Judebert.com (Entries tagged as java)

Interview Day 6: Unflattening a Bush

judebert@judebert.com (Judebert) — Sun, 20 Jan 2013 16:41:00 -0500

Oh, great. Yesterday I flattened a bush; today you want me to un-flatten it?

This is going to require a bit of recursion, I can just tell. That's fine, I love recursion.

I kept all the child pointers when I flattened the bush, so that will tell me where the children start. I'll just traverse the list, and anytime I find a child pointer, I'll disconnect the child and all its siblings, reconnecting them as the child of the parent.

Hmmm. Only one small problem: how will I know when to stop disconnecting the child? The first parent that I encounter will get the child, and the remainder of the list afterwards, as its children.

Maybe I'd better rewrite Flatten to use a different algorithm. Certainly the interviewer would have steered me toward a solution with recognizable ends. Wouldn't he?

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Interview Day 5: Flattening a Bush

judebert@judebert.com (Judebert) — Fri, 18 Jan 2013 21:42:26 -0500

Yesterday I made a linked list, in Java. Yeah, that's right.

Today I'm taking on a programming problem: given a doubly-linked list (with head and tail), where any element may have a child doubly-linked list (without head and tail), flatten the structure into a doubly-linked list.

That sounds vaguely tree-like, but trees don't usually have linked siblings. It's all just parents and children. I'm calling this a bush.

Let's look at the structure.

head->A<=>D<=>E<=>F<-tail
      |           |
      B<=>C       G
                  |
                  H

Without being told where to put stuff, I could just tack it all on the tail. But I think tacking it on after its parent is more elegant, if it can be implemented in similar complexity and space. In the bush I drew, I'd like the nodes to end up in alphabetical order.

I might even be able to do this without recursion.

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Interview Prep Day 2: Studying and Champagne

judebert@judebert.com (Judebert) — Wed, 16 Jan 2013 06:50:00 -0500

Today I only studied a little bit. I had too much other stuff to do. But I did manage to find a nice brain teaser from GeeksForGeeks.org.

Imagine a champagne pyramid, but in only two dimensions. Like this:

Level 1:    1
Level 2:   1 2
Level 3:  1 2 3

Of course, we could have as many levels of this "pyramid" as we liked.

All the glasses are the same. When you overfill Level 1 glass 1, the overflow goes half to the left (Level 2 glass 1) and half to the right (Level 2 glass 2). Every time a glass is overfilled, half the overflow goes to the left and half to the right.

Find a method to tell me how much champagne is in any glass, when I tell you how much I poured into the top glass.

For instance, if I say I poured 4 glassfuls, and I want to know about Level 3 glass 3, you'd tell me it had 0.25 glassfuls.

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Unsigned Subtraction in Old C++ Code

judebert@judebert.com (Judebert) — Wed, 18 Apr 2012 11:05:42 -0400

I'm porting some old C++ code to Java. How old? So old that it doesn't just assume a long is 32 bits, it actually checks to make sure and bails if not. That's 1997 or so, by my guess.

I can already hear the programmers in the audience asking, "Why not just interface with the library using JNI?" We decided to port the code because we must support Unix and 64-bit environments, and the library specifically fails on all our Unix machines. We do have the source, so we could modify it... but then we'd be forced to deliver and maintain it, which carries its own set of headaches. So I'm working on 20,000 SLOC.

(Anybody wondered why the site has been so quiet for so long? It's because I'm porting 20,000 SLOC of OLD C++ to Java.)

Most of the code has been pretty straightforward. There have been the usual problems with C++ unsigned types, which don't have any corresponding Java type, as well as a bit of pointer arithmetic confusion. When I get confused, I just write a quick test program and make sure the C++ does what I expect.

The thing that blew my mind today was the weird use of the "magic number" 0x80000000.

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Using SSL in Java, Part 2

judebert@judebert.com (Judebert) — Fri, 09 Mar 2012 09:15:39 -0500

In part 1, I explained how key-based SSL authentication works. In part 2, I intend to explain how to implement a certificate authorized SSL connection in Java.

If you haven't read part 1 yet, you might want to check it out. I'll still be here when you get back.

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Using SSL in Java, Part 1

judebert@judebert.com (Judebert) — Fri, 10 Feb 2012 13:40:43 -0500

Working (as I do) with secure equipment, it was inevitable that I'd eventually have to connect to a server using SSL. I didn't even get to write my own code; I had to use a Java client library that was provided to me.

The programmers of my client library weren't exactly forthcoming on where I could change its configuration, but at least they had provided me with the certificates I needed. I had to do a lot of research to find out how to use them, and I'm recording that information here.

After all, isn't learning supposed to be cooperative? Read on, and I'll show you how to work with SSL sockets in Java.

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