Read Bit String...!!!

MimisIVI has asked for the wisdom of the Perl Monks concerning the following question:

Replies are listed 'Best First'.
Re: Read Bit String...!!! by ikegami (Patriarch) on Apr 09, 2008 at 14:04 UTC
Things to try: Leave the string compressed and use `vec` Leave the string compressed and use masks (For example, `$compressed & "\x80"`) Replace `split(//, unpack("b", $compressed))` with `my @bits = unpack("b", $compressed) =~ /(.)/g` Skip assigning to an array using a regexp. (As in `while ($decompressed =~ /(.)/g) { ... }`) Skip assigning to an array using `substr`. (As in `for my $i (0..length($decompressed)-1) { my $bit = substr($decompressed, $i, 1); ... }`) ... If it's that slow, one has to wonder if you're using the optimal format for your data.	[reply] [d/l] [select]
Re^2: Read Bit String...!!! by Fletch (Bishop) on Apr 09, 2008 at 14:11 UTC
And after all that if it's still slow and you're sure its the optimal format but you still want to stay in Perl consider dropping into C via Inline::C or an XS module and do the bit twiddling there instead. The cake is a lie. The cake is a lie. The cake is a lie.	[reply]
Re^3: Read Bit String...!!! by ikegami (Patriarch) on Apr 09, 2008 at 14:19 UTC
Dropping to C is indeed an option, but bit twiddling should be pretty darn fast in Perl too. To generate 1 million random numbers and process their bits took 4 seconds: `my $stime = time; for (1..1_000_000) { my $i = rand(4294967295); while ($i) { $count++ if $i>>=2; } } my $etime = time; print($etime-$stime, " seconds to count the $count 1s in 32_000_000 bi +ts\n");` [download] `4 seconds to count the 14665902 1s in 32_000_000 bits` [download] Update: Oops! `if $i>>=2;` should be `if ($i>>=2) & 1;` as per reply.	[reply] [d/l] [select]
Re^4: Read Bit String...!!! by ysth (Canon) on Apr 10, 2008 at 04:24 UTC
Re: Read Bit String...!!! by stark (Pilgrim) on Apr 09, 2008 at 15:15 UTC
Have you looked at Bit::Vector? With the Interval_xxx funtions it should be possible...	[reply]
Re: Read Bit String...!!! by MimisIVI (Acolyte) on Apr 09, 2008 at 15:12 UTC
Hi guys, Ikegami i already use the three last solutions that you said and the regular expression is the best one... Now about the C solution the problem is that i dont know very well this language unfortunately and i dont have time to learn it.. I believe if i will work the bit string without unpack it will be the fastest solution .. Can you give me some help with the mask and vec functions??? About what i want to do with the bit string... In the string i save possitive compressed integers and to decode them i read from the start the string how many 1s exist until i will find the first 0 (lets assume N ones).Then the compressed number is (in binary) the next N bits after the 0, plus an additional bit in the begining of the next N digits. Like this way for the next bits.. For example lets assume that i compressed in the below bit string just one number... BitString: 1110,001 N=3 (the first 1s until the first 0) The next N digits are : 001 So the number in binary is 1001 by add one bit in the start of the next N bits... Can you tell me an optimal way to decode this bit string...?? Thanks Guys!!! Mimis	[reply]
Re^2: Read Bit String...!!! by tachyon-II (Chaplain) on Apr 09, 2008 at 15:31 UTC
I believe if i will work the bit string without unpack it will be the fastest solution .. Why? I believe that you believe but why do you believe? I believe because either I have faith, or because I have read informed expert discussion, or because I have tested and know it to be true. For other monks information please refer to this to see what MimisIVI actually wants to do. I believe he is barking up the wrong tree, seeking the solution to a problem that does not actually exist, and trying to optimise a bad algorithm.	[reply]