From: Rusty Russell <rusty@rustcorp.com.au>

[NOTE: this patch is for _gzip_! --Wayne]

It's pending for actual gzip inclusion!  The maintainer was dug up and
everything: it'll even be on by default.

But here's the old patch,
Rusty.
--
  Anyone who quotes me in their sig is an idiot. -- Rusty Russell.

--- gzip-1.2.4.orig/deflate.c	Fri Aug 13 22:35:31 1993
+++ gzip-1.2.4-rsync/deflate.c	Sat Dec 30 15:33:25 2000
@@ -121,6 +121,14 @@
 #endif
 /* Matches of length 3 are discarded if their distance exceeds TOO_FAR */
 
+#ifndef RSYNC_WIN
+#  define RSYNC_WIN 4096
+#endif
+/* Size of rsync window, must be < MAX_DIST */
+
+#define RSYNC_SUM_MATCH(sum) ((sum) % RSYNC_WIN == 0)
+/* Whether window sum matches magic value */
+
 /* ===========================================================================
  * Local data used by the "longest match" routines.
  */
@@ -202,6 +210,8 @@
 unsigned near good_match;
 /* Use a faster search when the previous match is longer than this */
 
+local ulg rsync_sum;  /* rolling sum of rsync window */
+local ulg rsync_chunk_end; /* next rsync sequence point */
 
 /* Values for max_lazy_match, good_match and max_chain_length, depending on
  * the desired pack level (0..9). The values given below have been tuned to
@@ -300,6 +310,10 @@
 #endif
     /* prev will be initialized on the fly */
 
+    /* rsync params */
+    rsync_chunk_end = 0xFFFFFFFFUL;
+    rsync_sum = 0;
+
     /* Set the default configuration parameters:
      */
     max_lazy_match   = configuration_table[pack_level].max_lazy;
@@ -536,6 +550,8 @@
         memcpy((char*)window, (char*)window+WSIZE, (unsigned)WSIZE);
         match_start -= WSIZE;
         strstart    -= WSIZE; /* we now have strstart >= MAX_DIST: */
+	if (rsync_chunk_end != 0xFFFFFFFFUL)
+	    rsync_chunk_end -= WSIZE;
 
         block_start -= (long) WSIZE;
 
@@ -563,13 +579,46 @@
     }
 }
 
+local void rsync_roll(start, num)
+    unsigned start;
+    unsigned num;
+{
+    unsigned i;
+
+    if (start < RSYNC_WIN) {
+	/* before window fills. */
+	for (i = start; i < RSYNC_WIN; i++) {
+	    if (i == start + num) return;
+	    rsync_sum += (ulg)window[i];
+	}
+	num -= (RSYNC_WIN - start);
+	start = RSYNC_WIN;
+    }
+
+    /* buffer after window full */
+    for (i = start; i < start+num; i++) {
+	/* New character in */
+	rsync_sum += (ulg)window[i];
+	/* Old character out */
+	rsync_sum -= (ulg)window[i - RSYNC_WIN];
+	if (rsync_chunk_end == 0xFFFFFFFFUL && RSYNC_SUM_MATCH(rsync_sum))
+	    rsync_chunk_end = i;
+    }
+}
+
+/* ===========================================================================
+ * Set rsync_chunk_end if window sum matches magic value.
+ */
+#define RSYNC_ROLL(s, n) \
+   do { if (rsync) rsync_roll((s), (n)); } while(0)
+
 /* ===========================================================================
  * Flush the current block, with given end-of-file flag.
  * IN assertion: strstart is set to the end of the current match.
  */
 #define FLUSH_BLOCK(eof) \
    flush_block(block_start >= 0L ? (char*)&window[(unsigned)block_start] : \
-                (char*)NULL, (long)strstart - block_start, (eof))
+                (char*)NULL, (long)strstart - block_start, flush-1, (eof))
 
