Other packages in the module: Bio::DB::Qual Bio::Seq::PrimaryQual::Qual

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Summary

Bio::DB::Qual -- Fast indexed access to a directory of quality files

Package variables

Included modules

Inherit

Bio::DB::SeqI Bio::Root::Root

Synopsis

  use Bio::DB::Qual;

  # create database from directory of qual files
  my $db      = Bio::DB::Qual->new('/path/to/qual/files');
  my @ids     = $db->ids;

  # simple access (for those without Bioperl)
  my @qual    = @{$db->qual('CHROMOSOME_I',4_000_000 => 4_100_000)};
  my @revqual = @{$db->qual('CHROMOSOME_I',4_100_000 => 4_000_000)};
  my $length  = $db->length('CHROMOSOME_I');
  my $header  = $db->header('CHROMOSOME_I');

  # Bioperl-style access
  my $obj     = $db->get_Qual_by_id('CHROMOSOME_I');
  my @qual    = @{$obj->qual};
  my @subqual = @{$obj->subqual(4_000_000 => 4_100_000)};
  my $length  = $obj->length;
  # (etc)

  # Bio::SeqIO-style access
  my $stream  = $db->get_PrimaryQual_stream;
  while (my $qual = $stream->next_seq) {
    # Bio::Seq::PrimaryQual operations
  }

  my $fh = Bio::DB::Qual->newFh('/path/to/qual/files');
  while (my $qual = <$fh>) {
    # Bio::Seq::PrimaryQual operations
  }

  # tied hash access
  tie %qualities,'Bio::DB::Qual','/path/to/qual/files';
  print $qualities{'CHROMOSOME_I:1,20000'};

Description

Bio::DB::Qual provides indexed access to one or more qual files. It provides
random access to each quality score entry without having to read the file from
the beginning. Access to subqualities (portions of a quality score) is provided,
although contrary to Bio::DB::Fasta, the full quality score has to be brought in
memory.
When you initialize the module, you point it at a single qual file or a
directory of multiple such files. The first time it is run, the module generates
an index of the contents of the file or directory using the AnyDBM module
(Berkeley DB* preferred, followed by GDBM_File, NDBM_File, and SDBM_File).
Thereafter it uses the index file to find the file and offset for any requested
quality score. If one of the source qual files is updated, the module reindexes
just that one file. (You can also force reindexing manually). For improved
performance, the module keeps a cache of open filehandles, closing less-recently
used ones when the cache is full.
The qual files may contain decimal quality scores. Entries may have any line
length up to 65,536 characters, and different line lengths are allowed in the
same file. However, within a quality score entry, all lines must be the same
length except for the last. An error will be thrown if this is not the case.
The module uses /^>(\S+)/ to extract the primary ID of each quality score
from the qual header. During indexing, you may pass a callback routine to modify
this primary ID. For example, you may wish to extract a portion of the
gi|gb|abc|xyz prefixes that are commonly used. The original header line can be
recovered later.
*Berkeley DB can be obtained free from www.sleepycat.com. After it is installed
you will need to install the BerkeleyDB Perl module.

Methods

Methods description

 Title   : new
 Usage   : my $db = Bio::DB::Qual->new( $path, @options);
 Function: initialize a new Bio::DB::Qual object
 Returns : new Bio::DB::Qual object
 Args    : path to dir of qual files or a single qual filename

 -glob         Glob expression to use    *.{qual,QUAL,qa,QA}
               for searching for qual
               files in directories. 

 -makeid       A code subroutine for     none
               transforming qual IDs.

 -maxopen      Maximum size of           32
               filehandle cache.

 -debug        Turn on status            0
               messages.

 -reindex      Force the index to be     0
               rebuilt.

 -dbmargs      Additional arguments      none
               to pass to the DBM
               routines when tied
               (scalar or array ref).

