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#include <stdio.h>
 
int main()
{
    signed char x = -128;
 
    if (x < 0) {
        x = -x;
    }
    printf("x = %d\n", (int) x);
    return 0;
}

I’ve been reading Matters Computational, an excellent (free) book by Jörg Arndt about programming and algorithms. It surprised me in the very first pages with this pitfall in two’s complement – there is always a number that is equal to its own negative, besides zero. The code above prints −128!

In hindsight, it’s pretty obvious. A signed char can hold values from −128 to 127 — that is, there are 127 positive numbers and 128 negative numbers! Therefore, it’s impossible to the unary negative operator to be one-to-one. The smallest negative number will always be mapped to itself. Of course, this also applies to int, etc.

The main implication of this fact is that, after the innocent-looking code

if (x < 0) x = -x;

x is not guaranteed to be positive!

(…) They [Netscape] did it by making the single worst strategic mistake that any software company can make:

They decided to rewrite the code from scratch.

I agree, mostly. Many projects market that they’ve been “rewritten from scratch” as if it was something marvelous, and most of time, it’s just a sign that the new version probably has more bugs than the previous. But, wait, I just rewrote Quivi (a image viewer and comic / manga reader) from scratch! Well, why?

Well, I thought I really had to justify this. So here are the reasons:

• It is a small project. Rewriting it certainly isn’t as hard as rewriting a browser!
• I couldn’t stand C++ anymore (the programming language I used before). Of course, this isn’t C++ fault per se. It has its uses — the right tool for the right job, and so on. But to write an desktop application? It’s overkill. If you manage to pull it of, hey, kudos to you. But I had no motivation to work on it anymore.
• I love Python (the programming language I use now), and of course, I’m not the only one. Programming is fun in Python, so much that had the motivation to rewrite Quivi from scratch in the first place! And of course, it will be much more easier for me to keep working on Quivi.
• I could change GUI libraries. SmartWin is a nice library, and uses templates in very interesting ways; when programming Quivi I ended up involved with its development too. But it has its bugs, and because it uses templates extensively, it’s awful slow to compile an application that uses it. I’ve changed to wxPython, which is a very mature GUI library – and cross platform to boot.

Of course, there are some downsides with the change. The whole software package is much bigger due to the dependencies (the installer jumped from 900K to 5MB!). And the program is a little bit slower, mainly when starting up, and uses more memory (9MB to 30MB with no images loaded). But I think it was a good enough trade-off.

Rewriting from scratch must be considered carefully, and in this case, I think it was a good idea. Quivi is a hobby project, and I guess the main point of it is to have fun writing it, and to make users happy. I hope I can do both with this new version (which will be released soon).