--- a/deflate.c
+++ b/deflate.c
@@ -131,6 +131,14 @@
 #endif
 /* Matches of length 3 are discarded if their distance exceeds TOO_FAR */
 
+#ifndef RSYNC_WIN
+#  define RSYNC_WIN 8192
+#endif
+/* Size of rsync window, must be < MAX_DIST */
+
+#define RSYNC_SUM_MATCH(sum) (((sum) & (RSYNC_WIN - 1)) == 0)
+/* Whether window sum matches magic value */
+
 /* ===========================================================================
  * Local data used by the "longest match" routines.
  */
@@ -212,6 +220,8 @@
 unsigned 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
@@ -314,6 +324,10 @@
 #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;
@@ -550,6 +564,8 @@
         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;
 
@@ -579,13 +595,44 @@
     }
 }
 
+local void rsync_roll(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
@@ -596,7 +643,7 @@
 local off_t deflate_fast()
 {
     IPos hash_head; /* head of the hash chain */
-    int flush;      /* set if current block must be flushed */
+    int flush = 0;      /* set if current block must be flushed, 2=>and padded  */
     unsigned match_length = 0;  /* length of best match */
 
     prev_length = MIN_MATCH-1;
@@ -626,7 +673,8 @@
 
             lookahead -= match_length;
 
-            /* Insert new strings in the hash table only if the 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.
              */
             if (match_length <= max_insert_length) {
@@ -654,9 +702,14 @@
             /* 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) {
+            flush = 2;
+            rsync_chunk_end = 0xFFFFFFFFUL;
+        }
         if (flush) FLUSH_BLOCK(0), block_start = strstart;
 
         /* Make sure that we always have enough lookahead, except
@@ -679,7 +732,7 @@
 {
     IPos hash_head;          /* head of hash chain */
     IPos prev_match;         /* previous match */
-    int flush;               /* set if current block must be flushed */
+    int flush = 0;           /* set if current block must be flushed */
     int match_available = 0; /* set if previous match exists */
     register unsigned match_length = MIN_MATCH-1; /* length of best match */
 
@@ -730,6 +783,7 @@
              */
             lookahead -= prev_length-1;
             prev_length -= 2;
+	    RSYNC_ROLL(strstart, prev_length+1);
             do {
                 strstart++;
                 INSERT_STRING(strstart, hash_head);
@@ -742,24 +796,38 @@
             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) {
+		rsync_chunk_end = 0xFFFFFFFFUL;
+		flush = 2;
+		FLUSH_BLOCK(0), block_start = strstart;
+	    }
             match_available = 1;
+	    RSYNC_ROLL(strstart, 1);
             strstart++;
             lookahead--;
         }
--- a/gzip.c
+++ b/gzip.c
@@ -215,6 +215,7 @@
 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 */
 
 static int handled_sig[] =
   {
@@ -275,7 +276,7 @@
     {"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 }
 };
 
@@ -359,6 +360,7 @@
  "  -Z, --lzw         produce output compatible with old compress",
  "  -b, --bits=BITS   max number of bits per code (implies -Z)",
 #endif
+ "  --rsyncable       Make rsync-friendly archive",
  "",
  "With no FILE, or when FILE is -, read standard input.",
  "",
@@ -489,8 +491,11 @@
 #else
             recursive = 1;
 #endif
-            break;
-        case 'S':
+	    break;
+        case 'R':
+	    rsync = 1; break;
+
+	case 'S':
 #ifdef NO_MULTIPLE_DOTS
             if (*optarg == '.') optarg++;
 #endif
--- a/gzip.h
+++ b/gzip.h
@@ -140,6 +140,7 @@
 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 off_t bytes_in;   /* number of input bytes */
 extern off_t bytes_out;  /* number of output bytes */
@@ -287,7 +288,7 @@
         /* in trees.c */
 extern void ct_init     (ush *attr, int *method);
 extern int  ct_tally    (int dist, int lc);
-extern off_t flush_block (char *buf, ulg stored_len, int eof);
+extern off_t flush_block (char *buf, ulg stored_len, int pad, int eof);
 
         /* in bits.c */
 extern void     bi_init    (file_t zipfile);
--- a/gzip.1
+++ b/gzip.1
@@ -5,6 +5,7 @@
 .ll +8
 .B gzip
 .RB [ " \-acdfhklLnNrtvV19 " ]
+.RB [ --rsyncable ]
 .RB [ \-S\ suffix ]
 [
 .I "name \&..."
@@ -287,6 +288,16 @@
 .I gunzip
 ).
 .TP
+.B --rsyncable
+While compressing, synchronize the output occasionally based on the input.
+This increases size by less than 1 percent most cases, but means that the
+.BR rsync (1)
+program can take advantage of similarities in the uncompressed input
+when syncronizing two files compressed with this flag.
+.I gunzip
+cannot tell the difference between a compressed file created with this option,
+and one created without it.
+.TP
 .B \-S .suf   --suffix .suf
 When compressing, use suffix .suf instead of .gz.
 Any non-empty suffix can be given, but suffixes
--- a/trees.c
+++ b/trees.c
@@ -856,9 +856,10 @@
  * trees or store, and output the encoded block to the zip file. This function
  * returns the total compressed length for the file so far.
  */
-off_t flush_block(buf, stored_len, eof)
+off_t 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 */
@@ -951,6 +952,10 @@
         Assert (input_len == bytes_in, "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 */
     }
 
     return compressed_len >> 3;
