[PATCH] Don't use floor() in _bitset_with_random_bits

[Przemyslaw Iskra <sparky@pld-linux.org>]

Use _even_rand() function instead of floor() in
_bitset_with_random_bits(). floor() function is missing in dietlibc (on
architectures other than x86). Moreover using floor() to clip rand
results does not assure even result distribution.
_even_rand() uses integer arithmetic only and is designed to return evenly
distributed results.

Signed-off-by: Przemyslaw Iskra <sparky@pld-linux.org>
---
 configure.in            |    3 +--
 lib/metadata/metadata.c |   16 +++++++++++++++-
 2 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/configure.in b/configure.in
index bd56136..6f6c67e 100644
--- a/configure.in
+++ b/configure.in
@@ -125,8 +125,7 @@ AC_STRUCT_TM
 
 ################################################################################
 dnl -- Check for functions
-AC_SEARCH_LIBS([floor], [m], , [AC_MSG_ERROR(bailing out)])
-AC_CHECK_FUNCS([floor ftruncate gethostname getpagesize \
+AC_CHECK_FUNCS([ftruncate gethostname getpagesize \
   gettimeofday memset mkdir mkfifo rmdir munmap nl_langinfo setenv setlocale \
   strcasecmp strchr strcspn strspn strdup strncasecmp strerror strrchr \
   strstr strtol strtoul uname], , [AC_MSG_ERROR(bailing out)])
diff --git a/lib/metadata/metadata.c b/lib/metadata/metadata.c
index 07222a7..6ee7731 100644
--- a/lib/metadata/metadata.c
+++ b/lib/metadata/metadata.c
@@ -1018,6 +1018,20 @@ static int _recalc_extents(uint32_t *extents, const char *desc1,
 	return 1;
 }
 
+/* return random integer in [0,max) interval */
+static unsigned _even_rand( unsigned *seed, unsigned max )
+{
+	unsigned r, ret;
+
+	/* make sure distribution is even */
+	do {
+		r = (unsigned) rand_r( seed );
+		ret = r % max;
+	} while ( r - ret >= RAND_MAX - max );
+
+	return ret;
+}
+
 static dm_bitset_t _bitset_with_random_bits(struct dm_pool *mem, uint32_t num_bits,
 					    uint32_t num_set_bits, unsigned *seed)
 {
@@ -1040,7 +1054,7 @@ static dm_bitset_t _bitset_with_random_bits(struct dm_pool *mem, uint32_t num_bi
 	/* Perform loop num_set_bits times, selecting one bit each time */
 	while (i++ < num_bits) {
 		/* Select a random bit between 0 and (i-1) inclusive. */
-		bit_selected = (unsigned) floor(i * (rand_r(seed) / (RAND_MAX + 1.0)));
+		bit_selected = _even_rand( seed, i );
 
 		/*
 		 * If the bit was already set, set the new bit that became
-- 
1.7.1



-- 
 ____  Sparky{PI] -- Przemyslaw _  ___  _  _  ......... LANG...Pl,Ca,Es,En
/____) ___  ___  _ _ || Iskra  |  | _ \| |  | : WWW...ppcrcd.pld-linux.org
\____\| -_)'___| ||^'||//\\// <   |  _/| |  | : WWW2..............rsget.pl
(____/||   (_-_|_||  ||\\ ||   |_ |_|  |_| _| : Mail..sparky@pld-linux.org