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org-davep-2bit

Introduction

org-davep-2bit is a small Common Lisp library that provides code for reading data from 2bit format files. These are files that can hold DNA sequences in a compressed format. You'll see examples of this if you look at the human genome data, for example. I believe there are other bodies of code out there for doing this, I wrote this one with two main motivations in mind:

  1. To de-rust my rusty Common Lisp -- I'd not written anything in CL for a while and wanted to refresh my memory about CL itself, and also using SLIME in Emacs.
  2. I wanted to write a 2bit reader that could provide different routes to reading the code. So, not just reading from a local file, but also reading via HTTP(S) from a remote file (as you'll see happen in a web-based genome browser such as Biodalliance).

Note that, as of the time of writing, the code is designed such that the latter can happen, but only reading from a local file has been implemented. Adding support for remote reading of 2bit files is left as an exercise for me to tinker with over time.

Usage

The library is broken up into two main classes, a reader class and a sequence class. The sequence class (called 2bit-sequence) is what provides access to the content of an actual sequence within the 2bit file, whereas the reader class does the work of reading data from a 2bit source.

Note that all code lives within a package called org.davep.2bit. See packages.lisp for a list of the exported symbols.

The reader class

For now the main reader class to use is file-reader. A helper function called make-file-reader is available to quickly and easily create an instance of a file-reader. For example:

CL-USER> (in-package :2bit)
#<PACKAGE "ORG.DAVEP.2BIT">
2BIT> (make-file-reader "hg38.2bit")
#<FILE-READER "hg38.2bit" :masking T>

The reader class offers the following methods:

open-reader

Given an instance of a reader, this method opens the connection to the source data. For example:

CL-USER> (in-package :2bit)
#<PACKAGE "ORG.DAVEP.2BIT">
2BIT> (make-file-reader "hg38.2bit")
#<FILE-READER "hg38.2bit" :masking T>
2BIT> (open-reader *)
#<FILE-READER "hg38.2bit" :masking T>

Once the reader is open the content can be accessed (see below).

close-reader

Given an instance of a reader, this method closes the connection to the source data. For example:

CL-USER> (in-package :2bit)
#<PACKAGE "ORG.DAVEP.2BIT">
2BIT> (make-file-reader "hg38.2bit")
#<FILE-READER "hg38.2bit" :masking T>
2BIT> (open-reader *)
#<FILE-READER "hg38.2bit" :masking T>
2BIT> (close-reader *)
T
2BIT>

sequences

Given an open reader object, this method returns a list of all of names of all of the sequences held in the 2bit data. For example:

2BIT> (open-reader (make-file-reader "hg38.2bit"))
#<FILE-READER "hg38.2bit" :masking T>
2BIT> (sequences *)
("chr1" "chr10" "chr11" "chr11_KI270721v1_random" "chr12" "chr13" "chr14"
 [...SNIP...] "chrY_KI270740v1_random")
2BIT>

seq

Given an open reader object, this method provides access to a particular named sequence. For example:

2BIT> (open-reader (make-file-reader "hg38.2bit"))
#<FILE-READER "hg38.2bit" :masking T>
2BIT> (seq * "chr1")
#<2BIT-SEQUENCE "chr1" 10487>
2BIT>

The sequence class

As mentioned above, access to the actual sequence data is done via the 2bit-sequence class -- this is what is returned by the call to seq mentioned above. Currently there is just one main method associated with this class:

bases

This method is used to get bases from the sequence. The parameters are the sequence to pull the bases from, the start location within the sequence, and the end location within the sequence. Note that, as is the usual convention, what's returned is inclusive of the start location but exclusive of the end location.

For example:

2BIT> (open-reader (make-file-reader "hg38.2bit"))
#<FILE-READER "hg38.2bit" :masking T>
2BIT> (seq * "chr1")
#<2BIT-SEQUENCE "chr1" 10487>
2BIT> (bases * 10000 10010)
"taaccctaac"
2BIT>

Helper macro

Note that, when creating and using a reader object yourself, it's very important that you open it, only use it while open, and then close it once you're finished. No sequence access can happen on a reader that is no longer open. Closing once you're finished is just good resource use.

To help with smaller calls to the data a with-2bit-file macro is made available. For example:

2BIT> (with-2bit-file (human "hg38.2bit")
        (format t "~S" (bases (seq human "chr1") 10000 10010)))
"taaccctaac"
NIL
2BIT>

This takes care of opening the reader for you, and closing it again afterwards.