 /* ===========================================================================
  * Processes a new input file and return its compressed length. This
@@ -580,7 +629,7 @@
 local ulg deflate_fast()
 {
     IPos hash_head; /* head of the hash chain */
-    int flush;      /* set if current block must be flushed */
+    int flush;      /* set if current block must be flushed, 2=>and padded  */
     unsigned match_length = 0;  /* length of best match */
 
     prev_length = MIN_MATCH-1;
@@ -609,6 +658,7 @@
 
             lookahead -= match_length;
 
+	    RSYNC_ROLL(strstart, match_length);
 	    /* Insert new strings in the hash table only if the match length
              * is not too large. This saves time but degrades compression.
              */
@@ -637,9 +687,14 @@
             /* No match, output a literal byte */
             Tracevv((stderr,"%c",window[strstart]));
             flush = ct_tally (0, window[strstart]);
+	    RSYNC_ROLL(strstart, 1);
             lookahead--;
 	    strstart++; 
         }
+	if (rsync && strstart > rsync_chunk_end) {
+	    rsync_chunk_end = 0xFFFFFFFFUL;
+	    flush = 2;
+	} 
         if (flush) FLUSH_BLOCK(0), block_start = strstart;
 
         /* Make sure that we always have enough lookahead, except
@@ -715,6 +770,7 @@
              */
             lookahead -= prev_length-1;
             prev_length -= 2;
+	    RSYNC_ROLL(strstart, prev_length+1);
             do {
                 strstart++;
                 INSERT_STRING(strstart, hash_head);
@@ -727,24 +783,39 @@
             match_available = 0;
             match_length = MIN_MATCH-1;
             strstart++;
-            if (flush) FLUSH_BLOCK(0), block_start = strstart;
 
+	    if (rsync && strstart > rsync_chunk_end) {
+		rsync_chunk_end = 0xFFFFFFFFUL;
+		flush = 2;
+	    }
+            if (flush) FLUSH_BLOCK(0), block_start = strstart;
         } else if (match_available) {
             /* If there was no match at the previous position, output a
              * single literal. If there was a match but the current match
              * is longer, truncate the previous match to a single literal.
              */
             Tracevv((stderr,"%c",window[strstart-1]));
-            if (ct_tally (0, window[strstart-1])) {
-                FLUSH_BLOCK(0), block_start = strstart;
-            }
+	    flush = ct_tally (0, window[strstart-1]);
+	    if (rsync && strstart > rsync_chunk_end) {
+		rsync_chunk_end = 0xFFFFFFFFUL;
+		flush = 2;
+	    }
+            if (flush) FLUSH_BLOCK(0), block_start = strstart;
+	    RSYNC_ROLL(strstart, 1);
             strstart++;
             lookahead--;
         } else {
             /* There is no previous match to compare with, wait for
              * the next step to decide.
              */
+	    if (rsync && strstart > rsync_chunk_end) {
+		/* Reset huffman tree */
+		rsync_chunk_end = 0xFFFFFFFFUL;
+		flush = 2;
+		FLUSH_BLOCK(0), block_start = strstart;
+	    }
             match_available = 1;
+	    RSYNC_ROLL(strstart, 1);
             strstart++;
             lookahead--;
         }
--- gzip-1.2.4.orig/gzip.c	Thu Aug 19 23:39:43 1993
+++ gzip-1.2.4-rsync/gzip.c	Fri Dec 29 21:20:54 2000
@@ -239,6 +239,7 @@
 unsigned insize;           /* valid bytes in inbuf */
 unsigned inptr;            /* index of next byte to be processed in inbuf */
 unsigned outcnt;           /* bytes in output buffer */
+int rsync = 0;             /* make ryncable chunks */
 
 struct option longopts[] =
 {
@@ -268,6 +269,7 @@
     {"best",       0, 0, '9'}, /* compress better */
     {"lzw",        0, 0, 'Z'}, /* make output compatible with old compress */
     {"bits",       1, 0, 'b'}, /* max number of bits per code (implies -Z) */
+    {"rsyncable",  0, 0, 'R'}, /* make rsync-friendly archive */
     { 0, 0, 0, 0 }
 };
 
