[Sensors] Improvements in shared buffer managing

device/generic_sensor/public/cpp/sensor_reading.h

+// 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.
+
+#ifndef DEVICE_GENERIC_SENSOR_PUBLIC_CPP_SENSOR_READING_H_
+#define DEVICE_GENERIC_SENSOR_PUBLIC_CPP_SENSOR_READING_H_
+
+#include "device/base/synchronization/one_writer_seqlock.h"
+#include "device/generic_sensor/public/interfaces/sensor.mojom.h"
+
+namespace device {
+
+// This class is guarantied to have a fixed size of 64 bits on every platform.
+// It is introduce to simplify sensors shared buffer memory calculation.
+template <typename Data>
+class SensorReadingField {
+ public:
+  static_assert(sizeof(Data) <= sizeof(int64_t),
+                "The field size must be <= 64 bits.");
+  SensorReadingField() = default;
+  SensorReadingField(Data value) { storage_.value = value; }
+  SensorReadingField& operator=(Data value) {
+    storage_.value = value;
+    return *this;
+  }
+  Data& value() { return storage_.value; }
+  operator Data() const { return storage_.value; }
+
+ private:
+  union Storage {
+    int64_t unused;
+    Data value;
+    Storage() { new (&value) Data(); }
+    ~Storage() { value.~Data(); }
+  };
+  Storage storage_;
+};
+
+// This structure represents sensor reading data: timestamp and 3 values.
+struct SensorReading {
+  SensorReading();
+  ~SensorReading();
+  SensorReading(const SensorReading& other);
+  SensorReadingField<double> timestamp;
+  SensorReadingField<double> values[3];
+};
+
+// This structure represents sensor reading buffer: sensor reading and seqlock
+// for synchronization.
+struct SensorReadingSharedBuffer {
+  SensorReadingSharedBuffer();
+  ~SensorReadingSharedBuffer();
+  SensorReadingField<OneWriterSeqLock> seqlock;
+  SensorReading reading;
+
+  // Gets the shared reading buffer offset for the given sensor type.
+  static uint64_t GetOffset(mojom::SensorType type);
+};
+
+}  // namespace device
+
+#endif  // DEVICE_GENERIC_SENSOR_PUBLIC_CPP_SENSOR_READING_H_