flate

commit 44ccdf16a1e951aff295edcbc76b23b4117a36e8
parent 2e11080e70d83bc8d30c95a37c3191db79609a00
Author: nsz <nszabolcs@gmail.com>
Date:   Tue, 11 Aug 2009 14:19:18 +0200

deflate += comments
Diffstat:
deflate.c
 | 
31
++++++++++++++++++++++---------

1 file changed, 22 insertions(+), 9 deletions(-)
diff --git a/deflate.c b/deflate.c
@@ -79,7 +79,7 @@ typedef struct {
 	ushort chain[MaxDist];  /* hash chain */
 	LzCode lzbuf[LzSize];   /* literal run length, match len, match dist */
 	LzCode *lz;             /* current pos in lzbuf */
-	int run;                /* literal run length */
+	int nlit;               /* literal run length */
 	ushort lfreq[Nlitlen];
 	ushort dfreq[Ndist];
 	uchar dstbuf[DstSize];
@@ -277,6 +277,7 @@ static void putbits(State *s, uint bits, int n) {
 	}
 }
 
+/* run length encode literal and dist code lengths into codes and extra */
 static int clencodes(uchar *codes, uchar *extra, uchar *llen, int nlit, uchar *dlen, int ndist) {
 	int i, c, r, rr;
 	int n = 0;
@@ -286,7 +287,6 @@ static int clencodes(uchar *codes, uchar *extra, uchar *llen, int nlit, uchar *d
 	for (i = 0; i < ndist; i++)
 		codes[nlit + i] = dlen[i];
 	for (i = 0; i < nlit + ndist;) {
-		/* run length encoding */
 		c = codes[i];
 		for (r = 1; i + r < nlit + ndist && codes[i + r] == c; r++);
 		i += r;
@@ -316,6 +316,7 @@ static int clencodes(uchar *codes, uchar *extra, uchar *llen, int nlit, uchar *d
 	return n;
 }
 
+/* compress block data into s->dstbuf using given codes */
 static void putblock(State *s, ushort *lcode, uchar *llen, ushort *dcode, uchar *dlen) {
 	int n;
 	LzCode *lz;
@@ -336,6 +337,7 @@ static void putblock(State *s, ushort *lcode, uchar *llen, ushort *dcode, uchar 
 	putbits(s, lcode[EOB], llen[EOB]);
 }
 
+/* build code trees and select dynamic/fixed/uncompressed block compression */
 static void deflate_block(State *s) {
 	uchar codes[Nlitlen + Ndist], extra[Nlitlen + Ndist];
 	uchar llen[Nlitlen], dlen[Ndist], clen[Nclen];
@@ -365,7 +367,7 @@ static void deflate_block(State *s) {
 	huffcodes(ccode, clen, Nclen);
 	for (nclen = Nclen; nclen > 4 && clen[clenorder[nclen-1]] == 0; nclen--);
 
-	/* calc size */
+	/* calc compressed size */
 	uncsize = 3 + 16 + 8 * blocklen + (16 - 3 - s->nbits) % 8; /* byte aligned */
 	fixsize = 3;
 	dynsize = 3 + 5 + 5 + 4 + 3 * nclen;
@@ -443,6 +445,7 @@ fprintf(stderr, "blen: %d [%d,%d] lzlen: %d dynlen: %d (tree: %d, rate: %.3f) fi
 */
 }
 
+/* find n in base */
 static int bisect(ushort *base, int len, int n) {
 	int lo = 0;
 	int hi = len;
@@ -458,15 +461,17 @@ static int bisect(ushort *base, int len, int n) {
 	return lo - 1;
 }
 
+/* add literal run length to lzbuf */
 static void flushlit(State *s) {
-	if (s->run) {
+	if (s->nlit) {
 		s->lz->bits = LzLitFlag;
-		s->lz->n = s->run;
+		s->lz->n = s->nlit;
 		s->lz++;
-		s->run = 0;
+		s->nlit = 0;
 	}
 }
 
+/* add match to lzbuf and update freq counts */
 static void recordmatch(State *s, Match m) {
 	int n;
 
@@ -483,16 +488,18 @@ static void recordmatch(State *s, Match m) {
 	s->dfreq[n]++;
 }
 
+/* update literal run length */
 static void recordlit(State *s, int c) {
-	s->run++;
+	s->nlit++;
 	s->lfreq[c]++;
 }
 
-/* multiplicative hash */
+/* multiplicative hash (using a prime close to golden ratio * 2^32) */
 static int gethash(uchar *p) {
 	return (0x9e3779b1 * ((p[0]<<16) + (p[1]<<8) + p[2]) >> (32 - HashBits)) % HashSize;
 }
 
+/* update hash chain at the current position */
 static int updatechain(State *s) {
 	int h, i;
 
@@ -507,6 +514,7 @@ static int updatechain(State *s) {
 	return i;
 }
 
+/* find longest match, mpos is the next in the hash chain */
 static Match getmatch(State *s, int mpos) {
 	Match m = {0, MinMatch-1};
 	int len;
@@ -538,16 +546,18 @@ static Match getmatch(State *s, int mpos) {
 	return m;
 }
 
+/* start a new block */
 static void newblock(State *s) {
 	s->startpos = s->pos;
 	s->dst = s->dstbegin = s->dstbuf;
 	s->lz = s->lzbuf;
-	s->run = 0;
+	s->nlit = 0;
 	memset(s->lfreq, 0, sizeof(s->lfreq));
 	memset(s->dfreq, 0, sizeof(s->dfreq));
 	s->lfreq[EOB]++;
 }
 
+/* shift and fill win from s->src */
 static int fillwin(State *s) {
 	int n, k;
 
@@ -576,6 +586,7 @@ static int fillwin(State *s) {
 	return 1;
 }
 
+/* check if the current block should be ended */
 static int endofblock(State *s) {
 	if (s->avail == 0)
 		return 1;
@@ -585,6 +596,7 @@ static int endofblock(State *s) {
 	return 0;
 }
 
+/* deflate from s->src into s->dstbuf */
 static int deflate_state(State *s) {
 	Match m;
 	int head;
@@ -636,6 +648,7 @@ static int deflate_state(State *s) {
 	}
 }
 
+/* alloc and init state */
 static State *alloc_state(void) {
 	State *s = malloc(sizeof(State));
 	int i;