[PATCH 1/8] Add virRandom() API to generate numbers with non-power-of-2 limit

["Daniel P. Berrange" <berrange@xxxxxxxxxx>]

From: "Daniel P. Berrange" <berrange@xxxxxxxxxx>

The current virRandomBits() API is only usable if the caller wants
a random number in the range [0, (n-1)] where n is a power of two.
This adds a virRandom() API which works for upper limits which are
not a power of two. It works by using virRandomBits() to generate
a number to the next nearest power of 2 limit, and then scales it
down.

Signed-off-by: Daniel P. Berrange <berrange@xxxxxxxxxx>
---
 src/libvirt_private.syms |  1 +
 src/util/virrandom.c     | 34 ++++++++++++++++++++++++++++++++++
 src/util/virrandom.h     |  1 +
 3 files changed, 36 insertions(+)

diff --git a/src/libvirt_private.syms b/src/libvirt_private.syms
index c023dbf..d406785 100644
--- a/src/libvirt_private.syms
+++ b/src/libvirt_private.syms
@@ -1640,6 +1640,7 @@ virPidFileDeletePath;
 
 
 # virrandom.h
+virRandom;
 virRandomBits;
 virRandomGenerateWWN;
 
diff --git a/src/util/virrandom.c b/src/util/virrandom.c
index 50bed46..260cc82 100644
--- a/src/util/virrandom.c
+++ b/src/util/virrandom.c
@@ -108,6 +108,40 @@ uint64_t virRandomBits(int nbits)
     return ret;
 }
 
+
+/**
+ * virRandom:
+ * @max: Upper bound on random number (not inclusive)
+ *
+ * Generate an evenly distributed random number between [0,max-1]
+ * If @max is a power of 2, then use virRandomBits instead
+ *
+ * Return: a random number with @nbits entropy
+ */
+uint64_t virRandom(unsigned long long max)
+{
+    int bits = 0;
+    unsigned long long tmp = max - 1;
+    uint64_t ret;
+
+    while (tmp) {
+        tmp >>= 1;
+        bits++;
+    }
+
+    ret = virRandomBits(bits);
+
+    if ((1 << bits) != max) {
+        double d = ret;
+        d *= max;
+        d /= (1 << bits);
+        ret = (uint64_t)d;
+    }
+
+    return ret;
+}
+
+
 #define QUMRANET_OUI "001a4a"
 #define VMWARE_OUI "000569"
 #define MICROSOFT_OUI "0050f2"
diff --git a/src/util/virrandom.h b/src/util/virrandom.h
index 29a055d..16d9fc7 100644
--- a/src/util/virrandom.h
+++ b/src/util/virrandom.h
@@ -25,6 +25,7 @@
 # include "internal.h"
 
 uint64_t virRandomBits(int nbits);
+uint64_t virRandom(unsigned long long max);
 int virRandomGenerateWWN(char **wwn, const char *virt_type);
 
 #endif /* __VIR_RANDOM_H__ */
-- 
1.7.11.2

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