[gambit-list] Re: Re: Regarding garbage collection

Marc Feeley feeley at iro.umontreal.ca
Thu Sep 10 12:57:36 EDT 2009

On 10-Sep-09, at 11:32 AM, Adrien Piérard wrote:

> 2009/9/11 Bradley Lucier <lucier at 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?

You can use 1 bit per letter if there are 2 letters.  You simply need  
a 1 bit at the top end to indicate the length of the bit string.  Then  
you simply use integer-length (minus one) to get the length of the bit  

 > (map integer-length '(1 2 3 4 5 6 7 8 9 10 11 12))
(1 2 2 3 3 3 3 4 4 4 4 4)

Note that your representation has the same asymptotic space efficiency  
as a u8vector where each byte contains 8 letters.  I'm pretty sure an  
explicit u8vector representation would be faster (don't be fooled by  
"appending a letter is just a shift"... the shift is going to be O(n)  
not O(1) because these are bignums).


P.S. Get some sleep!

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