 Title   : newFh
 Usage   : my $fh = Bio::DB::Qual->newFh('/path/to/qual/files');
 Function: gets a new Fh for a file or directory containing several files
 Returns : filehandle object
 Args    : none

 Title   : index_dir
 Usage   : $db->index_dir($dir)
 Function: set the index dir and load all files in the dir
 Returns : hashref of qual offsets in each file
 Args    : dirname, boolean to force a reload of all files

 Title   : get_Qual_by_id
 Usage   : my $qual = $db->get_Qual_by_id($id)
 Function: Bio::DB::RandomAccessI method implemented
 Returns : Bio::PrimarySeqI object
 Args    : id

 Title   : set_pack_method
 Usage   : $db->set_pack_method( @files )
 Function: Determines whether data packing uses 32 or 64 bit integers
 Returns : 1 for success
 Args    : one or more file paths

 Title   : index_file
 Usage   : $db->index_file($filename)
 Function: (re)loads a quality score file and indexes quality score offsets in
           the file
 Returns : qual offsets in the file
 Args    : filename, boolean to force reloading a file

 Title   : dbmargs
 Usage   : my @args = $db->dbmargs;
 Function: gets stored dbm arguments
 Returns : array
 Args    : none

 Title   : index_name
 Usage   : my $indexname = $db->index_name($path,$isdir);
 Function: returns the name of the index for a specific path 
 Returns : string
 Args    : path to check, boolean if it is a dir

 Title   : calculate_offsets
 Usage   : $db->calculate_offsets($filename,$offsets);
 Function: calculates the quality score offsets in a file based on ID
 Returns : offset hash for each file
 Args    : file to process, $offsets - hashref of id to offset storage

 Title   : get_all_ids
 Usage   : my @ids = $db->get_all_ids
 Function: gets all the stored ids in all indexes
 Returns : list of ids
 Args    : none

 Title   : length
 Usage   : $qualdb->length($seqid);
 Function: gets the number of quality values in a quality score
 Returns : scalar
 Args    : ID of a quality score

 Title   : subqual
 Usage   : my @qualarr = @{$qualdb->subqual($id,$start,$stop)};
 Function: returns a subqual of a quality score in the database
 Returns : subquality array reference
 Args    : id of quality score, starting quality value number, ending quality
           value number

 Title   : header
 Usage   : $qualdb->header($id);
 Function: returns the header of a quality score in the database
 Returns : header string
 Args    : id of quality score

 Title   : get_PrimaryQual_stream
 Usage   : $qualdb->get_PrimaryQual_stream
 Function: get a SeqIO-like stream of quality scores 
 Returns : stream object
 Args    : none

Methods code

BEGIN {

  @AnyDBM_File::ISA = qw(DB_File GDBM_File NDBM_File SDBM_File)

}

sub new {

    my ($class, $path) = @_;
    my %opts  = @_;

    my $self = bless {
        debug      => $opts{-debug},
        makeid     => $opts{-makeid},
        glob       => $opts{-glob}    || '*.{qual,QUAL,qa,QA}',
        maxopen    => $opts{-maxopen} || 32,
        dbmargs    => $opts{-dbmargs} || undef,
        fhcache    => {},
        cacheseq   => {},
        curopen    => 0,
        openseq    => 1,
        dirname    => undef,
        offsets    => undef,
    }, $class;
    my ($offsets,$dirname);

    if (-d $path) {
        # because Win32 glob() is broken with respect to long file names that
        # contain whitespace.
        $path = Win32::GetShortPathName($path)
        if $^O =~ /^MSWin/i && eval 'use Win32; 1';
        $offsets = $self->index_dir($path,$opts{-reindex});
        $dirname = $path;
    } elsif (-f _) {
        $offsets = $self->index_file($path,$opts{-reindex});
        $dirname = dirname($path);
    } else {
        $self->throw( "$path: invalid file or dirname");
    }
    @{$self}{qw(dirname offsets)} = ($dirname,$offsets);
    $self;

}

sub newFh {

    my $class = shift;
    my $self  = $class->new(@_);
    require Symbol;
    my $fh = Symbol::gensym or return;
    tie $$fh,'Bio::DB::Qual::Stream',$self or return;
    $fh;

}

sub _open_index {

    my $self = shift;
    my ($index,$write) = @_;
    my %offsets;
    my $flags = $write ? O_CREAT|O_RDWR : O_RDONLY;
    my @dbmargs = $self->dbmargs;
    tie %offsets,'AnyDBM_File',$index,$flags,0644,@dbmargs 
        or $self->throw( "Can't open cache file $index: $!");
    return\% offsets;

}

sub _close_index {

    my ($self, $index) = @_;
    untie %$index;

