Seagate STD124000N Product Manual page 79

Data Compression
Product Manual
The following steps describe a simplified version of the operation of the algorithm for
decompressing data.
1. From a reset dictionary point, (which contains only control codes and encoded
bytes) codewords are fetched from the input stream and looked up in the
dictionary.
2. New dictionary codes are built by combining the previously received codewords.
(The dictionary created during compression is recreated, guaranteeing that any
codeword received is contained in the dictionary.)
Codewords that are encoded bytes are output directly. Codewords that are
dictionary codes lead the algorithm through a series of bytes and codewords that
point to other dictionary entries. Bytes are stacked until an encoded byte occurs;
then, the stack is output.
The following table illustrates the reverse process of compression, showing
simplified decompression operation.
Input
Code Byte
Value Value
(R) R
(I) I
(N) N
(T) T
(IN) N
— I
(TI) I
— T
— —
(N) N
The following table shows the dictionary based on the table above and the table on
page 65.
Codeword Byte Value
(RI) I
(IN) N
(NT) T
(TI) I
(INT) T
(TIN) N
Pointer Root? LIFO
— Y R
— Y I
— Y N
— Y T
(I) N N
— Y NI
(T) N I
— Y IT
— — —
— Y N
Code Value
(Pointer)
(R)
(I)
(N)
(T)
(IN)
(TI)
Chapter 7
Output
Entry Byte
— R
RI I
IN N
NT T
— —
TI I
— —
INT T
— I
TIN N
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