请问下面函数(红色区域)的功能是什么？
#include <stdio.h>#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#define N 4096 /* size of ring buffer */
#define F 18 /* upper limit for match_length */
#define THRESHOLD 2 /* encode string into position and length
if match_length is greater than this */
#define NIL N /* index for root of binary search trees */
unsigned long int
textsize = 0, /* text size counter */
codesize = 0, /* code size counter */
printcount = 0; /* counter for reporting progress every 1K bytes */
unsigned char
text_buf[N + F - 1]; /* ring buffer of size N,
with extra F-1 bytes to facilitate string comparison */
int match_position, match_length, /* of longest match. These are
set by the InsertNode() procedure. */
lson[N + 1], rson[N + 257], dad[N + 1]; /* left & right children &
parents -- These constitute binary search trees. */
FILE *infile, *outfile; /* input & output files */
void InitTree(void) /* initialize trees */
{
int i;
/* For i = 0 to N - 1, rson[i] and lson[i] will be the right and
left children of node i. These nodes need not be initialized.
Also, dad[i] is the parent of node i. These are initialized to
NIL (= N), which stands for 'not used.'
For i = 0 to 255, rson[N + i + 1] is the root of the tree
for strings that begin with character i. These are initialized
to NIL. Note there are 256 trees. */
for (i = N + 1; i <= N + 256; i++) rson[i] = NIL;
for (i = 0; i < N; i++) dad[i] = NIL;
}
void InsertNode(int r)
/* Inserts string of length F, text_buf[r..r+F-1], into one of the
trees (text_buf[r]'th tree) and returns the longest-match position
and length via the global variables match_position and match_length.
If match_length = F, then removes the old node in favor of the new
one, because the old one will be deleted sooner.
Note r plays double role, as tree node and position in buffer. */
{
int i, p, cmp;
unsigned char *key;
cmp = 1; key = &text_buf[r]; p = N + 1 + key[0];
rson[r] = lson[r] = NIL; match_length = 0;
for ( ; ; ) {
if (cmp >= 0) {
if (rson[p] != NIL) p = rson[p];
else { rson[p] = r; dad[r] = p; return; }
} else {
if (lson[p] != NIL) p = lson[p];
else { lson[p] = r; dad[r] = p; return; }
}
for (i = 1; i < F; i++)
if ((cmp = key[i] - text_buf[p + i]) != 0) break;
if (i > match_length) {
match_position = p;
if ((match_length = i) >= F) break;
}
}
dad[r] = dad[p]; lson[r] = lson[p]; rson[r] = rson[p];
dad[lson[p]] = r; dad[rson[p]] = r;
if (rson[dad[p]] == p) rson[dad[p]] = r;
else lson[dad[p]] = r;
dad[p] = NIL; /* remove p */
}
void DeleteNode(int p) /* deletes node p from tree */
{
int q;
if (dad[p] == NIL) return; /* not in tree */
if (rson[p] == NIL) q = lson[p];
else if (lson[p] == NIL) q = rson[p];
else {
q = lson[p];
if (rson[q] != NIL) {
do { q = rson[q]; } while (rson[q] != NIL);
rson[dad[q]] = lson[q]; dad[lson[q]] = dad[q];
lson[q] = lson[p]; dad[lson[p]] = q;
}
rson[q] = rson[p]; dad[rson[p]] = q;
}
dad[q] = dad[p];
if (rson[dad[p]] == p) rson[dad[p]] = q; else lson[dad[p]] = q;
dad[p] = NIL;
}
void Encode(void)
{
int i, c, len, r, s, last_match_length, code_buf_ptr;
unsigned char code_buf[17], mask;
InitTree(); /* initialize trees */
code_buf[0] = 0; /* code_buf[1..16] saves eight units of code, and
code_buf[0] works as eight flags, "1" representing that the unit
is an unencoded letter (1 byte), "0" a position-and-length pair
(2 bytes). Thus, eight units require at most 16 bytes of code. */
code_buf_ptr = mask = 1;
s = 0; r = N - F;
for (i = s; i < r; i++) text_buf[i] = ' '; /* Clear the buffer with
any character that will appear often. */
for (len = 0; len < F && (c = getc(infile)) != EOF; len++)
text_buf[r + len] = c; /* Read F bytes into the last F bytes of
the buffer */
if ((textsize = len) == 0) return; /* text of size zero */