}

sub index_dir {

    my ($self, $dir, $force_reindex) = @_;

    # find all qual files
    my @files = glob("$dir/$self->{glob}");
    $self->throw("No qual files found in $dir") unless @files;

    # get name of index
    my $index = $self->index_name($dir,1);

    # if caller has requested reindexing, then unlink
    # the index file.
    unlink $index if $force_reindex;

    # get the modification time of the index
    my $indextime   = 0;
    for my $suffix('','.pag','.dir') {
        $indextime ||= (stat("${index}${suffix}"))[9];
    }
    $indextime ||= 0;  # prevent some uninit variable warnings

    # get the most recent modification time of any of the contents
    my $modtime = 0;
    my %modtime;
    $self->set_pack_method( @files );
    foreach (@files) {
        my $m = (stat($_))[9];
        $modtime{$_} = $m;
        $modtime = $m if defined $m && $modtime < $m;
    }

    my $reindex      = $force_reindex || $indextime < $modtime;
    $self->{offsets} = $self->_open_index($index,$reindex) or return;

    # no indexing needed
    return $self->{offsets} unless $reindex;

    # otherwise reindex contents of changed files
    $self->{indexing} = $index;
    foreach (@files) {
        next if( defined $indextime && $modtime{$_} <= $indextime);
        $self->calculate_offsets($_,$self->{offsets});
    }
    delete $self->{indexing};

    # we've been having troubles with corrupted index files on Windows systems,
    # so possibly closing and reopening will help
    $self->_close_index($self->{offsets});

    return $self->{offsets}  = $self->_open_index($index);

}

sub get_Qual_by_id {

    my ($self, $id) = @_;
    return unless exists $self->{offsets}{$id};
    return Bio::Seq::PrimaryQual::Qual->new($self,$id);

}

sub set_pack_method {

    my $self = shift;
    # Find the maximum file size:
    my ($maxsize) = sort { $b <=> $a } map { -s $_ } @_;
    my $fourGB    = (2 ** 32) - 1;

    if ($maxsize > $fourGB) {
        # At least one file exceeds 4Gb - we will need to use 64 bit ints
        $self->{packmeth}   =\& _packBig;
        $self->{unpackmeth} =\& _unpackBig;
    } else {
        $self->{packmeth}   =\& _pack;
        $self->{unpackmeth} =\& _unpack;
    }
    return 1;

}

sub index_file {

    my ($self, $file, $force_reindex) = @_;

    $self->set_pack_method( $file );
    my $index = $self->index_name($file);
    # if caller has requested reindexing, then unlink the index
    unlink $index if $force_reindex;

    # get the modification time of the index
    my $indextime = (stat($index))[9] || 0;
    my $modtime   = (stat($file) )[9] || 0;

    my $reindex = $force_reindex || $indextime < $modtime;
    my $offsets = $self->_open_index($index,$reindex) or return;
    $self->{offsets} = $offsets;

    return $self->{offsets} unless $reindex;

    $self->{indexing} = $index;
    $self->calculate_offsets($file,$offsets);
    delete $self->{indexing};
    return $self->{offsets};

}

sub dbmargs {

    my $self = shift;
    my $args = $self->{dbmargs} or return;
    return ref($args) eq 'ARRAY' ? @$args : $args;

}

sub index_name {

    my $self  = shift;
    my ($path,$isdir) = @_;
    unless ($path) {
        my $dir = $self->{dirname} or return;
        return $self->index_name($dir,-d $dir);
    } 
    return "$path/directory.index" if $isdir;
    return "$path.index";

}

sub calculate_offsets {

    my $self = shift;
    my ($file,$offsets) = @_;
    my $base = $self->path2fileno(basename($file));

    my $fh = IO::File->new($file) or $self->throw("Can't open $file: $!");
    binmode $fh;
    warn "Indexing $file\n" if $self->{debug};
    my ( $offset,$id, $linelength, $firstline, $count, $termination_length,
         $qual_lines, $last_line,  %offsets );
    my ( $l3_len, $l2_len, $l_len ) = ( 0, 0, 0 );

