From: Christian Brauner <christian.brauner@ubuntu.com>
Date: Wed, 18 Oct 2017 19:20:37 +0000 (+0200)
Subject: utils: add lxc_find_next_power2()
X-Git-Tag: lxc-2.0.10~615
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=6b38945f9847ff2aad1a8b37b59d923cbc61f3eb;p=thirdparty%2Flxc.git

utils: add lxc_find_next_power2()

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---

diff --git a/src/lxc/utils.c b/src/lxc/utils.c
index 887afdd03..bb3ac4e9b 100644
--- a/src/lxc/utils.c
+++ b/src/lxc/utils.c
@@ -2515,3 +2515,21 @@ int parse_byte_size_string(const char *s, int64_t *converted)
 	*converted = overflow;
 	return 0;
 }
+
+uint64_t lxc_find_next_power2(uint64_t n)
+{
+	/* 0 is not valid input. We return 0 to the caller since 0 is not a
+	 * valid power of two.
+	 */
+	if (n == 0)
+		return 0;
+
+	if (!(n & (n - 1)))
+		return n;
+
+	while (n & (n - 1))
+		n = n & (n - 1);
+
+	n = n << 1;
+	return n;
+}
diff --git a/src/lxc/utils.h b/src/lxc/utils.h
index 62c2d3c72..c96ce8ae6 100644
--- a/src/lxc/utils.h
+++ b/src/lxc/utils.h
@@ -462,4 +462,14 @@ extern bool lxc_nic_exists(char *nic);
 extern int lxc_make_tmpfile(char *template, bool rm);
 extern uint64_t lxc_getpagesize(void);
 
+/* If n is not a power of 2 this function will return the next power of 2
+ * greater than that number. Note that this function always returns the *next*
+ * power of 2 *greater* that number not the *nearest*. For example, passing 1025
+ * as argument this function will return 2048 although the closest power of 2
+ * would be 1024.
+ * If the caller passes in 0 they will receive 0 in return since this is invalid
+ * input and 0 is not a power of 2.
+ */
+extern uint64_t lxc_find_next_power2(uint64_t n);
+
 #endif /* __LXC_UTILS_H */