xz compression detection: avoid the need to seek

function                                             old     new   delta
unpack_unxz                                           12      67     +55
unpack_xz_stream                                    2357    2373     +16
xmalloc_read                                         197     199      +2
setup_unzip_on_fd                                    118      99     -19
rpm2cpio_main                                        222     203     -19
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/2 up/down: 73/-38)             Total: 35 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 14 insertions, 9 deletions

diff --git a/include/unarchive.h b/include/unarchive.h
index b4cf16082..3b8e1575a 100644
--- a/include/unarchive.h
+++ b/include/unarchive.h
@@ -7,16 +7,21 @@ PUSH_AND_SET_FUNCTION_VISIBILITY_TO_HIDDEN
 enum {
 #if BB_BIG_ENDIAN
         COMPRESS_MAGIC = 0x1f9d,
-        GZIP_MAGIC = 0x1f8b,
-        BZIP2_MAGIC = ('B'<<8) + 'Z',
-        XZ_MAGIC1 = (0xfd<<8) + '7',
-        XZ_MAGIC2 = ((((('z'<<8) + 'X')<<8) + 'Z')<<8) + 0,
+        GZIP_MAGIC  = 0x1f8b,
+        BZIP2_MAGIC = 'B' * 256 + 'Z',
+        XZ_MAGIC1   = 0xfd * 256 + '7',
+        XZ_MAGIC2   = (('z' * 256 + 'X') * 256 + 'Z') * 256 + 0,
+        /* Different form: 32 bits, then 16 bits: */
+        XZ_MAGIC1a  = ((0xfd * 256 + '7') * 256 + 'z') * 256 + 'X',
+        XZ_MAGIC2a  = 'Z' * 256 + 0,
 #else
         COMPRESS_MAGIC = 0x9d1f,
-        GZIP_MAGIC = 0x8b1f,
-        BZIP2_MAGIC = ('Z'<<8) + 'B',
-        XZ_MAGIC1 = ('7'<<8) + 0xfd,
-        XZ_MAGIC2 = (((((0<<8) + 'Z')<<8) + 'X')<<8) + 'z',
+        GZIP_MAGIC  = 0x8b1f,
+        BZIP2_MAGIC = 'Z' * 256 + 'B',
+        XZ_MAGIC1   = '7' * 256 + 0xfd,
+        XZ_MAGIC2   = ((0 * 256 + 'Z') * 256 + 'X') * 256 + 'z',
+        XZ_MAGIC1a  = (('X' * 256 + 'z') * 256 + '7') * 256 + 0xfd,
+        XZ_MAGIC2a  = 0 * 256 + 'Z',
 #endif
 };
 
@@ -196,7 +201,7 @@ typedef struct inflate_unzip_result {
 } inflate_unzip_result;
 
 IF_DESKTOP(long long) int inflate_unzip(inflate_unzip_result *res, off_t compr_size, int src_fd, int dst_fd) FAST_FUNC;
-/* xz unpacker takes .xz stream from offset 0 */
+/* xz unpacker takes .xz stream from offset 6 */
 IF_DESKTOP(long long) int unpack_xz_stream(int src_fd, int dst_fd) FAST_FUNC;
 /* lzma unpacker takes .lzma stream from offset 0 */
 IF_DESKTOP(long long) int unpack_lzma_stream(int src_fd, int dst_fd) FAST_FUNC;