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Re: Parsing .2bit DNA files

by pc88mxer (Vicar)
on Mar 05, 2008 at 21:10 UTC ( #672281=note: print w/replies, xml ) Need Help??

in reply to Parsing .2bit DNA files

I'll propose a theory on why the N's are not inserted. Basically, it simplifies I/O. If these files are large, they may find it more efficient to read the entire file into memory with one (or a few) I/O operations than to read it in small segments. Even if you have a scatter/gather I/O capability, generally you are limited to block (or at least byte) boundaries. Reading the whole file in and inserting bits in the middle afterwards is obviously going to be too inefficient for large files.

Also, it simplifies random access of the base stream directly from the file. You don't have to keep track of where you really are based on all the previous insertions.

The idea that .2bit files are more compressed is probably in relationship to the earlier .nib format which uses 4 bits per base. Clearly the frequency of N and M bases is such that the 2bit format is much more compact.

I think that a virtual api to a 2bit file would be useful. So you could do something like:

my $twobit = new TwoBitFile('some/path.2bit'); # returns 1000 bases starting from base 40,000,000: my $bases = $twobit->range(40_000_000, 1000);
and minimal disk I/O is performed.

Update: Being able to treat the 2bit file as a virtual string with the ability to run regular expressions on it would also be really cool. Is that something we can do in Perl6?

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Re^2: Parsing .2bit DNA files
by Limbic~Region (Chancellor) on Mar 05, 2008 at 23:23 UTC
    With regards to the N blocks, your hypothesis sounds valid. With regards to the virtual API, I was actually thinking of turning it into an iterator. Calling the iterator without any parameters would fetch the next X bytes where X is some default. You could then allow for named parameters to fetch forward by Y, rewind, select a range, etc. It should have gone into the TODO but wasn't really necessary.

    With regards to Perl 6 and applying a regular expressions on streams - the answer is yes unless things have changed since last I looked.

    Cheers - L~R

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