Add HAVE_LOG2 build macro (#783)

* Add HAVE_LOG2 build macro

Fixes #781

* Rename macro to BROTLI_HAVE_LOG2 and move comment for visibility

2 files changed, 15 insertions, 5 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index a8ea872..2520278 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -110,13 +110,16 @@ if(NOT LOG2_RES)
   CHECK_FUNCTION_EXISTS(log2 LOG2_LIBM_RES)
   if(LOG2_LIBM_RES)
     set(LIBM_LIBRARY "m")
+    add_definitions(-DBROTLI_HAVE_LOG2=1)
   else()
-    message(FATAL_ERROR "log2() not found")
+    add_definitions(-DBROTLI_HAVE_LOG2=0)
   endif()
 
   set(CMAKE_REQUIRED_LIBRARIES "${orig_req_libs}")
   unset(LOG2_LIBM_RES)
   unset(orig_req_libs)
+else()
+  add_definitions(-DBROTLI_HAVE_LOG2=1)
 endif()
 unset(LOG2_RES)
 
diff --git a/c/enc/fast_log.h b/c/enc/fast_log.h
index cade123..b1268e0 100644
--- a/c/enc/fast_log.h
+++ b/c/enc/fast_log.h
@@ -123,6 +123,16 @@ static const float kLog2Table[] = {
   7.9943534368588578f
 };
 
+/* Visual Studio 2012 and Android API levels < 18 do not have the log2()
+ * function defined, so we use log() and a multiplication instead. */
+#ifndef BROTLI_HAVE_LOG2
+#if ((defined(_MSC_VER) && _MSC_VER <= 1700) || (defined(__ANDROID_API__) && __ANDROID_API__ < 18))
+#define BROTLI_HAVE_LOG2 0
+#else
+#define BROTLI_HAVE_LOG2 1
+#endif
+#endif
+
 #define LOG_2_INV 1.4426950408889634
 
 /* Faster logarithm for small integers, with the property of log2(0) == 0. */
@@ -130,10 +140,7 @@ static BROTLI_INLINE double FastLog2(size_t v) {
   if (v < sizeof(kLog2Table) / sizeof(kLog2Table[0])) {
     return kLog2Table[v];
   }
-#if (defined(_MSC_VER) && _MSC_VER <= 1700) || \
-    (defined(__ANDROID_API__) && __ANDROID_API__ < 18)
-  /* Visual Studio 2012 and Android API levels < 18 do not have the log2()
-   * function defined, so we use log() and a multiplication instead. */
+#if !(BROTLI_HAVE_LOG2)
   return log((double)v) * LOG_2_INV;
 #else
   return log2((double)v);