Tuesday, May 28, 2013

Pretty ugly hacks

Recently I've (re-)discovered the more hacky kind of programming. It started after that I implemented my first bloom-filter. Somehow I ended up reading about all kinds of bit twiddling hacks, and finally I found myself in the realm of xor double linked lists.

Doubled linked list is a data structure where each node in the list has two pointers; one to the next node, and one to the previous node. The size of each node is thus 3 pointers. An xor linked list has the same logical function, but a compress representation -- it only requires 2/3 of the size of a normal linked list. This is achieved by observing that the only way to reach a node in a double linked list is either from the back or from the front of the list. Thus, it's enough to store the xor-sum of the pointer to the next node and the previous node, since either of those pointers is known. Anyway, the linked Wikipedia article explains it much better.

Well, the neighboring country to xor-lists is Tagged Pointer-land. In Tagger Pointer it's ok to do all kinds of crazy things to pointers as long as you know what you're doing. For instance you can exploit that dynamically allocated memory on most systems are aligned to 16-bits, 32-bits, 64-bit or even 128-bits borders. What can this be used for? Distinguishing between native integer values and arbitrary-precision integers for instance, or for putting small data structures (e.g., short strings) in the "pointer" rather than having to dynamically allocating a few bytes of memory.

It's actually pretty useful stuff, although -- as the title says -- pretty ugly. That is, the pretty kind of ugly.

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