Solipsism Gradient

Rainer Brockerhoff wrote:

…Imagine my surprise when I learned that Peter had just published the complete Hex Fiend source code, and also started up a Wiki to explain details…

That said, I haven’t had time to look at details of his data backend yet, but that too looks like it will save me at least a month of tinkering.

Well, I’ve now had a little time to look at the Hex Fiend code.

As I expected, it builds a tree of referenced/changed byte ranges for an edited file. There’s a generic “ByteSlice” class with concrete subclasses that represent either a range of bytes inside a file or a range of bytes in memory – the latter would be the result of an editing operation like typing or pasting in stuff. That much I’m already doing myself, albeit with different names.

The interesting part comes when an edited file is saved. Hex Fiend goes to great lengths in optimizing writing time, allocated RAM, and disk space; it uses threaded AVL trees and lots of neuron grease, and while I understood very generally what’s supposed to be going on, the details are extremely daunting. My hat’s off to the wizard. And he spent similar care on optimized searching, too.

Now, just lifting all that code and plopping it into XRay II just wouldn’t be cost-effective. Yes, the result is that you can open a 240GB file on a 250GB disk, swap huge chunks of it around, and insert random bytes in the middle, and save it with no problem. Do I see this situation arising frequently for XRay II users? Frankly, no. Remember, the idea is to do structured editing of file contents, not necessarily pure hex editing… and Hex Fiend is already terrific at this (not to mention, free). I see the hex editing panels in XRay II more suited to editing small amounts of data and as a convenience to view raw file contents without necessarily changing them.

So, falling back on the old method of saving an edited file to a temporary file, then swapping it with the original if creation succeeded, means that complexity will go way down at the expense of speed (noticeable only for really huge files) and of the necessity of extra free space (same).

A second problem is that, for Hex Fiend, a file is just a sequence of bytes – no structure. For me, on the contrary, changing a byte in one place – meaning editing a representation of (say) an ID3 tag in a music file, or a QuickTime atom in a movie file – will usually mean that elsewhere in the file one (or even several) count or length fields will also have to change, preserving the file’s integrity. So my data representation tree also needs to reflect a particular file’s format as decoded by a plugin – and there may be several plugins seeing the file in different ways – and the nodes need to be more intelligent, notifying each other when necessary.

Still, seeing the Hex Fiend code has given me assurance that I can do it myself, so that’s good…

So, there I was in my trusty BBEdit (thanks Rich!) twiddling HTML code for a friend’s website, trying to get stuff aligned “just so”, checking out the CSS, cleaning up some redundant markup – and of course reloading the browser after every change to be absolutely sure it worked. My friend, who knows how to operate her Mac but is otherwise non-technical, watched this with astonishment… after we were finished (took most of an evening) she exclaimed, in German, “Das ist ja zum Mäusemelken!”

I burst out laughing at this colloquial expression I hadn’t encountered before, and which literally means “that’s for milking mice!”, but actually is used in the sense of “that’s extremely exasperating!”. Or so Google tells me. However, in the sense of twiddling with HTML code – or Objective-C code for that matter – it struck me that it can be taken literally. To quote Tom Digby’s “Little Tiny Eyes” (found via Tim Bray):

“OH YOU NEED LITTLE TEENY EYES FOR READING LITTLE TEENY PRINT

LIKE YOU NEED LITTLE TEENY HANDS FOR MILKING MICE.”

Yep, you have to position those little teeny hands just so and be extremely patient because it’ll take a long time to do it right and get enough… and it takes a very peculiar and determined kind of person to sit there N hours per day, day in and day out, just twiddling away at the little bits to get everything running “just so”.

And that is why my main page has the subtitle “finely crafted software for the Macintosh”. Hm. Should I change this to “Digital Mice Milking” or something? But I suppose that, just because I put this title on this post, that Google will already put me somewhere on its mousemilking results, forever after confusing hapless NIH researchers. But then, that’s the price of fame…

As expected, I’ve worked a lot on XRay II during the trip. I’d hoped to get the raw data updating backend working, but unfortunately debugging the plugin interface took much more time than I had anticipated. In particular, I ran into edge cases on my automatic view resizing algorithms and had to refactor them completely; it turned out that I had 3 different cases – for NSTextFields, for my outline views, and for plugin views – which were fighting each other and thus had to be folded into a single model. Anyway, the new scheme seems conceptually sound and the only thing missing are some optimizations.

To test all this out I started work on a QuickTime plugin. Currently this has a movie preview pane and a QuickTime atoms pane. The preview pane was actually working well when we started out on the trip but it turned out to be surprisingly hard to adapt to the view resizing scheme, so I’ll have to redo it. I’m using Tiger’s new QTMovie view which does a lot of work for you, but at some point it’s unavoidable to dip into the old Carbon QuickTime API which is, to put it mildly, a confusing patchwork.

No doubt there are sound historical reasons for this, but it’s a huge pain to get it all working in a modern Cocoa environment. For instance, the movie inside a QTMovie view doesn’t necessarily obey Cocoa’s clipping rules while it is playing – especially if it’s a QTVR panorama. QTVRs also seem to override Cocoa’s cursors even if the view itself is hidden or clipped out.

The second pane does a hierarchical display of QuickTime atoms. There’s a huge and confusingly documented roster of possible QuickTime atoms and they may be nested in often unintuitive ways. So far, I’ve been just using them to debug my nested container views and the autoresizing scheme, but it’s clear that this will probably be an extreme test case for both; other plugins won’t stress these aspects so much.

With all this, the data updating scheme took a back seat. Briefly, XRay II plugins will interact with the file system over an abstraction class called XRayItem. An item can represent an actual file system item, the entire contents of one such an item’s data or resource forks, or specific subparts of those. The idea is to have plugins chop XRayItems up into smaller pieces and pass them to other plugins to format and display them. Ultimately, once a data portion is changed by some editing action by the user the changed data are cached and the changes are passed upward the chain so other plugins can update their own representations accordingly. Then, when the changes are saved, they have to be collected and written out into a reasonably efficient manner.

Turns out this is not as easy as it sounds. Starting with just the default plugins that show the hex view of a file’s data fork, the user might want to open a 20GB file, select half of it, cut it to the clipboard, past it back at the end of the file (or even another file), go back to the middle and change a single byte, then do a search/replace loop over the result. Since, on Tiger, a typical Cocoa program can allocate just a little over 2GB of RAM (and not necessarily in a single chunk), this becomes a non-trivial memory management problem.

Of course, for the vast majority of files up to a certain limit – let’s say up to 128MB or so – keeping all that in RAM and changing it there would be the simplest and not-to-slow solution, and I’ll certainly have a fallback implementation for that. And if this were a Leopard-only app with a 64-bit version, this limit could be pushed a lot higher – but it has to run on Tiger and on 32-bit systems too.

So I was putting this off while tinkering with the other parts of the app, and seriously considering asking Peter Ammon for more hints, as I knew he’d solved the problem in his excellent HexFiend hex editor. Imagine my surprise when I learned that Peter had just published the complete HexFiend source code, and also started up a Wiki to explain details. Thanks a lot, Peter. Every Cocoa developer should download this gem, there’s lots of cool stuff inside from a Cocoa team insider; I’ve already learned how to filter out all fixed-width fonts, for instance.

That said, I haven’t had time to look at details of his data backend yet, but that too looks like it will save me at least a month of tinkering.