[Libguestfs] [PATCH nbdkit 3/5] common/include: Add next_power_of_2 function

[Richard W.M. Jones]

It takes a 64 bit integer and finds the next power of 2,
eg. next_power_of_2 (510) => 512 (2^9)

Taken from https://jameshfisher.com/2018/03/30/round-up-power-2/
with some fixes.
---
 common/include/ispowerof2.h      |  9 +++++++++
 common/include/test-ispowerof2.c | 14 ++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/common/include/ispowerof2.h b/common/include/ispowerof2.h
index f067caf07..57ebcc4fd 100644
--- a/common/include/ispowerof2.h
+++ b/common/include/ispowerof2.h
@@ -59,4 +59,13 @@ log_2_bits (unsigned long v)
   return SIZEOF_LONG*8 - __builtin_clzl (v) - 1;
 }
 
+/* Round up to next power of 2.
+ * https://jameshfisher.com/2018/03/30/round-up-power-2/
+ */
+static inline uint64_t
+next_power_of_2 (uint64_t x)
+{
+  return x == 1 ? 1 : UINT64_C(1) << (64 - __builtin_clzll (x-1));
+}
+
 #endif /* NBDKIT_ISPOWEROF2_H */
diff --git a/common/include/test-ispowerof2.c b/common/include/test-ispowerof2.c
index 9620192f0..fe37c4a32 100644
--- a/common/include/test-ispowerof2.c
+++ b/common/include/test-ispowerof2.c
@@ -68,5 +68,19 @@ main (void)
   assert (log_2_bits (0x8000000000000000) == 63);
 #endif
 
+  /* Test next power of 2. */
+  assert (next_power_of_2 (1) == 1);
+  assert (next_power_of_2 (3) == 4);
+  assert (next_power_of_2 (8) == 8);
+  assert (next_power_of_2 (9) == 16);
+  assert (next_power_of_2 (0xffff) == 0x10000);
+  assert (next_power_of_2 (0x10000) == 0x10000);
+  assert (next_power_of_2 (UINT64_C ( 0xffffffff)) == 0x100000000);
+  assert (next_power_of_2 (UINT64_C (0x100000000)) == 0x100000000);
+  assert (next_power_of_2 (UINT64_C (0x200000001)) == 0x400000000);
+  assert (next_power_of_2 (UINT64_C (0x6ffffffff)) == 0x800000000);
+  assert (next_power_of_2 (UINT64_C (0x700000001)) == 0x800000000);
+  assert (next_power_of_2 (UINT64_C (0x800000000)) == 0x800000000);
+
   exit (EXIT_SUCCESS);
 }
-- 
2.39.2