perl 读取文件中某行

[perl]代码库

#-----------------------------
# looking for line number $DESIRED_LINE_NUMBER
$. = 0;
do { $LINE = <HANDLE> } until $. == $DESIRED_LINE_NUMBER || eof;
#-----------------------------
@lines = <HANDLE>;
$LINE = $lines[$DESIRED_LINE_NUMBER];
#-----------------------------
# usage: build_index(*DATA_HANDLE, *INDEX_HANDLE)
sub build_index {
    my $data_file  = shift;
    my $index_file = shift;
    my $offset     = 0;

    while (<$data_file>) {
        print $index_file pack("N", $offset);
        $offset = tell($data_file);
    }
}

# usage: line_with_index(*DATA_HANDLE, *INDEX_HANDLE, $LINE_NUMBER)
# returns line or undef if LINE_NUMBER was out of range
sub line_with_index {
    my $data_file   = shift;
    my $index_file  = shift;
    my $line_number = shift;

    my $size;               # size of an index entry
    my $i_offset;           # offset into the index of the entry
    my $entry;              # index entry
    my $d_offset;           # offset into the data file

    $size = length(pack("N", 0));
    $i_offset = $size * ($line_number-1);
    seek($index_file, $i_offset, 0) or return;
    read($index_file, $entry, $size);
    $d_offset = unpack("N", $entry);
    seek($data_file, $d_offset, 0);
    return scalar(<$data_file>);
}

# usage:
open(FILE, "< $file")         or die "Can't open $file for reading: $!\n";
open(INDEX, "+>$file.idx")
        or die "Can't open $file.idx for read/write: $!\n";
build_index(*FILE, *INDEX);
$line = line_with_index(*FILE, *INDEX, $seeking);
#-----------------------------
use DB_File;
use Fcntl;

$tie = tie(@lines, $FILE, "DB_File", O_RDWR, 0666, $DB_RECNO) or die 
    "Cannot open file $FILE: $!\n";
# extract it
$line = $lines[$sought - 1];
#-----------------------------
# download the following standalone program
#!/usr/bin/perl -w
# print_line-v1 - linear style

@ARGV == 2 or die "usage: print_line FILENAME LINE_NUMBER\n";

($filename, $line_number) = @ARGV;
open(INFILE, "< $filename") or die "Can't open $filename for reading: $!\n";
while (<INFILE>) {
    $line = $_;
    last if $. == $line_number;
}
if ($. != $line_number) {
    die "Didn't find line $line_number in $filename\n";
}
print;

#-----------------------------
# download the following standalone program
#!/usr/bin/perl -w
# print_line-v2 - index style
# build_index and line_with_index from above
@ARGV == 2 or
    die "usage: print_line FILENAME LINE_NUMBER";

($filename, $line_number) = @ARGV;
open(ORIG, "< $filename") 
        or die "Can't open $filename for reading: $!";

# open the index and build it if necessary
# there's a race condition here: two copies of this
# program can notice there's no index for the file and
# try to build one.  This would be easily solved with
# locking
$indexname = "$filename.index";
sysopen(IDX, $indexname, O_CREAT|O_RDWR)
         or die "Can't open $indexname for read/write: $!";
build_index(*ORIG, *IDX) if -z $indexname;  # XXX: race unless lock

$line = line_with_index(*ORIG, *IDX, $line_number);
die "Didn't find line $line_number in $filename" unless defined $line;
print $line;

#-----------------------------
# download the following standalone program
#!/usr/bin/perl -w
# print_line-v3 - DB_File style
use DB_File;
use Fcntl;

@ARGV == 2 or
    die "usage: print_line FILENAME LINE_NUMBER\n";

($filename, $line_number) = @ARGV;
$tie = tie(@lines, "DB_File", $filename, O_RDWR, 0666, $DB_RECNO)
        or die "Cannot open file $filename: $!\n";

unless ($line_number < $tie->length) {
    die "Didn't find line $line_number in $filename\n"
}

print $lines[$line_number-1];                        # easy, eh?

#-----------------------------