for (i = 1; i <= F; i++) InsertNode(r - i); /* Insert the F strings,
each of which begins with one or more 'space' characters. Note
the order in which these strings are inserted. This way,
degenerate trees will be less likely to occur. */
InsertNode(r); /* Finally, insert the whole string just read. The
global variables match_length and match_position are set. */
do {
if (match_length > len) match_length = len; /* match_length
may be spuriously long near the end of text. */
if (match_length <= THRESHOLD) {
match_length = 1; /* Not long enough match. Send one byte. */
code_buf[0] |= mask; /* 'send one byte' flag */
code_buf[code_buf_ptr++] = text_buf[r]; /* Send uncoded. */
} else {
code_buf[code_buf_ptr++] = (unsigned char) match_position;
code_buf[code_buf_ptr++] = (unsigned char)
(((match_position >> 4) & 0xf0)
| (match_length - (THRESHOLD + 1))); /* Send position and
length pair. Note match_length > THRESHOLD. */
}
if ((mask <<= 1) == 0) { /* Shift mask left one bit. */
for (i = 0; i < code_buf_ptr; i++) /* Send at most 8 units of */
putc(code_buf[i], outfile); /* code together */
codesize += code_buf_ptr;
code_buf[0] = 0; code_buf_ptr = mask = 1;
}
last_match_length = match_length;
for (i = 0; i < last_match_length &&
(c = getc(infile)) != EOF; i++) {
DeleteNode(s); /* Delete old strings and */
text_buf[s] = c; /* read new bytes */
if (s < F - 1) text_buf[s + N] = c; /* If the position is
near the end of buffer, extend the buffer to make
string comparison easier. */
s = (s + 1) & (N - 1); r = (r + 1) & (N - 1);
/* Since this is a ring buffer, increment the position
modulo N. */
InsertNode(r); /* Register the string in text_buf[r..r+F-1] */
}
if ((textsize += i) > printcount) {
printf("%12ld\r", textsize); printcount += 1024;
/* Reports progress each time the textsize exceeds
multiples of 1024. */
}
while (i++ < last_match_length) { /* After the end of text, */
DeleteNode(s); /* no need to read, but */
s = (s + 1) & (N - 1); r = (r + 1) & (N - 1);
if (--len) InsertNode(r); /* buffer may not be empty. */
}
} while (len > 0); /* until length of string to be processed is zero */
if (code_buf_ptr > 1) { /* Send remaining code. */
for (i = 0; i < code_buf_ptr; i++) putc(code_buf[i], outfile);
codesize += code_buf_ptr;
}
printf("In : %ld bytes\n", textsize); /* Encoding is done. */
printf("Out: %ld bytes\n", codesize);
printf("Out/In: %.3f\n", (double)codesize / textsize);
}
void Decode(void) /* Just the reverse of Encode(). */
{
int i, j, k, r, c;
unsigned int flags;
for (i = 0; i < N - F; i++) text_buf[i] = ' ';
r = N - F; flags = 0;
for ( ; ; ) {
if (((flags >>= 1) & 256) == 0) {
if ((c = getc(infile)) == EOF) break;
flags = c | 0xff00; /* uses higher byte cleverly */
} /* to count eight */
if (flags & 1) {
if ((c = getc(infile)) == EOF) break;
putc(c, outfile); text_buf[r++] = c; r &= (N - 1);
} else {
if ((i = getc(infile)) == EOF) break;
if ((j = getc(infile)) == EOF) break;
i |= ((j & 0xf0) << 4); j = (j & 0x0f) + THRESHOLD;
for (k = 0; k <= j; k++) {
c = text_buf[(i + k) & (N - 1)];
putc(c, outfile); text_buf[r++] = c; r &= (N - 1);
}
}
}
}
int main(int argc, char *argv[])
{
char *s;
if (argc != 4) {
printf("'lzss e file1 file2' encodes file1 into file2.\n"
"'lzss d file2 file1' decodes file2 into file1.\n"
"%d\n",argc);
return EXIT_FAILURE;
}
printf("ok\n");
if ((s = argv[1], s[1] || strpbrk(s, "DEde") == NULL)
|| (s = argv[2], (infile = fopen(s, "rb")) == NULL)
|| (s = argv[3], (outfile = fopen(s, "wb")) == NULL)) {
printf("??? %s\n", s); return EXIT_FAILURE;
}
if (toupper(*argv[1]) == 'E') Encode(); else Decode();
fclose(infile); fclose(outfile);
return EXIT_SUCCESS;
}
[ 本帖最后由 maoniu 于 2010-4-21 17:40 编辑 ]
