utils: improve gst_util_ceil_log2

According to the following comparison of algorithms, the value
for 0 and 1 was giving an incorrect result.

https://gist.github.com/ceyusa/6061b33ac109a68bcd222f6919968c9a

More information here:
https://github.com/rofrol/codeforces/blob/master/ceil_log2.c

Use a different algorithm which offers better result and keep the
performance.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7429>

girs/Gst-1.0.gir

@@ -57259,7 +57259,7 @@ element or %NULL if nothing was found</doc>
       </parameters>
     </function>
     <function name="util_ceil_log2" c:identifier="gst_util_ceil_log2" version="1.24">
-      <doc xml:space="preserve" filename="../subprojects/gstreamer/gst/gstutils.c">Return a max num of log2.</doc>
+      <doc xml:space="preserve" filename="../subprojects/gstreamer/gst/gstutils.c">Returns smallest integral value not less than log2(v).</doc>
       <source-position filename="../subprojects/gstreamer/gst/gstutils.h"/>
       <return-value transfer-ownership="none">
         <doc xml:space="preserve" filename="../subprojects/gstreamer/gst/gstutils.c">a computed #guint val.</doc>

subprojects/gstreamer/gst/gstutils.c

@@ -4484,7 +4484,7 @@ gst_bit_storage_uint64 (guint64 in)
  * gst_util_ceil_log2:
  * @v: a #guint32 value.
  *
- * Return a max num of log2.
+ * Returns smallest integral value not less than log2(v).
  *
  * Returns: a computed #guint val.
  *
@@ -4493,25 +4493,29 @@ gst_bit_storage_uint64 (guint64 in)
 guint
 gst_util_ceil_log2 (guint32 v)
 {
-  /* Compute Ceil(Log2(v)) */
+  static const unsigned int t[6] = {
-  /* Derived from branchless code for integer log2(v) from:
+    0x00000000ull,
-     <http://graphics.stanford.edu/~seander/bithacks.html#IntegerLog> */
+    0xFFFF0000ull,
-  guint r, shift;
+    0x0000FF00ull,
+    0x000000F0ull,
+    0x0000000Cull,
+    0x000000002ull
+  };
 
-  v--;
+  g_return_val_if_fail (v != 0, -1);
-  r = (v > 0xFFFF) << 4;
+
-  v >>= r;
+  int y = (((v & (v - 1)) == 0) ? 0 : 1);
-  shift = (v > 0xFF) << 3;
+  int j = 32;
-  v >>= shift;
+  int i;
-  r |= shift;
+
-  shift = (v > 0xF) << 2;
+  for (i = 0; i < 6; i++) {
-  v >>= shift;
+    int k = (((v & t[i]) == 0) ? 0 : j);
-  r |= shift;
+    y += k;
-  shift = (v > 0x3) << 1;
+    v >>= k;
-  v >>= shift;
+    j >>= 1;
-  r |= shift;
+  }
-  r |= (v >> 1);
+
-  return r + 1;
+  return y;
 }
 
 /**

subprojects/gstreamer/tests/check/gst/gstutils.c

@@ -1990,6 +1990,21 @@ GST_START_TEST (test_regression)
 
 GST_END_TEST;
 
+// 10 Values
+static const int ceil_log2_values[] = {
+  -1, 0, 1, 2, 2, 3, 3, 3, 3, 4
+};
+
+GST_START_TEST (test_ceil_log2)
+{
+  int i;
+  for (i = 1; i < 10; i++) {
+    fail_unless_equals_int (gst_util_ceil_log2 (i), ceil_log2_values[i]);
+  }
+}
+
+GST_END_TEST;
+
 GST_START_TEST (test_mark_as_plugin_api)
 {
   GstPluginAPIFlags api_flags;
@@ -2053,6 +2068,8 @@ gst_utils_suite (void)
 
   tcase_add_test (tc_chain, test_mark_as_plugin_api);
 
+  tcase_add_test (tc_chain, test_ceil_log2);
+
   return s;
 }