*: simplify checks for gz/bz2/xz magic

function                                             old     new   delta
send_tree                                            360     355      -5
rpm2cpio_main                                        249     220     -29
setup_unzip_on_fd                                    150     118     -32
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-66)             Total: -66 bytes

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

3 files changed, 41 insertions, 15 deletions

diff --git a/archival/rpm2cpio.c b/archival/rpm2cpio.c
index 598ec8670..1c67dcc6e 100644
--- a/archival/rpm2cpio.c
+++ b/archival/rpm2cpio.c
@@ -68,26 +68,31 @@ int rpm2cpio_main(int argc UNUSED_PARAM, char **argv)
 #else
         /* BLOAT */
         {
-                unsigned char magic[8];
+                union {
+                        uint8_t b[4];
+                        uint16_t b16[2];
+                        uint32_t b32[1];
+                } magic;
                 IF_DESKTOP(long long) int FAST_FUNC (*unpack)(int src_fd, int dst_fd);
 
-                xread(rpm_fd, &magic, 2);
-                if (magic[0] == 0x1f && magic[1] == 0x8b) {
+                xread(rpm_fd, magic.b16, sizeof(magic.b16));
+                if (magic.b16[0] == GZIP_MAGIC) {
                         unpack = unpack_gz_stream;
                 } else
                 if (ENABLE_FEATURE_SEAMLESS_BZ2
-                 && magic[0] == 'B' && magic[1] == 'Z'
+                 && magic.b16[0] == BZIP2_MAGIC
                 ) {
                         unpack = unpack_bz2_stream;
                 } else
                 if (ENABLE_FEATURE_SEAMLESS_XZ
-                 && magic[0] == 0xfd && magic[1] == '7'
+                 && magic.b16[0] == XZ_MAGIC1
                 ) {
                         /* .xz signature: 0xfd, '7', 'z', 'X', 'Z', 0x00 */
                         /* More info at: http://tukaani.org/xz/xz-file-format.txt */
-                        xread(rpm_fd, magic + 2, 4);
-                        if (strcmp((char*)magic + 2, "zXZ") != 0)
+                        xread(rpm_fd, magic.b32, sizeof(magic.b32));
+                        if (magic.b32[0] != XZ_MAGIC2)
                                 goto no_magic;
+                        /* unpack_xz_stream wants fd at position 0 */
                         xlseek(rpm_fd, -6, SEEK_CUR);
                         unpack = unpack_xz_stream;
                 } else {
diff --git a/include/unarchive.h b/include/unarchive.h
index aa7ecec55..8009de282 100644
--- a/include/unarchive.h
+++ b/include/unarchive.h
@@ -4,6 +4,22 @@
 
 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,
+#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',
+#endif
+};
+
 typedef struct file_header_t {
         char *name;
         char *link_target;
diff --git a/libbb/read.c b/libbb/read.c
index cd6bbeb13..b1eb3f24b 100644
--- a/libbb/read.c
+++ b/libbb/read.c
@@ -312,7 +312,11 @@ void* FAST_FUNC xmalloc_xopen_read_close(const char *filename, size_t *maxsz_p)
 void FAST_FUNC setup_unzip_on_fd(int fd /*, int fail_if_not_detected*/)
 {
         const int fail_if_not_detected = 1;
-        unsigned char magic[8];
+        union {
+                uint8_t b[4];
+                uint16_t b16[2];
+                uint32_t b32[1];
+        } magic;
         int offset = -2;
 # if BB_MMU
         IF_DESKTOP(long long) int FAST_FUNC (*xformer)(int src_fd, int dst_fd);
@@ -324,9 +328,9 @@ void FAST_FUNC setup_unzip_on_fd(int fd /*, int fail_if_not_detected*/)
 
         /* .gz and .bz2 both have 2-byte signature, and their
          * unpack_XXX_stream wants this header skipped. */
-        xread(fd, magic, 2);
+        xread(fd, magic.b16, sizeof(magic.b16));
         if (ENABLE_FEATURE_SEAMLESS_GZ
-         && magic[0] == 0x1f && magic[1] == 0x8b
+         && magic.b16[0] == GZIP_MAGIC
         ) {
 # if BB_MMU
                 xformer = unpack_gz_stream;
@@ -336,7 +340,7 @@ void FAST_FUNC setup_unzip_on_fd(int fd /*, int fail_if_not_detected*/)
                 goto found_magic;
         }
         if (ENABLE_FEATURE_SEAMLESS_BZ2
-         && magic[0] == 'B' && magic[1] == 'Z'
+         && magic.b16[0] == BZIP2_MAGIC
         ) {
 # if BB_MMU
                 xformer = unpack_bz2_stream;
@@ -346,19 +350,20 @@ void FAST_FUNC setup_unzip_on_fd(int fd /*, int fail_if_not_detected*/)
                 goto found_magic;
         }
         if (ENABLE_FEATURE_SEAMLESS_XZ
-         && magic[0] == 0xfd && magic[1] == '7'
+         && magic.b16[0] == XZ_MAGIC1
         ) {
                 /* .xz signature: 0xfd, '7', 'z', 'X', 'Z', 0x00 */
                 /* More info at: http://tukaani.org/xz/xz-file-format.txt */
                 offset = -6;
-                xread(fd, magic + 2, 4);
-                if (strcmp((char*)magic + 2, "zXZ") == 0) {
+                xread(fd, magic.b32, sizeof(magic.b32));
+                if (magic.b32[0] == XZ_MAGIC2) {
 # if BB_MMU
                         xformer = unpack_xz_stream;
+                        /* unpack_xz_stream wants fd at position 0 */
+                        xlseek(fd, offset, SEEK_CUR);
 # else
                         xformer_prog = "unxz";
 # endif
-                        xlseek(fd, offset, SEEK_CUR);
                         goto found_magic;
                 }
         }