BGZF Example

BGZIP allows for quick random access to a bgzip file by creating an index. As an example, here is a way to compress a file with bgzip and access a random portion of it:

$ ls -la
total 685104
drwxrwxr-x 2 abetusk abetusk      4096 Jun 12 17:33 .
drwxrwxr-x 5 abetusk abetusk      4096 Jun 12 17:33 ..
-rw-rw-r-- 1 abetusk abetusk 701533731 Jun 12 17:33 hu826751.gff
$ bgzip -i hu826751.gff 
$ ls
hu826751.gff.gz  hu826751.gff.gz.gzi
$ bgzip -h

Version: 1.4.1
Usage:   bgzip [OPTIONS] [FILE] ...
Options:
   -b, --offset INT        decompress at virtual file pointer (0-based uncompressed offset)
   -c, --stdout            write on standard output, keep original files unchanged
   -d, --decompress        decompress
   -f, --force             overwrite files without asking
   -h, --help              give this help
   -i, --index             compress and create BGZF index
   -I, --index-name FILE   name of BGZF index file [file.gz.gzi]
   -r, --reindex           (re)index compressed file
   -g, --rebgzip           use an index file to bgzip a file
   -s, --size INT          decompress INT bytes (uncompressed size)
   -@, --threads INT       number of compression threads to use [1]

$ time bgzip -b 100000000 -s 100 hu826751.gff.gz
54      .       +       .       alleles C/T;db_xref dbsnp.120:rs11035863;ref_allele C
chr11   CGI     REF     40509955        40510029        .       +       .
real    0m0.009s
user    0m0.004s
sys     0m0.004s

C Example

Here's a C example:

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

#include "bgzf.h"

#include <vector>
#include <string>

int main(int argc, char **argv) {
  int i, j, k, r;
  BGZF *bgzfp;
  std::string ifn, idx_fn;
  int64_t pos=-1;
  char buf[1024];
  ssize_t s;
  size_t buflen=1024;

  if (argc<2) {
    printf("provide bgzip file\n");
    exit(-1);
  }
  ifn = argv[1];

  printf("loading bgzip file %s\n", ifn.c_str());
  bgzfp = bgzf_open(ifn.c_str(), "r");
  if (!bgzfp) {
    fprintf(stderr, "error opening file %s\n", ifn.c_str());
    exit(1);
  }

  printf("loading index bgzip file %s%s\n", ifn.c_str(), ".gzi");
  r = bgzf_index_load(bgzfp, ifn.c_str(), ".gzi");
  printf("got %i\n", r);

  r = bgzf_useek(bgzfp, 100000000, SEEK_SET);
  printf("got %i\n", r);
  if (r<0) {
    perror("...");
    exit(-1);
  }
  s = bgzf_read(bgzfp, buf, sizeof(char)*buflen);
  printf("...%i\n", (int)s);

  printf("---\n");
  for (i=0; i<s; i++) { printf("%c", buf[i]); }
  printf("\n---\n");

  bgzf_close(bgzfp);

}

To compile:

g++ -I $HTSDIR/htslib-1.4.1/htslib -lhts bgzf-example.cpp -o bgzf-example -L $HTSLIB/htslib-1.4.1 -lhts

To run:

LD_LIBRARY_PATH=$HOME/htslib-1.4.1 ./bgzf-example hu826751.gff.gz

Which assumes the hu826751.gff.gz.gzi file is in the same directory as the hu826751.gff.gz file.

This assumes htslib is installed under the directory pointed to by the HTSLIB environment variable.

See the htslib repo for details on how to download and install.

2017-06-12