[core] Moved util::log2 to its own header

- Added util::{MIN,MAX}_ZOOM_F to avoid consecutive conversions from
double to float
- Move util::log2 to its own header (part of mbgl/math)

1 files changed, 25 insertions, 0 deletions

diff --git a/src/mbgl/math/log2.cpp b/src/mbgl/math/log2.cpp
new file mode 100644
index 0000000000..222e67dbd7
--- /dev/null
+++ b/src/mbgl/math/log2.cpp
@@ -0,0 +1,25 @@
+#include <mbgl/math/log2.hpp>
+
+namespace mbgl {
+namespace util {
+
+// From http://stackoverflow.com/questions/3272424/compute-fast-log-base-2-ceiling
+uint32_t ceil_log2(uint64_t x) {
+    static const uint64_t t[6] = {0xFFFFFFFF00000000, 0x00000000FFFF0000,
+                                  0x000000000000FF00, 0x00000000000000F0,
+                                  0x000000000000000C, 0x0000000000000002};
+    uint32_t y = (((x & (x - 1)) == 0) ? 0 : 1);
+    uint32_t j = 32;
+
+    for (const auto& i : t) {
+        const uint32_t k = (((x & i) == 0) ? 0 : j);
+        y += k;
+        x >>= k;
+        j >>= 1;
+    }
+
+    return y;
+}
+
+} // namespace util
+} // namespace mbgl