Re^5: Efficient 7bit compression

Are you talking about shrinking in place, so you don't have the overhead of having the compressed and uncompressed strings in memory at the same time?

my $str =<<'EOS';
There once was a man from Nantucket,
Who kept all his cash in a bucket.
His daughter, named Nan,
Ran off with a man.
And as for the bucket, Nantucket.
EOS

sub shrink_in_place {
    for (my $i=0; $i<length($_[0]); $i+=7) {
        for (substr($_[0], $i, 8)) {
            $_ = pack('B*', grep s/.(.{7})/$1/g, unpack('B*', $_));
        }
    }
}

sub grow_in_place {
    for (my $i=0; $i<length($_[0]); $i+=8) {
        for (substr($_[0], $i, 7)) {
            $_ = pack('B*', grep s/(.{7})/0$1/g, unpack('B*', $_));
        }
    }
}

print "$str";
printf "Length is %d\n", length($str);
shrink_in_place($str);
printf "Shrunk length is %d\n", length($str);
grow_in_place($str);
print "Restored: $str";
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Caution: Contents may have been coded under pressure.

Comment on Re^5: Efficient 7bit compression Download Code

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Re^6: Efficient 7bit compression by Limbic~Region (Chancellor) on Mar 14, 2005 at 16:43 UTC
Roy Johnson, Are you talking about shrinking in place, so you don't have the overhead of having the compressed and uncompressed strings in memory at the same time? No - but neat. What I was talking about was with regards to $unpacked = ''. Instead, pre-size it and use the 4 arg substr to fill it up instead of growing it each time. As I said before, I didn't get my vec() approach to work but I would have also been able to do this in the shrink() sub as well. Cheers - L~R	[reply] [d/l]
Re^7: Efficient 7bit compression by Roy Johnson (Monsignor) on Mar 14, 2005 at 18:15 UTC
I've been playing with it, and I see the problem. vec is horrible, because it reverses everything. When you pack them together by sevens and take them apart by eights, it's a mess to try to get everything reversed. Here's a version with pre-sized return strings, using vec (but there's no need for substr). No packing, so no reversed-bits madness to deal with. `sub shrink { my $shrunk = "\0" x (length($_[0]) * 7/8); my $bit = 0; for my $orig_bit (0..(8length($_[0])-1)) { next if $orig_bit % 8 == 7; vec($shrunk, $bit++, 1) = vec($_[0], $orig_bit, 1); } $shrunk; } sub grow { my $grown = "\0" x (length($_[0]) 8/7); my $orig_bit = 0; for my $bit (0..(8length($_[0])-1)) { vec($grown, $orig_bit++, 1) = vec($_[0], $bit, 1); vec($grown, $orig_bit++, 1) = 0 if $orig_bit % 8 == 7; } $grown; }` [download] Caution:* Contents may have been coded under pressure.	[reply] [d/l]