    while (<$fh>) { # don't try this at home
        # account for crlf-terminated Windows files      
        $termination_length ||= /\r\n$/ ? 2 : 1;
        if (/^>(\S+)/) {
            print STDERR "indexed $count quality scores...\n" 
            if $self->{debug} && (++$count%1000) == 0;
            my $pos = tell($fh);
            if ($id) {
                my $qualstrlength = $pos - $offset - length($_);
                $qualstrlength -= $termination_length * $qual_lines;
                $offsets->{$id} = &{$self->{packmeth}}(
                    $offset,
                    $qualstrlength,
                    $linelength,
                    $firstline,
                    $base
                );
            }
            $id = ref($self->{makeid}) eq 'CODE' ? $self->{makeid}->($_) : $1;
            ($offset, $firstline, $linelength) = ($pos, length($_), 0);
            $self->_check_linelength($linelength);
            ($l3_len, $l2_len, $l_len) = (0, 0, 0);
            $qual_lines = 0;
        } else {
            $l3_len = $l2_len;
            $l2_len = $l_len;
            $l_len = length($_);
            # need to check every line :(
            if (DIE_ON_MISSMATCHED_LINES &&
                $l3_len > 0 &&
                $l2_len > 0 &&
                $l3_len != $l2_len
            ) {
                my $fap = substr($_, 0, 20)."..";
                $self->throw("Each line of the qual entry must be the same ".
                "length except the last. Line above #$. '$fap' is $l2_len != ".
                "$l3_len chars.");
            }
            $linelength ||= length($_);
            $qual_lines++;
        }
        $last_line = $_;
    }

    $self->_check_linelength($linelength);
    # deal with last entry
    if ($id) {
        my $pos = tell($fh);
        my $qualstrlength = $pos - $offset;
      
        if ($linelength == 0) {
            $qualstrlength = 0;
        } else {
            if ($last_line !~ /\s$/) {
                $qual_lines--;
            }
            $qualstrlength -= $termination_length * $qual_lines;
        }
        $offsets->{$id} = &{$self->{packmeth}}(
            $offset,
            $qualstrlength,
            $linelength,
            $firstline,
            $base
        );
    }
    $offsets->{__termination_length} = $termination_length;
    return\% offsets;

}

sub get_all_ids {

 grep {!/^__/} keys %{shift->{offsets}}

}

sub offset {

    my ($self, $id) = @_;
    my $offset = $self->{offsets}{$id} or return;
    (&{$self->{unpackmeth}}($offset))[0];

}

sub length {

    # the NUMBER of quality values
    my ($self, $id) = @_;
    my $len = scalar(@{$self->subqual($id)});
    return $len;

}

sub lengthstr {

    # the length of the quality STRING
    my ($self, $id) = @_;
    my $offset = $self->{offsets}{$id} or return;
    (&{$self->{unpackmeth}}($offset))[1];

}

sub linelen {

    my ($self, $id) = @_;
    my $offset = $self->{offsets}{$id} or return;
    (&{$self->{unpackmeth}}($offset))[2];

}

sub headerlen {

    my ($self, $id) = @_;
    my $offset = $self->{offsets}{$id} or return;
    (&{$self->{unpackmeth}}($offset))[3];

}

sub header_offset {

    my ($self, $id) = @_;
    return unless $self->{offsets}{$id};
    return $self->offset($id) - $self->headerlen($id);

}

sub file {

    my ($self, $id) = @_;
    my $offset = $self->{offsets}{$id} or return;
    $self->fileno2path((&{$self->{unpackmeth}}($offset))[4]);

}

sub fileno2path {

    my ($self, $no) = @_;
    return $self->{offsets}{"__file_$no"};

}

sub path2fileno {

    my ($self, $path) = @_;
    if ( !defined $self->{offsets}{"__path_$path"} ) {
        my $fileno  = ($self->{offsets}{"__path_$path"} = 0+ $self->{fileno}++);
        $self->{offsets}{"__file_$fileno"} = $path;
    }
    return $self->{offsets}{"__path_$path"}

}

sub _check_linelength {

    my ($self, $linelength) = @_;
    return unless defined $linelength;
    $self->throw(
        "Each line of the qual file must be less than 65,536 characters.Line ".
        "$. is $linelength chars."
    ) if $linelength > 65535;

}

sub subqual {

    my ($self, $id, $start, $stop) = @_;

    # Quality values in a quality score can have 1 or 2 digits and are separated
    # by one (or several?) spaces. Thus contrary to Bio::DB::Fasta, here there
    # is no easy way match the position of a quality value to its position in
    # the quality string.
    # As a consequence, if a subqual of the quality is requested, we still need
    # to grab the full quality string first - performance penalty for big
    # quality scores :(
    # I think there is no way around starting at the begining of the quality
    # score but maybe there is a resource-efficient way of starting at the
    # begining of the quality score and stopping when the the position of the
    # last quality value requested is reached??

