On Fri, 2009-09-11 at 00:32 +0900, Adrien Piérard wrote:
2009/9/11 Bradley Lucier lucier@math.purdue.edu:
Or home-grown extensible u8vectors? Or strings (after configuring with --enable-char-size=1)? How many distinct "characters" are being distinguished?
Warning: *Beware of the following question, for I should already be sleeping* Supposing I have an alphabet of 2 letters (to make things simple), I can code it with two bits. Despite the possible algorithmic and computational pain somehow, since we have bignums, how about encoding a string of this alphabet into a number, using bitwise operations? Appending a char to a string is a SHIFT of two bits, then an OR. Referencing should be a matter of LOG (to get the size), and then shifting and an AND 3.
I'm still awake enough to encode just 2 letters and not three on 2 bits (for concatenating "01" to "00" would not work as expected). I guess that it relies on the representation of bignums in memory.
What would be such a misuse of bignums worth?
Bignums are not mutable at the "user" level, but they are mutable in the internal implementation of course.
At one point I wrapped integers in operations that implemented mutable sets of nonnegative integers (sets either with a finite number of elements (nonnegative exact integers) or all but a finite number of elements (negative exact integers)) as fixnum/bignum bitmaps. The ultimate extensible bit-vectors, to interpret as you wish.
The trouble is, such an implementation of sets of nonnegative integers is efficient only if the chance of a bit being set in your application is about 1/2. If you have very sparse sets, or sets whose elements tend to cluster, or any other kind of non-uniformity in your sets, other data structures are much more efficient. And if you keep adding elements on the end, it's quite inefficient (you don't increase the size by 20% at a time, you add 64 bits at a time; you'd probably want to start a set with a sentinel element if you know how big it will be eventually).
If I remember correctly, Marc didn't think they were worth inserting into the runtime. Perhaps I should resurrect them as an SRFI (but that's *so* much work) or just dump the code into the dumping grounds (if I can find it again).
Brad