web video company that is most likely a hoax

[malloc]

http://investorshub.advfn.com/boards/read_msg.aspx?message_i... should provide enough info about this.

On: Inspired by XKCD:903, Wikipedia steps to philosophy

[malloc]

minor correction: 20 clicks

On: A $4.8M model airport with computer controlled vehicles

[malloc]

Miniature company with team of ~200 builds airport model... how is this hacker news related?

[fhars → malloc]

The hacker movement grew out of the MIT model railroad club, so model railroads are by definition on topic :-)

[malloc → fhars]

http://ycombinator.com/newswelcome.html : ("A crap link is one that's only superficially interesting. Stories on HN don't have to be about hacking, because good hackers aren't only interested in hacking, but they do have to be deeply interesting.")

On: Think Stats, using python to learn stats

[malloc]

anyone knows of a similar resource for physics?

[ylem → malloc]

What area of physics do you want to know about? There are some free books (for example I know of one on small angle neutron scattering that you can find at NIST)--also there are many lecture notes online. For texts, though do you think it would be better to have a static PDF, or a wiki, where an author could add what they could and the community could improve it?

On: Torrent meta-search engine

[malloc]

damn you [developer of this], had this idea pending for the longest time. yeah, I know this crowd knows this line too much.

Anyway, my approach would be different by a single feature: movie sync. I've already watched many of the movies listed, and would be great to give you my imdb rating public url and you filtering those out. You could even implement a basic recommendation system using my ratings to give best matches.

Hope you can keep this service up.

On: Ask HN: What advice messed up your life?

[malloc]

mind telling more about your experience on this? how far would you say "getting out of your comfort zone" did you reach? how did programming affected your life (seems like you meant you spent too much time doing so before getting to that... how was it?)

[noname123 → malloc]

Personally after college, I had four goals: 1) be able to bench-press 200lb, 2) play guitar in a band/be able to improvise, solo, play at performance tempo etc., 3) be able to play pick-up basketball and win more games for my team than lose, 4) to approach women and other people in bar and other urban scenario's and f-close or the social equivalent.

No, I haven't completely achieved all of my goals although I have gone very far in all of them. But I'm not as anxious about them nor about admitting on my shortcomings on my goals in public; like in programming, at a point you develop a sense of confidence that even on a long programming project, it's only a matter a time that you will finish it. It's just a sequence of iterations of coding & debugging.

How did programming affect my life? It made me a nerd with inferiority-superiority complex who on one hand views average human beings as automatons who let their sense of social insecurity get in their way; and who on the other hand is insecure himself, requires constant social re-assurances, stroking of the ego and (most importantly) sexual gratification. And for that, I'm eternally indebted to programming.

On: Show HN: FlyByChat, the site I'm working on. Looking for feedback.

[malloc]

Looks great, the site. Would add sound notification on message arrival (optional).

On: Ask HN: What to do after graduation with 100k?

[malloc]

I find this discussion of great interest. For those suggestion he goes traveling, taking some time off, what destiny would you recommend?

Be specific please.

Personally, I can't get satisfied with landscape/water, so its hard to find cool places :D

On: Low Level Bit Hacks You Absolutely Must Know

[malloc]

it doesn't deserve the time...

first only works if a is a different var than b

second it can be much slower than typical swap...

and finaly, it isn't clear what it does if your code is for others to read

[scottdw2 → malloc]

You are write on the time issue (but it does work if a and b are the same)

The reason becomes very apparent if you write out the assembly necessary to compile the xor, vs what is needed to compile a classic swap.

The xor would look something like:

load $1, a load $2, b xor $3, $1, $2 xor $4, $3, $2 xor $5, $4, $3 store b, $4 store a, $5

where as the classic swap would look something like this:

load $1, [a] load $2, [b] store b, $1 store a, $2

which is much better.

If you take the pipeline into account, then the difference between the swap and the xor can be huge because their is high level of dependence between the instructions.

On a theoretical classic 5-stage pipeline, the swap approach ends up needing about 10 cycles, where as the xor needs about 18.

In a real processor the difference would probably be much worse.

[ori_b → scottdw2]

If you've hit memory, you've already lost. Running instructions will basically be line noise compared to that. Similarly, function call overhead will probably dwarf actually doing the xors.

However, with a good register allocator and inlined functions, your point becomes even stronger. The compiler can simply remove all instructions associated with the naieve swapping version, and simply record that before the swap %eax holds b, and after it, %eax now holds a. (Even the most naieve of code generators will do this sort of thing if the values in the swap don't fall outside of a basic block). The xors are far harder to optimize.

[patcdr → malloc]

It still works fine if a and b are the same value.

Here's the general proof:

a ^= b (a = a ^ b, b = b)

b ^= a (a = a ^ b, b = b ^ (a ^ b) = a)

a ^= b (a = b ^ (a ^ b) = b ,b = a)

Now, let's replace all references initial values with constant c:

a ^= b (a = c ^ c = 0, b = c)

b ^= a (a = 0, b = c ^ 0 = c)

a ^= b (a = c ^ 0 = c, b = c)

Notice that at the end, you still end up with a = b = c. There are plenty of reasons not to use this approach, but that ain't one of them.

[jibiki → patcdr]

He meant if they are the same variable, not the same value. (E.g., you pass pointers to your function.)

  int swap(int * a, int * b)
    {
    (*a)^=(*b);
    (*b)^=(*a);
    (*a)^=(*b);
    }
  int x = 15;
  int *y = &x;
  int *z = &x;

  swap(y,z);  //Now *y == 0

[gjm11 → patcdr]

The case in which it breaks is when a and b are the same variable, not when they have the same value.

[yters → malloc]

Why would I want to swap the same var with itself?

[ori_b → yters]

Assume you had a sorting algorithm that swaps values, and in some cases would swap two of the same values. It's easier (and often faster, thanks to the high cost of branch misprediction) to simply assume the swap of something with itself is a no-op than to explicitly check the address of the value to make sure it isn't the same. Or you could have just forgotten to make the check, and allowed the user to pass in their own swap functions (say, to deep-copy structs in C).

[sofal → yters]

Your code should be robust enough to provide an intuitive default behavior for corner cases like that. That way the application code doesn't need to be cluttered with special conditions.

[scott_s → yters]

You wouldn't, but let's say you wrote a function with the signature

  fast_swap_values(unsigned long*, unsigned long*)

Users of the function could accidentally pass in the same addresses.

[pkrumins → malloc]

that is true, hackers, that is true.

[kragen → pkrumins]

But they cannot help their neighbors!

[pkrumins → kragen]

that's not good, hackers, that's not good!

[duskwuff → pkrumins]

Obligatory: http://www.jwz.org/hacks/rms-deathmetal.mp3

(JWZ's subtitle: "Why Cooperation With RMS Is Impossible".)