User's Manual

Data compression
49
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 a
simplified decompression operation.
Input
Code
Value
Byte
Value
Pointer
Root?
LIFO
Entry
Output
Byte
(R)
R
—
Y
R
—
R
(I)
I
—
Y
I
RI
I
(N)
N
—
Y
N
IN
N
(T)
T
—
Y
T
NT
T
(IN)
N
(I)
N
N
—
—
—
I
—
Y
NI
TI
I
(TI)
I
(T)
N
I
—
—
—
T
—
Y
IT
INT
T
—
—
—
—
—
—
I
(N)
N
—
Y
N
TIN
N
The following table shows the dictionary based on the preceding table.
Codeword
Byte Value
Code Value
(Pointer)
(RI)
I
(R)
(IN)
N
(I)
(NT)
T
(N)
(TI)
I
(T)
(INT)
T
(IN)
(TIN)
N
(TI)