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The C Users' Group Library 1994 August
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wc-cdrom-cusersgrouplibrary-1994-08.iso
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v_08_06
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8n06051a
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1989-06-11
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Listing 2. The byte-wide decoding routines
/* decode routines for group 3 files
char g3_init (void) - initializes the g3 decode routines
char g3_reset (void) - may be called from within one of
the routines g3_white (), g3_black (), g3_error (), or
g3_EOL () to set the decoding state to zero
char g3_decode (void) - decodes a group 3 file
the above routines return nonzero on error, else zero
USER MUST PROVIDE THE FOLLOWING ROUTINES:
short g3_next_byte (void) - returns the next byte in the
image, -1 at EOF
void g3_white (short runlength) - writes a white run of
the specified length to the output file
void g3_black (short runlength) - writes a black run of
the specified length to the output file
void g3_EOL (void) - processes the end-of-line character
void g3_error (void) - process an invalid code
*/
static char initialized = 0, reset = 0;
static unsigned char action_table [220299];
static long branch_table [216] [256];
static short state = 0;
static short column, row;
extern unsigned char reverse_byte [];
char g3_init () {
if (!initialized) /* if this is the first time this
routine is called */ {
long bytes_read;
int action_handle, jump_handle;
... open the file containing action_table and read
into memory ...
... open the file containing branch_table and read
into memory ...
initialized = 1;
}
state = 0; reset = 0;
return (0);
}
char g3_reset () {
state = 0; reset = 1;
return (0);
}
char g3_decode () {
char rcode;
unsigned char *p_action;
short g3byte;
if (!initialized)
if (rcode = g3_init ()) return (rcode);
while ((g3byte = g3_next_byte ()) != -1)
/* while not end-of-image */ {
unsigned char code_byte;
p_action = &action_table [branch_table [state] [g3byte]];
while ((code_byte = *(p_action++)) != 255 && !reset)
/* until state change code */ {
if (!code_byte) g3_EOL ();
else if (code_byte == 1) g3_error ();
else if (code_byte < 67) g3_white (code_byte-2);
else if (code_byte < 106) g3_white ((code_byte-65)*64);
else if (code_byte < 171) g3_black (code_byte-106);
else g3_black ((code_byte-169)*64);
}
state = *p_action;
if (reset) /* if reset flag set during above */ {
state = 0; reset = 0;
}
}
}