@@ -357,6 +359,7 @@
  " -Z --lzw         produce output compatible with old compress",
  " -b --bits maxbits   max number of bits per code (implies -Z)",
 #endif
+ "    --rsyncable   Make rsync-friendly archive",
  " file...          files to (de)compress. If none given, use standard input.",
   0};
     char **p = help_msg;
@@ -516,6 +519,9 @@
 #else
 	    recursive = 1; break;
 #endif
+	case 'R':
+	    rsync = 1; break;
+
 	case 'S':
 #ifdef NO_MULTIPLE_DOTS
             if (*optarg == '.') optarg++;
--- gzip-1.2.4.orig/gzip.h	Fri Aug 13 22:35:33 1993
+++ gzip-1.2.4-rsync/gzip.h	Sat Dec 30 15:26:56 2000
@@ -131,6 +131,7 @@
 extern unsigned insize; /* valid bytes in inbuf */
 extern unsigned inptr;  /* index of next byte to be processed in inbuf */
 extern unsigned outcnt; /* bytes in output buffer */
+extern int rsync;  /* deflate into rsyncable chunks */
 
 extern long bytes_in;   /* number of input bytes */
 extern long bytes_out;  /* number of output bytes */
@@ -282,7 +283,7 @@
         /* in trees.c */
 void ct_init     OF((ush *attr, int *method));
 int  ct_tally    OF((int dist, int lc));
-ulg  flush_block OF((char *buf, ulg stored_len, int eof));
+ulg  flush_block OF((char *buf, ulg stored_len, int pad, int eof));
 
         /* in bits.c */
 void     bi_init    OF((file_t zipfile));
--- gzip-1.2.4.orig/gzip.texi	Thu Aug 19 06:42:50 1993
+++ gzip-1.2.4-rsync/gzip.texi	Fri Dec 29 21:20:54 2000
@@ -316,6 +316,14 @@
 into the directory and compress all the files it finds there (or
 decompress them in the case of @code{gunzip}).
 
+@item --rsyncable
+While compressing, synchronize the output occasionally based on the
+input.  This reduces compression by about 1 percent most cases, but
+means that the @code{rsync} program can take advantage of similarities
+in the uncompressed input when syncronizing two files compressed with
+this flag.  @code{gunzip} cannot tell the difference between a
+compressed file created with this option, and one created without it.
+
 @item --suffix @var{suf}
 @itemx -S @var{suf}
 Use suffix @samp{@var{suf}} instead of @samp{.gz}. Any suffix can be
--- gzip-1.2.4.orig/trees.c	Wed Aug 18 03:36:32 1993
+++ gzip-1.2.4-rsync/trees.c	Sat Dec 30 15:37:00 2000
@@ -850,9 +850,10 @@
  * trees or store, and output the encoded block to the zip file. This function
  * returns the total compressed length for the file so far.
  */
-ulg flush_block(buf, stored_len, eof)
+ulg flush_block(buf, stored_len, pad, eof)
     char *buf;        /* input block, or NULL if too old */
     ulg stored_len;   /* length of input block */
+    int pad;          /* pad output to byte boundary */
     int eof;          /* true if this is the last block for a file */
 {
     ulg opt_lenb, static_lenb; /* opt_len and static_len in bytes */
@@ -944,6 +967,10 @@
         Assert (input_len == isize, "bad input size");
         bi_windup();
         compressed_len += 7;  /* align on byte boundary */
+    } else if (pad && (compressed_len % 8) != 0) {
+        send_bits((STORED_BLOCK<<1)+eof, 3);  /* send block type */
+        compressed_len = (compressed_len + 3 + 7) & ~7L;
+        copy_block(buf, 0, 1); /* with header */
     }
     Tracev((stderr,"\ncomprlen %lu(%lu) ", compressed_len>>3,
            compressed_len-7*eof));