archive-zip: use deflateInit2() to ask for raw compressed data

We use the function git_deflate_init() -- which wraps the zlib function
deflateInit() -- to initialize compression of ZIP file entries.  This
results in compressed data prefixed with a two-bytes long header and
followed by a four-bytes trailer.  ZIP file entries consist of ZIP
headers and raw compressed data instead, so we remove the zlib wrapper
before writing the result.

We can ask zlib for the the raw compressed data without the unwanted
parts in the first place by using deflateInit2() and specifying a
negative number of bits to size the window.  For that purpose, factor
out the function do_git_deflate_init() and add git_deflate_init_raw(),
which wraps it.  Then use the latter in archive-zip.c and get rid of
the code that stripped the zlib header and trailer.

Also rename the helper function zlib_deflate() to zlib_deflate_raw()
to reflect the change.

Thus we avoid generating data that we throw away anyway, the code
becomes shorter and some magic constants are removed.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 14 insertions, 22 deletions

diff --git a/archive-zip.c b/archive-zip.c
index a8d1193..b2c4fe0 100644
--- a/archive-zip.c
+++ b/archive-zip.c
@@ -111,8 +111,9 @@ static void copy_le32(unsigned char *dest, unsigned int n)
 	dest[3] = 0xff & (n >> 030);
 }
 
-static void *zlib_deflate(void *data, unsigned long size,
-		int compression_level, unsigned long *compressed_size)
+static void *zlib_deflate_raw(void *data, unsigned long size,
+			      int compression_level,
+			      unsigned long *compressed_size)
 {
 	git_zstream stream;
 	unsigned long maxsize;
@@ -120,7 +121,7 @@ static void *zlib_deflate(void *data, unsigned long size,
 	int result;
 
 	memset(&stream, 0, sizeof(stream));
-	git_deflate_init(&stream, compression_level);
+	git_deflate_init_raw(&stream, compression_level);
 	maxsize = git_deflate_bound(&stream, size);
 	buffer = xmalloc(maxsize);
 
@@ -265,14 +266,11 @@ static int write_zip_entry(struct archiver_args *args,
 	}
 
 	if (buffer && method == 8) {
-		deflated = zlib_deflate(buffer, size, args->compression_level,
-				&compressed_size);
-		if (deflated && compressed_size - 6 < size) {
-			/* ZLIB --> raw compressed data (see RFC 1950) */
-			/* CMF and FLG ... */
-			out = (unsigned char *)deflated + 2;
-			compressed_size -= 6;	/* ... and ADLER32 */
-		} else {
+		out = deflated = zlib_deflate_raw(buffer, size,
+						  args->compression_level,
+						  &compressed_size);
+		if (!out || compressed_size >= size) {
+			out = buffer;
 			method = 0;
 			compressed_size = size;
 		}
@@ -353,7 +351,7 @@ static int write_zip_entry(struct archiver_args *args,
 		unsigned char compressed[STREAM_BUFFER_SIZE * 2];
 
 		memset(&zstream, 0, sizeof(zstream));
-		git_deflate_init(&zstream, args->compression_level);
+		git_deflate_init_raw(&zstream, args->compression_level);
 
 		compressed_size = 0;
 		zstream.next_out = compressed;
@@ -370,13 +368,10 @@ static int write_zip_entry(struct archiver_args *args,
 			result = git_deflate(&zstream, 0);
 			if (result != Z_OK)
 				die("deflate error (%d)", result);
-			out = compressed;
-			if (!compressed_size)
-				out += 2;
-			out_len = zstream.next_out - out;
+			out_len = zstream.next_out - compressed;
 
 			if (out_len > 0) {
-				write_or_die(1, out, out_len);
+				write_or_die(1, compressed, out_len);
 				compressed_size += out_len;
 				zstream.next_out = compressed;
 				zstream.avail_out = sizeof(compressed);
@@ -394,11 +389,8 @@ static int write_zip_entry(struct archiver_args *args,
 			die("deflate error (%d)", result);
 
 		git_deflate_end(&zstream);
-		out = compressed;
-		if (!compressed_size)
-			out += 2;
-		out_len = zstream.next_out - out - 4;
-		write_or_die(1, out, out_len);
+		out_len = zstream.next_out - compressed;
+		write_or_die(1, compressed, out_len);
 		compressed_size += out_len;
 		zip_offset += compressed_size;