Programmer's block - Journal of Omnifarious
Jun. 27th, 2009
12:22 am - Programmer's block
Skein has an interesting tree mode in which it's possible to parallelize the hash function calculation to a significant degree. I wanted to write a general interface for this so I could make a command line utility that used it to test it against sha256sum command.
Applying a tree hash to an existing file is a no-brainer. But I wanted to be able to handle much more general cases in which the leaf data may not be available on a random-access basis. In particular if the file is coming in on stdin or something similar.
I was having difficulty coming up with a general extensible interface for this. Partly the interface for the system for handling leaf data needed a way to allocate chunks of leaf data to work on, and then release them. This would allow for a sliding window type approach to fetching leaf data.
My biggest and most recent breakthrough was realizing that the leaf data objects were like ::std::auto_ptr objects. I didn't want to force heap use, so I needed the data about a leaf to be copyable. But I didn't want to have to have any kind of silly reference counts or anything like that. So that meant I needed it to be moveable, not copyable. Just like auto_ptr. But auto_ptr is a klduge in C++. In C++0x there is a very nice concept called rvalue references that let you implement move semantics very cleanly.
It took me awhile to realize I wanted move semantics. I kept on beating on the interface and coming up with usage scenarios that were just awkward and broken. Once I figured it out, things went a lot easier.
Here is a link to what I finally came up with: skeintreepp.hpp.