Fix undefined reference to log2 on android

webrtc/modules/video_coding/utility/quality_scaler.cc

 /*
  *  Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #include "webrtc/modules/video_coding/utility/quality_scaler.h"
 
+#include <math.h>

[nisse-webrtc, 2016/09/15 08:22:12]

Why switching from cmath to math.h? Just to drop the std:: prefix?

[kthelgason, 2016/09/15 08:25:15]

Yep.

+
 #include <algorithm>
-#include <cmath>
+
+// Android does not support log2

[nisse-webrtc, 2016/09/15 08:22:12]

[citation needed]

Is there any documentation or bug report about this? 

Both my local man-page and http://en.cppreference.com/w/c/numeric/math/log2
claims that log2 is part of c99, which ought to be supported by Android.

Wild guess, but have you checked if only changing to math.h solves the problem?
I'm thinking, maybe c99 log2 works, but c++ std::log2 doesn't? They might
correspond to different symbols when linking.

[kthelgason, 2016/09/15 08:25:15]

I found some references to this on the internet, including:
https://code.google.com/p/android/issues/detail?id=212634
I had the same hunch, unfortunately that didn't work when I tried it.

+#if defined(WEBRTC_ANDROID)
+#define log2(x) (log(x) / log(2))
+#endif
 
 namespace webrtc {
 
 namespace {
 // Threshold constant used until first downscale (to permit fast rampup).
 static const int kMeasureSecondsFastUpscale = 2;
 static const int kMeasureSecondsUpscale = 5;
 static const int kMeasureSecondsDownscale = 5;
 static const int kFramedropPercentThreshold = 60;
 // Min width/height to downscale to, set to not go below QVGA, but with some
@@ skipping 150 matching lines @@
   scaled_buffer->ScaleFrom(frame);
 
   return scaled_buffer;
 }
 
 void QualityScaler::UpdateTargetResolution(int width, int height) {
   if (width < kMinDownscaleDimension || height < kMinDownscaleDimension) {
     maximum_shift_ = 0;
   } else {
     maximum_shift_ = static_cast<int>(
-        std::log2(std::min(width, height) / kMinDownscaleDimension));
+        log2(std::min(width, height) / kMinDownscaleDimension));
   }
   target_res_ = Resolution{width, height};
 }
 
 void QualityScaler::ClearSamples() {
   framedrop_percent_.Reset();
   average_qp_.Reset();
 }
 
 
 }  // namespace webrtc