[sensors] [mac] Implement ambient light sensor for macOS

device/sensors/public/cpp/device_util_mac.cc

Index: device/sensors/public/cpp/device_util_mac.cc
diff --git a/device/sensors/public/cpp/device_util_mac.cc b/device/sensors/public/cpp/device_util_mac.cc
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index 0000000000000000000000000000000000000000..91325ef0e1fbbaf445c35a76258e779b50d206ae
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+++ b/device/sensors/public/cpp/device_util_mac.cc
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+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "device/sensors/public/cpp/device_util_mac.h"
+
+#include <math.h>
+
+namespace device {
+
+double LMUvalueToLux(uint64_t raw_value) {
+  // Conversion formula from regression.
+  // https://bugzilla.mozilla.org/show_bug.cgi?id=793728
+  // Let x = raw_value, then
+  // lux = -2.978303814*(10^-27)*x^4 + 2.635687683*(10^-19)*x^3 -
+  //       3.459747434*(10^-12)*x^2 + 3.905829689*(10^-5)*x - 0.1932594532
+
+  static const long double k4 = pow(10.L, -7);
+  static const long double k3 = pow(10.L, -4);
+  static const long double k2 = pow(10.L, -2);
+  static const long double k1 = pow(10.L, 5);
+  long double scaled_value = raw_value / k1;
+
+  long double lux_value =
+      (-3 * k4 * pow(scaled_value, 4)) + (2.6 * k3 * pow(scaled_value, 3)) +
+      (-3.4 * k2 * pow(scaled_value, 2)) + (3.9 * scaled_value) - 0.19;
+
+  double lux = ceil(static_cast<double>(lux_value));
+  return lux > 0 ? lux : 0;
+}
+
+}  // namespace device