    # position of the quality values
    if ($id =~ /^(.+):([\d_]+)(?:,|-|\.\.)([\d_]+)$/) {
        ($id, $start, $stop) = ($1,$2,$3);
        $start =~ s/_//g;
        $stop  =~ s/_//g;
    }

    # position in quality string
    my $string_start = 1;
    my $string_stop = $self->lengthstr($id);

    # fetch full quality string
    my $fh = $self->fh($id) or return;
    my $filestart = $self->caloffset($id, $string_start);
    my $filestop  = $self->caloffset($id, $string_stop);
    seek($fh,$filestart,0);
    my $data;
    read($fh, $data, $filestop-$filestart+1);

    # process quality score
    $data =~ s/\n//g;
    $data =~ s/\r//g;
    my $reverse = 0;
    if ($stop && $start && $stop < $start) {
        $reverse = 1;
        my $tmp = $start;
        $start = $stop;
        $stop = $tmp; 
    }
    my $subqual = 0;
    $subqual = 1 if ( $start || $stop );
    my @data;
    if ( $subqual || $reverse ) {
        @data = split / /, $data, $stop+1;
        my $length = scalar(@data);
        $start = 1       if $start < 1;
        $stop  = $length if $stop  > $length;
        pop @data if ($stop != $length);
        splice @data, 0, $start-1;
        @data = reverse(@data) if $reverse;
        $data = join ' ', @data;
    } else {
        @data = split / /, $data;
    }

    return\@ data;

}

sub fh {

    my ($self, $id) = @_;
    my $file = $self->file($id) or return;
    $self->fhcache("$self->{dirname}/$file") ||
        $self->throw("Can't open file $file");

}

sub header {

    my ($self, $id) = @_;
    my ($offset,$seqlength,$linelength,$firstline,$file) 
        = &{$self->{unpackmeth}}($self->{offsets}{$id}) or return;
    $offset -= $firstline;
    my $data;
    my $fh = $self->fh($id) or return;
    seek($fh,$offset,0);
    read($fh,$data,$firstline);
    chomp $data;
    substr($data,0,1) = '';
    $data;

}

sub caloffset {

    my ($self, $id, $a) = @_;
    $a--;
    my ($offset,$seqlength,$linelength,$firstline,$file)
        = &{$self->{unpackmeth}}($self->{offsets}{$id});
    $a = 0            if $a < 0;
    $a = $seqlength-1 if $a >= $seqlength;
    my $tl = $self->{offsets}{__termination_length};
    $offset + $linelength * int($a/($linelength-$tl)) + $a % ($linelength-$tl);

}

sub fhcache {

    my ($self, $path) = @_;
    if (!$self->{fhcache}{$path}) {
        if ($self->{curopen} >= $self->{maxopen}) {
            my @lru = sort {$self->{cacheseq}{$a} <=> $self->{cacheseq}{$b};}
                keys %{$self->{fhcache}};
            splice(@lru, $self->{maxopen} / 3);
            $self->{curopen} -= @lru;
            for (@lru) {
                delete $self->{fhcache}{$_};
            }
        }
        $self->{fhcache}{$path} = IO::File->new($path) || return;
        binmode $self->{fhcache}{$path};
        $self->{curopen}++;
    }
    $self->{cacheseq}{$path}++;
    $self->{fhcache}{$path}

}

sub _pack {

    pack STRUCT, @_;

}

sub _packBig {

    pack STRUCTBIG, @_;

}

sub _unpack {

    unpack STRUCT, shift;

}

sub _unpackBig {

    unpack STRUCTBIG, shift;

}

sub get_PrimaryQual_stream {

    my $self = shift;
    return Bio::DB::Qual::Stream->new($self);

