text
stringlengths 0
99.6k
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out the prefix 6 and make "c" the new prefix.
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At this point our string table looks like this:
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string code prefix+extension
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------ ---- ----------------
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a 1
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b 2
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c 3
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ab 4 1+2
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ba 5 2+1
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aba 6 4+1
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abac 7 6+1
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Or if you prefer trees:
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a=1 - b=4 - a=6 - c=7
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/
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root - b=2 - a=5
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\
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c=3
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This goes on until the string table is full. If there
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is enough repetition in the input file, long sequences of
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characters can be replaced by short 9-12 bit codes and
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significant savings can be achieved.
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ARC VERSION 2.20 PAGE - 37
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ARC starts out by initializing its string table to 256
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entries; the byte values 0 to 255. Since the first 256 codes
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generated will take on values between 256 and 511, we only
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need nine bits for the first 256 codes. The next 512 codes
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will be between 512 and 1023, so we only need 10 bits for
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the next 512 codes. The size of the code keeps growing like
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this until it reaches 12 bits and the string table is full.
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This significantly improves the compression ratio when
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crunching small files.
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Once the string table is full, if a character is
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encountered for the first time, it will have to be sent to
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the output file as a 12 bit code; a 50% loss! We have found
|
that the string table usually fills up at about 60-70 CBM
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disk blocks for text, and at about 40 for machine language.
|
You may notice that ML programs of 40 blocks or less usually
|
crunch, wheras longer ML programs tend to be squashed. If
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you are crunching text files, the compression ratio is
|
usually about 2.00. Once the string table has become full,
|
the compression ratio will start to diminish because of
|
this.
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ARC reserves two codes when it crunches a file. Code
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256 is reserved to indicate the end of file. This is
|
necessary since ARC doesn't know a files length until after
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it has been archived using the single pass crunch option.
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Code 257 is reserved for future versions of ARC, which will
|
use it as a signal to tell the decompressor to reset the
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string table once the compression ratio starts to fall off
|
on large files.
|
Another amazing thing about crunching is the fact that
|
it is not very efficient! No attempt is made to find the
|
most often occuring character sequences in the file. The LZW
|
approach simply takes things as they come. A long and very
|
infrequently occuring character sequence could be taking up
|
valuable space in the string table, when other frequently
|
occurring sequences have to be coded as individual codes
|
once the table is full. When a file is very long, the string
|
table may have given a good compression ratio near the
|
beginning of the file but the string table may no longer
|
reflect the files characteristics near the end of the file.
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Despite the fact that crunching gives quite good
|
compression ratios, there is lots of room for improvement.
|
We could be more selective about the strings we allow in the
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table, we could purge the table periodically for
|
infrequently used codes, or we could simply use a larger
|
string table.
|
ARC VERSION 2.20 PAGE - 38
|
Archive File Format
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Each archive entry consists of a short header followed
|
by the compressed file. If the file is squeezed or squashed,
|
the Huffman encoding table appears immediately following the
|
header and before the file data. The archive header consists
|
of the following bytes:
|
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