[PATCH 07/16] compat: add endinanness helpers

[Vicent Marti <tanoku@gmail.com>]

The POSIX standard doesn't currently define a `nothll`/`htonll`
function pair to perform network-to-host and host-to-network
swaps of 64-bit data. These 64-bit swaps are necessary for the on-disk
storage of EWAH bitmaps if they are not in native byte order.
---
 git-compat-util.h |   28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/git-compat-util.h b/git-compat-util.h
index ff193f4..bc9b591 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -710,4 +710,32 @@ void warn_on_inaccessible(const char *path);
 /* Get the passwd entry for the UID of the current process. */
 struct passwd *xgetpwuid_self(void);
 
+#include <endian.h>
+
+#ifndef __BYTE_ORDER
+# if defined(BYTE_ORDER) && defined(LITTLE_ENDIAN) && defined(BIG_ENDIAN)
+#  define __BYTE_ORDER BYTE_ORDER
+#  define __LITTLE_ENDIAN LITTLE_ENDIAN
+#  define __BIG_ENDIAN BIG_ENDIAN
+# else
+#  error "Cannot determine endianness"
+# endif
+#endif
+
+#if __BYTE_ORDER == __BIG_ENDIAN
+# define ntohll(n) (n)
+# define htonll(n) (n)
+#elif __BYTE_ORDER == __LITTLE_ENDIAN
+# if defined(__GNUC__) && defined(__GLIBC__)
+#  include <byteswap.h>
+#  define ntohll(n) bswap_64(n)
+#  define htonll(n) bswap_64(n)
+# else /* GNUC & GLIBC */
+#  define ntohll(n) ( (((unsigned long long)ntohl(n)) << 32) + ntohl(n >> 32) )
+#  define htonll(n) ( (((unsigned long long)htonl(n)) << 32) + htonl(n >> 32) )
+# endif /* GNUC & GLIBC */
+#else /* __BYTE_ORDER */
+# error "Can't define htonll or ntohll!"
+#endif
+
 #endif
-- 
1.7.9.5