}

sub TIEHASH {

    my $self = shift;
    return $self->new(@_);

}

sub FETCH {

    shift->subqual(@_);

}

sub STORE {

    shift->throw("Read-only database");

}

sub DELETE {

    shift->throw("Read-only database");

}

sub CLEAR {

    shift->throw("Read-only database");

}

sub EXISTS {

    defined shift->offset(@_);

}

sub NEXTKEY {

 tied(%{shift->{offsets}})->NEXTKEY(@_);

}

sub DESTROY {

    my $self = shift;
    if ($self->{indexing}) {  # killed prematurely, so index file is no good!
        warn "indexing was interrupted, so deleting $self->{indexing}";
        unlink $self->{indexing};
    }
}

#-------------------------------------------------------------
# Bio::Seq::PrimaryQual compatibility
#
# Usage is the same as in Bio::Seq::PrimaryQual

package Bio::Seq::PrimaryQual::Qual;
use overload '""' => 'display_id';

use base qw(Bio::Root::Root Bio::Seq::PrimaryQual);

}

General documentation

The two constructors for this class are new() and newFh(). The former creates a
Bio::DB::Qual object which is accessed via method calls. The latter creates a
tied filehandle which can be used Bio::SeqIO-style to fetch quality score
objects in a data stream. There is also a tied hash interface.
  Create a new Bio::DB::Qual object from the Qual file or files indicated by
$qual_path. Indexing will be performed automatically if needed. If successful,
new() will return the database accessor object. Otherwise it will return undef.
  $qual_path may be an individual qual file, or may refer to a directory
containing one or more of such files. Following the path, you may pass a series
of name=>value options or a hash with these same name=>value pairs.
Valid options are:

 Option Name   Description               Default
 -----------   -----------               -------

 -glob         Glob expression to use    *.{qa,QA,qual,QUAL}
               for searching for qual
               files in directories. 

 -makeid       A code subroutine for     None
               transforming qual IDs.

 -maxopen      Maximum size of           32
               filehandle cache.

 -debug        Turn on status            0
               messages.

 -reindex      Force the index to be     0
               rebuilt.

 -dbmargs      Additional arguments      none
               to pass to the DBM
               routines when tied
               (scalar or array ref).

  -dbmargs can be used to control the format of the index. For example, you can
pass $DB_BTREE to this argument so as to force the IDs to be sorted and
retrieved alphabetically. Note that you must use the same arguments every time
you open the index!
  -reindex can be used to force the index to be recreated from scratch.
  Create a tied filehandle opened on a Bio::DB::Qual object. Reading from this
filehandle with <> will return a stream of quality objects,
Bio::SeqIO-style.
The -makeid option gives you a chance to modify quality score IDs during
indexing. The option value should be a code reference that will take a scalar
argument and return a scalar result, like this:

  $db = Bio::DB::Qual->new("file.qual",-makeid=>\&make_my_id);

  sub make_my_id {
    my $description_line = shift;
    # get a different id from the quality header, e.g.
    $description_line =~ /(\S+)$/;
    return $1;
  }

make_my_id() will be called with the full qual id line (including the ">"
symbol!). For example:

  >A12345.3 Predicted C. elegans protein egl-2

By default, this module will use the regular expression /^>(\S+)/ to extract
"A12345.3" for use as the ID.If you pass a -makeid callback, you can extract any
portion of this, such as the "egl-2" symbol.
The -makeid option is ignored after the index is constructed.

The following object methods are provided.
          Return a quality score array reference given an ID and optionally a start and
stop position (the quality value number) in the quality score. If $stop is less
than $start, then the reverse complement of the quality score is returned (this
violates Bio::Seq conventions).
          For your convenience, subqualities can be indicated with any of the following
compound IDs:

   $db->qual("$id:$start,$stop")

   $db->qual("$id:$start..$stop")

   $db->qual("$id:$start-$stop")

