*: 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;
 		}
 	}