Re: Efficient 7bit compression

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Re^2: Efficient 7bit compression by Limbic~Region (Chancellor) on Mar 14, 2005 at 15:52 UTC
Roy Johnson, This looks similar to my unshown trading memory for time solution. The $bitstr will temporarily be 8 times larger than the original $str. I outlined 2 kinds of efficiency in my post (RAM & runtime). Now if there was a way to have our cake and eat it too. Thanks! Cheers - L~R	[reply]
Re^3: Efficient 7bit compression by Roy Johnson (Monsignor) on Mar 14, 2005 at 16:05 UTC
This has constant overhead by expanding only 8 chars at a time. `sub shrink { my $packed = ''; while ($_[0] =~ /(.{1,8})/gs) { my $bitstr = unpack('B', $1); $bitstr =~ s/.(.{7})/$1/g; $packed .= pack('B', $bitstr); } $packed; } sub grow { my $unpacked = ''; while ($_[0] =~ /(.{1,7})/gs) { my $bitstr = unpack('B', $1); $bitstr =~ s/(.{7})/0$1/g; $unpacked .= pack('B', $bitstr); } $unpacked; }` [download] Caution: Contents may have been coded under pressure.	[reply] [d/l]
Re^4: Efficient 7bit compression by Limbic~Region (Chancellor) on Mar 14, 2005 at 16:14 UTC
Roy Johnson, I tried a similar approach only using vec() and starting 1 bit back from the end of the string each time - I got bizarre results. An idea I had that I would have pursued if I could have made it work would be to pre-size the strings and use 4arg substr instead of growing it each time. Cheers - L~R	[reply]
Re^5: Efficient 7bit compression by Roy Johnson (Monsignor) on Mar 14, 2005 at 16:36 UTC
Re^6: Efficient 7bit compression by Limbic~Region (Chancellor) on Mar 14, 2005 at 16:43 UTC
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Re^3: Efficient 7bit compression by Ven'Tatsu (Deacon) on Mar 14, 2005 at 19:43 UTC
I'm not sure how this fares in reguard to speed, but it only uses 2-5 scalars, tries to avoid generating intermidiate lists, and modifies their arguments inplace. sub shrink { my $dbit = 0; for (my $sbit = 0; $sbit < length($_[0]) * 8; $sbit++) { next if $sbit % 8 == 7; vec($_[0], $dbit++, 1) = vec($_[0], $sbit, 1); } my $dlen = length($_[0]) * 7 / 8; $dlen++ unless $dlen == int($dlen); $dlen = int($dlen); my $extra = length($_[0]) - $dlen; if ($extra > 0) { substr($_[0], $dlen, $extra, ''); } for (my $pbit = $dbit; $pbit < $dlen * 8; $pbit++) { vec($_[0], $pbit, 1) = 0; } } sub grow { my $sbit = int(length($_[0]) * 8 / 7) * 7 - 1; for (my $dbit = int(length($_[0]) * 8 / 7) * 8 - 1; $dbit >= 0; $d +bit--) { vec($_[0], $dbit, 1) = $dbit % 8 == 7 ? 0 : vec($_[0], $sbit-- +, 1); } } [download] I have the nagging fealing there is a better way to implement `ceil` (near the middle of `shrink`). If padding the compressed string with 0 bits is not needed then the second for loop of `shrink` can be omitted to save (on average) 4 bits of time.	[reply] [d/l] [select]
Re^4: Efficient 7bit compression by Roy Johnson (Monsignor) on Mar 14, 2005 at 20:22 UTC
...a better way to implement ceil... `my $dlen = int((7 + length($_[0]) * 7)/8); substr($_[0], $dlen) = ''; vec ($_[0], $dbit++, 1) = 0 while ($dbit < $dlen * 8); }` [download] This replaces your shrink function from the definition of $dlen on. Caution: Contents may have been coded under pressure.	[reply] [d/l]


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