          Return the length of the indicated quality score, i.e. the number of quality
values.
          Return the header line for the ID, including the initial ">".
          Return the name of the file in which the indicated quality score can be found.
          Return the offset of the indicated quality score from the beginning of the file
in which it is located. The offset points to the beginning of the quality
score, not the beginning of the header line.
          Return the length of the header line for the indicated quality score.
          Return the offset of the header line for the indicated quality score from the
beginning of the file in which it is located.
          Return the path to the index file.
          Return the path to the Qual file(s).
For BioPerl-style access, the following methods are provided:
    Return a Bio::Seq::PrimaryQual object, which obeys the Bio::PrimarySeqI
conventions. To recover the quality score, call $qual->qual().
    Note that get_Qual_by_id() does not bring the entire quality score into memory
until requested. Internally, the returned object uses the accessor to generate
subqualities as needed.
    These methods all do the same thing as get_Qual_by_id().
    Return a Bio::DB::Qual::Stream object, which supports a single method
next_seq(). Each call to next_seq() returns a new Bio::Seq::PrimaryQual object,
until no more quality scores remain.
See Bio::Seq::PrimaryQual and Bio::PrimarySeqI for methods provided by the
quality objects returned from get_Qual_by_id() and get_PrimaryQual_stream().

This module provides two tied interfaces, one which allows you to treat the
quality score database as a hash, and the other which allows you to treat the
database as an I/O stream.

The tied hash interface is very straightforward.
 Tie %db to Bio::DB::Qual using the indicated path to the Qual files. The
optional @args list is the same set of named argument/value pairs used by
Bio::DB::Qual->new().
 If successful, tie() will return the tied object. Otherwise it will return
undef.
Once tied, you can use the hash to retrieve an individual quality score by its
ID, like this:

  my $qual = $db{CHROMOSOME_I};

You may select a subquality by appending the comma-separated range to the
quality score ID in the format "$id:$start,$stop". For example, here is the
first 1000 quality values of the quality score with ID "CHROMOSOME_I":

  my $qual = $db{'CHROMOSOME_I:1,1000'};

(The regular expression used to parse this format allows quality score IDs to
contain colons.)
When selecting subqualities, if $start > stop, then the reverse complement
will be returned.
The keys() and values() functions will return the IDs and their quality scores,
respectively. In addition, each() can be used to iterate over the entire data
set:

 while (my ($id,$quality) = each %db) {
    print "$id => $quality\n";
 }

When dealing with very large quality scores, you can avoid bringing them into
memory by calling each() in a scalar context. This returns the key only. You can
then use tied(%db) to recover the Bio::DB::Qual object and call its methods.

 while (my $id = each %db) {
    print "$id => $db{$quality:1,100}\n";
    print "$id => ",tied(%db)->length($id),"\n";
 }

You may, in addition invoke Bio::DB::Qual the FIRSTKEY and NEXTKEY tied hash
methods directly.
  Return the first ID in the database.
  Given an ID, return the next quality score ID.
This allows you to write the following iterative loop using just the object-
oriented interface:

 my $db = Bio::DB::Qual->new('/path/to/qual/files');
 for (my $id=$db->FIRSTKEY; $id; $id=$db->NEXTKEY($id)) {
    # do something with quality
 }

The Bio::DB::Qual->newFh() method creates a tied filehandle from which you
can read Bio::Seq::PrimaryQual quality score objects sequentially. The following
bit of code will iterate sequentially over all quality scores in the database:

 my $fh = Bio::DB::Qual->newFh('/path/to/qual/files');
 while (my $qual = <$fh>) {
   print $qual->id,' => ',$qual->length,"\n";
 }

When no more quality scores remain to be retrieved, the stream will return
undef.

When a quality score is deleted from one of the qual files, this deletion is not
detected by the module and removed from the index. As a result, a "ghost" entry
will remain in the index and will return garbage results if accessed. Currently,
the only way to accommodate deletions is to rebuild the entire index, either by
deleting it manually, or by passing -reindex=>1 to new() when
initializing the module.
All quality score lines for a given quality score must have the same length
except for the last (not sure why there is this limitation). This is not
problematic for sequences but could be annoying for quality scores. A workaround
is to make sure the your quality scores fit on no more than 2 lines. Another
solution could be to padd them with blank spaces so that each line has the same
number of characters (maybe this padding should be implemented in
Bio::SeqIO::qual?).

Florent E Angly <florent . angly @ gmail-dot-com>.
Module largely based on and adapted from Bio::DB::Fasta by Lincoln Stein.
Copyright (c) 2007 Florent E Angly.
This library is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.